The origin of SNC meteorites: An alternative to Mars

Shergottites, Nakhlites, and Chassignites (SNC) are a small group of achondrites with crystallization ages of approximately 1.3 AE. Although it has recently been postulated the these meteorites came from Mars, the dynamical difficulties of ejecting large meteorites from a major planet have caused us to examine the alternative possibility that they crystallized from an impact melt formed on a large asteroid. The kinetic energy necessary to produce a crater of a given size is estimated; it is postulated that 25% of this energy is partitioned into heat, and the heat is distributed in this model in a pattern suggested by the impact melt distribution in Brent Crater and the radioactivity distribution in Cactus nucelear explosion crater. The time evolution of the temperature by heat conduction for several locations around the crater is computed. Crystallization times for the more deeply buried impact melts are form 5 × 10⁴ years for 60-km-diameter craters and increase for larger craters. These times are long enough for the observed cumulate textures to develop. Once solidified, these rocks may be ejected from the asteroid by subsequent cratering events. Since asteroidal escape velocities are low, ejection may be accomplished by shock pressures too low to produce petrologically detectable shock features. The SNC meteorites could thus have originated in the asteroid belt, their young crystallization ages being due to melting induced by impacts occurring on asteroids long after condensation from the solar nebula. This scenario avoids the dynamical difficulties of a major planet origin, but raises questions of how the SNC's acquired their chemical and REE characteristics. To date, there seems to be no internally consistent model for the origin of these strange meteorites. The impact melt hypothesis is offered as a rational alternative to a Martian origin. Neither hypothesis explains all the problems.     

Mars regolith versus SNC meteorites: Possible evidence for abundant crustal carbonates

Viking XRF analyses of the Martian regolith are compared with typical igneous rocks of the Earth, the Moon, the eucrite parent asteroid, and especially the shergottites, nakhlites, and Chassigny (SNC) meteorites, which are suspected to be basalts and mafic cumulates from Mars. A striking feature of the Martian regolith, compared to igneous rocks with similar molar (Mg + Fe)/Si ratios, is its extraordinarily low Ca/Si ratio. The regolith's low Ca/Si ratio is probably not a result of simple mixing (isochemical weathering) of SNC-like rocks with other igneous rocks, unless the regolith contains a large component of rock with an improbable combination of extremely low Ca/Si and (Mg + Fe)/Si, and yet low K₂O and Zr. Several other models might conceivably account for the low Ca/Si ratio, but I suggest that most of the “missing” Ca was removed from the regolith as Ca-carbonate. Formation of a mass of carbonate equivalent to a global shell 20 m thick would suffice to remove 1000 mbar of CO₂ from the Martian atmosphere. Thus, the peculiar Ca/Si ratio of the Martian regolith tends to support the hypothesis that the climate of Mars was once far warmer and wetter than it is today.     

Siderophile trace elements in ALH84001, other SNC meteorites and eucrites: Evidence of heterogeneity,possibly time-linked,in the mantle of Mars

Abstract— We report neutron activation analyses, including radiochemical determination of trace siderophile elements (Au, Ge, Ir, Ni, Os and Re), for three SNC/martian meteorites, and Os and Re results for numerous eucrites. Ratios such as Ga/Al in the SNC orthopyroxenite ALH84001 confirm its martian affinity—its many distinctive characteristics, most notably its near-primordial age, notwithstanding. To the list of ALH84001's idiosyncrasies can now be added extraordinarily low concentrations of Au, Ni and, especially, Re (17 pg/g), for a martian meteorite. We consider several possible origins for the anomalously low Re content in ALH84001, including metasomatism or alteration. The pyroxene-cumulate nature of this rock probably does not account for its low Re content. Other SNC meteorites are also cumulates. An examination of Re-Nd variations among terrestrial basalts and komatiites suggests that Re is compatible with mantle minerals in general and only incompatible with olivine (however, olivine dominates the mantle residuum, especially during komatiite genesis). Our preferred model is that the ALH84001 parent melt formed in a mantle source region that was far more Re-depleted, and/or at a substantially lower oxygen fugacity, than the sources of the young SNC meteorites. Such a contrast is consistent with models that replenish siderophile elements in planetary mantles by gradual admixture of late-accreting matter and similarly derive most planetary water (which serves as an oxidant) very late in accretion. According to this model, ALH84001 formed before the siderophile-rich matter and water had been mixed well into the martian interior. Possibly the martian mantle never became generally as Re-rich and/or oxidized as the source region(s) of the younger SNCs.     

The formation of weathering products on the LEW 85320 ordinary chondrite: Evidence from carbon and oxygen stable isotope compositions and implications for carbonates in SNC meteorites

Abstract— Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the δ¹³C and δ¹⁸O values of the two generations of bicarbonate (Antarctic and Texas) are different: δ¹³C = +7.9‰ and +4.2‰; δ¹⁸O = +17.9‰ and + 12.1‰ respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at ?2 ± 4 °C (Antarctic) and +16 ± 4 °C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapour or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar δ¹³C values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.     

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.     

