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.     

Cosmic-ray Composition as Observed by Ulysses

The cosmic ray isotopic composition measurements from the High Energy Telescope (HET) on the Ulysses spacecraft are reviewed. The source isotopic composition of key elements is found to be surprisingly like the Solar system abundances with the notable exception of ²²Ne. The average density of interstellar material cosmic rays traverse is found to be 0.25 atom cm^–3, corresponding to a confinement time of 20 Myr. Vanadium isotopic abundances are shown to be consistent with weak cosmic-ray reacceleration. The implications of these measurements are discussed.     

Martian Volatiles: Isotopic Composition, Origin, and Evolution

Information about the composition of volatiles in the Martian atmosphere and interior derives from Viking spacecraft and ground-based measurements, and especially from measurements of volatiles trapped in Martian meteorites, which contain several distinct components. One volatile component, found in impact glass in some shergottites, gives the most precise measurement to date of the composition of Martian atmospheric Ar, Kr, and Xe, and also contains significant amounts of atmospheric nitrogen showing elevated ¹⁵N/¹⁴N. Compared to Viking analyses, the ³⁶Ar/¹³²Xe and ⁸⁴Kr/¹³²Xe elemental ratios are larger in shergottites, the ¹²⁹Xe/¹³²Xe ratio is similar, and the ⁴⁰Ar/³⁶Ar and ³⁶Ar/³⁸Ar ratios are smaller. The isotopic composition of atmospheric Kr is very similar to solar Kr, whereas the isotopes of atmospheric Xe have been strongly mass fractionated in favor of heavier isotopes. The nakhlites and ALH84001 contain an atmospheric component elementally fractionated relative to the recent atmospheric component observed in shergottites. Several Martian meteorites also contain one or more Martian interior components that do not show the mass fractionation observed in atmospheric noble gases and nitrogen. The D/H ratio in the atmosphere is strongly mass fractionated, but meteorites contain a distinct Martian interior hydrogen component. The isotopic composition of Martian atmospheric carbon and oxygen have not been precisely measured, but these elements in meteorites appear to show much less variation in isotopic composition, presumably in part because of buffering of the atmospheric component by larger condensed reservoirs. However, differences in the oxygen isotopic composition between meteorite silicate minerals (on the one hand) and water and carbonates indicate a lack of recycling of these volatiles through the interior. Many models have been presented to explain the observed isotopic fractionation in Martian atmospheric N, H, and noble gases in terms of partial loss of the planetary atmosphere, either very early in Martian history, or over extended geological time. The number of variables in these models is large, and we cannot be certain of their detailed applicability. Evolutionary data based on the radiogenic isotopes (i.e., ⁴⁰Ar/³⁶Ar, ¹²⁹Xe/¹³²Xe, and ¹³⁶Xe/¹³²Xe ratios) are potentially important, but meteorite data do not yet permit their use in detailed chronologies. The sources of Mars' original volatiles are not well defined. Some Martian components require a solar-like isotopic composition, whereas volatiles other than the noble gases (C, N, and H₂O) may have been largely contributed by a carbonaceous (or cometary) veneer late in planet formation. Also, carbonaceous material may have been the source of moderate amounts of water early in Martian history.     

Element Abundances and Isotope Ratios in the Giant Planets and Titan

This paper reviews our present knowledge about elemental and isotopic ratios in the Giant Planets and Titan. These parameters can provide key information about the formation and evolution of these objects. Element abundances, especially after the results of the Galileo Probe Mass Spectrometer in Jupiter, strongly support the formation model invoking an initial core formation (Mizuno, 1980; Pollack et al., 1996). They also suggest that solar composition icy planetesimals (SCIPs) brought the heavy elements to Jupiter. The Jupiter value of D/H appears to be representative of the protosolar value, while the D/H enrichment observed on Uranus and Neptune is consistent with the formation scenario of these planets. The ¹⁵N/¹⁴N measurement in Jupiter seems to be representative of its protosolar value. Future measurements are expected to come from the Cassini and Herschel space mission, as well as the ALMA submillimeter observatory. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ptolemy – an Instrument to Measure Stable Isotopic Ratios of Key Volatiles on a Cometary Nucleus

