The Farthest Shore – Chapter Five The Universe and Us
From The Space Library
Chapter Five The Universe and Us
Charles Cockell, Giovanni Fazio, Lauren Fletcher, James Green, Mikhail Marov, Chris McKay, Michael Rycroft and Isabelle Scholl
“We shall not cease from exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time.”
--T. S. Eliot, from “Four Quartets”, Little Gidding, part 5, 1942
5.1 Space Sciences
Space science embodies an incredibly broad range of exciting topics, which we now address. Beginning with a discussion of our Universe, we will then explore our solar system, our home planet Earth and the Moon.
5.1.1 The Universe - Where Are We?
We live on planet Earth, one of eight planets orbiting around a star called the Sun. Our Earth, 6371 km in radius, is located at a mean distance of 149.7 million kilometers (1.497 x 108km) from the Sun, whose mass is a third of a million times greater than the Earth’s. Distances in the Universe are so large that we really need more suitable units than kilometers to measure them. The mean distance of the Earth from the Sun, called one Astronomical Unit (AU), is a convenient unit for measuring distances in the solar system. Another important unit of distance is the light year (ly), which is the distance traveled by light in one year, 9.46 x 1012(almost 1013 km. It takes light 8 minutes to travel from the Sun to the Earth; so, when we look at the Sun we are actually seeing it as it was 8 minutes ago.
After Mercury and Venus, Earth is the third planet from the Sun. Beyond the Earth’s orbit are the planets Mars (at 1.5 AU), the giant, gaseous planets Jupiter (at 5.2 AU) and Saturn (9.5 AU), and the giant icy planets Uranus (19.2 AU) and Neptune (30.0 AU). Thus, Neptune is 4.2 light hours from the Sun. Beyond the planet Neptune, at 30 AU to approximately 55 AU from the Sun, there is a band of small objects in orbit around the Sun, called the Kuiper Belt; these are a remnant of solar system formation. It is estimated that this belt contains as many as 70,000 bodies greater than 100 km is size. The dwarf planet Pluto, ~ 40 AU from the Sun, is now classified as a Kuiper Belt object (KBO). Outside the Kuiper Belt is a spherical shell of billions of icy objects called the Oort Cloud, beginning at about 2000 AU from the Sun. The outer part of the Oort Cloud, the boundary of the solar system, is about 1 ly from the Sun and about a quarter of the distance to the next star, Proxima Centauri. Proxima Centauri is a triple star system 4.2 ly away. Traveling to Proxima Centauri in a rocket at just above the escape velocity from the Earth (12 km/s) would take 100,000 years. The distance between stars in the vicinity of the Sun is vast - the average distance between stars compared to the diameter of the Sun is a factor of about 250 million. Within 20 ly from the Sun we know of 150 stars and brown dwarfs.
Stars are clustered into vast stellar groupings, or associations (“islands in the Universe”), in the shape of thin disks called galaxies (see Figure 5.1). Our solar system is located in one such galaxy, which can be seen in the night sky as a band of light called the Milky Way. There are ~ 300 billion stars in our Milky Way galaxy which is about 100,000 ly across. Located about 26,000 ly from its center, our solar system rotates once around the center every 220 million years, at a speed of 220 km/s. Galaxies are usually found in groups, our galaxy being one of more than 54 in the Local Group which is about a million ly in diameter. Other members of the group include the Andromeda galaxy, which is the most dominant, and the Large and Small Magellanic Clouds, which are dwarf galaxies.
As we look farther out into the Universe, we notice that groups of galaxies form into clusters, which may contain as many as 2000 members. These clusters can be as large as 10 million ly in diameter and have a mass of a million billion Suns (1015M sun). Nearby clusters include the Virgo Cluster, the Hercules Cluster and the Coma Cluster. Clusters can form into an even larger structure called a Supercluster more than a hundred million ly across. On even larger scales we see structure that looks like a large bottle full of soap bubbles, with the galaxies being confined to the surfaces of the bubbles, at distances beyond 1010ly. Insignificant as we humans may be on planet Earth, with our small brains we can contemplate and measure the enormity of the Universe and appreciate the beauty and the complexity of its structure.
5.1.2 The Solar System
Investigating the formation of the solar system is one of the most challenging problems of modern science; it is termed stellar-planetary cosmogony. Based on fundamental theoretical concepts and the variety of observational data available we consider the origin and evolution of stars and planets, trying to find the most plausible scenario.
It is generally accepted that, like other planetary systems, our solar system was born out of the original protoplanetary nebula and gas-dust disk around the forming star. The disk compressed, its density increased, and as gravity instabilities within the disk developed solid grains and dust clusters formed. After numerous collisions occurring over ~ 108years, planet embryos - planetesimals - became planets and other small bodies in the solar system. These processes are illustrated in Figure 5.2. The evident difference in the composition of the inner and outer planets is explained by the condensation of high temperature and low temperature materials from the protoplanetary disk depending on distance from the Sun. This fractionation is responsible for the rocky inner planets and the gaseous-icy outer planets, several of which have ring systems.
Key mechanical and chemical properties of the solar system place important constraints on scenarios of its origin and evolution. All the planets (except Venus and Uranus) and their satellites orbit the Sun in the same prograde (anticlockwise) direction, in the same direction as the Sun rotates around its axis. The orbits are nearly circular and have very small inclina-tions to the ecliptic, the imaginary plane containing the Earth’s circumsolar orbit. The satellites of planets are locked in resonance with the planet’s intrinsic rotation and therefore their same side faces the planet, as is the case for our Moon. Some of the outermost satellites, captured by the planet’s gravity field, have stranger orbits. There is a peculiar mass and angular momentum distribution in the solar system; while the Sun comprises 99.8% of the solar system mass, the planets carry nearly 98% of its angular momentum as a result of the disk’s evolution.
Chemical constraints involve there being similar abundances of chemical elements in the Sun and the most primitive meteorites, which are considered to be formed of the original material and the remnants of planetesimals. There is some evidence that the inner planets were formed of matter resembling that of chondritic meteorites, while the gaseous-icy giant planets preserved their original composition for the most part. The asteroids (between Mars and Jupiter) have a composition intermediate between the silicate/metal-rich inner planets and the volatile-rich outer planets. Comets are mainly composed of water ice and other frozen volatiles, and so these bodies encapsulate the most pristine matter from which the solar system was formed.
(Courtesy of G. Fazio)
Planets form in the general process of stellar origin and evolution, and can be viewed as being by-products of star formation. Planetary formation strongly depends on the mass M of the star and its position on the Hertzsprung-Russell (H-R) diagram (named after the two scientists who discovered the relationship between stellar luminosity and surface temperature or color). For a body to become a star (i.e. for nuclear fusion reactions to occur at the high pressure and temperature in its interior), M has to be 0.08 M sun. Bodies with M < 0.01 M sun are regarded as planets (this threshold is ten times greater than the mass of Jupiter), while bodies of intermediate mass are termed brown dwarfs. Chemical elements heavier than the cosmically most abundant elements of hydrogen and helium were produced in the interiors of the first stars during the course of stellar evolution, a process known as nucleosynthesis. It is only in regions of our galaxy where the first stars made of only hydrogen and helium lived, then died by blowing off their outer shells adding heavy elements to nearby nebula that creates the environment with the right materials to make solar system planets and smaller bodies.
The discoveries of protoplanetary accretion disks and extrasolar planets place some additional constraints on the theory of solar system formation. Numerous accretion disks around young stars, where planets could be born, and more than three thousand extrasolar planets or exoplanets have been discovered (for the latest count of the number of detected exoplanets see http://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html).
Exoplanets come in a large variety of sizes and orbits. At first, most of the exoplanet discoveries were of large Jupiter-sized planets that were close to their parent star because they caused easily measureable wobbles in their star’s motion. Using the transiting technique of planets blocking out small amounts of light from their parent star as they pass across their disk the NASA Kepler satellite has now discovered many different types of planets. Today, the most common type of planet is the super-earth (up to 10 times as massive as the earth and twice as large) and the mini-Neptunes. Most recently, the Trappist-1 system with its seven earth-sized terrestrial planets was found only 39 light-years away. Based on Kepler results, planetary scientists estimate that our Milky Way galaxy has 1.6 planets per star!
In our solar system beyond earth, the planet Mars, Jupiter’s satellite Europa and Saturn’s satellites Enceladus and Titan are thought to be the most probable abodes for primitive life forms. These solar system bodies have organics, energy, and liquids (water for all except Titan which has liquid methane) which is what we believe are the necessary ingredients for life. Could our solar system be unique?
The orbits of planets and satellites around the Sun indicate key elements of celestial mechanics. The fundamental Newtonian law of gravity governs the motions of the planets around the Sun and of the satellites around their planets, whereas Kepler’s three laws define the shapes and regularities of their orbits. Newton’s law states that the gravitational force F is F = G Mm/r2 , where M is the mass of the Sun, m is the mass of a planet, r is the distance between a planet and the Sun, and G = 6.67 x 10-11m3kg-1s-2is the universal constant of gravitation. Kepler’s laws of planetary orbits are as follows (for details, see also Chapter 8):
- A planet’s orbit is an ellipse, with the Sun (our own star) at one focus of the ellipse,
- The radius vector drawn from the Sun to a planet sweeps out equal areas in equal times, and
- The squares of the periodic times of revolution P are proportional to the cubes of the semi-major axes a of their orbits: (P1/P2)2= (a1/a2)3
The main characteristics of the planets and their satellites are summarized in Beatty, et al., 1999. All of the giant planets have rings populated by particles ranging from microns to meters in size, and also some satellites (called “shepherds”) that control the rings’ shapes and dimensions.
Since their origin some 4.55 billion years ago, the inner planets have experienced dramatic changes in the course of their evolution. Their heavy bombardment by asteroid-size bodies in their early epoch, about 4.0 billion years ago, and their internal heat source due to the decay of long-lived radioisotopes played significant roles. Impacts scarred the surface and left behind numerous craters while internal heating was responsible for tectonic processes and widespread volcanism. For the outer giant planets, the heat released from the interior exceeds the incident solar flux, mainly because of their continuing contraction. Planetary geology is closely related to the differentiation of the planetary interiors into shells (core, mantle, and crust). Impact structures are most clearly seen on Mercury and the Moon, which have no atmospheres. Heavily cratered terrains are preserved on the Martian surface though, in the presence of an atmosphere, ancient craters have been eroded by the weathering processes. On Mars, there are great ancient shield volcanoes at heights up to 24 km above the mean surface level, which (despite the relatively small size of the planet) are the tallest in the solar system. Another remarkable geologic feature is Valley Marineris, a valley hundreds of kilometers wide and 8 km deep, that extends along the equator for more than 3000 km (see Figure 5.3). It is poorly associated, however, with the global tectonics. On Venus, geological features were only revealed using the radar technique aboard the Venera 15 and 16 and Magellan missions, because of its very thick atmosphere and clouds which fully obscure the surface when observing in optical part of the spectrum (see Figure 5.4).
The atmospheric properties of our neighboring planets Venus and Mars differ dramatically from that of Earth and from each other. The pressure at the Venusian surface reaches 92 atmospheres and the temperature is 735 K, whereas at the surface of Mars the average pressure is only 0.006-atmosphere and the average temperature is about 220 K. The com-position of the atmospheres of both planets is mostly carbon dioxide, with relatively small admixtures of nitrogen and argon and a tiny mixing ratio of water vapor and oxygen (on Mars). Because the obliquity of Mars is nearly the same as that of Earth (25.19 degrees, compared to 23.45 degrees for Earth), there are pronounced seasonal variations resulting in temperature contrasts between summer and winter exceeding 100 K. The temperature at the winter pole drops to below the freezing point of CO2 and so “dry ice” deposits cover the polar caps, though they are mainly formed of water ice. Seasonal variations and the release of carbon dioxide from the polar regions is one of the important drivers of atmospheric circulation on Mars, involving both meridional and zonal wind patterns. By way of contrast, the atmospheric circulation on Venus is mainly characterized by “super-rotation”, such that the zonal wind speed at the surface of less than 1 m/s increases up to nearly 100 m/s near the upper cloud level at about 60 km altitude. Venusian clouds consist of quite concentrated sulfuric acid droplets, defining (in addition to the very hot and dense atmosphere) a hostile environment on our closest planet, which until recently, was thought to be Earth’s twin.