Precursors of Mars: Constraints from nitrogen and oxygen isotopic compositions of martian meteorites

Abstract— We present an approach to assess the nature of materials involved in the accretion of Mars by the planet's nitrogen (δ¹⁵N) and oxygen (Δ¹⁷O) isotopic compositions as derived from data on martian meteorites. δ¹⁵N for Mars has been derived from nitrogen and xenon systematics, while Δ¹⁷O has been taken from the literature data. These signatures indicate that Mars has most probably accreted from enstatite and ordinary chondritic materials in a ratio of 74:26 and may not have a significant contribution from the carbonaceous (CI, CM, or CV) chondrites. This is consistent with the chromium isotopic (?⁵³Cr) signatures of martian meteorites and the bulk planet Fe/Si ratio for Mars as suggested by the moment of inertia factor (I/MR²) obtained from the Mars Pathfinder data. Further, a simple homogeneous accretion from the above two types of materials is found to be consistent with the planet's moment of inertia factor and the bulk composition of the mantle. But, it requires a core with 6.7 wt% Si, which is consistent with the new results from the high pressure and temperature melting experiments and chemical data on the opaque minerals in enstatite chondrites.     

Constraints on the Martian cratering rate based on the SNC meteorites and implications for Mars climatic history

Two constraints placed upon the cratering flux at Mars by the SNC meteorites are examined: crystallization ages as a constraint on surface ages and cosmic ray exposure ages and number of impacts as a constraint on absolute rates. The crystallization ages of the SNC meteorites appear to constrain the Martian cratering rate to be 4xLunar or more if the parent lavas are in the north of Mars and the number of SNC ejecting impacts are small. If the SNCs result from a single impact that formed the Lyot basin then the cratering rate must be at least 7xLunar or higher to produce a basin age less than the SNC crystallization age because the basin ages are themselves determined by crater counting. Assuming multiple uncorrelated impacts for SNC ejection from Mars over 10 million years a cratering rate of approximately 4xLunar is also found for ejecting impacts that form craters over 12km in diameter. Therefore, both crystallization ages and ejection ages and number of impacts appear consistent with a 4xLunar cratering rate at Mars. The effect on Martian chronologies of such a high cratering rate is to place the SNC crystallization ages partly within the epoch of channel formation on Mars and to extend this liquid water epoch over much of Mars history.     

Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates

Heterogeneities in terrestrial samples for ¹⁸²W/¹⁸³W and ¹⁴²Nd/¹⁴⁴Nd are only preserved in Hadean and Archean rocks while heterogeneities in ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in ¹⁸²W/¹⁸³W and ¹⁴²Nd/¹⁴⁴Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different ¹⁸²W/¹⁸³W, ¹⁴²Nd/¹⁴⁴Nd, ¹²⁹Xe/¹³⁰Xe, ¹³⁶Xe/¹³⁰Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system ¹⁸²Hf‐¹⁸²W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using ¹⁸²Hf‐¹⁸²W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, ⁸⁷Rb‐⁸⁷Sr and ¹⁴⁷Sm‐¹⁴³Nd. The apparent absence of ¹⁸²W/¹⁸³W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the ¹⁸²W/¹⁸³W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.     

Ferric iron in SNC meteorites as determined by Mössbauer spectroscopy: Implications for martian landers and martian oxygen fugacity

Abstract— Mössbauer spectra of martian meteorites are currently of great interest due to the Mössbauer spectrometers on the Athena mission MER rovers as well as the European Space Agency Mars Express mission, with its Beagle 2 payload. Also, considerable current effort is being made to understand the oxygen fugacity of martian magmas because of the effect of fO₂ on mineral chemistry and crystallization processes. For these 2 reasons, the present study was conceived to acquire room temperature Mössbauer spectra of mineral separates and whole rock samples of 10 SNC meteorites. The results suggest that mineral identification using remote application of this technique will be most useful when the phases present have distinctive parameters arising from Fe in very different coordination polyhedra; for example, pyroxene coexisting with olivine can be discriminated easily, but opx versus cpx cannot. The MER goal of using Mössbauer spectroscopy to quantify the relative amounts of individual mineral species present will be difficult to satisfy if silicates are present because the lack of constraints on wt% FeO contents of individual silicate phases present will make modal calculations impossible. The remote Mössbauer spectroscopy will be most advantageous if the rocks analyzed are predominantly oxides with known stoichiometries, though these phases are not present in the SNCs. As for the detection of martian oxygen fugacity, no evidence exists in the SNC samples studied of a relationship between Fe³⁺ content and fO₂ as calculated by independent methods. Possibly, all of the Fe³⁺ observed in olivine is the result of dehydrogenation rather than oxidation, and this process may also be the source of all the Fe³⁺ observed in pyroxene. The observed Fe³⁺ in pyroxene also likely records an equilibrium between pyroxene and melt at such low fO₂ that little or no Fe³⁺ would be expected.     

