The Solar Noble Gas Record in Lunar Samples and Meteorites

Lunar soil and certain meteorites contain noble gases trapped from the solar wind at various times in the past. The progress in the last decade to decipher these precious archives of solar history is reviewed. The samples appear to contain two solar noble gas components with different isotopic composition. The solar wind component resides very close to grain surfaces and its isotopic composition is identical to that of present-day solar wind. Experimental evidence seems by now overwhelming that somewhat deeper inside the grains there exists a second, isotopically heavier component. To explain the origin of this component remains a challenge, because it is much too abundant to be readily reconciled with the known present day flux of solar particles with energies above those of the solar wind. The isotopic composition of solar wind noble gases may have changed slightly over the past few Ga, but such a change is not firmly established. The upper limit of ~5% per Ga for a secular increase of the 3He/4He ratio sets stringent limits on the amount of He that may have been brought from the solar interior to the surface (cf. Bochsler, 1992). Relative abundances of He, Ne, and Ar in present-day solar wind are the same as the long term average recorded in metallic Fe grains in meteorites within error limits of some 15-20%. Xe, and to a lesser extent Kr, are enriched in the solar wind similar to elements with a first ionisation potential < 10 eV, although Kr and Xe have higher FIPs. This can be explained if the ionisation time governs the FIP effect (Geiss and Bochsler, 1986). This revised version was published online in June 2006 with corrections to the Cover Date.     

On the Isotopic Composition of Primordial Xenon in Terrestrial Planet Atmospheres

Xenon plays a crucial role in models of atmospheric evolution in which noble gases are fractionated from their initial compositions to isotopically heavier distributions by early hydrodynamic escape of primordial planetary atmospheres. With the assumption that nonradiogenic Xe isotope ratios in present-day atmospheres were generated in this way, backward modeling from these ratios through the fractionating process can in principle identify likely parental Xe compositions and thus the probable sources of noble gases in pre-escape atmospheres. Applied to Earth, this approach simultaneously establishes the presence of an atmospheric Xe component due principally to fission of extinct ²⁴⁴Pu and identifies a composition called U-Xe as primordial Xe. Pu-Xe comprises 4.65±0.30% of atmospheric ¹³⁶Xe, and 6.8±0.5% of the present abundance of ¹²⁹Xe derives from decay of extinct ¹²⁹I. U-Xe is identical to the measured composition of solar-wind Xe except for deficits of the two heaviest isotopes – an unexpected difference since the modeling otherwise points to solar wind compositions for the lighter noble gases in the primordial terrestrial atmosphere. Evidence for the presence of U-Xe is not restricted to the early Earth; modeling based on a purely meteoritic data set defines a parental component in chondrites and achondrites with the same isotopic distribution. Results of experimental efforts to measure this composition directly in meteorites are promising but not yet conclusive. U-Xe also appears as a possible base component in interstellar silicon carbide, here with superimposed excesses of ¹³⁴Xe and ¹³⁶Xe six-fold larger than those in the solar wind. These compositional differences imply mixing of U-Xe with a nucleogenetic heavy-isotope component whose relative abundance in the solar accretion disk and in pre-solar environments varied both spatially and temporally. In contrast to Earth, the U-Xe signature on Mars was apparently overwhelmed by local accretion of materials rich in either chondritic Xe or solar-wind Xe. Data currently in hand from SNC meteorites on the composition of the present atmosphere are insufficiently precise to constrain a modeling choice between these two candidates for primordial martian Xe. They likewise do not permit definitive resolution of a ²⁴⁴Pu component in the atmosphere although its presence is allowed within current measurement uncertainties. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar and Solar-Wind Composition Results from the Genesis Mission

The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition, particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different solar-wind regimes, and ¹⁵N/¹⁴N and ¹⁸O/¹⁷O/¹⁶O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being measured.     