In terms of its natural environment, Mars is another extreme, though more favorable in its weather and climate and, therefore, much more accessible for exploration and future human expansion throughout the solar system. This planet could harbor life beyond Earth at the microbial level; extant or extinct life could be found. Unlike Venus where an assumed early ocean was lost soon after the “runaway greenhouse effect” responsible for its contemporary, furnace-like conditions developed, ancient Mars had plenty of water until a catastrophic drought happened about 3.5 billion years ago. There is evidence that contemporary Mars still has some water (the depth of its original ocean being estimated to have been nearly 0.5 km) stored as permafrost and in deep-seated ice layers (termed “lenses”). The recent NASA Phoenix lander close to the North Pole of Mars (see Fig. 5.3) revealed features which confirm such a scenario, as well as the earlier NASA Odyssey mission which showed the existence of subsurface water at about 1 m depth, unevenly distributed over the planet. Our general understanding is that ancient Mars had a much more pleasant climate than nowadays, when water coursed its surface until its quite dense atmosphere was lost into space and water ice was buried beneath thick sand-dust sediments.
The gaseous giant planets with their numerous satellites and rings are completely different. These planets also underwent differentiation of their interiors and, as a result, they have rather large rocky cores and extensive gaseous-icy mantles, the outermost shell being referred to as an atmosphere. Their effective temperatures range from 135 K (Jupiter) to 38 K (Neptune). Because of the continuing contraction of these massive planets, the heat released from their interiors is about two or three times the incident solar flux. This internal heat (which is many orders of magnitude greater than that for the terrestrial planets) is responsible for specific features of their atmospheric circulation. These include, at certain latitudes, zones and belts with very strong shear turbulence, which leads to the formation of eddies. Spectacular examples of long-lived eddies (anticyclones) are the Great Red Spot (GRS) on Jupiter and the Great Dark Spot (GDS) on Neptune. The dimensions of the GRS have varied from 10,000 to 14,000 km in the north-south direction and 24,000 to 40,000 km in the east-west direction, which is nearly three times the diameter of the Earth. The GRS also rotates counter-clockwise with a period of seven days. The atmospheres of the giant planets consist mostly of hydrogen, helium, and hydrogen-bearing compounds, such as water, ammonia, and methane.
Of special interest are the satellites of the giant planets studied at close quarters by space missions such as Voyager 1 and 2. Of particular interest are the Galilean satellites of Jupiter. The closest to the planet, Io, exhibits widespread volcanism due to heating via the tidal forces exerted on it, in its eccentric orbit, by other satellites orbiting Jupiter. Io’s surface is covered with sulfur deposits from volcanic eruptions, which give Io its yellow-orange color. Europa also experiences tidal heating which melts the ice about 10-15 km below its icy shell, giving rise to an electrically conducting salty ocean ~ 50 km deep. That could be warm enough to be home to complex organic molecules. Therefore, Europa is on the short list of future space missions to search for signs of life in the solar system. Ganymede is less subject to tidal heating; its icy surface is more cratered than Europa’s, though it possesses an even stronger magnetic field. In turn, Callisto, having a heavily cratered terrain is regarded as a dead body, with impact features preserved throughout the history of the solar system, though it also has magnetic field comparable in strength to that of Europa. Its origin is still an enigma.
Saturn’s Titan and Neptune’s Triton are intriguing. Their bulk density is comparable to that of Ganymede and Callisto, which means that they are formed half of rocks and half of water ice. Titan is unique, with its very dense atmosphere composed of nitrogen and argon at a surface pressure of 1.6 atmospheres and a temperature of 94 K. Since the acceleration due to gravity on Titan is only one seventh of Earth’s, to create such a high surface pressure the mass of gas on Titan is nearly ten times the mass of the Earth’s atmosphere. Liquid methane and other hydrocarbons exist on Titan’s surface; the recent Huygens lander of the Cassini mission indeed found methane lakes. Titan is thus another target in the search for life elsewhere in the solar system.
Unlike Titan, Triton is deprived of an atmosphere and the temperature of its nitrogen-methane surface is only 38 K; similar conditions are found on Pluto. A peculiar phenomenon on Triton, discovered during the Voyager fly-by, is the geyser-like eruptions of liquid nitrogen; this is called cryovolcanism (i.e. cold volcanism). Its probable source relates to the dissipation of tidal energy generated by the gravity interactions of Triton with Neptune. Like Pluto, Triton could have been captured from the Kuiper belt by Neptune’s gravity.
The most abundant, and dynamic, objects in the solar system are small bodies -asteroids, comets, meteoroids, and meteor dust. Asteroids are mostly located in a main belt between the orbits of Mars and Jupiter (from 2.7 to 3.2 AU). They range in size from about one thousand kilometers (Ceres) to Ida, 52 km long and shown in Figure 5.5, to only several meters; bodies less than l m in size are called meteoroids. The number of asteroids larger than 1 km is estimated to be ~105, though only about half this number has been cataloged. Three special groups of asteroids, which have eccentric orbits approaching, or even crossing, the Earth’s orbit (Apollo, Amor, and Aten) are referred to as Near Earth Objects (NEO). If one of these asteroids a few km in size impacted the Earth it would constitute a terrible threat to our home planet. Meteorites are collisional fragments of asteroids, which have fallen onto the Earth’s surface. Asteroids and meteorites are commonly classified by their composition as stony, iron, and iron-stony (a mixture). The most primitive stony meteorites (carbonaceous chondrites) come from the outer part of the main asteroid belt. Encapsulating primordial material, they give us crucial information about conditions existing when the solar system was formed.
Comets are regarded as the most pristine bodies in the solar system. Comets are classified into short period (orbiting the Sun with a period less than 200 years), and long period (having a period more than 200 years). The main family of comets is located in the Oort cloud (at 104to 105AU) and in the Kuiper belt beyond the orbit of Neptune (from 30 to 100 AU from the Sun). Comets are icy bodies (see Figure 5.6) having a nucleus, atmosphere (coma), and tail (always pointing away from the Sun). Their very porous nucleus, in the “dirty snow ball” model of a comet, is composed mainly of water ice with other frozen gases and dust. The coma and tail are produced as the ices sublime when the comet traveling along its very eccentric orbit approaches the Sun; they can be partially ionized. There two types of tail: one is mainly composed of fine dust pushed outwards by the solar radiation pressure, and the second (consisting mainly of ionized particles) is strongly influenced by the solar wind plasma and the interplanetary magnetic field.
Interplanetary dust particles ranging in size from millimeters to nanometers are either collisional fragments of asteroids/meteoroids or “cosmic” particles formed by sublimation from a cometary nucleus. The fragments are randomly distributed in interplanetary space, the smallest particles slowly drifting towards the Sun due to the Poynting-Robertson effect, while the cosmic particles form around cometary orbits. Periodically the Earth meets some of these dust particles in its orbit around the Sun when this closely intersects the comet’s orbit. This phenomenon is well known via the meteor showers named after the direction of the constellation in the skies where the meteors come from (e.g., Aquarids, Perseids, Quadrantids, etc.).
In conclusion, we can state that the study of solar system bodies gives us insight into the problem of its origin and evolution and also serves as a powerful tool to increase our knowledge of the Earth. In other words, the planetary sciences allow us to understand most comprehensively the nature of our home planet and our place in the Universe. Small bodies can carry prebiotic matter and thus help to answer the fundamental question of the origin of life on Earth.
5.1.3. The Earth and the Moon
Earth is the third planet from the Sun and the largest of the group of four terrestrial planets. The Earth’s orbit has very small eccentricity (0.017), which means that its orbit is almost circular. The Earth moves along its orbit at 29.8 km/s, and the period Р orb = 365.256 days (1 year). The sidereal period of rotation around its axis (relative to the stars, or 1 sidereal day) is P rot = 23 hr 56 min 4.99 s. The inclination of the Earth’s equator to the plane of its circumsolar orbit (the ecliptic plane) is 23°27’, which leads to significant seasonal changes on the surface of our planet.
The Moon, as seen in Figure 5.7, is the brightest object in our skies after the Sun. The semi-major axis of the Moon’s orbit is 383,398 km, and its eccentricity is 0.055. The inclination of the Moon’s orbit to the ecliptic is 5°09’. Its sidereal period of revolution around the Earth is 27 days 7 hr 43 min, while its synodic period (relative to the Sun, and corresponding to the lunar phase changes) is 29 days 12 hr 44 min.
The Earth-Moon system is unique in the solar system, having the smallest planet-to- satellite ratio of 81. This means that the two bodies exert significant mutual gravity forces on one another. On the Earth, these are clearly manifested by the ocean tides, which are larger than those caused by the Sun. In turn, the Earth locks the Moon’s orbit and its intrinsic rotation in resonance, such that period of lunar rotation equals the sidereal period. This means that the Moon orbits the Earth in exactly the same time as it takes to complete one rotation around its axis. This is why the Moon always presents the same aspect to the Earth and why we can only see the “farside of the Moon” from a Luna-Zond orbiter or an Apollo capsule. The tidal energy lost by the Earth to the Moon is responsible for the Moon moving steadily away from Earth by about 3 cm per year.
The Earth is not a significant body on the cosmic scale. Its equatorial radius is Re = 6,378 km, its polar radius is Rp = 6,356 km and, hence, its oblateness is 0.0034. The pear-shaped figure of the Earth is called the geoid. Its mass is Ме = 5.974 x 1024kg, the mean density is 5.515 x 103kg/m3, and the mean acceleration due to gravity at the surface is g = 9.78 m/s2(a bit more at the poles than at the equator). The Earth’s surface has continents and oceans, the latter occupying nearly two thirds of the whole surface. The age of the Earth is 4.55±0.07 billion years.
The main geological mechanism operating on Earth is plate tectonics, which means that its outer shell (the lithosphere) is not homogeneous but split into several large plates. This mechanism involves a spreading zone, where hot lava ascends from the upper mantle pushing the lithosphere plates apart and making “cracks” (rifts) between them, and subduction zones where some plates covered with sediments are slowly pushed deep under the continents. The spreading zone coincides with the global system of mid ocean ridges at the bottom of all the oceans. Both the spreading and subduction zones are associated with powerful volcanic activity and earthquakes. Plate tectonics are responsible for the drift of continents, which continuously move away from each other, as was first suggested by the German scientist Alfred Wegener in 1912. Reconstruction of the process back in time led to the conclusion that, ~250 million years ago, there was one super-continent Pangea which then disintegrated into several pieces giving rise to the present continents. In support of this model, we may compare the shapes of the eastern part of South America and the western part of Africa, noting that they fit together like two pieces of a jigsaw. The model has been confirmed by studies of the bottom of the ocean and of the magnetic properties of recent lava flows there.
The oceans comprise nearly 97% of all the water on Earth (the hydrosphere), about 1021kg, and cover 361 million km2. The remaining 3% is fresh water in rivers, lakes, and glaciers and a small amount in the atmosphere, as well as in the polar icecaps of the Arctic and Antarctica. The mean depth of the ocean is 3,900 m, while the maximum depth is 11,000 m in the Marian trough in the Pacific.
