Chemical Evolution across Space & Time - American Chemical Society


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Chapter 7

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Chemical Diversity and Abundances across the Solar System John S. Lewis Department of Planetary Sciences and Lunar and Planetary Laboratory, University of Arizona, Tucson,AZ85721

Chemical zoning of the Solar System, first noted by Harrison Brown in 1950, displays three major compositional classes distinguished by volatility, ranging from rocky material close to the Sun through ice-rock mixtures in more distant small bodies, to massive gas-giant planets rich in "permanent gases", notably hydrogen and helium. Spacecraft exploration of the Solar System, laboratory studies of asteroidal, lunar, and Martian materials, and theoretical studies of accretion, condensation, melting, differentiation and partitioning of rocky and icy materials combine to reveal many additional features of these categories. The oxidation state gradient of the terrestrial planets, the chemical complexity of planetary atmospheres and the asteroid belt, the spectra (indeed, the very existence!) of the Centaur and Trans-Neptunian Objects, and compositional zoning in Jovian satellite systems combine to provide a more detailed picture of genetic processes soon to be tested by observations of other stellar systems.

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Introduction Both conditions of origin and evolutionary pathways have profoundly shaped the material of Earth and other Solar System bodies. Our first datum, Earth, has provided us for over 2000 years with many ideas of what other worlds must be like; indeed, the literature on the "plurality of worlds", the απεροι κόσμοι of Greek philosophy, saw a vast universe with Earthlike worlds at every star, whereas Aristotle held that Earth was the only world. But which of the features of Earth are universal, foreshadowing other worlds not yet discovered, and which are idiosyncratic, rare and possibly altogether absent on other worlds? When we see and begin to study other "Earth-like" worlds, such as Mercury, Mars, and Venus, their differences are attributed either to different circumstances of origin, or to divergent evolution from a common starting point. This cosmic version of the nature/nurture debate is at the heart of many current disputes about Solar System bodies and processes. As we shall see, such an "either-or" dichotomy has little value, since both conditions of origin and "evolutionary" paths are both closely linked to distance from the Sun. There is a further confusion inherent in the usage of the word "evolution" in astronomy and geology, where it is generally used to imply the ageing of specific single bodies rather than progressive change in a population of bodies. Thus the "chemical evolution of the Galaxy" makes sense, whereas the "evolution of Venus" does not. We would be better advised to speak of "historical chemical and structural changes" of Solar System bodies rather than their "evolution". I shall call this "history" for the sake of conciseness. The history of the material of the Solar System is illustrative of the allpervading war between entropy (abetted by temperature) and enthalpy, in a sense that would be very familiar to J. Willard Gibbs. Raw preplanetary material, an inheritance from the interstellar medium, may have been chemically and physically rather uniform on centimeter scales and larger, but even that material hosted a wide variety of small grains with different astrophysical provenances and histories, and different chemical and isotopic compositions: although it was highly heterogeneous on the micrometer scale, it may well have been grossly homogeneous on macroscopic scales. The entropy of this mixture was very high. But even this limited degree of uniformity breaks down quickly as a shrinking interstellar cloud gives rise to an accretion disk with profound gradients of temperature, pressure, and density. In the inner part of the disk temperatures were high enough to allow a close approach to thermodynamic equilibrium. In the outer regions, species that condense at low temperatures may have approached equilibrium, but high-temperature condensate grains inherited from the protostellar cloud would survive with little equilibration. It is in this cloud that the history of the Solar System as a distinct entity finally begins. That history can be considered to consist of three eras. Thefirst,the nebular phase, transpires in a flattened, disk-shaped gas and dust cloud of interstellar

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origin, surrounding the forming Sun. The second, accretionary, phase, comprises the physical and chemical processes associated with the assembly and growth of young planets. The third, which overlaps somewhat with the accretionary phase, is the differentiation and outgassing phase, during which heat sources such as accretion energy and radioactive decay melt planetary material and allow it to separate into layers of different density and composition, during which the minor and trace elements partition themselves between metals, sulfides, silicates and volatiles according to their chemical affinities.