The Accretion, Composition and Early Differentiation of Mars

The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest history of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestrial planets. Judging by the data for Martian meteorites it has Rb/Sr 0.07 and K/U 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as rich in Fe as the mantle of the Earth, the Martian core being small (20% by mass). This is thought to be because conditions were more oxidizing during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophile on Mars. The very different apparent behavior of high field strength (HFS) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile element abundance patterns have been interpreted as reflecting strong partitioning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proportions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed — Mars would be expected to be more depleted in volatiles than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late accretionary events, such as giant impacts. This might also explain the non-chondritic Si/Mg ratio of the silicate portion of the Earth. The lower density of Mars is consistent with this interpretation, as are isotopic data. ⁸⁷Rb-⁸⁷Sr, ¹²⁹I-¹²⁹Xe, ¹⁴⁶Sm-¹⁴²Nd, ¹⁸²Hf-¹⁸²W, ¹⁸⁷Re-¹⁸⁷Os, ²³⁵U-²⁰⁷Pb and ²³⁸U-²⁰⁶Pb isotopic data for Martian meteorites all provide evidence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have proved complicated to interpret in terms of ²⁴⁴Pu decay and timing because of fractionation thought to be caused by hydrodynamic escape. There are, as yet, no resolvable isotopic heterogeneities identified in Martian meteorites resulting from ⁹²Nb decay to ⁹²Zr, consistent with the paucity of perovskite in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived ¹⁷⁶Lu-¹⁷⁶Hf system also preserves little record of early differentiation. In contrast W isotope data, Ba/W and time-integrated Re/Os ratios of Martian meteorites provide powerful evidence that the mantle retains remarkably early heterogeneities that are vestiges of core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differentiation, there is no evidence that Mars grew more quickly than the Earth at an equivalent size. Mars appears to have just stopped growing earlier because it did not undergo late stage (>20 Myr), impacts on the scale of the Moon-forming Giant Impact that affected the Earth.     

On Noble Gas Processing in the Solar Accretion Disk

Two fractionation models are applied to the problem of generating the widely distributed “Q-component” noble gases in meteorites from the solar-like isotopic and elemental compositions that presumably characterized the early solar accretion disk. Noble gas fractionation by mass-dependent dissipation of the solar nebula, as suggested by Ozima et al. (1998), is examined in the context of a model developed by Johnstone et al. (1998) for accretion disk photoevaporation driven by intense UV radiation from a neighboring giant star. Hydrodynamic escape of heavier species entrained in hydrogen outflow from the UV-heated outer regions of the disk can generate substantial noble gas fractionations, but they do not match the observed Q-component isotopic pattern and moreover require the physically unrealistic assumption that the fractionated gases are confined to the heated disk boundary zone, without mixing with the interior nebula, for long periods of time. It seems more likely that hydrodynamic outflow is actually established below this zone, in the body of the disk. In this case fractionations are governed by Rayleigh distillation of the entire remaining nebula, and are negligible at the time when disk erosion is halted by the gravitational potential of the young sun embedded in the disk. A “local” model of noble gas fractionation by hydrodynamic blowoff of transient, methane-rich atmospheres outgassed from the interiors of large primitive planetesimals (Pepin, 1991) is updated and assessed against current data. Degassed atmospheres are assumed to contain isotopically solar noble gases except for an additional nucleogenic Xe component that contributes primarily to the two heaviest isotopes; there is evidence that this same component is present at varying levels in other solar-system volatile reservoirs, possibly reflecting a compositional change with time in the solar nebula. Single fixed values for the two free parameters in the blowoff modeling equations can generate fractionated Xe, Kr, Ar and Ne compositions in the residual atmosphere that closely match observed meteoritic isotopic distributions, and Q-gas elemental ratios are approximated by adsorption of fractionated gases on planetesimal surface grains using plausible values of relative Henry Law constants. Additional requirements for adsorption of sufficient absolute amounts of Q-gases on carrier grains, and their subsequent ejection to space, mixing in the nebula, and dispersal into meteorite bodies, are examined in the context of current models for body sizes and dynamical evolution in an early mass-rich asteroid belt (Chambers and Wetherill, 2001). Despite its ability to replicate isotopic compositions, uncertainties about the environments in which the blowoff model can successfully operate suggest that there is, as yet, no entirely satisfactory understanding of how the Q-component noble gases might have evolved from solar-like precursor compositions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Galileo Probe Mass Spectrometer experiment