A fundamental goal of cometary studies is to determine the exact relationship between these bodies and the Solar System – the question(s) can be summarised as follows: did comets originate during the same events that spawned the Sun and planets, are they more primitive bodies that record a pre-solar history, or are they interstellar materials collected in relatively more recent times? Now, whatever the origin of comets, it is entirely possible that they could, in part, contain interstellar or pre-solar components – indeed, it seems rather likely in light of the fact that primitive meteorites contain such entities. These particular components are likely to be refractory (dust, macromolecular organic complexes, etc.). Of more relevance to the issues above are the volatile constituents, which make up the bulk of a comet's mass. Since these materials, by their very nature, volatilise during perihelion passage of a comet they can, in some instances, be detected and measured spectroscopically. Perhaps the most useful species for isotopic investigations are C₂, HCN and CN. Unfortunately, spectroscopic measurements can only currently be made with accuracies of ±10 to ±20%. As such it is very often not practical to conclude anything further than the fact that isotopic measurements are compatible with ‘`solar’' values, which tends to imply an origin from the margins of the solar accretion disk. But there is another problem with the spectroscopic measurements – since these are made on gaseous species in the coma (and relatively minor species at that) it is impossible to be certain that these represent the true nuclear values. In other words, if the processes of sublimation, active jetting, and photochemistry in the coma impart isotopic fractionation, the spectroscopic measurements could give a false impression of the true isotope ratios. What is required is an experiment capable of measuring isotopic ratios at the very surface of a comet. Herein we describe the Ptolemy instrument, which is included on the Philae lander as part of the Rosetta mission to 67P/Churyumov-Gerasimenko. The major objective of Ptolemy is a detailed appraisal of the nature and isotopic compositions of all materials present at the surface of a comet.     

The composition of heavy ions in solar energetic particle events

We review recent advances in determining the elemental, charge-state, and isotopic composition of 1 to 20 MeV per nucleon ions in solar energetic particle (SEP) events and outline our current understanding of the nature of solar and interplanetary processes which may explain the observations.The composition within individual SEP events may vary both with time and energy, and will in general be different from that in other SEP events. Average values of relative abundances measured in a large number of SEP events, however, are found to be roughly energy independent in the 1 to 20 MeV per nucleon range, and show a systematic deviation from photospheric abundances which seems to be organized in terms of the first ionization potential of the ion.Direct measurements of the charge states of SEPs have revealed the surprisingly common presence of energetic He⁺ along with heavy ions with typically coronal ionization states. High-resolution measurements of isotopic abundance ratios in a small number of SEP events show these to be consistent with the universal composition except for the puzzling overabundance of the SEP ²²Ne/²⁰Ne relative to this isotopes ratio in the solar wind. The broad spectrum of observed elemental abundance variations, which in their extreme result in composition anomalies characteristic of ³He-rich, heavy-ion rich and carbon-poor SEP events, along with direct measurements of the ionization states of SEPs provide essential information on the physical characteristics of, and conditions in the source regions, as well as important constraints to possible models for SEP production.It is concluded that SEP acceleration is a two-step process, beginning with plasma-wave heating of the ambient plasma in the lower corona, which may include pockets of cold material, and followed by acceleration to the observed energies by either flare-generated coronal shocks or Fermi-type processes in the corona. Interplanetary propagation as well as acceleration by interplanetary propagating shock will often further modify the composition of SEP events, especially at lower energies.     

The isotopic composition of anomalous cosmic rays from sampex

Measurements of the anomalous cosmic ray (ACR) isotopic composition have been made in three regions of the magnetosphere accessible from the polar Earth orbit of SAMPEX, including the interplanetary medium at high latitudes and geomagnetically trapped ACRs. At those latitudes where ACRs can penetrate the Earth's magnetic field while fully stripped galactic cosmic rays (GCRs) of similar energies are excluded, a pure ACR sample is observed to have the following composition: ¹⁵N/N < 0.023, ¹⁸O/¹⁶O < 0.0034, and ²²Ne/²⁰Ne = 0.077(+0.085, –0.023). We compare our values with those found by previous investigators and with those measured in other samples of solar and galactic material. In particular, a comparison of ²²Ne/²⁰Ne measurements from various sources implies that GCRs are not simply an accelerated sample of the local interstellar medium.     

Short-Lived Radionuclides in Meteorites: Constraints on Nebular Timescales for the Production of Solids

Variations in the abundances of short-lived radionuclides such as ²⁶Al (τ_1/2 ≈ 0.74 Ma) and ⁵³Mn (τ_1/2 ≈ 3.7 Ma) in meteoritic solids may be used to infer relative formation intervals of these solids in the nebula at precisions of less than 1 Ma. In a strict chronometric interpretation of the isotopic variations, whereby criteria such as spatial and temporal isotopic homogeneity and closed system isotopic evolution are met, solid formation occurred in the nebula for at least several million years. This is longer than some theoretical and astronomical estimates for the duration of the active nebula. The evidence for live ⁴¹Ca (τ_1/2 ≈ 0.10 Ma) in meteoritic inclusions further indicates that the onset of solid formation occurred quite early, perhaps within a few hundred thousand years after the onset of the collapse of the sun's parent molecular cloud. Failure of the chronometric interpretation may arise for a variety of reasons, including but not limited to, the late, inhomogeneous injection of material from a nearby stellar source or the local production of short-lived radionuclides by an energetic particle irradiation, e. g., from T Tauri (X-wind) or galactic cosmic ray sources. Although some isotopic evidence exists that the criteria required for a strict chronometric interpretation are not met by each of the short-lived chronometers, there is no compelling reason to shorten the interval of solid formation in the nebula to less than 1 Ma. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Genesis Solar Wind Concentrator

