Martian Origin Research Articles

The parent magma of the Nakhla (SNC) meteorite, inferred from magmatic inclusions

The Nakhla meteorite, one of the SNC group of putative Martian origin, is an igneous rock, a cumulate of augite and olivine, that does not represent a magma composition. Samples of its parent magma were trapped as magmatic inclusions in its cumulus olivine crystals. The inclusions range from 10–350 μm in diameter and consist of augite rich in Ti and Al, silica-rich glass, iron oxides, ilmenite, apatite, and rare amphibole. Each inclusion has a rind of augite and chromite against its host olivine, and a core of radiating sprays of augite set in glass and feldspar. These textures are consistent with an origin as trapped silicate liquid. Twenty four inclusions were analyzed by electron microprobe, by averaging series of rastered beam analyses. On average, the analyses are independent of the size of the inclusion, most clearly for inclusions larger than 70 μm diameter, and variations among analyses are consistent with small differences in the proportions of augite, glass, and iron oxides exposed on thin section surfaces. The average inclusion analysis is calculated to represent 28% host olivine and 72% inclusion composition. The inclusion rinds are calculated to be 13% of the inclusion composition, for which a small correction is made. The inclusion composition is then corrected for Mg Fe exchange with the host olivine (to chemical equilibrium with the cumulus augite cores) and crystallization of 13% olivine onto the host crystal to yield a magma composition with the proper MgO FeO ratio and with olivine and augite on its liquidus. This composition, NK93, is an estimate of Nakhla's parent magma originally trapped in the inclusions. NK93 is basaltic, rich in iron, and poor in aluminum compared to terrestrial basalts. Its abundances of Al, Ti, and Na are consistent with the geochemistry of the SNC meteorites as a whole. NK93 is similar to the parent magmas proposed in an independent study of magmatic inclusions in Nakhla, but unlike other proposed compositions. One proposed parent magma, based on mass balance and the assumption that Nakhla was a closed-system after crystal accumulation, is inconsistent with the magmatic inclusion studies and Nakhla's petrography. This suggests that Nakhla was not a closed-system, but was infiltrated by post-cumulus basaltic magmas. Another suggested parent magma was derived from element distribution between augite and basaltic magma, but that calculation relied on an incorrect augite composition. Using a correct augite composition in that calculation yields a parent magma much closer in composition to NK93. Thus, three independent approaches converge in requiring that Nakhla originally formed from a low-Al, low MgO FeO basalt like NK93.

An assessment of the nature and origins of the carbon‐bearing components in fines collected during the sawing of EET A79001

Saw dust obtained from the cutting of EET A79001, a meteorite with a presumed Martian origin, has been analyzed for its carbon content and stable isotopic composition in order to assess the sample's potential value for studies aimed at elucidating Martian surface processes. These fines, which are available in plentiful supply, are a useful source of material and are important since they are a representative whole‐rock sample of this complex meteorite. In addition, the fines give an indication of contamination sources which may affect not only the collected sawings but also the sawn surface of the meteorite. A number of contaminants were identified in the fines (e.g. organic materials, Teflon, diamond, and carbon associated with metal). It is considered that the levels of carbon contamination measured represent a worst possible case and hence provide a valuable reference with which to compare other analyses of this sample, or indeed, other Martian meteorites. Evidence was found for an indigenous carbonate component similar to that observed previously in other fractions of the meteorite. However, based on the results presented herein it is not possible to assert that isotopically distinct carbonate minerals of different origins are present in the meteorite. The levels of trapped Martian atmospheric CO2 calculated for the fines are unexpectedly high (∼1 ppm carbon), but this could be influenced by the presence of other, as yet unknown, carbonaceous materials.