The Galileo Probe Mass Spectrometer (GPMS) is a Probe instrument designed to measure the chemical and isotopic composition including vertical variations of the constituents in the atmosphere of Jupiter. The measurement will be performed by in situ sampling of the ambient atmosphere in the pressure range from approximately 150 mbar to 20 bar. In addition batch sampling will be performed for noble gas composition measurement and isotopic ratio determination and for sensitivity enhancement of non-reactive trace gases.The instrument consists of a gas sampling system which is connected to a quadrupole mass analyzer for molecular weight analysis. In addition two sample enrichment cells and one noble gas analysis cell are part of the sampling system. The mass range of the quadrupole analyzer is from 2 amu to 150 amu. The maximum dynamic range is 10⁸. The detector threshold ranges from 10 ppmv for H₂O to 1 ppbv for Kr and Xe. It is dependent on instrument background and ambient gas composition because of spectral interference. The threshold values are lowered through sample enrichment by a factor of 100 to 500 for stable hydrocarbons and by a factor of 10 for noble gases. The gas sampling system and the mass analyzer are sealed and evacuated until the measurement sequence is initiated after the Probe enters into the atmosphere of Jupiter. The instrument weighs 13.2 kg and the average power consumption is 13 W.The instrument follows a sampling sequence of 8192 steps and a sampling rate of two steps per second. The measurement period lasts appropriately 60 min through the nominal pressure and altitude range.     

Ages and Geologic Histories of Martian Meteorites

We review the radiometric ages of the 16 currently known Martian meteorites, classified as 11 shergottites (8 basaltic and 3 lherzolitic), 3 nakhlites (clinopyroxenites), Chassigny (a dunite), and the orthopyroxenite ALH84001. The basaltic shergottites represent surface lava flows, the others magmas that solidified at depth. Shock effects correlate with these compositional types, and, in each case, they can be attributed to a single shock event, most likely the meteorite's ejection from Mars. Peak pressures in the range 15 – 45 GPa appear to be a "launch window": shergottites experienced ～30 – 45 GPa, nakhlites ～20 ± 5 GPa, Chassigny ～35 GPa, and ALH84001 ～35 – 40 GPa. Two meteorites, lherzolitic shergottite Y-793605 and orthopyroxenite ALH84001, are monomict breccias, indicating a two-phase shock history in toto: monomict brecciation at depth in a first impact and later shock metamorphism in a second impact, probably the ejection event. Crystallization ages of shergottites show only two pronounced groups designated S₁ (～175 Myr), including 4 of 6 dated basalts and all 3 lherzolites, and S₂ (330 – 475 Myr), including two basaltic shergottites and probably a third according to preliminary data. Ejection ages of shergottites, defined as the sum of their cosmic ray exposure ages and their terrestrial residence ages, range from the oldest (～20 Myr) to the youngest (～0.7 Myr) values for Martian meteorites. Five groups are distinguished and designated S_Dho (one basalt, ～20 Myr), S_L (two lherzolites of overlapping ejection ages, 3.94 ± 0.40 Myr and 4.70 ± 0.50 Myr), S (four basalts and one lherzolite, ～2.7 – 3.1 Myr), S_DaG (two basalts, ～1.25 Myr), and S_E (the youngest basalt, 0.73 ± 0.15 Myr). Consequently, crystallization age group S₁ includes ejection age groups S_L, S_E and 4 of the 5 members of S, whereas S₂ includes the remaining member of S and one of the two members of S_DaG. Shock effects are different for basalts and lherzolites in group S/S₁. Similarities to the dated meteorite DaG476 suggest that the two shergottites that are not dated yet belong to group S₂. Whether or not S₂ is a single group is unclear at present. If crystallization age group S₁ represents a single ejection event, pre-exposure on the Martian surface is required to account for ejection ages of S_L that are greater than ejection ages of S, whereas secondary breakup in space is required to account for ejection ages of S_E less than those of S. Because one member of crystallization age group S₂ belongs to ejection group S, the maximum number of shergottite ejection events is 6, whereas the minimum number is 2. Crystallization ages of nakhlites and Chassigny are concordant at ～1.3 Gyr. These meteorites also have concordant ejection ages, i.e., they were ejected together in a single event (NC). Shock effects vary within group NC between the nakhlites and Chassigny. The orthopyroxenite ALH84001 is characterized by the oldest crystallization age of ～4.5 Gyr. Its secondary carbonates are ～3.9 Gyr old, an age corresponding to the time of Ar-outgassing from silicates. Carbonate formation appears to have coincided with impact metamorphism, either directly, or indirectly, perhaps via precipitation from a transient impact crater lake. The crystallization age and the ejection age of ALH84001, the second oldest ejection age at 15.0 ± 0.8 Myr, give evidence for another ejection event (O). Consequently, the total number of ejection events for the 16 Martian meteorites lies in the range 4 – 8. The Martian meteorites indicate that Martian magmatism has been active over most of Martian geologic history, in agreement with the inferred very young ages of flood basalt flows observed in Elysium and Amazonis Planitia with the Mars Orbital Camera (MOC) on the Mars Global Surveyor (MGS). The provenance of the youngest meteorites must be found among the youngest volcanic surfaces on Mars, i.e., in the Tharsis, Amazonis, and Elysium regions.     