The Earth’s interior has a complicated structure revealed by seismic soundings. The speed of propagation of longitudinal and transverse seismic waves depends on the density and elasticity of the rocks. The waves also experience reflection by and diffraction at the boundaries between different layers; transverse waves do not travel through liquids. The main zones are the upper crust, the partially melted upper and lower mantle, the liquid outer core and the inner solid core. The thickness of the crust is ~35 km under the continents and about half that under the oceans. The region between the crust and upper mantle is the lithosphere, to a depth of about 70 km, and above the asthenosphere extending to a depth of 250 km. The boundary between the crust and the upper mantle is the Mohorovicic boundary, or the Moho for short. The dozen large plates of the lithosphere “float” on top of the asthenosphere, therefore ensuring the ever-continuing action of global plate tectonics.
The crust, composed of bedrocks (basalts and granites) having a mean density ~3000 kg/m3, comprises less than 1% of the mass of the Earth. The mantle takes nearly 65% and the core 34%. The thickness of the mantle is 2900 km; it is composed mainly of silicate rocks, as well as silicate and magnesium oxides modified under high pressure. The outer radius of the liquid core, composed of molten iron and nickel, is 2250 km, and the inner solid core has a radius of 1220 km. The temperature reaches ~1800 К at a depthof100km,~5000Кatthemantle-coreboundary,and~8000Katthe center where the density exceeds 10,000 kg/m3. Today, the liquid core and partially melted mantle are accounted for by heating due to long-lived radionuclides – uranium, thorium, and potassium – in the Earth’s interior. Dynamo action in the liquid core is responsible for the significant magnetic field of the Earth. Its strength is ~31,000 nanoTesla at the magnetic equator and twice that at the poles. Near the North geographic pole is the South (North seeking) magnetic pole (with the angle between the geographic and magnetic axes being ~11.5o). In the last 10 million years the magnetic field has reversed its polarity 16 times.
The Earth has a unique atmosphere in the solar system, composed mainly of nitrogen and oxygen, and having five stratified layers defined according to their temperature; they are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The troposphere stretches from the surface to 12 km (somewhat higher at low latitudes and lower at high latitudes). The mean temperature at the surface is +15°C with variations from -85°C (inner regions of Antarctica) to +70°C (West Sahara). The temperature decreases with increasing height at roughly 6°/km. Small amounts of water vapor, carbon dioxide (now almost 0.04%) and methane are mainly responsible for the so-called greenhouse effect. This raises the Earth’s temperature by about 33 °C compared with what it would have been in the absence of these molecules (which absorb infra-red radiation).
The mean gas density at the surface when the temperature is 15.0°С and the relative humidity is zero is 1.225 kg/m3(at standard atmospheric pressure, 1013 hPa). In the stratosphere (12–50 km), the temperature rises from approximately –50° С at 12 km due to the absorption of solar ultraviolet (UV) radiation between 200 and 300 nm by molecular oxygen and ozone. The stratosphere shields all living fauna and flora on the planet from intense ultraviolet radiation and thereby prevents the destruction of the biosphere. Ozone raises the temperature of the upper stratosphere to nearly 0° С at 50 km. In the mesosphere above the temperature falls, reaching its minimum value (about – 90 ° С) at the mesopause at 85 km. Above this level, in the thermosphere, EUV (extreme UV, at wavelengths shorter than 200 nm) and soft X-ray radiation is absorbed, causing the heating and ionization of the atmospheric molecules and atoms. The temperature steadily increases to 800-1000 K at ~300 km altitude, but that varies from about 500 K at solar minimum to 1500 K at the maximum of the Sun’s 11-year cycle of activity. This causes atmospheric density variations by almost two orders of magnitude at 400 km altitude, and, hence, dramatically influences the lifetimes of Low Earth Orbiting satellites and the International Space Station. Also at these heights is the ionosphere, an electrically charged gas, with its the prominent D, E, F1 and F2 layers where the plasma density is high enough to reflect radio waves of short wavelength (at frequencies below ~ 10 MHz) and thus to enable long distance radio communications around the Earth. Above about 500 km altitude is the exosphere, mostly hydrogen and helium, where collisions between atoms are very rare and where upward moving atoms have sufficient thermal velocity to escape into outer space.
The Moon has a radius of 1,737 km and a mass of 7.348 x 1022kg: its bulk density of 3,350 kg/m3is comparable with the density of the Earth’s mantle. This fact hints at its origin resulting from a collision of a Mars-sized body with the Earth soon after the Earth was formed. The acceleration due to gravity on lunar surface is only 1.63 m/s2, which makes the Moon a very efficient launch pad for solar system exploration. The Moon’s gravity is too small for it to retain an atmosphere, whether or not there was one in the past. The Moon is a rather inhospitable world, with no atmosphere and very marked temperature variations from -170° C at night to +130° C during the day, with both day and night lasting nearly a fortnight. There is no water on the Moon now, but there could be some ice in the vicinity of the South Pole. (This statement is based on indirect measurements made by the “Lunar Prospector” spacecraft in 1999.) This ice could have been “delivered” by a cometary impact.
The Moon has high mountains and deep depressions, termed seas (mare). The surface is heavily cratered, the large craters being named after great astronomers such as Tycho, Ptolemy, and Copernicus. The craters were formed long ago by impacts: in the absence of an atmosphere (and so no winds) they experienced little erosion, by meteorites and the solar wind. The range of crater sizes stretches from centimeters to hundreds of km, with the total number of craters on the near side of the Moon bigger than 1 km exceeding 300,000. After an impact, the molten mantle close to the surface is believed to have flowed as lava into the basins. The mountains reach heights of 2 km (Carpathes) to 6 km (Apennines). The rocks brought back to Earth by the Apollo and Luna missions have been estimated to have been more than 4.4 billion years old when they solidified.
An intriguing question is whether the Moon has a liquid core According to modeling partially supported by Apollo seismic data, the thickness of the upper lunar crust ranges from 60 to 100 km and the thicknesses of the upper and middle mantle below are about 400 and 600 km, respectively. Together they constitute a strong solid lithosphere. The lower mantle goes down to ~ 1600 km and thus the radius of the core could be roughly only about 150 km. The very small magnetic field of the Moon (~ 10-4of Earth’s) could indicate either a solid core or be due to the lack of a dynamo in the slowly rotating body.
Using the Moon’s resources is on the agenda of the space faring nations. Metallic elements and their compounds in rocks could be useful for building lunar habitats. In parallel, oxygen and water could be produced from H2 and O2 locked into the rocks. The incident solar radiation ensures an essentially unlimited supply of energy. Some people value the opportunity to extract 3He isotope deposited by the solar wind on the Moon’s surface and bring it to Earth for use in controlled “pure” thermonuclear fusion reactors. There are many opportunities for research into planetary science and radio astronomy on the Moon, which could be referred to as investigations both on and from the Moon. In solar system exploration and exploitation, an exciting future lies ahead.
5.2 The Space Environment
Most of the known matter in the Universe exists as an ionized gas, termed a plasma; some aspects of the special behavior of plasmas are discussed. Plasma phenomena occurring on the Sun’s surface and in its outer atmosphere, the corona, are explained, as are the solar wind, the Earth’s magnetosphere created as an obstacle in the supersonic solar wind, and aurorae. Today, Sun-Earth physics has an operational application known as space weather.
5.2.1 The Electromagnetic Spectrum
Almost all processes occurring on the Earth are powered by solar energy. Energy radiated by the Sun is transferred to the Earth in the form of light, electromagnetic waves, i.e. oscillating electric and magnetic fields, which travel at the velocity of light, 3.0 x 108meters per second (also denoted m/s). The frequency of the oscillations, in Hertz (also Hz or cycles/s), and the wavelength of the radiation lambda (λ) given in meters are inversely related to each other.
In the visible part of the electromagnetic spectrum with which we are most familiar, blue light has a wavelength of ~ 400 nanometers, or nm (4 x 10-7m, or 0.4 micrometers, µm), and red light ~ 700 nm. The spectrum is illustrated in Figure 5.8. (Note that the symbol ~ means “approximately.”) This is shown on the right-hand side of the diagram in Figure 5.8. Radiation of even longer wavelengths comes in the form of infrared (IR) radiation, microwave radiation, and radio waves. Commercial radio broadcasts at ~ 100 MHz use Frequency Modulation (FM, see the expanded scale at the bottom left of Figure 1) to convey the information carried by audio frequency (~ kHz) sound waves to the listener. TV transmitters operate at Ultra High Frequency (UHF), almost up to 1000 MHz, or 1 GHz. The longest naturally occurring radio waves have a wavelength ~ 40,000 km, the circumference of the Earth, and a frequency of ~ 8 Hz. Extremely Low Frequency (ELF) radio signals from lightning excite resonances of the spherical cavity between the Earth and the ionosphere at this frequency.
At wavelengths shorter than the visible are ultraviolet radiation, X-rays and Gamma rays. Radiation of shorter wavelengths, i.e. of higher frequencies than the visible, is more energetic. For such radiation we often think of their particle-like behavior; photons are “packets of energy” which travel at the speed of light. The energy of a photon is directly proportional to its wavelength. Although a photon has zero mass, it does have momentum.
Electromagnetic waves may be reflected, refracted, absorbed, scattered or polarized by interacting with matter. For example, visible light is reflected by silvered glass (a mirror), refracted through a glass prism (as was first investigated by Isaac Newton around 1670) which splits the light into its constituent colors (red, orange, yellow, green, blue, indigo, and violet), absorbed by a filter, scattered by a rough surface or by particles whose size is comparable with the wavelength, and polarized by passing through a sheet of Polaroid. Radio waves are reflected by the ionosphere where and when their frequency equals the plasma frequency at a particular height in the ionosphere. Radio waves are partially absorbed when propagating through the ionosphere due to collisions between electrons in the ionospheric plasma and atmospheric atoms or molecules.
Atoms of any particular element radiate only at particular wavelengths. Astrophysically speaking, hydrogen is the most abundant gas, and it has a strong emission line at a wavelength of 121.6 nm. This is in the ultraviolet, and is termed the Lyman alpha line, the first line in the Lyman series of lines. When the Balmer alpha line emitted by hydrogen on the Sun’s surface (the photosphere) passes through the solar atmosphere above it (the chromosphere and the corona), it is partially absorbed, so that a dark absorption line is apparent in the solar spectrum at a wavelength of 656.3 nm (red).
The spectrum radiated by a hot body such as the Sun is a continuous spectrum. As Figure 5.9 shows, the amount of energy radiated as a function of wavelength, termed the spectral radiance and described by Planck’s radiation law, varies greatly with temperature T (in K, i.e. degrees C + 273). This figure is drawn for what is called a blackbody, i.e. a body that absorbs all of the energy which is incident upon it and which reradiates it totally. The Sun is a very good approximation to a blackbody with a temperature near 5800 K; the maximum of the curve occurs in the yellow part of the visible spectrum to which our eyes are most sensitive. The Earth and its atmosphere also behave like a black-body, with an absolute temperature of 288 K; the maximum of this curve is at a wavelength of ~ 10 µm, in what is broadly known as the thermal infra-red part of the spectrum. The area under any curve in Figure 5.9 is the total energy radiated by that blackbody; the Stefan Boltzmann law states that this is proportional to T4, the fourth power of the absolute temperature of the radiating body.
If the source of electromagnetic radiation is moving at some velocity relative to an observer, the wavelength of the radiation received by an observer differs from that emitted by the source by a small amount. This is called the Doppler effect (or Doppler shift). In everyday life this effect is evident if we listen to the siren of a police car. When the car is moving towards us the frequency of the siren increases (higher pitch) and the car is moving away from us the frequency decreases (lower pitch). The magnitude of the shift is proportional to the velocity of the car. Likewise for a light source moving toward us the wavelength of the light decreases or is blue shifted (frequency increases), and for a source moving away from us the light is redshifted (frequency decreases). A special case of this effect is called the cosmological redshift in which the light emitted by a distant galaxy is red shifted due to the expansion of the universe. Measurement of this cosmological redshift yields information on the rate of the expansion of the universe.