Phobos-2 results on the ionospheric plasma escape from Mars

The outflow of ions from the Martian ionosphere was studied by the ASPERA instrument on the Soviet Phobos-2 spacecraft. The Martian magnetosphere tail is found to be dominated by plasma of Martian origin, primarily atomic oxygen and exospheric hydrogen, but also with a substantial admixture of molecular ions. The ionospheric plasma escape may be related with several different processes, one which is similar to that near magnetized bodies (e.g. the Earth), and another that usually dominates near non-magnetized bodies (e.g. comets). The loss rate of the ionospheric ion outflow is very high, about 1 kg/s. This value corresponds to an erosion of the Martian atmosphere in about 100 million years. Thus, under the assumption that Mars was weakly magnetized also in the past, the solar wind interaction with its exosphere/ionosphere may have been a major reason for the observed dehydration of Mars.

The question of a Martian planetary magnetic field

Near Mars magnetic measurements from the Phobos-2 and Mars-2, -3, -5 spacecraft are investigated with respect to indications for magnetic fields of Martian origin. The presence of a part in the measured magnetic fields, corotating with Mars, is shown. Furthermore, it could be possible that this corotating magnetic field has a component on the planetary scale and a more local part, eventually connected with a magnetic anomaly in the Tharsis region.

Water in SNC meteorites: evidence for a martian hydrosphere.

The Shergotty-Nakhla-Chassigny (SNC) meteorites, purportedly of martian origin, contain 0.04 to 0.4 percent water by weight. Oxygen isotopic analysis can be used to determine whether this water is extraterrestrial or terrestrial. Such analysis reveals that a portion of the water is extraterrestrial and furthermore was not in oxygen isotopic equilibrium with the host rock. Lack of equilibrium between water and host rock implies that the lithosphere and hydrosphere of the SNC parent body formed two distinct oxygen isotopic reservoirs. If Mars was the parent body, the maintenance of two distinct reservoirs may result from the absence of plate tectonics on the planet.

Fall days of the SNC meteorites: Evidence for an SNC meteoroid stream, and a common site of origin

Abstract— Four of the SNC meteorites of putative Martian origin are falls. Two of these fell on October 3: Chassigny in 1815 and Zagami in 1962. The probability of this coincidence arising from random fall days is approximately 1 in 60. If this coincidence is not the result of chance, it suggests that some of the SNC meteorites are derived from a meteoroid stream. In that Chassigny and Zagami span nearly the full range of SNC lithologies and histories, the coincidence of fall days is consistent with suggestions that all of the SNCs came from a single site (impact crater) on their parent planet.

On the momentum transfer of the solar wind to the Martian topside ionosphere

Hot plasma measurements from the Soviet Phobos‐2 spacecraft in the Martian magnetosphere suggests that the solar wind interaction with Mars is cometary‐like, with mass loading of the solar wind and ion pick‐up occurring also outside the subsolar bow‐shock. The interaction is characterized by a pronounced decrease of the solar wind speed inside what has teen termed the mass‐loading boundary (MLB). Well outside the MLB, the ion pick‐up process acts in a “normal” sense. There ions pick up approximately the solar wind velocity‐independent of mass. Inside the MLB, the momentum loss of solar wind ions is more pronounced—heavy ions of Martian origin taking up most of the solar wind ion momentum flux. The heavy mass‐loading of solar wind ions in the innermost part of the Martian boundary layer (near the “magnetopause”) leads to a “loaded” ion pick‐up. The process can be understood as internal loading of an MHD‐dynamo, propelled by a driver plasma ‐ the solar wind. The ASPERA ion composition and momentum data is consistent with such a pick‐up process.Inside the “magnetopause” ions of Martian origin are accelerated up to energies close to those of the solar wind protons. We propose two types of acceleration processes, one similar to that acting within the Earth's auroral acceleration region (acting in the presence of an ambient magnetic field), another a pick‐up process acting within a limited spatial region.