The Nebular Phase A young prestellar nebula, rich in gases and polyatomic molecules, has such high infrared opacity that most of it convects and circulates in its own gravitational field as an adiabatic mixture of gas and dust in which changes in gravitational potential (GM/r) govern the thermal energy of the gas (kT). In general, most of such a nebula, excepting only regions where the opacity is low and radiative processes control temperatures, displays a strong temperature gradient in which Τ varies closely with 1/r. Where radiative control dominates, the luminosity (L) of the proto-Sun soon provides a radiant flux that drops off according to an inverse square law (L/47tr ), in which solid particles absorb that flux rather efficiently and re-radiate it in a steady state as thermal radiation (σΤ ). In this radiatively controlled regime, T varies as 1/r , and Τ is therefore proportional to l/r , a much weaker dependence. Nonetheless, from the perihelion of Mercury (0.31 Astronomical Units from the Sun) to the aphelion of Mars (1.68 AU), the temperature must still have varied by a factor of 2.3. Since the vapor pressures of solids vary exponentially with 1/T, this is a very significant difference. The conclusion is inescapable: the raw materials available for planetary accretion varied strongly with location in the pre-solar accretion disk, from fully vaporized close to the Sun to thoroughly baked and equilibrated solids in the terrestrial planet region to weakly heated and poorly equilibrated solids in the outer half of the asteroid belt. The main features predicted by chemical equilibrium models (which pertain directly to the inner disk) include successive condensation of refractory oxides and metals, metallic iron-nickel alloy, magnesium silicates, feldspars (aluminosilicates of Ca, Na, and K), FeS, FeO-bearing silicates, and -OH- and H 0-bearing silicates. Departures from local equilibrium in newly accreted solid bodies could be caused by any of three major factors: imperfect gas-phase equilibrium due to kinetic inhibition of reactions such as the reduction of CO and N to C H and N H ar low temperatures, radial mixing of materials originating at different distances from the Sun, and relict presolar grains originally formed in distant astrophysical settings such as nova or supernova shells or mass outflow 2

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from giant stars. Where temperatures are low and complete equilibration cannot be achieved, relict high-temperature grains survive nearly unaltered, as in the carbonaceous asteroids and comets, accompanied by organic matter, with lowtemperature condensates such as water ice which are in equilibrium with the ambient gases. As a result, bulk composition and density closely follow the predictions of perfect equilibrium, whereas refractory relict grains may be far out of equilibrium with the gas and other neighboring grains. This chemistry is reflected in detail in meteorites.

The Accretionary Phase Most discussions of the accretion history of the terrestrial planets are based on a series of models that begin with a population of 100-200 equal-mass moonsized bodies in moderately eccentric 2-dimensional orbits (/, 2). Further refinement of this model, including its generalization to three dimensions (3, 4) led to Wetherill's (5) accretion models and Lewis's (6) condensation-accretion models for Mercury and the terrestrial region. A logical consequence of the initial conditions assumed for these models is that disruptive collisions between large bodies occur , and that the absence of a substantial mass of smaller bodies removes the damping effects of accretion of small bodies on the migration of large bodies. Wide wandering of the growing planets occurs, largely erasing any initial gradient in composition, and leading to a highfrequencyof "pathological cases" in which the simulation produces "Mercuries" that may have originated nearly anywhere in the inner solar system. The effect of accretion of the vastgnumbers of smaller bodies omitted from these simulations may be simulated by using Gaussian accretion sampling models that allow for a substantial composition gradient across the terrestrial planet region. Such models predict a high-temperature, FeO-poor and volatile-poor composition for Mercury, similar to the highly reduced Mercury model proposed by Wasson (7). The terrestrial planet region, in addition to these condensation and accretion processes, can be shaped by radial migration of growing planets under the combined influence of dynamical interaction with the gas disk and gravitational interactions with other planets, especially their (moving) orbital resonances, in much the same way as suggested by Malhotra for Pluto's orbit (8). 5

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The Differentiation and Outgassing Phase The accompanying review by Fegley discusses both the release of volatiles from planetary interiors during melting and density-dependent differentiation and their subsequent compositional evolution (9). The present composition of planetary atmospheres reflects a richly complex history.

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Evidence From Across the Modern Solar System

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1. TheTerrestrial Planet Region Different amounts of compositional information of different sorts are available for each terrestrial planet: the only two lines of evidence available for all of them are bulk density and oxidation state. The bulk densities, however, must be corrected to "zero pressure" (actually, 1 atmosphere) to remove the effect of internal self compression and afford a directly relevant measure of composition (see Table I).