5.2.2 The Plasma Universe
What is a plasma? Fluorescent lights, lightning, rocket exhausts, thermonuclear fusion reactors, the solar system, stars, and galaxies all contain a substance that simultaneously obeys the laws of electromagnetism and fluid dynamics. In 1928 Irving Langmuir named this substance “plasma”; it is sometimes called the fourth state of matter, the other three being solid, liquid and gas. When a gas is heated it is completely ionized, forming a collection of electrically charged particles (both positive ions and a mixture of negative ions and electrons) possessing all the qualities of a fluid but with additional properties caused by the presence of electric and magnetic fields. A plasma is an ionized gas in which the particle dynamics due to long-range electromagnetic forces are more important than those due to short-range collisional interactions. Plasmas are usually very hot, or tenuous, or both. A plasma in space is an assembly of positive particles (protons, and other ions) and negative particles in one volume, in which the dominant interactions are collective, i.e. all the electrons act in unison together, as do all the protons and other ions. In some cases, the motion of a plasma can accurately be described by looking at how individual charged particles act.
Plasma physics is essential to explain most astrophysical and space phenomena. In fact, we live in a plasma Universe since more than 99% of the known matter (matter that we can observe) in the Universe is in the plasma state. Nearly all plasmas in the Universe also have neutral atoms that co-exist with the ionized plasma gas. For most of the plasma in the Universe - or in laboratories on Earth - collisions between the individual particles (electrons and ions) and neutrals (atoms and neutrons) occur very rarely. How plasmas behave, and their motions in space, are determined by the most important forces acting; they are gravitational and electromagnetic forces. Understanding charged particles moving in a gravity field, an electric field, and a magnetic field is crucial in both astrophysical and space physics. The force due to gravity depends directly only on the mass of the particle, thereby affecting neutrals as well as all the components of the ionized plasma. For charged particles experiencing an electric field, in the absence of any other force, electrons will move in one direction and the ions will move in the exact opposite direction, along the electric field direction. The motion of a charged particle in a magnetic field is very different. Charged particles moving in magnetic fields, in the absence of other forces, move in a spiral about the magnetic field direction, with electrons and ions spiraling in opposite directions.
The combined forces acting on a charged particle result in rather complicated motions. Figure 5.10 illustrates various motions of a charged particle under the influence of magnetic and electric fields. Panel A shows the motion of an electron with only a perpendicular velocity in a constant magnetic field. Panel B shows charged particle motion when it has both a parallel and perpendicular velocity. Panel C illustrates the bouncing motion that a charged particle will perform when in a magnetic field that changes (either increases or decreases) with time or with position. Panel D shows the drift motion of a charged particle along a shell in a very realistic astrophysical or space magnetic field, namely that due to a dipole. It is these motions of charged particles that have trapped high-energy plasma in the Earth’s magnetic field, so forming the Van Allen radiation belts.
Plasmas can also be treated as a collection of charged particles. This approach leads to other important ways of describing how a plasma behaves or interacts with fields and matter collectively. Because a plasma can act collectively, there is a unique way in which it moves when its individual charges are displaced and released to move back towards their original positions. These motions are called plasma oscillations . Their characteristic frequency of oscillation, called the plasma frequency, depends only on the density of the plasma. The higher that the density of the plasma is, the higher is the resulting frequency of its oscillation.
Magnetic field reconnection (also called annihilation ) is a way of describing what happens when two different plasmas and their associated magnetic fields collide. In general, plasmas with their separately frozen-in magnetic fields do not mix. But when these magnetic fields are in opposite directions the field lines can connect, allowing the plasmas to mix and some plasma caught up in the process to be accelerated, sometimes to extremely high velocities. Magnetic fields in a plasma behave like stretched rubber bands.
The effect of field connection is like that of cutting the bands and retying them to produce shorter but more relaxed segments. Panel A of Figure 5.11 shows the frozen-in magnetic field configuration before reconnection and panel B afterwards. The annihilation of magnetic fields may occur where oppositely directed fields are pushed together by outside forces, such as the solar wind impinging on the Earth’s dayside magnetosphere. Energy is released in this interaction as charged particles, which are energized near the “neutral line” between the two opposing fields. The annihilation process is believed to occur in many space plasma situations, such as in solar flares, in coronal mass ejections, at the Earth’s dayside magnetopause, and in the center of the Earth’s magnetotail. Charged particles accelerated via magnetic annihilation in solar flares are known to be a major hazard for human space travelers.
5.2.3 Our Closest Star, the Sun
As our closest star, the Sun is an object of immediate interest. It is an ordinary star, though special to us; it is not too hot star, and one that is middle-aged. The Sun has been a source of wonder, different beliefs and worship since the earliest times. From the God Ré for the ancient Egyptians, Shamash for the Babylonians, Surya for ancient Hindus, Helios for ancient Greeks, and Amaterasu (the Japanese Goddess of the Sun believed to be the ancestress of the royal family), among others, the Sun was both adored and feared by these peoples. Total solar eclipses were also a source of fear in ancient times. The oldest records of eclipses dating from around 800 BC were engraved on tablets found in Syria and China. The first recorded observations of the Sun, by the Chinese, date from 165 BC. Naked eye observations of sunspots were possible through dense fog or dust. In 1612 AD, Galileo and Kepler were the first astronomers to observe sunspots with a telescope.
Nowadays, the Sun is studied in three different ways, to address questions about the three main layers of its structure. The invisible interior is probed by helioseismology techniques, which, as for the Earth’s seismology, “listen” to the sound of oscillations inside the Sun. The visible atmosphere is observed with imaging and spectrographic techniques, whereas the extended atmosphere is probed by in situ instruments. Wavelengths used to observe the Sun range from Gamma rays to radio waves. There are several ground-based solar observatories around the globe, but space observatories are preferred because the Earth’s atmosphere is opaque to many wavelengths.
The Sun’s core (25% of the solar radius, see Figure 5.12) is where thermonuclear fusion occurs. The energy produced in the core is transported out to a distance of 0.7 solar radii by radiation. Energy is then transported to the solar surface by convection. The core and the radiative zone rotate as a solid body whereas in the convection zone the (angular) speed of rotation varies with latitude. The temperature is ~ 15 million K in the core, progressively decreasing to ~ 5,800 K at the surface; the density decreases by a factor of 16,000 from core to surface.
The solar atmosphere is split into four layers. Rotating with the convection zone, the photosphere is the deepest; it is the “visible” surface of the Sun. (However, it is extremely dangerous look directly at the Sun with the naked eye.) This layer goes out to a height of about 500 km, the temperature decreasing to 4,200 K; most of the Sun’s light comes from this layer. Solar features such as sunspots and faculae exist here. These are a manifestation of the Sun’s magnetic field and of solar activity. Sunspots (see Figure 5.12) are seen as dark areas because they are colder than their surrounding plasma, and are the sites of strong magnetic fields. Faculae are brighter regions where the magnetic field is even stronger than on the rest of the solar disk. The photosphere is observed at visible wavelengths with imagers, magnetographs and spectrographs.
The chromosphere is the layer immediately above the photosphere, with a thickness about of 1,500 km. Here the temperature starts to increase, reaching 10,000 K. It can be observed from the Earth in two different ways -with the naked eye at the very beginning of a total solar eclipse (before the corona becomes visible) and with some instruments at some specific wavelengths. Features appearing on the solar disk are filaments and prominences, two versions of the same phenomenon, i.e. enhanced plasma regions contained by magnetic field lines, the first dark on the solar disk, and the second bright beyond the solar limb (the Sun’s apparent edge) due to the contrast with the blackness of space. They are located along the neutral line of the magnetic field, i.e. at the region of opposite magnetic field polarities. This layer is observable from the ground at visible wavelengths where spectral lines of hydrogen and calcium are emitted, as well as at radio wavelengths. Coronographs are instruments built to simulate a total solar eclipse for making observations of this thin, and important, layer, either from space or from the ground.
The transition region is the next layer, where the temperature increases sharply up to 100,000 K. Here other solar features appear. Coronal holes, for example, are less bright areas, more or less extended on the solar disk, with lower temperatures and densities. Here, the magnetic field lines are open into the interplanetary medium, so that charged particles can escape from the solar atmosphere at high speed. This layer can only be observed from space, mainly in the ultraviolet and X-ray parts of the spectrum.
The corona extends to more than a million km (about 10 solar radii) from the Sun. Its temperature can exceed 2 MK, as its density continues to drop. The lower part of the corona can only be seen from Earth when the moon blocks out the light from the solar disk during a total solar eclipse. Violent events release energetic plasma into interplanetary space can also be observed sporadically; these are solar flares and coronal mass ejections (CMEs). Flares are sudden events always associated with features like sunspots or prominences. They can vary in severity and they evolve through different phases. Since they emit at different wavelengths, they can be observed from the ground (radio) as well from space (ultraviolet). Coronal mass ejections are huge bubbles of plasma and magnetic fields lines ejected from the Sun at speeds ranging from 500 to 2500 km/s, higher than the usual solar wind speed. They produce a bow shock ahead of them; if the CME is coming directly towards the Earth, it can first be felt acting on the Earth’s environment.
As mentioned earlier, different types of activity occur at different heights in the solar atmosphere. Among these phenomena, one that gave an early indication of the Sun’s varying activity was sunspots. Their changing positions on the solar disk were used to find the rotation speed of the photosphere at different latitudes, whereas their changing number demonstrates the cycle of solar activity. The number of sunspots seen on the disk increases and decreases, with an average periodicity of 11 years (see Figure 5.13), and sunspots move towards the solar equator as solar activity increases. Sunspots, arched magnetic fields in prominences, and differential rotation are closely connected, and indicate that the cause of the solar cycle is its magnetic field. The Sun’s magnetic field has its origin in its interior, being produced by a dynamo effect. As a consequence of differential rotation and the high electrical conductivity of the plasma in the convection zone, the magnetic field becomes twisted.
The average time for one rotation of the photosphere as seen from Earth is about 27.3 days, ranging from 26.3 days at the equator (whereas its sidereal rotation is about 25.3 days) and up to 30 days near the poles. The rotation rate varies also with altitude. The combination of the differential rotation and the magnetic field leads to the solar cycle, a mechanism that provokes the reversal of the Sun’s dipolar magnetic field every 11 years; therefore the true period of the cycle is ~ 22 years. The classical model explains that differential rotation changes the poloidal magnetic field at solar minimum (mainly North-South) into an intense toroidal magnetic field (mainly East-West). This intense field floats up through the photosphere because of its buoyancy, and emerges out of the photosphere as active regions containing sunspots. There is a complex, even chaotic, reorganization of magnetic fields at the maximum of the solar activity, before the poloidal field closes up, with its polarity reversed.
The expansion of the corona into interplanetary space takes the form of a continuous flow called the solar wind. The solar wind carries the solar magnetic field with it, away from the Sun; energetic charged particles move in helical paths along these field lines to interact with the various magnetic environments of the planets. The topical subject of the interaction of the solar wind with planetary magnetic environments is known as space weather.