Formation and Composition of Planetesimals

The composition of planetesimals depends upon the epoch and the location of their formation in the solar nebula. Meteorites produced in the hot inner nebula contain refractory compounds. Volatiles were present in icy planetesimals and cometesimals produced in the cold outer nebula. However, the mechanism responsible for their trapping is still controversial. We argue for a general scenario valid in all regions of the turbulent nebula where water condensed as a crystalline ice (Hersant et al., 2004). Volatiles were trapped in the form of clathrate hydrates in the continuously cooling nebula. The epoch of clathration of a given species depends upon the temperature and the pressure required for the stability of the clathrate hydrate. The efficiency of the mechanism depends upon the local amount of ice available. This scenario is the only one so far which proposes a quantitative interpretation of the non detection of N₂ in several comets of the Oort cloud (Iro et al., 2003). It may explain the large variation of the CO abundance observed in comets and predicts an Ar/O ratio much less than the upper limit of 0.1 times the solar ratio estimated on C/2001 A2 (Weaver et al., 2002). Under the assumption that the amount of water ice present at 5 AU was higher than the value corresponding to the solar O/H ratio by a factor 2.2 at least, the clathration scenario reproduces the quasi uniform enrichment with respect to solar of the Ar, Kr, Xe, C, N and S elements measured in Jupiter by the Galileo probe. The interpretation of the non-uniform enrichment in C, N and S in Saturn requires that ice was less abundant at 10 AU than at 5 AU so that CO and N₂ were not clathrated in the feeding zone of the planet while CH₄, NH₃ and H₂S were. As a result, the 14N/15N ratio in Saturn should be intermediate between that in Jupiter and the terrestrial ratio. Ar and Kr should be solar while Xe should be enriched by a factor 17. The enrichments in C, N and S in Uranus and Neptune suggest that available ice was able to form clathrates of CH₄, CO and the NH₃ hydrate, but not the clathrate of N₂. The enrichment of oxygen by a factor 440 in Neptune inferred by Lodders and Fegley (1994) from the detection of CO in the troposphere of the planet is higher by at least a factor 2.5 than the lower limit of O/H required for the clathration of CO and CH4 and for the hydration of NH₃. If CO detected by Encrenaz et al. (2004) in Uranus originates from the interior of the planet, the O/H ratio in the envelope must be around of order of 260 times the solar ratio, then also consistent with the trapping of detected volatiles by clathration. It is predicted that Ar and Kr are solar in the two planets while Xe would be enriched by a factor 30 to 70. Observational tests of the validity of the clathration scenario are proposed.     

Isotopic Fractionation by Gravitational Escape

Present natural data bases for abundances of the isotopic compositions of noble gases, carbon and nitrogen inventories can be found in the Sun, the solar wind, meteorites and the planetary atmospheres and crustal reservoirs. Mass distributions in the various volatile reservoirs provide boundary conditions which must be satisfied in modelling the history of the present atmospheres. Such boundary conditions are constraints posed by comparison of isotopic ratios in primordial volatile sources with the isotopic pattern which was found on the planets and their satellites. Observations from space missions and Earth-based spectroscopic telescope observations of Venus, Mars and Saturn's major satellite Titan show that the atmospheric evolution of these planetary bodies to their present states was affected by processes capable of fractionating their elements and isotopes. The isotope ratios of D/H in the atmospheres of Venus and Mars indicate evidence for their planetary water inventories. Venus' H₂O content may have been at least 0.3% of a terrestrial ocean. Analysis of the D/H ratio on Mars imply that a global H₂O ocean with a depth of ≤ 30 m was lost since the end of hydrodynamic escape. Calculations of the time evolution of the ¹⁵N/¹⁴N isotope anomalies in the atmospheres of Mars and Titan show that the Martian atmosphere was at least ≥ 20 times denser than at present and that the mass of Titan's early atmosphere was about 30 times greater than its present value. A detailed study of gravitational fractionation of isotopes in planetary atmospheres furthermore indicates a much higher solar wind mass flux of the early Sun during the first half billion years. This revised version was published online in August 2006 with corrections to the Cover Date.     

