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.     

Molecular Biosignatures

Life, as we know it, is based on carbon chemistry operating in an aqueous environment. Living organisms process chemicals, make copies of themselves, are autonomous and evolve in concert with the environment. All these characteristics are driven by, and operate through, carbon chemistry. The carbon chemistry of living systems is an exact branch of science and we have detailed knowledge of the basic metabolic and reproductive machinery of living organisms. We can recognise the residual biochemicals long after life has expired and otherwise lost most life-defining features. Carbon chemistry provides a tool for identifying extant and extinct life on Earth and, potentially, throughout the Universe. In recognizing that certain distinctive compounds isolable from living systems had related fossil derivatives, organic geochemists coined the term biological marker compound or biomarker (e.g. Eglinton et al. in Science 145:263–264, 1964) to describe them. In this terminology, biomarkers are metabolites or biochemicals by which we can identify particular kinds of living organisms as well as the molecular fossil derivatives by which we identify defunct counterparts. The terms biomarker and molecular biosignature are synonymous. A defining characteristic of terrestrial life is its metabolic versatility and adaptability and it is reasonable to expect that this is universal. Different physiologies operate for carbon acquisition, the garnering of energy and the storage and processing of information. As well as having a range of metabolisms, organisms build biomass suited to specific physical environments, habitats and their ecological imperatives. This overall ‘metabolic diversity’ manifests itself in an enormous variety of accompanying product molecules (i.e. natural products). The whole field of organic chemistry grew from their study and now provides tools to link metabolism (i.e. physiology) to the occurrence of biomarkers specific to, and diagnostic for, particular kinds of metabolism. Another characteristic of living things, also likely to be pervasive, is that an enormous diversity of large molecules are built from a relatively small subset of universal precursors. These include the four bases of DNA, 20 amino acids of proteins and two kinds of lipid building blocks. Third, life exploits the specificity inherent in the spatial, that is, the three-dimensional qualities of organic chemicals (stereochemistry). These characteristics then lead to some readily identifiable and measurable generic attributes that would be diagnostic as biosignatures. Measurable attributes of molecular biosignatures include:

1. Enantiomeric excess
2. Diastereoisomeric preference
3. Structural isomer preference
4. Repeating constitutional sub-units or atomic ratios
5. Systematic isotopic ordering at molecular and intramolecular levels
6. Uneven distribution patterns or clusters (e.g. C-number, concentration, δ ¹³C) of structurally related compounds.

In this paper we address details of the chemical and biosynthetic basis for these features, which largely arise as a consequence of construction from small, recurring sub-units. We also address how these attributes might become altered during diagenesis and planetary processing. Finally, we discuss the instrumental techniques and further developments needed to detect them.     

Updated Review of Planetary Atmospheric Electricity

This paper reviews the progress achieved in planetary atmospheric electricity, with focus on lightning observations by present operational spacecraft, aiming to fill the hiatus from the latest review published by Desch et al. (Rep. Prog. Phys. 65:955–997, 2002). The information is organized according to solid surface bodies (Earth, Venus, Mars and Titan) and gaseous planets (Jupiter, Saturn, Uranus and Neptune), and each section presents the latest results from space-based and ground-based observations as well as laboratory experiments. Finally, we review planned future space missions to Earth and other planets that will address some of the existing gaps in our knowledge.     

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.     

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.     

The Genesis Solar Wind Concentrator: Flight and Post-Flight Conditions and Modeling of Instrumental Fractionation

The Genesis mission Solar Wind Concentrator was built to enhance fluences of solar wind by an average of 20x over the 2.3 years that the mission exposed substrates to the solar wind. The Concentrator targets survived the hard landing upon return to Earth and were used to determine the isotopic composition of solar-wind—and hence solar—oxygen and nitrogen. Here we report on the flight operation of the instrument and on simulations of its performance. Concentration and fractionation patterns obtained from simulations are given for He, Li, N, O, Ne, Mg, Si, S, and Ar in SiC targets, and are compared with measured concentrations and isotope ratios for the noble gases. Carbon is also modeled for a Si target. Predicted differences in instrumental fractionation between elements are discussed. Additionally, as the Concentrator was designed only for ions ≤22 AMU, implications of analyzing elements as heavy as argon are discussed. Post-flight simulations of instrumental fractionation as a function of radial position on the targets incorporate solar-wind velocity and angular distributions measured in flight, and predict fractionation patterns for various elements and isotopes of interest. A tighter angular distribution, mostly due to better spacecraft spin stability than assumed in pre-flight modeling, results in a steeper isotopic fractionation gradient between the center and the perimeter of the targets. Using the distribution of solar-wind velocities encountered during flight, which are higher than those used in pre-flight modeling, results in elemental abundance patterns slightly less peaked at the center. Mean fractionations trend with atomic mass, with differences relative to the measured isotopes of neon of +4.1±0.9 ‰/amu for Li, between ?0.4 and +2.8 ‰/amu for C, +1.9±0.7‰/amu for N, +1.3±0.4 ‰/amu for O, ?7.5±0.4 ‰/amu for Mg, ?8.9±0.6 ‰/amu for Si, and ?22.0±0.7 ‰/amu for S (uncertainties reflect Monte Carlo statistics). The slopes of the fractionation trends depend to first order only on the relative differential mass ratio, Δm/m. This article and a companion paper (Reisenfeld et al. 2012, this issue) provide post-flight information necessary for the analysis of the Genesis solar wind samples, and thus serve to complement the Space Science Review volume, The Genesis Mission (v. 105, 2003).     