What happens at the edge of our solar system, at the outer boundary of the zone of influence of the Sun called the heliosphere? The heliosphere is like as a bubble that extends far beyond the planets, blown outwards by the solar wind. Eventually it is stopped by the interstellar medium; this happens where the pressures balance, at a rather ragged boundary called the heliopause. Before this, the solar wind travels at an average speed ranging from 200 to 800 km/s until it reaches the termination shock, investigated by the two NASA Voyager spacecraft, where the speed of the solar wind drops abruptly as it begins to feel the effects of the interstellar medium. The heliosheath is the outermost region of the heliosphere, just beyond the termination shock. Beyond the heliopause is another bow shock formed as the heliosphere plows its way through interstellar space.
5.2.4. Sun-Earth Interaction
The Sun affects the Earth’s environment in several complex ways that are investigated directly by scientific instruments aboard satellites. In the early days of the space age the first US satellite (Explorer 1) discovered the Van Allen radiation belts around the Earth; Explorer 3 and 4 explored these in more detail.[2]The discovery prompted one young researcher to exclaim – “my God, space is radioactive!” Then the magnetometer aboard a satellite in a highly elliptical orbit around the Earth, IMP 1, the first Interplanetary Monitoring Platform, investigated the detailed shape of the magnetopause.[3]This is the outer boundary of the magnetosphere, the region of the Earth’s magnetic field in space, which is illustrated in Fig.5.154.[4]
The geomagnetic field is compressed by the solar wind which flows away from the Sun at supersonic speeds, > 400 km/s, pulling the Sun’s magnetic field lines out with it. On the dayside of the Earth the magneto-pause is often located ~ 10 Earth radii (RE) upstream from the Earth’s center. When the Sun is active and producing much faster solar wind streams than usual (up to ~ 1000 km/s) the magnetopause lies closer to the Earth, at 7 RE or occasionally even at 6 RE. If the interplanetary magnetic field lines have a southward component, they can readily “connect” to the northward pointing geomagnetic field lines at the dayside magnetopause. As the solar wind flows past the magnetosphere, it pulls geomagnetic field lines on the night side out into a comet-like tail. The magnetotail extends beyond the Moon’s orbit, at ~ 240 RE from the Earth, on the night side.
The interplanetary plasma beyond the magnetosphere, in the magnetosheath, is rather turbulent until the bow shock is encountered a few RE further upstream. Here the magnetic field suddenly changes its magnitude and/or direction, and so do the solar wind velocity and temperature. The bow shock is rather like the shock wave formed ahead of a model aircraft in a supersonic wind tunnel. It is a region where charged particles are accelerated. An excellent introduction to this topic and to the entire physics of the magnetosphere is given in a book.[5]
ESA’s Cluster mission has four closely spaced satellites orbiting in formation in a different highly elliptical orbit (also shown in Figure 5.14). Properties of the plasma and energetic charged particles, and also of magnetic and electric field fluctuations and waves, are observed with a comprehensive suite of sophisticated instruments (see, for example, Gustafsson, et al.[6]). These are designed to study in detail the plasma physics of solar wind entry into the magnetosphere, especially in the vicinity of the cusp on the magnetopause.[7]On the night side they sometimes pass through the source region of strong natural electromagnetic waves,[8]at an audio frequency of a few kHz, between 0.2 and 0.4 times the local electron cyclotron frequency, f ce. These waves are termed chorus; they travel through the magnetosphere in the whistler mode.[9]A hot research topic nowadays[10]is the acceleration of electrons to very high energies (~ MeV) by chorus waves. These so-called “killer” electrons can penetrate satellites and cause damage to electronic circuits inside. The damage may be either temporary (called a “latch up”) or, even worse, permanent.
The northern lights, called the aurora borealis, and the southern lights, called the aurora australis, have been observed from above by astronauts on the Space Shuttle, as depicted in Figure 5.15. TV cameras sensitive to ultraviolet light on several satellites have also recorded Auroras, starting with Dynamics Explorer and, more recently, Polar. Images acquired by the instrumentation on these platforms clearly show the evolution, from minute to minute, of the location of the auroral oval, a ring of light around the geomagnetic pole at magnetic latitudes of ~ 67 degrees.[11]The aurora is produced when electrons with energies of ~ 10 keV moving down geomagnetic field lines strike the upper atmosphere.[12]They excite atmospheric atoms and molecules into higher energy states; these then fall back to their ground states, emitting visible light of a certain wavelength (or color). At times when the solar wind is disturbed by excessive solar activity, the auroral oval expands to lower latitudes. The initiation of a specific feature, e.g., a brightening, of the auroral oval, and its subsequent temporal and spatial development, are well investigated by studying sequences of such images.
The plasmapause[13]at ~ 4 RE from the center of the Earth is the geomagnetic field aligned outer boundary of the plasmasphere, which has recently been imaged by the IMAGE satellite (Imager for Magnetopause-to-Aurora Exploration).[14]The plasmasphere is filled by plasma flowing out of the ionosphere. Discovered experimentally in 1924 by radio experiments on the ground, the ionosphere at heights between ~ 80 and ~ 300 km reflects HF (High Frequency, 3 to 30 MHz) radio signals used for communications around the world. Radio signals at ~ 1.23 and 1.58 GHz from the GPS satellites,[15]which nowadays are essential for positioning and navigation on -and above -the Earth’s surface, pass through the ionosphere. The ionosphere adversely affects them if it is disturbed following solar activity.[16]
During strong auroral displays, huge electric currents (~ millions of Amperes) flow around the auroral oval in the ionosphere at heights of ~ 110 km. These are fed by currents flowing down geomagnetic flux tubes in certain Local Time regions, the circuit being completed by upward currents elsewhere. Power generation for this gigantic electric circuit relies upon the dynamo action of the solar wind speeding past the Earth’s magnetic field. The response of the near-Earth space environment to disturbances on the Sun is a most important topic, which is now termed “space weather” and which is discussed in the next section.[17]
5.2.5 Space Weather and its Effects
What caused the huge power outage in March 1989 in North America? Everything started with a major solar flare, which happened to be observed from the Kitt Peak Solar Observatory on 9 March. Several eruptions followed, sending X-radiation and clouds of ionized gas in the direction of the Earth, because of the flare’s central location on the solar disk, as seen from Earth. A positive effect of this eruption was a splendid aurora seen at as low as latitude as the Florida Keys! A negative effect was the breakdown of the electricity supply that paralyzed the Canadian province of Quebec and caused major damage to the power grid in the USA.
The first evidence of a link between solar activity and terrestrial disturbances goes back to 1859 when Richard Carrington observed a sudden and intense bright light on the Sun. Some 24 hours later there was a strong magnetic storm at the Earth. Before that, Von Humboldt, who observed strong and irregular variations of the Earth’s magnetic field correlated with bright displays of the northern lights, reported the first evidence of a magnetic storm in 1808. The main causes of magnetic storms are violent and sudden releases from the Sun of bubbles of plasma and strong magnetic fields that propagate far into interplanetary space. Such Coronal Mass Ejection (CME) events, and especially High Speed CMEs or multiple CMEs, are the major cause, but solar flares, filament eruptions, active regions and also enhanced solar wind speeds also contribute to powerful geomagnetic storms. As a CME cannot be observed directly from Earth, we need instruments aboard spacecraft to observe this phenomenon which was seen, for the first time, in December 1971 with a white light coronagraph on NASA’s OSO-7 spacecraft (the seventh Orbiting Solar Observatory). Later, the US National Space Weather Council (1995) clearly defines this phenomenon: “The term Space Weather generally refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health”. The National Oceanic and Atmospheric Administration (NOAA) classified storms as follows:
- Geomagnetic storms, disturbances in the geomagnetic field caused by gusts of solar wind.
- Solar Radiation storms: elevated levels of radiation that occur when the fluxes of energetic charged particles increase.
- Radio blackouts: disturbances of the ionosphere caused by X-rays emitted by the Sun.
Explosive solar events generate geomagnetic storms and cause ionospheric disturbances as well as magnetically induced currents, and also disrupt telecommunications and radio positioning as illustrated in Figure 5.16.
Explosive solar events have various effects on human beings as has been discovered from space exploration programs. Long-term exposure to the space environment can entail either short-term or delayed biological effects for astronauts living in space (e.g., on the ISS or the Space Shuttle) or carrying out extra vehicular activities (EVA) in space or on the surface of the Moon or Mars. Exposure to solar radiation (UV or X-rays) can lead to cancers or genetic changes. But current technologies cannot effectively protect astronauts against enhanced fluxes of energetic charged particles, especially heavy ions, during strong magnetic storms. Therefore, carrying out an EVA at the same time as a CME collides with the magnetosphere could lead to deadly events.
From the launch phase to their final orbit, spacecraft are not very well protected from radiation. Nowadays electronics components are becoming smaller and smaller, and therefore more vulnerable to energetic charged particles. Damage due to such radiation can be either transient or cumulative. Single effect upsets (SEU) are transient events that happen when highly energetic charged particles (> 50 MeV) penetrate spacecraft shielding, and then change the bit states of computer chips. This situation can lead to changes in onboard software and memory content, and later provoke a “latch-up” (temporary outage requiring reset action) or a permanent failure of an onboard computer. Noise in detectors (especially CCD detectors) can also be a temporary disturbance. It can lead to bad data acquired by an instrument, or even the loss of a spacecraft that uses a star tracker to acquire its position, since a bright point generated by an excess of photons could be misinterpreted as a referenced star. Bombardment by charged particles also has a cumulative effect on spacecraft components, causing degradation and accelerating the ageing process of spacecraft parts. Solar panels are one example of sensitive materials whose lifetime is dramatically decreased each time a proton event occurs.
When the shock ahead of the CME front arrives at the Earth’s magnetosphere, the magnetosphere is so compressed that a geostationary spacecraft is now in interplanetary space. The enhanced solar wind of the CME drags the spacecraft, moving it by ~ 1 meter typically but by up to 50 meters sometimes. At the same time the spacecraft’s magnetic sensors become confused and their data cannot be used for orienting the spacecraft.
LEO space debris tracking is also a critical issue, since the tracking process becomes inaccurate and must be reinitialized after each magnetic storm. For example, during the 1989 storm, the increased density of the atmosphere caused increased drag on an orbiting object so that it lost 30 km in altitude and there was a significant decrease in its lifetime too.
During a storm some terrestrial radio services become unreliable due to absorption by the ionosphere. Some of the many such applications here include aviation, coastal marine traffic, and search and rescue services. Mineral resource exploration and geophysical prospecting, using airborne magnetic surveys at high latitudes, is also upset during a magnetic storm. GPS signals are affected when solar activity causes sudden variations in the density of the ionosphere, which reduces the accuracy of positioning and navigation systems on the ground, in particular at high latitudes.
The Sun, a broadband radio source, is also highly variable. If the beam of a spacecraft antenna includes the Sun when an intense radio burst happens, the radio signal from the spacecraft can be overwhelmed. This could be critical when tracking a spacecraft, or when downloading telemetry from or sending commands to the spacecraft.
Commercial aviation today strongly relies on GPS and radio communications for several phases of flight. The risk of disturbances to radio propagation increases at high altitudes and high latitudes. Air routes crossing the Arctic have been developed for economic reasons, mostly from the US to China. However, when there is a magnetic storm, radio equipment on aircraft using these polar routes is very sensitive to radio disturbances, and communications can be interrupted. On these routes there is also an increased risk of exposure to radiation for crew and passengers alike, as there is too for avionic equipment.
When there is a storm alert, flights need to be re-routed to lower latitudes for obvious safety reasons. This automatically leads to delayed flights, higher fuel consumption and, of course, various financial losses. For example, in January 2005, four air routes were closed for the first time due to a strong storm. Several planes completely lost contact with the ground since no radio signals could get through, and no GPS techniques were operating; one airline announced a loss of more than 250 M$ in four days.
There is growing evidence indicating that changes of the geomagnetic field can affect biological systems. Several research groups have begun to study relations between geomagnetic activity and psychiatric disorders. Animal life also seems to be affected by these storms, in particular the navigational abilities of pigeons and dolphins.