Long-Term Evolution of the Martian Crust-Mantle System

Lacking plate tectonics and crustal recycling, the long-term evolution of the crust-mantle system of Mars is driven by mantle convection, partial melting, and silicate differentiation. Volcanic landforms such as lava flows, shield volcanoes, volcanic cones, pyroclastic deposits, and dikes are observed on the martian surface, and while activity was widespread during the late Noachian and Hesperian, volcanism became more and more restricted to the Tharsis and Elysium provinces in the Amazonian period. Martian igneous rocks are predominantly basaltic in composition, and remote sensing data, in-situ data, and analysis of the SNC meteorites indicate that magma source regions were located at depths between 80 and 150 km, with degrees of partial melting ranging from 5 to 15 %. Furthermore, magma storage at depth appears to be of limited importance, and secular cooling rates of 30 to 40 K?Gyr^?1 were derived from surface chemistry for the Hesperian and Amazonian periods. These estimates are in general agreement with numerical models of the thermo-chemical evolution of Mars, which predict source region depths of 100 to 200 km, degrees of partial melting between 5 and 20 %, and secular cooling rates of 40 to 50 K?Gyr^?1. In addition, these model predictions largely agree with elastic lithosphere thickness estimates derived from gravity and topography data. Major unknowns related to the evolution of the crust-mantle system are the age of the shergottites, the planet’s initial bulk mantle water content, and its average crustal thickness. Analysis of the SNC meteorites, estimates of the elastic lithosphere thickness, as well as the fact that tidal dissipation takes place in the martian mantle indicate that rheologically significant amounts of water of a few tens of ppm are still present in the interior. However, the exact amount is controversial and estimates range from only a few to more than 200 ppm. Owing to the uncertain formation age of the shergottites it is unclear whether these water contents correspond to the ancient or present mantle. It therefore remains to be investigated whether petrologically significant amounts of water of more than 100 ppm are or have been present in the deep interior. Although models suggest that about 50 % of the incompatible species (H₂O, K, Th, U) have been removed from the mantle, the amount of mantle differentiation remains uncertain because the average crustal thickness is merely constrained to within a factor of two.     

Core Formation and Mantle Differentiation on Mars

Geochemical investigation of Martian meteorites (SNC meteorites) yields important constraints on the chemical and geodynamical evolution of Mars. These samples may not be representative of the whole of Mars; however, they provide constraints on the early differentiation processes on Mars. The bulk composition of Martian samples implies the presence of a metallic core that formed concurrently as the planet accreted. The strong depletion of highly siderophile elements in the Martian mantle is only possible if Mars had a large scale magma ocean early in its history allowing efficient separation of a metallic melt from molten silicate. The solidification of the magma ocean created chemical heterogeneities whose ancient origin is manifested in the heterogeneous ¹⁴²Nd and ¹⁸²W abundances observed in different meteorite groups derived from Mars. The isotope anomalies measured in SNC meteorites imply major chemical fractionation within the Martian mantle during the life time of the short-lived isotopes ¹⁴⁶Sm and ¹⁸²Hf. The Hf-W data are consistent with very rapid accretion of Mars within a few million years or, alternatively, a more protracted accretion history involving several large impacts and incomplete metal-silicate equilibration during core formation. In contrast to Earth early-formed chemical heterogeneities are still preserved on Mars, albeit slightly modified by mixing processes. The preservation of such ancient chemical differences is only possible if Mars did not undergo efficient whole mantle convection or vigorous plate tectonic style processes after the first few tens of millions of years of its history.     

Noble Gas Isotopes on the Moon

We discuss some of the major noble gas components on the Moon and their implications on the history of Moon and Sun. He, Ar, and Rn have been detected in the tenuous lunar atmosphere. The Ar and Rn abundances suggest that a sizeable fraction of the lunar interior presently loses all its freshly produced radiogenic noble gases. Part of the radiogenic Ar and Xe (the latter from now extinct radioactive isotopes of I and Pu) outgassed from the lunar interior later became retrapped in the dust grains on the lunar surface. These “parentless” gases may also be used to constrain the degassing history of the Moon, although a quantitative understanding is lacking. The dust layer on the lunar surface contains large amounts of noble gases implanted by the solar wind. The lunar regolith therefore constitutes the best available archive of the solar history of the past four billion years. This revised version was published online in August 2006 with corrections to the Cover Date.     