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.     

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.     

Ceramic ChemCam Calibration Targets on Mars Science Laboratory

The ChemCam instrument on the Mars Science Laboratory rover Curiosity will use laser-induced breakdown spectroscopy (LIBS) to analyze major and minor element chemistry from sub-millimeter spot sizes, at ranges of ～1.5–7?m. To interpret the emission spectra obtained, ten calibration standards will be carried on the rover deck. Graphite, Ti?metal, and four glasses of igneous composition provide primary, homogeneous calibration targets for the laser. Four granular ceramic targets have been added to provide compositions closer to soils and sedimentary materials like those expected at the Gale Crater field site on Mars. Components used in making these ceramics include basalt, evaporite, and phyllosilicate materials that approximate the chemical compositions of detrital and authigenic constituents of clastic and evaporite sediments, including the elevated sulfate contents present in many Mars sediments and soils. Powdered components were sintered at low temperature (800?°C) with a small amount (9?wt.%) of lithium tetraborate flux to produce ceramics that retain volatile sulfur yet are durable enough for the mission. The ceramic targets are more heterogeneous than the pure element and homogenous glass standards but they provide standards with compositions more similar to the sedimentary rocks that will be Curiosity’s prime targets at Gale Crater.     

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.     

Chronology of SNC meteorites

In this paper a model is presented for the geochemical evolution of Mars which is constrained by the isotope systematics of Pb, Nd, and Sr determined for SNC meteorites (SNCs). The young magmatic crystallization ages (internal or mineral ages) of SNCs may indicate that these meteorites indeed stem from Mars. Internal ages and U-Pb and Pb-Pb systematics strongly suggest that they are the result of two magmatic processes. In addition, shock metamorphism is implied from observed petrographic shock features. For ALHA 77009 a shock-age < 15 Ma is obtained which is within uncertainty identical to the independently determined cosmic ray exposure age. It is therefore plausible that shock and exposure ages are identical for all SNCs. The Rb/Sr data of all common (non-SNC) meteorites form a 4.55 Ga isochron as do the Pb-Pb data (geochron). The SNC data fall close to these two isochrons. The Sr and Pb isotopic compositions in SNCs suggest that they formed in a recent (1.3-0.15 Ga) melting event from reservoirs which had been magmatically differentiated 4.3 ± 0.2 Ga ago. In a concordia diagram (U-Pb evolution plot) the SNC data reflect recent increase of the U/Pb ratio and the same two stage magmatic history as suggested by the other isotopic systems. The oxygen isotopic composition as well as the Nd isotopic systematics strongly suggest that the SNCs stem from one common reservoir which chemically differentiated 4.3 ± 0.2 Ga ago and then formed sub-reservoirs. In contrast to common meteorites, SNCs experienced an early magmatic differentiation where the Sm/Nd, U/Pb and Rb/Sr ratios have been strongly fractionated. In the recent magmatic process (1.3-0.15 Ga ago), in which the SNCs were formed as rocks, Sm/Nd and U/Pb were fractionated, while Rb/Sr remained similar to that of the source from which the magmas originated. During these melting events, mixing of components from different sub-reservoirs might have had occurred. At least three subreservoirs are necessary to explain the isotopic variations observed in SNCs. In contrast to the isotopic evolution of the Earth, Mars conserved remnants of the primary differentiation, a fact, which places important constraints on the tectonic evolution of Mars.     

Solar Variability and Climate Impact on Terrestrial Planets

Some possible factors of climate changes and of long term climate evolution are discussed with regard of the three terrestrial planets, Earth, Venus and Mars. Two positive feedback mechanisms involving liquid water, i.e., the albedo mechanism and the greenhouse effect of water vapour, are described. These feedback mechanisms respond to small external forcings, such as resulting from solar or astronomical constants variability, which might thus result in large influences on climatic changes on Earth. On Venus, reactions of the atmosphere with surface minerals play an important role in the climate system, but the involved time scales are much larger. On Mars, climate is changing through variations of the polar axis inclination over time scales of ～10⁵–10⁶ years. Growing evidence also exists that a major climatic change happened on Mars some 3.5 to 3.8 Gigayears ago, leading to the disappearance of liquid water on the planet surface by eliminating most of the CO₂ atmosphere greenhouse power. This change might be due to a large surge of the solar wind, or to atmospheric erosion by large bodies impacts. Indeed, except for their thermospheric temperature response, there is currently little evidence for an effect of long-term solar variability on the climate of Venus and Mars. This fact is possibly due to the absence of liquid water on these terrestrial planets.     