The primary goal of the Genesis Mission is to collect solar wind ions and, from their analysis, establish key isotopic ratios that will help constrain models of solar nebula formation and evolution. The ratios of primary interest include ¹⁷O/¹⁶O and ¹⁸O/¹⁶O to ±0.1%, ¹⁵N/¹⁴N to ±1%, and the Li, Be, and B elemental and isotopic abundances. The required accuracies in N and O ratios cannot be achieved without concentrating the solar wind and implanting it into low-background target materials that are returned to Earth for analysis. The Genesis Concentrator is designed to concentrate the heavy ion flux from the solar wind by an average factor of at least 20 and implant it into a target of ultra-pure, well-characterized materials. High-transparency grids held at high voltages are used near the aperture to reject >90% of the protons, avoiding damage to the target. Another set of grids and applied voltages are used to accelerate and focus the remaining ions to implant into the target. The design uses an energy-independent parabolic ion mirror to focus ions onto a 6.2 cm diameter target of materials selected to contain levels of O and other elements of interest established and documented to be below 10% of the levels expected from the concentrated solar wind. To optimize the concentration of the ions, voltages are constantly adjusted based on real-time solar wind speed and temperature measurements from the Genesis ion monitor. Construction of the Concentrator required new developments in ion optics; materials; and instrument testing and handling. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Isotopic Composition of the Solar Wind Inferred from In-Situ Spacecraft Measurements

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gyr ago from the protosolar nebula. Data from space missions and theoretical models indicate that the solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. In anticipation of results from the Genesis mission’s solar-wind implanted samples, we revisit solar wind isotopic abundance data from the high-resolution CELIAS/MTOF spectrometer on board SOHO. In particular, we evaluate the isotopic abundance ratios ¹⁵N/¹⁴N, ¹⁷O/¹⁶O, and ¹⁸O/¹⁶O in the solar wind, which are reference values for isotopic fractionation processes during the formation of terrestrial planets as well as for the Galactic chemical evolution. We also give isotopic abundance ratios for He, Ne, Ar, Mg, Si, Ca, and Fe measured in situ in the solar wind.     

Solar wind helium isotopic composition from SWICS/ULYSSES

This is the first study of the isotopic composition of solar wind helium with the SWICS time-of flight mass spectrometer. Although the design of SWICS is not optimized to measure³He abundances precisely,⁴He/³He flux ratios can be deduced from the data. The long term ratio is 2290±200, which agrees with the results obtained with the ICI magnetic mass spectrometer on ISEE-3 and with the Apollo SWC foil experiments.The ULYSSES spacecraft follows a trajectory which is ideal for the study of different solar wind types. During one year, from mid-1992 to mid-1993, it was in a range of heliographic latitudes where a recurrent fast stream from the southern polar coronal hole was observed every solar rotation. Solar wind bulk velocities ranged from 350 km/s to 950 km/s which would, in principle allow us to identify velocity-correlated compositional variations. Our investigation of solar wind helium, however, shows an isotopic ratio which does not depend on the solar wind speed.     

Stable Isotope Ratios as a Biomarker on Mars

As both Earth and Mars have had similar environmental conditions at least for some extended time early in their history (Jakosky and Phillips in Nature 412:237–244, 2001), the intriguing question arises whether life originated and evolved on Mars as it did on Earth (McKay and Stoker in Rev. Geophys. 27:189–214, 1989). Conceivably, early autotrophic life on Mars, like early life on Earth, used irreversible enzymatically enhanced metabolic processes that would have fractionated stable isotopes of the elements C, N, S, and Fe. Several important assumptions are made when such isotope fractionations are used as a biomarker. The purpose of this article is two-fold: (1) to discuss these assumptions for the case of carbon and to summarize new insights in abiologic reactions, and (2) to discuss the use of other stable isotope systems as a potential biomarker. It is concluded that isotopic biomarker studies on Mars will encounter several important obstacles. In the case of carbon isotopes, the most important obstacle is the absence of a contemporary abiologic carbon reservoir (such as carbonate deposits on Earth) to act as isotopic standard. The presence of a contemporary abiologic sulfate reservoir (evaporite deposits) suggests that sulfur isotopes can be used as a potential biomarker for sulfate-reducing bacteria. The best approach for tracing ancient life on Mars will be to combine several biomarker approaches; to search for complexity, and to combine small-scale isotopic variations with chemical, mineralogical, and morphological observations. An example of such a study can be a layer-specific correlation between δ ¹³C and δ ³⁴S within an ancient Martian evaporite, which morphologically resembles the typical setting of a shallow marine microbial mat.     