Plasma composition measurements of the Martian magnetosphere morphology

Plasma composition measurements from the Phobos‐2 spacecraft have revealed new data on the dynamics and composition of plasmas in the Martian magnetosphere—a magnetosphere being significantly wider and more dynamic than was anticipated before. The tail width is some 6–9 Mars radii at antisolar distances greater than ten Martian radii and it contains some features that are similar to those observed in the Geotail. The strong dominance of plasma of Martian origin implies a modest access of solar wind particles into the central tail. Heavy ions like O+ are in fact up to ten times more abundant then H+. Some plasma properties, such as the characteristics of the ionospheric ion outflow, the presence of a distinct plasma sheet and the overall width of the tail are arguments for a weak intrinsic magnetic field on Mars. However, yet other plasma characteristics, such as strong ion pick‐up processes, indicates a Venus‐like interaction. Thus, our present view based on primarily plasma measurements is that Mars constitutes a hybrid obstacle to the solar wind.The observation of two distinctly different plasma boundaries suggests that the magnetosphere of Mars is contained whithin an exterior / composition boundary—termed the Mass‐Loading Boundary (MLB), and an interior boundary—a “magnetopause”. MLB marks the boundary for a cometary‐like interaction and the “magnetopause” the limit of solar wind plasma entry—possibly also constituting the boundary for an intrinsic magnetic field.

Ions of martian origin and plasma sheet in the martian magnetosphere: initial results of the TAUS experiment

UNLIKE plasma instruments used on previous space missions to Mars, the TAUS instrument on Phobos 2 was designed so that the energy per charge and angular spectra of three species of ions could be measured separately. These species were H+ and He2+ characteristic of the solar wind, and 'heavy ions' collected in one integral channel covering the mass per charge (M/q) range 3 to infinite, which we anticipated to find predominantly in the near-martian regime. In all spacecraft orbits around Mars we found a sharp boundary, separating the shocked solar wind from the martian magnetosphere which was characterized by the absence of solar-wind-like plasma. As the plasma inside the magnetosphere, and particularly in the tail, was dominated by heavy ions with number densities orders of magnitude higher than found in the solar wind, we assumed it was mainly of martian origin. Typically, heavy ions of low tailward flow velocity were seen near the boundary of the magnetotail, whereas high-speed tailward plasma flows of such ions were detected deeper inside the tail, a region not investigated before. Near the centre of the martian magnetotail a plasma regime, comparable to the terrestrial as well as the venusian1 plasma sheet, was detected, characterized by highly supersonic tailward streams of heavy ions. The flux of planetary ions leaving Mars through its magnetotail is tentatively estimated to be of the order of a few times 1025 s-1. Such loss rates would be significant for the dissipation of the martian atmosphere on cosmological timescales.

Spectral mineralogy of terrestrial planets: scanning their surfaces remotely

AbstractSpectral measurements of sunlight reflected from planetary surfaces, when correlated with experimental visible-near-infrared spectra of rock-forming minerals, are being used to detect transition metal cations, to identify constituent minerals, and to determine modal mineralogies of regoliths on terrestrial planets. Such remote-sensed reflectance spectra measured through earth-based telescopes may have absorption bands in the one micron and two micron wavelength regions which originate from crystal field transitions within Fe2+ ions. Pyroxenes with Fe2+ in M2 positions dominate the spectra, and the resulting 1 μm versus 2 µm spectral determinative curve is used to identify compositions and structure-types of pyroxenes on surfaces of the Moon, Mercury, and asteroids, after correcting for experimentally-determined temperature-shifts of peak positions. Olivines and Fe2+-bearing plagioclase feldspars also give diagnostic peaks in the 1 µm region, while tetrahedral Fe2+ in glasses absorb in the 2 µm region as well. Opaque ilmenite, spinel and metallic iron phases mask all of these Fe2+ spectral features. Laboratory studies of mixed-mineral assemblages enable coexisting Fe2+ phases to be identified in remote-sensed reflectance spectra of regoliths. Thus, noritic rocks in the lunar highlands, troctolites in central peaks of impact craters such as Copernicus, and high-Ti and low-Ti mare basalts have been mapped on the Moon's surface by telescopic reflectance spectroscopy. The Venusian atmosphere prevents remote-sensed spectral measurements of its surface mineralogy, while atmospheric CO2 and ferric-bearing materials in the regolith on Mars interfere with pyroxene characterization in bright- and dark-region spectra. Reflectance spectral measurements of several meteorite types, including specimens from Antarctica, are consistent with a lunar highland origin for achondrite ALHA 81005 and a martian origin for shergottite EETA 79001, although source regions may not be outermost surfaces of the Moon and Mars. Correlations with asteroid reflectance spectra suggest that Vesta is the source of basaltic achondrites, while wide ranges of olivine/pyroxene ratios are inconsistent with an ordinary-chondrite surface composition of many asteroids. Visible-near-infrared spectrometers are destined for instrument payloads in future spacecraft missions to neighbouring solar system bodies.