Table I. Observed and Zero-Pressure Densities of Terrestrial Bodies Body

Observed density (Mg/m ) 5.43 5.24 5.515 3.93 3.34 2.2 1.7 3.57 3

Mercury Venus Earth Mars Moon Phobos* Deimos* Io

Zero-Pressure Density (Mg/m ) 5.30 4.00 4.05 3.74 3.32 2.2 1.7 3.54 3

*Phobos and Deimos are so small that they may contain substantial void space

These corrections are large for the terrestrial bodies with the largest masses and internal pressures, Venus and Earth, and smallest for the Moon and Mercury. Note that making precise corrections requires some prior knowledge of the compressibility, and hence the mineralogy and structure, of the interior. These estimates assume that the same basic minerals (Fe-Ni, FeS, and silicates) are present in each body and that all are fully differentiated. The choice of models can be further narrowed by consideration of the reduced principal moment of inertia, I/Mr which is close to that of a uniform sphere (I/Mr = 0.4) for undifferentiated and uncompressed (low-mass) bodies The Moon displays a very small degree of central mass concentration (10% with I/Mr = 0.3932±0.0002 consistent with a metallic core with mass no greater than about 2.5 to 3% of the mass of the Moon, and possibly as low as 0.5%. Mars' value of I/Mr = 0.3650±0.0012 (//), is small enough to require differentiation, but too large to fit models in which the core and mantle have the same uncompressed density as Earth's core and mantle. The easiest explanation of this difference is that much more of the iron in Mars is oxidized, lessening the 2

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mass and density of the core and raising the mass and density of the mantle. Such a model is compatible with core sulfur contents up to pure FeS composition (12) An alternative explanation, that Mars is incompletely differentiated, is ruled out by the study of Martian surface rocks and dirt, Martian meteorites, and planetary thermal history models. There is also direct evidence regarding the oxidation state of terrestrial bodies and a gradient in FeO content with distancefromthe Sun (Table II).

Table IL FeO Contents of Terrestrial Bodies FeO Content -2% 7%? 8% 12% 18%

Body Mercury Venus Earth Moon Mars

The FeO content of Mars given here is consistent with the geochemical models of Dreibus and Wânke (13) and with laboratory studies of Mars-derived SNC meteorites. Since FeO content is a sensitive marker of the temperature of origin, the original location of bulk lunar material appears to have been between the orbits of Earth and Mars. The FeO content of Mercury's crust is probably best quoted as 1.5±1.5%, since it remains without any firm detection, and its presence cannot be verified at the 3% level (14). Temperatures in the solar accretion disk dropped off inversely with heliocentric distance (T~l/r) reflecting the inter-conversion of gravitational potential energy, GM/r, and thermal energy, kT, in the turbulent gas disk. The thermodynamic activity of FeO varies according to log a o ~ Î/T ~ r. Fe

2. The Asteroid Belt Much of the progress in understanding the compositions of asteroids has come from comparing the spectra of known meteorite types and meteoritic mineral separates to astronomical spectra of asteroids. Over 20 spectrally distinct classes of asteroids have been found, ranging from the essentially FeOfree differentiated silicates of the Ε class (dominated by the FeO-free mineral enstatite, MgSi0 ) at the inner edge of the asteroid belt to black, highly oxidized and often water-rich carbonaceous C-, D-, and P-class material in the outer half of the belt, which are rich in polymeric organic matter made by the inability of CO and N to equilibrate to N H and C H at low temperatures (15). 3