Isotopic analysis of cometary organic matter

Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of ¹³C/¹²C values in those particles is quite uncertain. The D/H ratio in H₂O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case.     

HF-W Chronometry and Inner Solar System Accretion Rates

Models for the mechanisms of accretion of the terrestrial planets are re-examined using the experimental technique of high-precision isotope ratio mass spectrometry of tungsten (W). The decay of ¹⁸²Hf to ¹⁸²W (via ¹⁸²Ta) provides a new kind of radiometric chronometer of planet formation processes. Hafnium and W, the parent and daughter trace elements, are highly refractory; however, Hf is lithophile and strongly partitioned into the silicate portion of a planet, whereas W is moderately siderophile and preferentially partitioned into a coexisting metallic phase. More than 90% of terrestrial W has gone into the Earth's core during its formation. The residual silicate portion, the Earth's primitive mantle, has a Hf/W ratio in the range 10−40, an order of magnitude higher than chondritic (∼1.3). Tungsten isotopic data for the Earth and the Moon suggest that we can date a major event of planet formation: The Moon formed about 50 Myrs after the start of the solar system, providing strong support for the Giant Impact Theory of lunar origin. Recent simulations of this event imply that the Earth was probably only half formed at the time. From this we can deduce the planetary accretion rate. Tungsten isotope data for Mars provide evidence of a much shorter accretion interval, perhaps as little as 10 Myrs, but the rates for the Earth over the same time interval could have been comparable. The large W isotopic heterogeneities on Mars could only have been produced within the first 30 Myrs of the solar system. Large-scale mixing, e.g. from convective overturn, as is thought to drive the Earth's plates, must be absent from Mars. Limitations of the method such as 1) cosmogenic ¹⁸²Ta effects on lunar samples, 2) incomplete mixing of debris to cause W isotope heterogeneity on the Moon, and 3) initial ¹⁸²Hf/¹⁸⁰Hf heterogeneities of the early solar system are critically discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Isotopic Criteria for Identification of Organic Carbon on Earth and Meteorites

Contrary to the often stated view, an enrichment of organic material in the light isotope is not a conclusive evidence of its life-related origin. The β¹³C - σ¹³C correlation is a special feature of biological systems. Therefore it can be used as a criterion for identification of organic carbon. A survey of the available isotopic data for organic compounds in meteorites shows that they do not comply with the β¹³C - σ¹³C correlation. The prevalence of amino and hydroxy acids in purines and sugars found in carbonaceous meteorites indicates that condensation of HCN and HCHO passed through cyanohydrin reaction, while biological evolution proceeds through formation of adenosine triphosphate (ATP). This, in addition to the isotope criterion, indicates that the organic compounds in carbonaceous chondrites are not life-related substances. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

OB Associations, Wolf–Rayet Stars, and the Origin of Galactic Cosmic Rays

We have measured the isotopic abundances of neon and a number of other species in the galactic cosmic rays (GCRs) using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the ACE spacecraft. Our data are compared to recent results from two-component (Wolf–Rayet material plus solar-like mixtures) Wolf–Rayet (WR) models. The three largest deviations of galactic cosmic ray isotope ratios from solar-system ratios predicted by these models, ¹²C/¹⁶O, ²²Ne/²⁰Ne, and ⁵⁸Fe/⁵⁶Fe, are very close to those observed. All of the isotopic ratios that we have measured are consistent with a GCR source consisting of ∼20% of WR material mixed with ∼80% material with solar-system composition. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that OB associations within superbubbles are the likely source of at least a substantial fraction of GCRs. In previous work it has been shown that the primary ⁵⁹Ni (which decays only by electron-capture) in GCRs has decayed, indicating a time interval between nucleosynthesis and acceleration of >10⁵ y. It has been suggested that in the OB association environment, ejecta from supernovae might be accelerated by the high velocity WR winds on a time scale that is short compared to the half-life of ⁵⁹Ni. Thus the ⁵⁹Ni might not have time to decay and this would cast doubt upon the OB association origin of cosmic rays. In this paper we suggest a scenario that should allow much of the ⁵⁹Ni to decay in the OB association environment and conclude that the hypothesis of the OB association origin of cosmic rays appears to be viable.     

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.