Testing General Relativity with Atomic Clocks

We discuss perspectives for new tests of general relativity which are based on recent technological developments as well as new ideas. We focus our attention on tests performed with atomic clocks and do not repeat arguments present in the other contributions to the present issue (Space Sci. Rev. 2009, This Issue). In particular, we present the scientific motivations of the space projects ACES (Salomon et al. in CR Acad. Sci. IV-2:1313, 2001) and SAGAS (Wolf et al. in Exp. Astron. 23:651, 2009).     

Morphological Biosignatures in Early Terrestrial and Extraterrestrial Materials

Biosignatures in early terrestrial rocks are highly relevant in the search for traces of life on Mars because the early geological environments of the two planets were, in many respects, similar and, thus, the potential habitats for early life forms were similar. However, the identification and interpretation of biosignatures in ancient terrestrial rocks has proven contentious over the last few years. Recently, new investigations using very detailed field studies combined with highly sophisticated analytical techniques have begun to document a large range of biosignatures in Early Archaean rocks. Early life on Earth was diversified, widespread and relatively evolved, but its traces are generally, but not always, small and subtle. In this contribution I use a few examples of morphological biosignatures from the Early-Mid Archaean to demonstrate their variety in terms of size and type: macroscopic stromatolites from the 3.443 Ga Strelley Pool Chert, Pilbara; a meso-microscopic microbial mat from the 3.333 Ga Josefsdal Chert, Barberton; microscopic microbial colonies and a biofilm from the 3.446 Ga Kitty’s Gap Chert, Pilbara; and microscopic microbial corrosion pits in the glassy rinds of 3.22–3.48 Ga pillow lavas from Barberton. Some macroscopic and microscopic structures may be identifiable in an in situ robotic mission to Mars and in situ methods of organic molecule detection may be able to reveal organic traces of life. However, it is concluded that it will probably be necessary to return suitably chosen Martian rocks to Earth for the reliable identification of signs of life, since multiple observational and analytical methods will be necessary, especially if Martian life is significantly different from terrestrial life.     

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.     

An Overview of the Lunar Crater Observation and Sensing Satellite (LCROSS)

The Lunar Crater Observation Sensing Satellite (LCROSS), an accompanying payload to the Lunar Reconnaissance Orbiter (LRO) mission (Vondrak et al. 2010), was launched with LRO on 18 June 2009. The principle goal of the LCROSS mission was to shed light on the nature of the materials contained within permanently shadowed lunar craters. These Permanently Shadowed Regions (PSRs) are of considerable interest due to the very low temperatures, <120?K, found within the shadowed regions (Paige et al. 2010a, 2010b) and the possibility of accumulated, cold-trapped volatiles contained therein. Two previous lunar missions, Clementine and Lunar Prospector, have made measurements that indicate the possibility of water ice associated with these PSRs. LCROSS used the spent LRO Earth-lunar transfer rocket stage, an Atlas V Centaur upper stage, as a kinetic impactor, impacting a PSR on 9 October 2009 and throwing ejecta up into sunlight where it was observed. This impactor was guided to its target by a Shepherding Spacecraft (SSC) which also contained a number of instruments that observed the lunar impact. A?campaign of terrestrial ground, Earth orbital and lunar orbital assets were also coordinated to observe the impact and subsequent crater and ejecta blanket. After observing the Centaur impact, the SSC became an impactor itself. The principal measurement goals of the LCROSS mission were to establish the form and concentration of the hydrogen-bearing material observed by Lunar Prospector, characterization of regolith within a PSR (including composition and physical properties), and the characterization of the perturbation to the lunar exosphere caused by the impact itself.     

Vibrational relaxation of CO2 (m,nl, p) in a CO2-N2 mixture. Part 1: Survey of available data

The knowledge of the vibrational relaxation reaction rates of diatomic and triatomic molecules is required for the modelling of numerous gaseous mixtures such as the Earth atmosphere (involving radiation transfer phenomena), exhaust plumes, afterbody wakes, Mars atmosphere, shock waves and chemical lasers. It enables a better understanding of the chemical reaction product rates and provides the populations of the radiating levels. In this study, we consider the few lowest vibrational levels of a CO₂-N₂ mixture and the V-T and V-V energy transfer processes between these levels. The reaction rates given by experimental and theoretical studies and by different surveys of data supplied by a bibliographic search are presented. Then, comparisons of the data available for the most important processes are shown. The usual assumptions concerning the vibrational relaxation of CO₂ (m, n^l, p) are recalled and briefly discussed.     

Atom-Based Test of the Equivalence Principle

We describe a test of the equivalence principle with quantum probe particles based on atom interferometry. For the measurement, a light pulse atom interferometer based on the diffraction of atoms from effective absorption gratings of light has been developed. A differential measurement of the Earth’s gravitational acceleration g for the two rubidium isotopes ⁸⁵Rb and ⁸⁷Rb has been performed, yielding a difference Δg/g=(1.2±1.7)×10^?7. In addition, the dependence of the free fall on the relative orientation of the electron to the nuclear spin was studied by using atoms in two different hyperfine states. The determined difference in the gravitational acceleration is Δg/g=(0.4±1.2)×10^?7. Within their experimental accuracy, both measurements are consistent with a free atomic fall that is independent from internal composition and spin orientation.     

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.     

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.