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3. The Meteorite Connection The approximately 50 known classes of meteorite, excluding those known to have originated from the Moon or Mars, span a wide range of compositions. Most meteorites are stones. The remaining classes of meteorites, the irons and the stony-irons, are products of melting and differentiation. They consist mainly of metallic iron-nickel alloys, sulfides, carbides, phosphides, and igneous silicates. (See the chapter by M . Lipschutz in this volume). The stone meteorites, which constitute the overwhelming majority of those falling on Earth, fall into two general categories. First, there are the most abundant type of stones, which are fairly homogeneous mixtures of grains of silicate, oxide, sulfide, metal and other minerals with widely different densities and melting points, often far out of chemical equilibrium with each other, and showing no signs of melting and density-dependent differentiation. Since most of these meteorites contain small glassy spherules called chondrules, the meteorites themselves are called chondrites. In contrast, those stony meteorites that have undergone melting, equilibration, and density-dependent differentiation are called achondrites. The chondrites are subdivided into about a dozen classes distinguished by their content of metal, FeO, water and other volatiles. Some, with very low volatile and FeO contents, are clearly of relatively high-temperature origin. Since their dominant silicate mineral (as in the Ε asteroids) is the FeO-poor silicate enstatite (MgSi0 ), they are termed enstatite (E) chondrites. Their low FeO content betrays a high-temperature origin. Certain other chondrites are rich in water-bearing minerals, organic compounds, magnetite (Fe 0 ), and watersoluble salts. They are called carbonaceous (C) chondrites. (The D and Ρ classes of asteroids, which appear to be "super-carbonaceous", have no counterpart among the meteorites known on Earth.) The C chondrites, having formed at the lowest temperatures, contain the most complete inventory of volatile and moderately volatile elements. They are often referred to as the most "primitive" meteorites, meaning that they are closest to the parental solar material in composition. The word "primitive" does not imply that planetary bodies and the other meteorite classes are derived from or descendedfromthem. Many workers, taking the word literally, have assumed that the planets were originally made of C-type material, which unfortunately is the class of chondrites that is the least similar to the present terrestrial planets, and have than expended heroic efforts trying to "evolve" C-type material into the present Earth. Workers often speak of "chondritic" compositional models for the terrestrial planets, but some use the phrase to denote "C chondrites" and some take it to mean "a mix of chondrite classes" or "material mineralogically similar to chondrites, but not exactly the same as any single class". This confusion is a perennial source of confusion/ Caveat lector. 3

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4. The Giant Planets The four giant planets have hydrogen- and helium-rich compositions reminiscent of the Sun, but all of them clearly depart from strict solar composition in that their densities are too high and the few heavier elements whose tropospheric abundances can be measured all show clear evidence of enrichment. For all four giant planets we have spectroscopic compositional data on the few compounds that remain uncondensed in the visible portion of their atmospheres, above their main cloud layers. These include ammonia, methane, phosphine, and germane. For Jupiter, these volatile elements (C, N , S, Ρ and Ge) are enriched relative to their solar abundances by about a factor of five. For Saturn, with no detection of germane, the enhancement of C, N , and Ρ is about a factor of 10. For Uranus and Neptune the methane enrichment factor is at least 60, consonant with their much higher uncompressed densities. The Galileo entry probe was sent to Jupiter to sound the atmosphere down through the main cloud layers, to profile the clouds and measure the abundances of the important cloud-forming condensates ammonia, water vapor and hydrogen sulfide in warm, upwelling, condensate-free air. Improbably, and unfortunately, it fell into and sampled a rare region of subsiding, cloud-free, and extremely dry stratospheric air.

5. Satellite Systems of the Giant Planets The giant planets display "miniature solar systems" of close moons in loweccentricity, low-inclination orbits, families of highly-inclined moons at intermediate distances, and vast swarms of small retrograde moons at greater distances. Clear evidence of radial compositional trends in the inner satellite system of Jupiter suggests a close analogy with composition gradients observed in the solar system at large. The inclined orbits of intermediate-distance moons and the retrograde outer moons resemble the transition from the coplanar inner Solar System to the inclined plutinos to the randomly-orbiting long-period comets in the Oort cloud.

6. Planetary Sub-nebulae: the Galilean Family The four Galilean satellites of Jupiter show a strong density gradient. The closest of these satellites to Jupiter is Io (Jl), which has a Moon-like size and density, suggestive of highly oxidized rocky material with no condensed ices. Io has a volcanically active surface dominated by sulfur volcanism in a trace atmosphere largely composed of S0 . Hydrogen compounds are rare or absent. The next satellite outward from Jupiter, Europa (J2), has a lower density 2

Zaikowski and Friedrich; Chemical Evolution across Space & Time ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

138 suggestive of a surface layer of ice tens of kilometers thick. The surface has the visible and near-IR spectrum of water ice. Surface imaging of Europa by the Voyager flybys and the Galileo orbiter suggest a thin (10 km?), mobile ice crust floating on a deep (50 km?) aqueous ocean. The outermost Galilean satellites, Ganymede (J3) and Callisto (J4), have much lower densities suggestive of massive amounts of water ice, approximately in the proportions expected for full condensation of rock-forming elements and H 0 from a parent gas of approximately solar composition. Ganymede's surface displays convincing evidence of large-scale tectonic activity, whereas the smaller and more distant Callisto has a heavily cratered ancient surface and little suggestion of internal dynamics. Both, like Europa, have the spectrum of water ice.