Effect of an impact-generated gas cloud on entrained surface boulders and spall ejecta

The purpose of this investigation is to study the effect of an impact-generated gas cloud on the size-velocity distribution of large, solid ejecta fragments that become entrained in it, with particular reference to the possible Martian origin of the SNC meteorites. The entrainment both of loose surface boulders and of the early-time, large-size, high-velocity spall component of the crater ejecta is modeled numerically. Surface boulders from as far as 40 km from the center of impact can be accelerated by the high velocity leading edge of the gas cloud to velocities in excess of Martian escape velocity (5 km/s), but are generally crushed by the acceleration. Spall fragments become entrained later and nearer the center of the gas cloud, where gas velocities are much less. High velocity spalls are decelerated by the gas and low velocity spalls are accelerated by the gas, but no spalls ejected at < 5km/s are accelerated to velocities 5km/s. An impact-generated gas cloud is thus expected to scour the pre-existing surface of loose material and to change the size-velocity distribution of spall ejecta, but does not sufficiently enhance the velocities of crater ejecta to explain the Martian origin of SNC meteorites as large rocks.

Effect of an impact‐generated gas cloud on the acceleration of solid ejecta

A large amount of petrologic, geochemical, isotopic, and rare gas evidence strongly indicates that the shergottite, nakhlite, and Chassigny (SNC) meteorites originated on a parent body large enough to have sustained recent geological activity, probably Mars. The most straightforward interpretation of the cosmic ray exposure evidence requires that the protoshergottites were at least 5 and, more likely, 15 m in diameter; this large size combined with the 5 km/s escape velocity for Mars presents a formidable dynamic problem. This study investigates the possibility that the entrainment of large blocks of solid ejecta within the rapidly expanding vapor cloud produced by an impact provides sufficient additional acceleration to overcome this problem. A large number of impact parameters, such as impactor and target material, impact velocity, times of ejection, and crater size, were varied to determine their relative effects. Ejection time was found to be the single most important variable: Rocks entrained at very early times are invariably crushed by high dynamic pressures, and rocks ejected at late times, after the gas density has become very low, are little affected by the gas. Only rocks ejected within a fairly narrow window of intermediate times have their velocities significantly changed. Rocks initially ejected at very high velocities, such as those predicted by the Melosh spallation model, are strongly decelerated by the gas. Rocks injected into the gas stream with velocities much different from the average gas velocity at times early enough that the gas density is still significant are often subjected to dynamic pressures great enough to crush them. Thus the net effect of a large, rapidly expanding, impact‐generated gas cloud on solid ejecta is to homogenize the velocity distribution and to reduce the average size. For 30‐km diameter craters the only rocks with final velocities greater than 5 km/s were those whose initial velocities were greater than or approximately equal to 5 km/s. Since the average final velocity increases with crater size, it may be necessary to invoke a crater much larger than 30 km to explain the Martian origin of SNC meteorites, if the proto‐SNCs had diameters of greater than 1 m. On the other hand, if the proto‐SNCs were only a few tens of centimeters or less, gas acceleration of ejecta from a 30‐km‐diameter crater is consistent with the Martian origin of these unusual meteorites.