The electrical power grid infrastructure, particularly at high latitudes, is affected by storms. Because transformers can be destroyed by geomagnetically induced currents to cause power outages, there can be a domino effect on the mesh of transformers directly connected to the one that fails. The corrosion of oil and natural gas pipelines is another concern following a geomagnetic storm.
Because many Earth applications rely so heavily on the space environment nowadays, there is a need to predict space weather events that may endanger humans and their services. There are many different users of space weather applications, with the requirements mainly being for:
- Space exploration, aviation, and satellite operators, and grid operators, etc., who need to know, in real-time, about upcoming events and also about clear periods,
- Launchers who tend to delay rocket launches when there is a high probability of a solar storm,
- Spacecraft re-insurance companies who need to know if the risk was predictable or not,
- Spacecraft designers who need to assess risks, analyze them and mitigate their effects by studying the database of past failures,
- Space tourism passengers who would not risk to fly if a solar event is forecast,
- Military activities around the world and in space, and
- Airlines for which radiation monitoring is critical.
All these customers’ needs require a better understanding of the space environment, the Earth’s magnetic environment and geomagnetic activity, and on the capability to develop better models.
5.3 Space Observations beyond the solar system
Telescopes looking outward from orbit around the Earth observe the Universe across the electromagnetic spectrum, from radio waves to the infrared (IR), through the visible and ultraviolet (UV), to X-rays and gamma-rays; much new astrophysical knowledge has been found. Instruments on satellites make in situ observations of the properties of the plasma environment around the satellite. Instruments looking downward from Low Earth Orbit (LEO) or from a similar orbit around a planet observe the planetary surface and its atmosphere.
5.3.1 Background
Astronomers explore the Universe by observing the electromagnetic radiation emitted by the planets, stars, galaxies, the interstellar medium, and the intergalactic medium. However, most of this radiation is absorbed by the Earth’s atmosphere (see Figure 5.17) and never reaches the surface of the Earth. As a result, for most of the history of mankind, our view of the Universe has been limited to the very narrow optical (visible) band of the electromagnetic spectrum. Another atmospheric window exists at radio wavelengths and, although radio waves from our galaxy were first detected in 1933, it was not till after World War II that techniques were available to explore the radio spectrum fully. A third partial window exists at infrared wavelengths. The first infrared observations were of the Sun in the early 1800’s, but, due to the difficulty of developing sensitive detectors, it was not until the 1960’s that ground-based infrared astronomy made a significant impact.
Since the 1960’s, spacecraft carrying telescopes above the atmosphere have opened up the entire electromagnetic spectrum for astronomers to observe, and have permitted observations free from the distorting effects of the Earth’s atmosphere. Over the past half-century space telescopes have revolutionized our knowledge of our Universe.
Compared to ground-based observations, observing with space telescopes is not easy and is very expensive. However, for observing most of the electromagnetic spectrum there is no alternative. Instruments in space have to work for long periods and unattended, in many cases in difficult environments, due primarily to the extremes of temperature and damaging radiation experienced. The mass, power and volume available for instruments are very limited on spacecraft. It takes many years to design, build, test, and launch a telescope into space. Instrumentation placed at the focus of space telescopes consists of spectrometers (both narrow and/or broadband), interferometers, and imagers. To find extra solar planets, other telescope techniques used in space include accumulating intensity data over specific wavelength regions, and producing light curves as functions of time to detect a planet occulting (or slightly blocking) light emitted by the observed star.
5.3.2 Space Telescope
Telescopes are a means of collecting electromagnetic radiation from distant objects and focusing it on a detector. The detector absorbs the radiation and converts it into an electrical signal from which an image of the source can be produced or from which the intensity of the source as a function of wavelength can be measured. Telescopes can be classified as either “reflectors” (using mirrors) or “refractors” (using lenses). Today, most large telescopes in space are reflectors, and the primary collecting surface is usually parabolic in shape to convert a parallel beam of radiation to a single, sharply focused spot of light.
The size of a telescope is described by the diameter, D, of the primary mirror. Most large space telescopes are of the Cassegrain design, in which light from the primary mirror is reflected back by a secondary mirror (hyperbolic), through a hole in the primary mirror, and focused behind the primary mirror. The advantage of the Cassegrain design is that the focal plane detectors can be mounted on the mirror support structure. To obtain good image quality over a large field of view, a modification called the Ritchey-Chretian (RC) design is used in which both the primary and secondary mirrors are hyperbolic. The Hubble Space Telescope (HST) launched in 1990 is an example of this type of telescope (see Figure 5.18). The angular resolution of a telescope is proportional to the observed wavelength divided by the diameter of the telescope mirror (λ/D).
5.3.3 Radio Astronomy
Although the atmosphere is transparent to most of the radio spectrum, one advantage that space offers is the ability to perform very long baseline interferometry at these wavelengths. Using two telescopes, one on the surface of the Earth and another in a highly elliptical orbit (e.g., apogee altitude of 21,400 km and a perigee altitude of 580 km), and measuring both the amplitude and the relative phase of the same radio signal at both telescopes, an angular resolution can be achieved which is proportional to the wavelength divided by the separation distance of the telescopes. This technique provides a very significant increase in angular resolution at radio wavelengths, even better than can be achieved by ground-based interferometers. Examples of this type of mission are the Japanese satellite Halca (a.k.a. VSOP and Muses-B) and the Russian mission RadioAstron, which were used in conjunction with a ground-based interferometer.
5.3.4 Infrared and Sub-millimeter Astronomy
Space offers tremendous advantages for infrared astronomy. Because the atmosphere absorbs most of the infrared spectrum, an infrared telescope in space can observe the entire infrared spectrum. Also the telescope mirror can be cooled (usually by liquid helium), to reduce the unwanted infrared background radiation by a factor of a million. Instruments at the focal plane include cryogenically cooled infrared array detectors, both for imaging and spectroscopy. Our knowledge of the birth and evolution of galaxies in the very early Universe, the nature of exoplanets, and the birth and evolution of stars in our galaxy has increased significantly because of observations with space infrared telescopes with even modest mirror sizes (< 1 m in diameter). (The earliest infrared space telescopes were flown aboard rockets and high-altitude balloons and aircraft.) Examples of satellite missions include NASA’s Infrared Astronomy Satellite (IRAS), ESA’s Infrared Satellite Observatory (ISO), NASA’s Spitzer Space Telescope, and the Japanese mission AKARI. Spitzer was placed in a solar orbit, gradually drifting away from the Earth, to reduce the heat load on the telescope from the Earth and the Moon and to permit more efficient observations. NASA’s WISE, a mid-infrared all-sky survey, was launched in 2010 into a polar orbit. SOFIA is a Boeing 747 aircraft with a 2.7-meter telescope that initiated its science program in 2009. ESA’s Planck and Herschel missions were launched together in 2009 into L2 Lagrangian point orbits. Herschel observed the sub-millimeter part of the spectrum and Planck measured the cosmic microwave background radiation produced by the Big Bang. Planned for launch in 2018 is the James Webb Space Telescope (JWST), which will have a 6.5-meter diameter mirror; this will also orbit around the L2 Lagrangian point.
5.3.5 Optical and Ultraviolet Astronomy
Even though optical astronomy can be done from the Earth’s surface, an orbiting optical telescope has the advantage that it is not be affected by the blurring effects of the Earth’s atmosphere, and hence it achieves a much higher resolution and sensitivity. The Hubble Space Telescope (HST), launched in 1990, has produced a series of very important astronomical discoveries at both optical and ultraviolet wavelengths, and is a prime example of what can be achieved from space. Examples of these discoveries include accurate distance measurements to nearby galaxies, refined estimates of the Hubble constant, which specifies the rate of expansion of the Universe, new insights into galaxy evolution, the deepest image of our Universe ever made, and evidence for massive black holes in the centers of galaxies. The HST Cassegrain telescope has a 2.4 m diameter mirror; at its focal plane is a series of four instrument bays for astronomical cameras, photometers, spectrometers, and polarimeters. Because the telescope can be serviced by the Space Shuttle, the focal plane instruments have been replaced and/or repaired on several occasions.
Ultraviolet radiation at wavelengths less than approximately 320 nm is absorbed by the Earth’s atmosphere. Therefore ultraviolet observations must be carried out from rockets or satellites and many such telescopes have been flown. These include astronomical missions with photometers, cameras, low and high-resolution spectrographs, and polarimeters. HST remains the primary ultraviolet observatory, but numerous smaller missions of moderate size have been flown including IUE, EUVE, FUSE and GALEX. Ultraviolet space astronomy has been especially important in improving our understanding of the nature of the interstellar medium, the nature of stellar atmospheres, properties of the interstellar dust, the compo-sition of comets, and the properties of active galactic nuclei.
5.3.6 X-Ray Astronomy
X-rays are completely absorbed by the atmosphere, so telescopes and instruments to observe celestial X-rays were first flown on balloons and rockets, but are now flown primarily on spacecraft. X-rays originate from an extremely hot gas, are associated with very energetic phenomena, and arise from such objects as neutron stars and black holes. Normal Cassegrain telescopes cannot be used to focus X-rays because X-rays impinging perpendicular to any material are absorbed rather than reflected. X-rays can be focused by selecting a mirror material that reflects the radiation if it is incident on the mirror at small, or grazing, incident angles. The materials usually used are gold or nickel, and the critical reflection angles are of the order of 1 degree. The highest resolution telescopes use a reflection from a parabolic mirror followed by a reflection from a hyperbolic mirror. The Chandra X-ray Observatory (CXO) is the prime X-ray mission today, consisting of nested sets of circular grazing incidence parabolic and hyperbolic mirrors. Other mission examples include the XMM-Newton observatory, Rossi X-ray Timing Explorer (RXTE), ASCA, ROSAT, BeppoSAX, and Swift. X-ray detectors are required to determine the location of an X-ray photon in two dimensions and have reasonable detection efficiency. X-ray detectors at the focal plane of these telescopes include imaging proportional counters, charge-coupled device (CCD) detectors, similar to those in visible-light cameras, micro-calorimeters that can detect one X-ray photon at a time and measure its energy, and transition edge sensors, that are a more advanced type of micro-calorimeter. Both these latter sensors need to be cooled to liquid helium temperatures.
5.3.7 Gamma-ray Astronomy
Gamma rays are the most energetic and shortest wavelength photons in the electromagnetic spectrum. Because the atmosphere absorbs most all of them, cosmic gamma-ray observations must be implemented using high-altitude balloons and satellites. Gamma rays are so penetrating that they simply pass through most materials and cannot be reflected by a mirror like optical or X-ray photons. Some gamma-ray telescopes act as “light buckets” and detect photons incident on the sensitive area of the detector. These use scintillators or solid-state detectors to detect a gamma ray and convert it into an electronic signal. Another class of detectors is based on the nature of the gamma-ray interaction process, using either imaging pair-production or Compton scattering to determine the gamma-ray arrival direction or a coded-mask in front of the detector to allow an image to be reconstructed. Gamma ray detectors usually have poor angular and spectral resolution. The recent satellite missions include INTEGRAL, AGILE, and Fermi. Gamma rays are generated in the most violent parts of the Universe, being the result of explosions or high-energy collisions. One of the most spectacular observations has been the detection of gamma-ray bursts, lasting from fractions of a second to some minutes. They appear to come from the distant Universe; therefore, they are some of the most energetic events ever observed. NASA’s Swift satellite is devoted to detecting these bursts and identifying their source.