Isotopic Composition of H,HE and NE in the Protosolar Cloud

Observations and measurements in the solar wind, the Jovian atmosphere and the gases trapped in lunar surface material provide the main evidence from which the isotopic composition of H, He and Ne in the Protosolar Cloud (PSC) is derived. These measurements and observations are reviewed and the corrections are discussed that are needed for obtaining from them the PSC isotopic ratios. The D/H, ³He/⁴He (D+³He)/H, ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios adopted for the PSC are presented. Protosolar abundances provide the basis for the interpretation of isotopic ratios measured in the various solar system objects. In this article we discuss constraints derived from the PSC abundances on solar mixing, the origin of atmospheric neon, and the nature of the “SEP” component of neon trapped at the lunar surface. We also discuss constraints on the galactic evolution provided by the isotopic abundances of H and He in the PSC. This revised version was published online in August 2006 with corrections to the Cover Date.     

Water Reservoirs in Small Planetary Bodies: Meteorites,Asteroids, and Comets

Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed \(\leq 4\) Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter’s orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn’s orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources of the noble gases accreted by Earth and Venus. Possible constraints that these observations can place on models of giant planet formation and migration are explored.     

Chemistry,accretion, and evolution of Mars

The high FeO concentrations measured by VIKING for the Martian soils correspond to all probability to a FeO-rich mantle. In general, the VIKING XRF-data indicate a mafic crust with a considerably smaller degree of fractionation compared to the terrestrial crust.In recent years evidence has been collected which points towards Mars being the parent body of SNC-meteorites and, hence, these meteorites have become a valuable source of information about the chemistry of Mars. Using element correlations observed in SNC-meteorites and general cosmochemical constraints, it is possible to estimated the bulk composition of Mars. Normalized to Si and Cl, the mean abundance value for the elements Ga, Fe, Na, P, K, F, and Rb in the Martian mantle is found to be 0.35 and thus exceeds the terrestrial value by about a factor of two. Aside pressure effects and the H₂O poverty, the high P and K content of the Martian mantle may lead to magmatic processes different from those on Earth.The composition of the Earth's mantle can successfully be described by a two component model. Component A: highly reduced and almost free of all elements more volatile than Na; component B: oxidized and containing all elements in Cl-abundances including volatile elements. The same two components can be used as building blocks for Mars, if one assumes that, contrary to the inhomogeneous accretion of the Earth, Mars accreted almost homogeneously. The striking depletion of all elements with chalcophile character indicates that chemical equilibrium between component A and B was achieved on Mars which lead to the formation of significant amounts of FeS which, on segregation, extracted the elements according to their sulphide-silicate partition coefficients. While for the Earth a mixing ratio AB = 8515 was derived, the Mars ratio of 6040 reflects the higher concentrations of moderately volatile elements like Na, K, and sulphur on Mars. A homogeneous accretion of Mars could also explain the obvious low abundances of water and primordial rare gases.     

Composition of Light Solar Wind Noble Gases in the Bulk Metallic Glass flown on the Genesis Mission

We discuss data of light noble gases from the solar wind implanted into a metallic glass target flown on the Genesis mission. Helium and neon isotopic compositions of the bulk solar wind trapped in this target during 887 days of exposure to the solar wind do not deviate significantly from the values in foils of the Apollo Solar Wind Composition experiments, which have been exposed for hours to days. In general, the depth profile of the Ne isotopic composition is similar to those often found in lunar soils, and essentially very well reproduced by ion-implantation modelling, adopting the measured velocity distribution of solar particles during the Genesis exposure and assuming a uniform isotopic composition of solar wind neon. The results confirm that contributions from high-energy particles to the solar wind fluence are negligible, which is consistent with in-situ observations. This makes the enigmatic “SEP-Ne” component, apparently present in lunar grains at relatively large depth, obsolete. ²⁰Ne/ ²²Ne ratios in gas trapped very near the metallic glass surface are up to 10% higher than predicted by ion implantation simulations. We attribute this superficially trapped gas to very low-speed, current-sheet-related solar wind, which has been fractionated in the corona due to inefficient Coulomb drag.     