The Shergotty Consortium and SNC meteorites: An overview

The Shergotty meteorite has a multi-phase (magmatic and shock) history. While the Shergotty picture is complex, consortium studies have advanced our knowledge and understanding of Shergotty and shergottites, nakhlites and Chassigny (SNC) meteorites. Martian origin for the SNC meteorites is strongly favored by several workers from the evidence of trapped noble gases and nitrogen compositions in glasses (lithology C) of the EETA 79001 meteorite, which compare well with the Martian atmosphere analysis made by the Viking Spacecraft. The parent body is about 2 to 4 times richer in volatiles (Cl, Br, Na, K, Rb, Zn, F, Pb, etc.) than the Earth. Consortium studies on Shergotty show very low thermoluminescence, no deformation of tracks, cosmic ray exposure age of about 2.5 million years (m.y.), a pre-atmospheric size of about 12 cm radius, and apparently one shock event at 30 GPa pressure that converted plagioclase to maskelynite. The crystallization age of Shergotty by the Sm-Nd method is 360 ± 16 m.y. The Rb-Sr age for Shergotty is reported as 166 m.y. and the Pb-U age as about 200 m.y. Interpretations of age-dating and exposure scenarios are controversial and may require further studies.At least two scenarios for the ejection of SNC meteorites are possible: 1) ejection as a large body (>6 m size) by a single impact on Mars and then multiple breakup in the asteroidal belt at about 11 m.y. for Chassigny and nakhlites, at 2.5 m.y. for Shergotty, Zagami and ALHA 77005, and at 0.6 m.y. for EETA 79001; and 2) ejection of small objects (<0.5 m size) by multiple impacts on the Martian terrain at 11, 2.5 and 0.6 m.y. with no breakup in space.

The case for a martian origin of the shergottites, II. Trapped and indigenous gas components in EETA 79001 glass

Analysis of nitrogen and light noble gases in a large sample of glass (lithology C) from the antarctic shergottite EETA 79001 yields a minimum δ 15N > +300‰ for the isotopic composition of nitrogen trapped in the glass. The new data fall on the mixing line through the martian atmospheric composition defined by δ 15N vs. 40Ar/ 14N for two smaller samples analyzed previously. The results from all three samples are consistent with a two-component nitrogen system in which ∼ 84 ppb of trapped martian atmospheric N is mixed in variable proportions with another, more thermally labile N component during stepped heating. This second component, which appears to be indigenous to the glass rather than adsorbed from air and is present in amounts that vary by more than a factor of 3 from sample to sample, may represent volatiles from the martian interior. Data from crystalline phases of several SNC meteorites indicate that the indigenous gas may have δ 15N < −35‰ and 36Ar/ 14N≲ 3 × 10 −6 , similar to the enstatite chondrites. Neon compositions in EETA 79001 glass samples suggest an earth-like value of ∼ 10.1 ± 0.7 for the unknown 20Ne/ 22Ne ratio in the martian atmosphere. The nitrogen-argon correlation systematics yield trapped 40Ar/ 36Ar= 2260 ± 200 , within error of the Viking value. There is evidence that 36Ar/ 38Ar in the martian atmosphere is 4.1 ± 0.2, strikingly different from terrestrial or typical chondritic ratios near 5.3. Attribution of this low value to excess 38Ar generated over martian history by galactic cosmic-ray (GCR) spallation of surface materials would be difficult for a number of reasons, among them the excessive GCR fluences required and the absence of a corresponding 21Ne excess.

SNC meteorites: Clues to Martian petrologic evolution?

The shergottites, nakhlites, and Chassigny (SNC meteorites) are apparently cumulate mafic and ultramafic rocks that crystallized at shallow levels in the crust of their parent body. The mineralogy and chemistry of these meteorites are remarkably like equivalent terrestrial rocks, although their ratios of Fe/(Fe + Mg) and certain incompatible elements and their oxygen isotopic compositions are distinctive. All have crystallization ages of 1.3 b.y. or younger and formed from magmas produced by partial melting of previously fractionated source regions. Isotope systematics suggest that the SNC parent body had a complex and protracted thermal history spanning most of geologic time. Some meteorites have been severely shock metamorphosed, and all were ejected from their parent body at relatively recent times, possibly in several impact events. Late crystallization ages, complex petrogenesis, and possible evidence for a large gravitational field suggest that these meteorites are derived from a large planet. Trapped gases in shergottite shock melts have compositions similar to the composition measured in the Martian atmosphere. Ejection of Martian meteorites may have been accomplished by acceleration of near‐surface spalls or other mechanisms not fully understood. If SNC meteorites are of Martian origin, they provide important information on planetary composition and evolution. The bulk composition and redox state of the Martian mantle, as constrained by shergottite phase equilibria, must be more earthlike than most current models. Planetary thermal models should benefit from data on the abundances of radioactive heat sources, the melting behavior of the mantle, and the timing of planetary differentiation. Calculated depletion of chalcophile elements in source regions indicates a core dominated by sulfides, and paleomagnetic measurements suggest the presence of a weak magnetic field within the last several hundred thousand years. Concentrations of volatile elements indicate that the SNC parent body was not volatile depleted, and trapped atmospheric components measured in shock melts may be useful in understanding planetary degassing. By providing comparisons for spectral reflectance data and Viking soil analyses, these meteorites may also constrain surface mineralogy and weathering mechanisms.