5.3.8 Space Physics
In the field of space physics, space borne instruments primarily make measurements in situ but imaging systems at specific wavelengths are used to observe aurora and the Sun. The main instruments in this area consist of static magnetic and electric fields (DC), electromagnetic radiation with frequencies from Hz to MHz, and a variety of charged particle measurements. Charged particles (ions and electrons) are measured in specific directions with respect to the magnetic field (called pitch angles) over specific energy ranges extending from eV to GeV. Ion measurements, typically from mass spectrometers, can be made as total ions or delineated species such as hydrogen, helium, oxygen, etc., at various states of ionization. Recently, a number of space physics instruments have started to observe “hot neutral” ions which have their origin in charge exchange processes between a cold neutral atmospheric ion giving up an electron to a fast moving ion (typically accelerated within a magnetosphere) in a collision. These neutral atom detection instruments can create pseudo images of large spatial regions. Magnetospheric radio sounding has also been used to find the density distributions of remote regions of plasma to very low densities (~ 10^5 /m^3) and to very great distances from the Earth (up to 30,000 km).
5.3.9 Earth Observation
Earth science uses both active and passive remote sensing techniques to make observations of the integrated Earth system (i.e. the land, oceans, ice sheets, and atmosphere), especially from Low Earth Orbit. Passive sensors detect scattered, reflected and/or absorbed solar radiation and emitted thermal radiation from the Earth to acquire spectral, spatial and temporal measurements of the solid Earth, its ice caps, the oceans, vegetation and the atmosphere. Typical instrument pointing is nadir (straight down) or side-scan to cover large land areas, while some instruments use solar and lunar occultations in a limb-scanning mode to probe the atmosphere with high vertical resolution. Limb scanning is typically used to make measurements of the vertical structure of aerosols, ozone, water vapor, and other important trace gases in the upper troposphere and stratosphere. Active remote sensing employs pulses of electromagnetic energy generated by the orbiting instrument (typically lasers and microwaves) transmitted to the Earth and scattered, reflected or absorbed and re-radiated by the atmosphere, oceans, or land surfaces. A tiny fraction of the scattered or reflected energy is collected by a telescope and detected. Typical lidar and radar measurements include sounding and profiling of the atmosphere to deduce the atmospheric composition, cloud and aerosol distributions, major/minor trace gas content and wind speeds and directions. Lidar and radar are also used to map the topography of the solid Earth, glaciers and polar ice caps, profile vegetation, measure sea surface heights, and probe the subsurface structure with ground penetrating radar.
5.3.10 Planetary Observation
For planetary science many of the same instruments used for Earth science and space physics are applicable. However, in addition to orbital remote sensing techniques, planetary landers and rovers carry out in situ measurements. These include geophysical instruments such as seismometers and heat flow probes, mass spectrometers and gas chromatographs for compositional and organics analysis, neutron and gamma ray spectrometers as well as X-ray diffraction and X-ray fluorescence instruments to discover the elemental composition of a sample. Compositional analysis can also be done with laser ablation spectroscopy and laser induced breakdown spectroscopy from several meters away from the rover on the planetary surface. Further, for planetary bodies with atmospheres, acoustic anemometers and lidars can measure vector winds and profile clouds and aerosols from the surface up to > 10 km altitude. Radio science occultation techniques use an orbiting spacecraft’s Radio Frequency (RF) transmitter to make measurements from which the ionospheric and atmospheric prop-erties of a planetary body may be derived. In such cases the spacecraft moves behind the planet while using its RF signal to communicate back to Earth. The planet’s ionosphere and atmosphere cause dispersion and refraction of the RF signal that can be measured and used to better understand that intervening environment.
5.4 Astrobiology
Astrobiology crosses the boundaries of all disciplines in order to answer some of the most fundamental questions: “Is there life outside of our planet?”, “Does it look (biologically) like us?” and, “Are there other intelligent life-forms in the Universe?” Here we will start with an introduction to Astrobiology and set the conditions for life as we know it and dis-cuss why this is an important endeavor. Finally, we will present an overview of some of the most exciting opportunities available in the Astrobiology research community.
5.4.1 What is Astrobiology?
Carl Sagan (Cosmos, 1985) wrote: “The nature of life on Earth and the search for life elsewhere are two sides of the same question: the search for who we are .” Thus astrobiology is the study of the origin, evolution, distribution, and future of life in the Universe. It addresses three basic questions: how does life begin and evolve, does life exist elsewhere in the Universe, and what is the future of life on Earth and beyond? By its very nature, astrobiology research is international, interdisciplinary, and intercultural and has many societal implications beyond the purely scientific. Its success depends critically upon the close coordination of diverse scientific disciplines such as molecular biology, ecology, planetary science, astronomy, information science, space exploration technologies, and other related disciplines. At the same time, astrobiology encourages planetary stewardship, the recognition of ethical issues associated with exploration and societal implications of discovering other examples of life, and envisioning the future of life on Earth and in space. Astrobiology has a strong emphasis on education and public outreach; it offers a crucial opportunity to educate and inspire the next generation of scientists, technologists, and informed citizens.
Seven science goals have been developed in order to try to begin to answer these questions that have been asked in various ways for generations: understanding the nature and distribution of habitable environments in the Universe, exploring for habitable environments and life in our solar system, understanding the emergence of life, determining how early life on Earth interacted with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth.
5.4.2 Conditions for Life
The search for life begins with an understanding of what are the basic necessities for life. These lead to the determination of the requisite conditions to support these basic necessities and ultimately define specific parameters that serve as our guide for locating and detecting life or the signatures of life. The most basic necessity to sustain life as we know it is access to liquid water, but the existence of liquid water alone does not guarantee that life will develop. Additional requirements include elements such as carbon, nitrogen and phosphorus and an energy source. The energy source may either use inorganic compounds (for chemoautotrophic life forms) or organic compounds (heterotrophic life forms), or light (phototrophic life forms).
Habitability, or the factors which when combined create the necessary conditions to support life, depends upon the characteristics of both the star system and the planetary body. The size and type of the central star most directly affect this, with the intensity of radiation output defining a minimum and maximum orbital radius from the star within which liquid water exists on a planetary surface. The Habitable Zone (HZ) of our solar system is from about 0.9 to ~ 1.5 AU. Only the Earth lies clearly within this boundary, with Mars being on its extreme outer edge. Distances less than the minimum result in super-heated surfaces incapable of supporting liquid water, such as found on Mercury, or conditions which cause a runaway greenhouse gas situation such as found on Venus. At distances greater than the maximum conditions are too cold to support liquid water (at least on open surfaces) as, for example, on the moons around the gas-giants.
A planets’ capability to support liquid water can be modified to a certain extent by the type and characteristics of the planet (i.e. factors which include its size, geological activity, mineralogical composition, magnetosphere, and the type and thickness of its atmosphere). Furthermore, liquid water can exist well outside this classical habitable zone. The tidal heating of a moon around a planet outside the habitable zone can sustain liquid water, such as is exemplified by the Jovian moon, Europa.
While the minimum and maximum distances vary from star-system to star-system, generally, the larger the star, the hotter and quicker it burns. This could mean that life will neither have the appropriate conditions nor the necessary time to develop. Planetologists interested in finding potential habitable planets in other star systems therefore focus their searches on stars that are known to have the correct characteristics. The gas-giants so far discovered around large stars may have habitable moons and, thus, these stars cannot be automatically eliminated from the search for far-away habitable planetary bodies.
5.4.3. Why is Astrobiology an Active Science Area Now?
In his book A Pale Blue Dot, Carl Sagan noted: “The Earth is the only world known so far to harbor life.” The search for life in our solar system is a continuous process. There have been many ideas over the years, such as hidden cities under the thick layers of the clouds of Venus and the infamous canals of Mars first drawn by Giovanni Schiaparelli in 1877 and supposed by Percival Lowell in 1895 to be of artificial origin. In both cases, theories and myths were over-ridden as technology improved direct observations and after spacecraft had landed on these planets in the 1960’s. The Venus missions, principally led by the former USSR, determined that the surface conditions were too harsh for life to survive due to high pressures, temperatures, and acidity. The US Mars Viking missions included one experiment designed to detect CO2 respiration of active microbial life, and a second to detect organic carbon by super-heating a soil sample and measuring vaporized compounds via Gas Chromatograph-Mass Spectrometry (GC-MS). The results of these, while still controversial, have determined that neither life nor life signatures were detected. In more than forty years of searching for life within our solar system, the results continue to be frustratingly negative; however, recent results across many areas give plenty of optimism that our search for extra-terrestrial life, both inside and outside our solar system, is advancing, thus fueling our desire to continue the search.
The origin and rise of life on Earth is the subject of much research from scientific, philosophical and religious perspectives. From the scientific point of view, the conditions on early Earth that led to the increase of life has to be understood in order to judge the capacity for the rise of life in similar situations on other planetary bodies, both in and beyond our solar system. Currently it is estimated that life originated on Earth approximately 3.9 billion years ago. This is significant because it is a mere 700 million years after the formation of the solar system and barely 100 million years after the Earth’s surface cooled enough to allow the formation of stable permanent oceans and the accumulation of organic compounds. Also 3.9 billion years ago was at the very end of the period of heavy bombardment during which large meteorites were colliding with the Earth and continuously re-vaporizing the oceans. That would possibly have destroyed any build-up of organic compounds (either those which were arriving to the Earth’s surface from the very same meteorites or those which were being naturally created by Earth’s reducing atmosphere of the time). This is a very surprising revelation, indicating that the genesis of life occurred very quickly once there were stable liquid oceans and access to organic compounds and an energy source (in this case, light from the Sun).
This is very important because it is believed that there was also liquid water on Mars, Europa, and Enceladus, all of which had access to the same organic compounds arriving from meteorites, and all at the same time as when life arose on Earth. Even if life does not exist on any of these planetary bodies today, it would appear that conditions and time were sufficient for the rise of life there. The discovery of fossils or other ancient life signatures would be of great significance to our understanding of the conditions leading to the genesis of life and the ease of its genesis. This begs the question: is the genesis of life common or is the genesis of life rare and, perhaps, unique to Earth?
The great number of thriving ecosystems in extreme environments on Earth demonstrates the durability and adaptability of life. The limits of life on Earth are broad; life is found, or has been found to be able to survive, under many of the most extreme environments imaginable: high and low temperatures, high and low pressures, high acidity (e.g. pH ~ 0), high salt, and high radiation. Hyperthermophilic and anaerobic chemoautotrophic bacteria have been found at the highest temperature and pressure environments at the deep-sea vents at the bottom of the Marianas Trench in the Indian Ocean, at a pressure of more than 1200 atmosphere and at 113˚C, and 3 km underground in the East Driefontein Gold Mine of South Africa. Endoliths (microorganisms that inhabit the interior of rocks) and hypoliths (that inhabit the underside of rocks) have been found under quartz rocks in the hyper-arid Taylor dry valleys of Antarctica and the hyper-arid Atacama Desert of Chile and Peru. The ammonia-oxidizing bacterium, Nitrosomonas cryotolerans , has been shown to be active in -18˚C ice cores and thus to explain high N2O levels found in ice cores from Lake Vostok, Antarctica.
The Deinococcus radiodurans bacterium is an example of a polyextremophile, which is one of the most radioresistant organisms known; it can survive cold, dehydration, vacuum, and acid. But D. radiodurans is not the only organism that is able to survive in a vacuum as the Streptococcus mitus was found in the Surveyor III camera after three years, unprotected, on the lunar surface. Recent experiments on NASA’s Long Duration Exposure Facility and the European Space Agency’s BioPan space experiments have shown that Bacillus subtilis spores and halophiles in the active (vegetative) state can survive direct exposure to the raw conditions of space. The use of extreme environments as “analog” environments for other planets gives astrobiology a strong field component. No single environment equates exactly in all physical and chemical extremes to other planets, but some environments can be used to study the response of microorganisms to certain types of extremes. For example, the Atacama Desert (see above) is used as an environment to study life at extreme desiccation. Asteroid and comet impact craters are used to study the habitats created in these environments for microorganisms; volcanic environments yield insights into how microorganisms can access minerals as a source of nutrients and energy. The information from all of these environments provides astrobiologists with a means to focus the search for life elsewhere.