Atmosphere Impact Losses

Determining the origin of volatiles on terrestrial planets and quantifying atmospheric loss during planet formation is crucial for understanding the history and evolution of planetary atmospheres. Using geochemical observations of noble gases and major volatiles we determine what the present day inventory of volatiles tells us about the sources, the accretion process and the early differentiation of the Earth. We further quantify the key volatile loss mechanisms and the atmospheric loss history during Earth’s formation. Volatiles were accreted throughout the Earth’s formation, but Earth’s early accretion history was volatile poor. Although nebular Ne and possible H in the deep mantle might be a fingerprint of this early accretion, most of the mantle does not remember this signature implying that volatile loss occurred during accretion. Present day geochemistry of volatiles shows no evidence of hydrodynamic escape as the isotopic compositions of most volatiles are chondritic. This suggests that atmospheric loss generated by impacts played a major role during Earth’s formation. While many of the volatiles have chondritic isotopic ratios, their relative abundances are certainly not chondritic again suggesting volatile loss tied to impacts. Geochemical evidence of atmospheric loss comes from the \({}^{3}\mathrm{He}/{}^{22}\mathrm{Ne}\), halogen ratios (e.g., F/Cl) and low H/N ratios. In addition, the geochemical ratios indicate that most of the water could have been delivered prior to the Moon forming impact and that the Moon forming impact did not drive off the ocean. Given the importance of impacts in determining the volatile budget of the Earth we examine the contributions to atmospheric loss from both small and large impacts. We find that atmospheric mass loss due to impacts can be characterized into three different regimes: 1) Giant Impacts, that create a strong shock transversing the whole planet and that can lead to atmospheric loss globally. 2) Large enough impactors (\(m_{\mathit{cap}} \gtrsim \sqrt{2} \rho_{0} (\pi h R)^{3/2}\), \(r_{\mathit{cap}}\sim25~\mbox{km}\) for the current Earth), that are able to eject all the atmosphere above the tangent plane of the impact site, where \(h\), \(R\) and \(\rho_{0}\) are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. 3) Small impactors (\(m_{\mathit{min}}>4 \pi\rho_{0} h^{3}\), \(r_{\mathit {min}}\sim 1~\mbox{km}\) for the current Earth), that are only able to eject a fraction of the atmospheric mass above the tangent plane. We demonstrate that per unit impactor mass, small impactors with \(r_{\mathit{min}} < r < r_{\mathit{cap}}\) are the most efficient impactors in eroding the atmosphere. In fact for the current atmospheric mass of the Earth, they are more than five orders of magnitude more efficient (per unit impactor mass) than giant impacts, implying that atmospheric mass loss must have been common. The enormous atmospheric mass loss efficiency of small impactors is due to the fact that most of their impact energy and momentum is directly available for local mass loss, where as in the giant impact regime a lot of energy and momentum is ’wasted’ by having to create a strong shock that can transverse the entirety of the planet such that global atmospheric loss can be achieved. In the absence of any volatile delivery and outgassing, we show that the population of late impactors inferred from the lunar cratering record containing 0.1% \(M_{\oplus }\) is able to erode the entire current Earth’s atmosphere implying that an interplay of erosion, outgassing and volatile delivery is likely responsible for determining the atmospheric mass and composition of the early Earth. Combining geochemical observations with impact models suggest an interesting synergy between small and big impacts, where giant impacts create large magma oceans and small and larger impacts drive the atmospheric loss.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