Are all the ‘martian’ meteorites from Mars?

The shergottites, nakhlites and Chassigny, collectively known as the SNC meteorites, seem to define a petrological and geochemical sequence1. Together with their young radiometric solidification ages of only ∼1,300 Myr (refs 2–9), this has been interpreted as evidence for magmatic activity on their parent body late in its history. Such late magmatism is most easily envisaged for planetary-sized bodies and it has been suggested that SNC meteorites are derived from Mars (see resf. 10 and refs therein). This suggestion is supported by the apparent similarities between the noble gas and nitrogen composition of the present martian atmosphere11,12 and gases trapped in shock-produced glass phases of the shergottite EETA 79001 (refs 13, 14). Our analyses of trapped argon, krypton and xenon in Shergotty, Nakhla and Chassigny reported here show, however, that these meteorites do not contain unadulterated noble gases from the martian atmosphere, fractionated or not. They indicate that for Nakhla and Chassigny, in particular, a martian origin is only possible if the martian atmosphere contains a higher fraction of radiogenic 129Xe and of fissiogenic xenon than does xenon in martian rocks. This is the opposite of the situation on Earth.

The formation age of the Brachina meteorite

Nuclear particle tracks were studied in various phases from the Brachina meteorite, which was classified until recently as a chassignite. Fission tracks due to the decay of 244Pu (T 12 = 82m.y.) were observed and indicate that Brachina formed ∼ 4.5 b.y. ago in a parent body which was most probably asteroidal in size. Contrary to what has been previously suggested [7], there is no need to postulate a Martian origin for this meteorite. This conclusion is supported by independent evidence obtained by other groups.

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites

Isotopic concentrations of the noble gases have been measured in several different phases of Elephant Moraine A79001 and in whole rock samples of Zagami and Allan Hills A77005, three meteorites which belong to the rare group of SNC achondrites that may have originated from the planet Mars. Shocked phases of EETA79001 contain a trapped Ar, Kr, and Xe component characterized by 84Kr 132Xe ∼15, 40Ar 36Ar > 2000 , 129Xe 132Xe ≥ 2 , and 4He 40Ar ≤ 0.1 . These elemental and isotopic ratios are unlike those for any other noble gas component except analyses of the Martian atmosphere made by Viking spacecraft. The isotopic composition of the trapped Kr shows an approximate 1% per mass unit enrichment of lighter isotopes compared to terrestrial Kr, and the traped Xe may show either a fission component or a fractionated enrichment of heavier isotopes compared to terrestrial Xe. It is hypothesized that these gases represent a portion of the Martian atmosphere which was shock-implanted into EETA79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. Cosmic ray-produced gases in the eight known SNC meteorites form three distinct groups with exposure ages of ∼11 MY (Chassigny and the nakhlites), ∼2.6 MY (Shergotty, Zagami, and ALHA77005), and ∼0.5 MY (EETA79001). These ages suggest three distinct events and cannot have been produced by irradiation for a common time under greatly different shielding. Comparison of cosmogenic 3He 21 Ne measured in EETA79001 with two independent models for the production of this ratio as a function of shielding indicates that this meteorite was irradiated in space as a relatively small object. If the SNC meteorites were ejected from Mars ∼ 180 My ago, the shock age of the shergottites, they must have been relatively large objects (>6 meters diameter) which experienced at least three space collisions to initiate cosmic ray exposure. Ejection from Mars by three events at the times of initiation of cosmic ray exposure would permit the ejected objects to have been much smaller (<1 meter diameter), but would require three such events on 1.3 Gy Martian terraine in the past ∼10 MY and would not explain the common 180 MY shock age seen in all four shergottites.