The expansion of the limits of life on Earth has had a dramatic affect on the expanding the search for life in the solar system. While the many missions led by the former USSR found the surface of Venus to be uninhabitable by life as we know it, the conditions on Venus for its first 1-2 billion years may have sustained liquid water on its surface. Life could have had the chance to develop and, over time, communities could have grown in extreme niches as conditions changed to the current state. Some scientists believe that there are regions of the atmosphere where microbial communities could float at pressures, temperatures, and highly acidic conditions which are not all that dissimilar to conditions found in highly acidic thermal baths such as the Norris Geyser Basin in Yellowstone National Park, USA.
Mars, at the outer limit of the HZ, is a barren desert with no visible life, much like the Dry Valleys of Antarctica and the hyper-arid Atacama Desert in Chile and Peru. While Mars is unlikely to have life directly on the surface today, there is evidence that liquid water once existed on the surface of Mars for many millions of years at the same time as life first developed on the Earth. There is also evidence of liquid water from time to time in more recent years, which, if there are sufficient nutrient and energy supplies, could support a thriving subsurface microbiological community.
Jupiter’s moon, Europa, is located far beyond the traditional HZ, but there is significant evidence that tidal forces cause enough internal heating to maintain a liquid ocean below the icy surface. The environment could be similar to that expected to be found in the liquid Lake Vostok, Antarctica, 4 km under the surface of the central Antarctic ice sheet.
Another active field of research is the search for, and study of, planets orbiting distant stars – extra-solar planets. As of early 2008, the count for the discovery of extra-solar planets stood at 277. There are a few Earth-like planets such as OGLE-2005-BLG-390LB, which is 5.5 times the mass of Earth, located more than 20,000 light years away towards the center of the galaxy. Earth-like planets are expected to be in at least 1% of the star-systems that contain planetary systems. Further, it has recently been shown that organic material is found widely in the outer solar system, in comets and meteorites, and within the interstellar medium. This would indicate that the three basic conditions for life -stable planets, access to organic matter, and an energy source (light) - might be very common throughout our galaxy.
5.4.4 Opportunities in Astrobiology Research
Astrobiology research covers many diverse areas, but the major focus of astrobiology can be summarized in five searches:
- The search for the origin of life on Earth. Understanding the origin of life on Earth, a major scientific question of all time, would provide us with a sound basis for considering how life might have begun on other worlds. Evidence about the origin of life comes from several fields. Laboratory simulations such as the Miller-Urey synthesis of organics under possible early Earth conditions test direct chemical models for the origin of life. The geological record provides evidence on both the conditions existing on the Earth and the timing when life began. Finally, the biochemical record of phylogenetics and metabolic pathways provides information about the nature of early life.
- The search for the limits of life. Understanding the biochemical and ecological limits to life on Earth provides a basis for assessing the habitability of environments on other worlds. There are two complementary aspects of this search. The first is to understand the biochemical limits of organisms. These include temperature, UV and ionizing radiation, acid-base properties, and salinity. The second aspect, understanding under what range of conditions communities can survive, has seen remarkable discoveries in recent decades. Ecosystems have been found in unexpected places, such as deep below the surface in basaltic rocks, at the bottom of glaciers, and in deep-sea vents.
- The search for life on other worlds in our solar system. A key goal for astrobiology is the detection of a second genesis of life. Having a second example of life would allow us to compare the biochemistry of our life against this new alien life. Such a comparison may provide deep insights into what aspects of our biochemistry are essential and what aspects are accidental. It may also allow us to address questions about the nature of life at the level of life itself. Indeed, there may be a number of questions about life that we will not be able to answer until we have another example for comparison with ours. In our solar system the most likely targets for a search for life are, as mentioned earlier, Mars, Europa, and Enceladus. On Mars we might find living organisms in water reservoirs, if they still exist. Another location where we might find intact and preserved organisms, although probably dead, is in the ancient ice-rich ground in the polar regions. In both of these cases we would be able to study the biochemistry of the life that we find and compare it with the biochemistry of Earth life to determine if it indeed represents a second genesis. We cannot assume that life on Mars is a second genesis without proof, because we know that, as meteorites, rocks can exchange between Mars and Earth, and presumably in the other direction as well. These rocks could have exchanged microbial life between these worlds, making Earth life and Mars life identical. There is good evidence that Europa and Enceladus have liquid water below their icy surfaces. On Europa ocean water may be carried to the surface through the ridges seen on the surface ice. On Enceladus, the geyser of water jetting out from the South Pole may come from a deep, pressurized aquifer. In both cases, any biological material in the subsurface water might be carried to the surface, providing a target for collection on a space mission. As on Mars, the direct biochemical analysis of any organic material collected may allow us to determine if it represents biologically produced organics and if the biology that produced them has a different biochemistry.
- The search for evidence of life on other Earth-like planets around other stars by looking for oxygen or ozone. Current technology cannot directly determine the presence of life on such a planet; however, the detection of O2 or O3 in the atmosphere by spectroscopic techniques could be a strong indication of life. That could provide additional indications of life that has advanced considerably in terms of environmental evolution.
- The Search for Extraterrestrial Intelligence (SETI). The long-standing search for radio signals from extraterrestrial civilizations continues and at an ever increasing rate due to advances in computer technology. However, no signal has been detected after forty years of effort. Experts in the SETI methods suggest that, if the current exponential growth rate in search capabilities continues, then the effective search will be completed in about another forty years. It will never be possible completely to remove the possibility of an undetected signal, and so the main SETI search may continue indefinitely.
Footnotes
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- ^ Ness, N. F., C. S. Scearce and J. B. Seek, 1964, Initial results of the IMP-1 magnetic field experiment, Journal of Geophysical Research, 69, 3531- 3569.
- ^ Goldstein, M.L., 2005, Magnetospheric physics: Turbulence on a small scale, Nature, 436, 782- 783.
- ^ Kivelson, M. G. and C. T. Russell (eds), 1995, Introduction to space physics, Cambridge University Press, 568 pp.
- ^ Gustafsson, G., R. Bostrom, G. Holmgren et al., 1997, The electric field and wave experiment for the Cluster mission, Space Science Reviews, 79, 137- 156.
- ^ Fritz, T.A. and S. F. Fung (eds), 2005, The magnetospheric cusps: Structure and dynamics, Surveys in Geophysics, 26, 1- 414.
- ^ Santolik, O. and D.A. Gurnett, 2003, Transverse dimensions of chorus in the source region, Geophysical research Letters, 30(2), 1031, doi:10.1029/2002GL016178.
- ^ Green, J. L. and U. S. Inan, 2007, Lightning effects on space plasmas and applications, Chapter 4 in Plasma Physics Applied, Research Signposts, C. Grabbe (ed.), 12 pp.
- ^ Rodger, C. J. and M. A. Clilverd, 2008, Magnetospheric physics: Hiss from the chorus, Nature, 452, 41- 42.
- ^ Akasofu, S.-I., 1968, Polar and magnetospheric substorms, D. Reidel, Dordrecht, Holland, 280 pp.
- ^ Rees, M. H., 1989, Physics and chemistry of the upper atmosphere, Cambridge University Press, 289 pp.
- ^ Lemaire, J. F. and K. I. Gringauz, 1998, The Earth’s plasmasphere, Cambridge University Press, 350 pp.
- ^ Burch, J. L., 2003, The first two years of IMAGE, Space Science Reviews, 109, 1- 24.
- ^ Kaplan, E. D. (ed.), 1996, Understanding GPS: Principles and applications, Artech House Publishers, Boston, 554 pp.
- ^ Hunsucker, R. D. and J. K. Hargreaves, 2003, The high-latitude ionosphere and its effects on radio propagation, Cambridge University Press, 617 pp.
- ^ Lilenstein, J. (ed.), 2007, Space weather; research towards applications in Europe, Springer, Dordrecht, The Netherlands, 330 pp.
- ^ Additional Readings, Astrobiology:
M. Marov (1986). Planets of the solar system. Nauka, Moscow. (Translated into German, Verlag –Nauka, 1987, and Spanish, MIR-Editorial, 1986). D. Morrison and T. Owen (1988). The Planetary System. Addison-Wesley Publ. Co., Menlo Park /NewYork /Wokingam /Amsterdam /Bonn /Sydney /Singapore /Tokyo /Madrid /Bogota /Santiago /San Juan.
M. Marov and D. Grinspoon (1998). The planet Venus. Yale University Press.
Encyclopedia of the solar system (1999). Eds. P. Weisman, L.-A. McFadden, and T. Johnson, Academic Press, San Diego/ London/Boston/New York Sydney/Tokyo /Toronto.
J. Kelly Beatty, C.C. Peterson, and A. Chaikin (1999). The New solar system. Cambridge University Press.
Lada, and N. D. Kylafis (1999). The Origin of Stars and Planetary Systems. Kluwer Academic Publishers, Dordrecht/Boston/London.
Benz, R. Kallenbach, and G. Lugmair, Eds. (2000). From Dust to Terrestrial Planets. Kluwer Academic Publishers, Dordrecht/Boston/London.
Marov (2002) Collisions in the solar system: Implications for Planetary Atmospheres Origin. Space Times, AAS, issue 5, v. 41, pp. 10-15.
Williams, and N. Thomas (2001). Solar and Extra-Solar Planetary Systems. Springer, city.
Marov, H. Rickman, eds (2002) Collisions in the solar system. Kluwer Academic Publishers, Dordrecht/Boston/London.
Lewis, J. S. (1997) Physics and Chemistry of the solar system (revised ed.), New York: Academic Press.
Wilhelms, D. E. (1993) To a Rocky Moon: a Geologist’s History of Lunar Exploration, Tucson, AZ: University of Arizona Press.
Spudis, P. D. (1996) The Once and Future Moon. Washington: Smithsonian Institution Press.
King, L.C. (1967). Morphology of the Earth, 2nd Ed. Oliver and Boyd Ltd., Edinburgh.
Des Marais, David J., Joseph A. Nuth, Louis J. Allamandola, Alan
P. Boss, Jack D. Farmer, Tori M. Hoehler, Bruce M. Jakosky, Victoria S. Meadows, Andrew Pohorille, Bruce Runnegar, Alfred M. Spormann.(2008). The NASA Astrobiology Roadmap. Astrobiology . 8(4): 715-730. doi:10.1089/ast.2008.0819.
Drake, Frank and Dava Sobel (1993). Is Anyone Out There? Souvenir Press Ltd.
Gilmour, Iain, and Mark A. Sephton (2004) An Introduction to Astrobiology . Cambridge University Press.
Lunine, Jonathan (2004). Astrobiology: A Multi-disciplinary Approach . Benjamin Cummings.
Sagan, Carl (1985). Cosmos . Ballantine Books.
Sagan, Carl (1994). Pale Blue Dot: A Vision of the Human Future in Space . Random House.
Woodruff, T. Sullivan III, and John Baross (2007). Planets and Life: The Emerging Science of Astrobiology . Cambridge University Press. Online Astrobiology
Resources:
- NASA: http://astrobiology.nasa.gov/
- European Astrobiology Network Association (EANA): http:// www.astrobiologia.pl/eana/
- Astrobiology Magazine: http://www.astrobio.net/news/
- International Journal of Astrobiology: http://journals.cambridge. org/action/displayJournal?jid=IJA
The Farthest Shore – Chapter Six Satellites Serving Humankind