Origin,age, and composition of meteorites

This paper attempts to bring together and evaluate all significant evidence on the origin of meteorites.The iron meteorites seem to have formed at low pressures. Laboratory evidence shows that the absence of a Widmanstätten pattern in meteorites with > 16% Ni cannot be attributed to high pressures, but to supercooling or an unusually fast cooling rate for these meteorites, which prevented the development of a pattern. The presence of tridymite in the Steinbach siderophyre provides further, direct proof that the Widmanstätten pattern can form at pressures less than 3 kb. Neither diamond, nor cliftonite, nor cohenite are reliable pressure indicators in meteorites. Diamonds were formed by shock while cliftonite may have been derived from a cubic carbide such as Fe₄C. Cohenite is apparently stabilized by kinetic rather than thermodynamic factors. Several lines of evidence suggest that the irons come from more than one parent body, perhaps as many as four.The frequency of pallasites is perfectly consistent with an origin in the transition zone between core and mantle of the parent body. Hybrid meteorites such as Brenham are not necessarily derived from the metal-silicate interface, but probably resulted from dendrite growth in the solidifying melt.Ordinary chondrites definitely are equilibrium assemblages rather than chance conglomerates. According to the best available evidence, Prior's rules seem to be valid. The metal particles in chondrites differentiated into kamacite and taenite in their present location, rather than in a remote earlier environment. Trace element abundances in ordinary and carbonaceous chondrites suggest that these meteorites accreted from two types of matter: an undepleted fraction that separated from its complement of gases at low temperatures, and a depleted fraction that lost its gases at high temperatures. These two fractions of primitive meteoritic matter are tentatively identified with the matrix and chondrules-plus-metal, respectively. New restrictive limits are placed on the iron-silicate fractionation in chondrites. No direct evolutionary path exists that connects the currently accepted solar abundances of Fe and Ni and the observed Fe/Si and Ni/Si ratios in chondrites. Apparently the solar abundance of iron is in error. The iron-silicate fractionation seems to have occurred while chondritic matter was in a more strongly reduced state than its present one.The U-He and K-Ar ages of hypersthene chondrites are systematically shorter than those of bronzite chondrites. Short ages are correlated with shock effects, and it seems that the hypersthene chondrites suffered reheating and partial-to-complete outgassing 0.4 AE ago. The cosmic-ray exposure ages of all classes of meteorites cluster distinctly, indicating that the meteorites were produced in a few discrete major collisions rather than by a quasi-continuum of smaller ones. The dates of the principal breakups are: irons, 0.6 and 0.9 AE; aubrites, 45 m.y.; bronzite chondrites, 4 m.y.; hypersthene chondrites, 0.025, 3, 7–13, and 16–31 m.y. All four clusters of hypersthene chondrites show evidence of severe outgassing 0.4 AE ago, which implies that most or all hypersthene chondrites come from the same parent body.As already noted by Signer and Suess, two distinct types of primordial gas occur in meteorites. Differentiated meteorites always contain unfractionated gas, while relatively undifferentiated meteorites contain fractionated gas. The former component is invariably associated with shock effects, and seems to have been derived from the solar wind. The latter component is correlated with other volatiles and seems to be a truly primitive constituent of meteoritic matter. Isotopic anomalies in the fractionated gas suggest that meteoritic matter was irradiated with 10¹⁷ protons/cm² at a very early stage of its history.There is very little doubt that most, if not all, meteorites come from the asteroid belt rather than from the moon. The orbits and geocentric velocities of stony meteorites resemble those of the Apollo asteroids (most of which are former members of the asteroid belt that have strayed into terrestrial space), but disagree strongly with the calculated orbits and velocities for lunar ejecta. Öpik's conclusions about the difficulty of accelerating lunar debris to escape velocity represent a further argument against a lunar origin of stony meteorites.The most likely parent bodies of the meteorites are the 34 asteroids which cross the orbit of Mars. Collisional debris from these objects will remain in Mars-crossing orbits, and perturbations by Mars will inject some fraction of this material into terrestrial space. Most of the Mars asteroids, comprising 98% of the mass and 92% of the cross-section, belong to three Hirayama families (Phocaea, Desiderata, and Aethra), and an additional, previously unrecognized family. These families were apparently produced by disruption of parent asteroids ca. 104, 105, and 46 km in diameter. The size distribution and light curves of asteroids indicate that the larger asteroids are original accretions, rather than collision fragments. There is no reason to believe that the meteorites ever resided in bodies larger than Ceres (d = 770 km).Various theories on the origin of the meteorites are critically reviewed in the light of the preceding evidence. Wood's theory, which postulates a high-temperature and a low-temperature variety of primordial matter, is in best accord with the evidence. Apparently the asteroids accreted from varying proportions of these two types of material, and were then heated by extinct radioactivity produced in the early irradiation.     

Presolar Isotopic Signatures in Meteorites and Comets: New Insights from the Rosetta Mission to Comet 67P/Churyumov–Gerasimenko

Comets are considered the most primitive planetary bodies in our Solar System, i.e., they should have best preserved the solid components of the matter from which our Solar System formed. ESA’s recent Rosetta mission to Jupiter family comet 67P/Churyumov–Gerasimenko (67P/CG) has provided a wealth of isotope data which expanded the existing data sets on isotopic compositions of comets considerably. In this paper we review our current knowledge on the isotopic compositions of H, C, N, O, Si, S, Ar, and Xe in primitive Solar System materials studied in terrestrial laboratories and how the Rosetta data acquired with the ROSINA (Rosetta Orbiter Sensor for Ion and Neutral Analysis) and COSIMA (COmetary Secondary Ion Mass Analyzer) mass spectrometer fit into this picture. The H, Si, S, and Xe isotope data of comet 67P/CG suggest that this comet might be particularly primitive and might have preserved large amounts of unprocessed presolar matter. We address the question whether the refractory Si component of 67P/CG contains a presolar isotopic fingerprint from a nearby Type II supernova (SN) and discuss to which extent C and O isotope anomalies originating from presolar grains should be observable in dust from 67P/CG. Finally, we explore whether the isotopic fingerprint of a potential late SN contribution to the formation site of 67P/CG in the solar nebula can be seen in the volatile component of 67P/CG.