The case for a martian origin of the shergottites: nitrogen and noble gases in EETA 79001

Nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite EETA 79001. A nitrogen component trapped in the glass, but not present in the matrix, has a δ 15N value at least as high as +190‰. Ratios of 40Ar/ 14N and 15N/ 14N in the glass are consistent with dilution of a martian atmospheric component ( δ 15N = 620 ± 160‰, 40Ar/ 14N= 0.33 ± 0.03 ) by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock. Trapped noble gases in the glass reproduce, within error, the elemental and isotopic compositions measured in Mars' atmosphere by Viking, and are in general agreement with previous measurements except for much lower abundances of neutron-generated krypton and xenon isotopes. The most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the EETA 79001 glass, and that this meteorite, and thus the shergottites and probably the nakhlites and chassignites as well, originated on Mars. Nitrogen in the non-glassy matrix of EETA 79001 amounts to less than 0.5 ppm and has a spallation-corrected δ 15N value in the range 0 to −20‰; it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen. Spallogenic noble gases yield single-stage exposure ages between 400,000 and 900,000 years, depending on irradiation geometry. Trapped argon may have an unusually low 36Ar/ 38Ar ratio. Trapped krypton, except for a small excess at 80Kr, is smoothly mass-fractionated with respect to either terrestrial or chondritic Kr. The trapped xenon composition is consistent with addition of neutron-capture, radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric Xe. The elemental 84Kr/ 132Xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio.

Petrogenesis of the SNC (shergottites, nakhlites, chassignites) meteorites: Implications for their origin from a large dynamic planet, possibly Mars

Abundances of major, minor, and trace elements were determined in the whole rock and mineral separates of the two distinct lithologies present in the Antarctic achondrite Elephant Moraine A (EETA) 79001 via sequential instrumental (INAA) and radiochemical neutron activation analysis (RNAA). Additional chalcophile, siderophile, and volatile trace elements are reported for the shergottites ALHA 77005, Shergotty, and Zagami. The rare earth element (REE) abundances of each of the lithologies of EETA 79001 (A and B) exhibit chondritic normalized patterns similar to ALHA 77005. The close relationship of the Antarctic shergottites indicates that ALHA 77005 is a residual source produced by incongruent melting of a source similar in bulk composition to EETA 79001A and that EETA 79001B and the interstitial phases in EETA 79001A are the melts produced by such melting episodes. Incorporating previous isotopic studies and our chemical data, we suggest that either fractionation of or contamination by feldspar could account for the different shergottite sources. The large ion lithophile (LIL) trace element abundances of the shergottites require variable but extensive degrees of nonmodal melting of isotopically constrained parent sources. The extreme extent of such melting is hypothesized to be represented by ALHA 77005 as a residual composition. Derivation of a pre‐1.3 AE(109yr) fractionation sequence is suggested in which ol, ol + opx, opx + cpx, and opx + cpx + plag sequentially produce residual liquids capable of accounting for the precursor sources for the SNC meteorites. On the basis of chemical, isotopic, and petrologic considerations we conclude that the SNC sources are consistent with their derivation by extensive fractionation of a primitive magma initially produced from a source having chondritic refractory LIL trace element abundances, Fe′(Fe/Fe+Mg atomic)≲0.3, and a mineralogy primarily of ol and pyx. Our model‐dependent deductions suggest that this magma could be derived without plagioclase or garnet left in the residuum. Petrogenetic and age relationships among SNC meteorites suggest a single complex‐provenance on a dynamic planet not unlike the earth. Considering that each of these meteorites possesses chemical, isotopic, and petrologic features consistent with data presently available from Mars, we conclude that a Martian origin is quite probable.