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7. Titan Titan has recently been studied by the Cassini Saturn orbiter and the Huygens entry probe. Placing Titan in the context of the other regular Saturnian satellites is not easy, since all of them apparently formed at such low temperatures that ices, including water ice, ammonia hydrate, and methane clathrate hydrate were fully condensed, thus causing density and composition differences to be too small to be discerned. Titan itself has an atmosphere that is denser than Earth's, dominated by a nitrogen/methane meteorology that involves an evaporation/precipitation cycle, methane rain at least at higher latitudes, and mature drainage systems and lakes. Photochemical processing of the methane in Titan's atmosphere by solar UV must have produced a layer of condensed hydrocarbons and C-H-N compounds at least 100 m in thickness over the age of the Solar System. Titan serves as a test case for theories of the inorganic origin of organic matter. Pluto, its largest moon Charon, and Neptune's largest moon Triton appear to be colder cousins of nitrogen- and methane-rich Titan. Their formation temperatures must have been low enough for retention of ammonia- or nitrogenbearing ices.

8. Centaurs, Trans-Neptunian Objects and Comets Centaurs have unstable orbits that cross those of the Jovian planets. It is reasonable to search for a source of bodies to replenish Centaurs as they are lost to planetary impacts and to violent orbital perturbations that may convert them into outer (retrograde; transient) planetary satellites or periodic comets, or even expel them from the Solar System. Our ability to identify genetic relationships between Centaurs, Kuiper-Belt Objects (KBOs; "iceteroids" that orbit beyond Neptune), the distant irregular and retrograde moons of the giant planets, Trojan

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139 asteroids and periodic ("short-period") comets is limited by the very small amount of compositional data we have on each of these classes. Comets are well established as mixtures of ices and small unequilibrated rocky particles, but their dynamically related cousins are poorly studied. It is clear that there are two broad spectral groups present among Centaurs, one group neutral or slightly red in color and the other very red. Although KBOs do not show a bimodal color pattern, those with perihelia beyond 40 A U and those with inclinations less than about 4.5° are all red. To date, no clear relationship between membership in these spectral groups and orbital parameters has been established for Centaurs. Therefore the significance of their color dichotomy is not understood.

9. Asteroids and Comets as Sources of Volatiles for Terrestrial Planets Since Newton there have been countless suggestions of profound effects on Earth attendant upon comet impacts. Many have suggested that Earth's water supply is a secondary feature introduced by late "veneering" with cometary material, which also should introduce a wide range of other volatile materials in grossly solar proportions. However, the serious mismatch between the observed isotopic (D:H) compositions of terrestrial and cometary water (all comets studied have had D:H ratios about twice as high as in terrestrial water) argues strongly against this explanation. The alternative carrier of water, the C-type and related classes of asteroids, seems increasingly plausible in light of the isotopic similarity to terrestrial water. The question of whether most of this water was brought in during the accretion phase or as a "veneer" is unresolved; however, the presence of about 1% ferric iron in Earth's mantle, which has a circulation time scale of about a billion years, could be attributed to oxidation of FeO by ancient water. The Fe 0 content could also be attributed to high-pressure disproportionation of FeO into metallic iron and ferric iron near the core-mantle boundary. In any case, no single source of volatiles satisfies both the water and noble gas data, and more complicated scenarios must be considered. 2

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Conclusions The tendency to approach chemical equilibrium was a driving force in determining the composition of sold materials in the early Solar System. Equilibrium between gases and dust during the nebular phase could be closely approached at the high temperatures found close to the Sun, in the terrestrial planet region. However, reduction of the stable high-temperature gases N and CO must have been kinetically inhibited at low temperatures far from the Sun where C H and N H are thermodynamically stable. Thus CO and N were available for incorporation into ices, whereas the reduced gases were in limited 2

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140 supply when cometary ices were condensed. At all distances beyond the heart of the asteroid belt the process of equilibration between preexisting grains and gases was very incomplete. Thus, outer Solar System bodies from carbonaceous asteroids through the Centaurs, TNOs, outer retrograde satellites, and Trojan bodies on the orbits of the giant planets are repositories of ancient presolar grains that bear the chemical signatures of their own condensation environment. The regular (inner) satellite systems of the giant planets, formed in low-entropy (much denser) subnebula around their parent planets, should bear the chemical evidence of much better equilibration, most notably abundant methane and ammonia, in their interiors.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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