Antarctic Dry Valleys and indigenous weathering in Mars meteorites: Implications for water and life on Mars

The Dry Valleys of Antarctica are an excellent analog of the environment at the surface of Mars. Soil formation histories involving slow processes of sublimation and migration of water-soluble ions in polar desert environments are characteristic of both Mars and the Dry Valleys. At the present time, the environment in the Dry Valleys is probably the most similar to that in the mid-latitudes on Mars although similar conditions may be found in areas of the polar regions during their respective Mars summers. It is thought that Mars is currently in an interglacial period, and that subsurface water ice is sublimating poleward. Because the Mars sublimation zones seem to be the most similar to the Antarctic Dry Valleys, the Dry Valleys-type Mars climate is migrating towards the poles. Mars has likely undergone drastic obliquity changes, which means that the Dry Valleys analog to Mars may be valid for large parts of Mars, including the polar regions, at different times in geologic history. Dry Valleys soils contain traces of silicate alteration products and secondary salts much like those found in Mars meteorites. A martian origin for some of the meteorite secondary phases has been verified previously; it can be based on the presence of shock effects and other features which could not have formed after the rocks were ejected from Mars, or demonstrable modification of a feature by the passage of the meteorite through Earth's atmosphere (proving the feature to be pre-terrestrial). The martian weathering products provide critical information for deciphering the near-surface history of Mars. Definite martian secondary phases include Ca-carbonate, Ca-sulfate, and Mg-sulfate. These salts are also found in soils from the Dry Valleys of Antarctica. Results of earlier Wright Valley work are consistent with what is now known about Mars based on meteorite and orbital data. Results from recent and current Mars missions support this inference. Aqueous processes are active even in permanently frozen Antarctic Dry Valleys soils, and similar processes are probably also occurring on Mars today, especially at the mid-latitudes. Both weathering products and life in Dry Valleys soils are distributed heterogeneously. Such variations should be taken into account in future studies of martian soils and also in the search for possible life on Mars.     

The thermodynamics of dust formation: Evidence from meteorites

A substantial fraction of interstellar dust probably formed in the nebulae around protostars, a setting similar to that envisioned for meteoritic material. From studies of the mineralogy and composition of meteorites it is possible to obtain quantitative information on the conditions that prevailed in the nebula. For example, pressures in the range 10^–3 to 10^–6 atm are indicated. At these pressures the kinetics of nucleation and grain growth are favorable.The fact that the gas associated with interstellar dust has solar H/S ratios indicates that FeS, which forms at 680 K, independent of pressure, is not present in the dust. Since iron only becomes oxidized at even lower temperatures, also via pressure-independent reactions, oxidized iron is not expected in the dust. If most interstellar dust forms in nebulae and is ejected back into space, a relatively high temperature is implied, 700K. Dust formation around stars with high C/O ratios is expected to produce minerals found in the highly reduced enstatite chondrites.High-temperature fractionation processes ( 1000 K) played an important role in the nebula. Much of the Al, Ca, Ti, etc., evidently condensed and accreted into cm-sized objects, some of which are found in carbonaceous chondrites. These objects are explicable in terms of formation from a cooling neutral gas with cosmic composition. Their most important distinguishing characteristics are low volatile and low Si contents, coupled with high refractory element contents constrains formation via isothermal compression to grain temperature 1000 K.Invited contribution to the proceedings of a workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars

We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured δ³⁷Cl values range from ?3.8 to +8.6‰. The olivine‐phyric shergottites are the isotopically lightest samples, with δ³⁷Cl mostly ranging from ?4 to ?2‰. Samples with evidence for a crustal component have positive δ³⁷Cl values, with an extreme value of 8.6‰. Most of the basaltic shergottites have intermediate δ³⁷Cl values of ?1 to 0‰, except for Shergotty, which is similar to the olivine‐phyric shergottites. We interpret these data as due to mixing of a two‐component system. The first component is the mantle value of ?4 to ?3‰. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a ³⁷Cl‐enriched crustal component. We speculate that preferential loss of ³⁵Cl to space has resulted in a high δ³⁷Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low δ³⁷Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0‰. We are not aware of any parent‐body process that could lower the δ³⁷Cl value of the Martian mantle to ?4 to ?3‰. Instead, we propose that this low δ³⁷Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher δ³⁷Cl values seen in many chondrites are explained by later incorporation of ³⁷Cl‐enriched HCl‐hydrate.     

Mars: Evidence for dynamic processes from mariners 6 and 7

Mariners 6 and 7 photographs of the equatorial region of Mars document a three-stage evolution of that part of the Martian surface: (1) High- and intermediate-albedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyon-like reentrants and featureless terrain along the Hellas-Hellespontus boundary.Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.     

Chemical layering in the upper mantle of Mars: Evidence from olivine‐hosted melt inclusions in Tissint

Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine‐hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine–phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo_76‐70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo_66‐55). REE‐plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole‐rock. Model calculations indicate two‐stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in ~10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole‐rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE‐rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.     

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ³⁷Cl values of magmatic irons range from ?7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ³⁷Cl values ranging from ?6.1 to ?3.2‰. The anomalously high and low δ³⁷Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ³⁷Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ³⁷Cl values, whereas volatilization is known to increase δ³⁷Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <?5‰ and down to ?7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ³⁷Cl values down to ?6, and ?3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ³⁷Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ³⁷Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ³⁷Cl value by the loss of approximately 60% of the initial Cl content.     

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Abstract— Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45–55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic‐ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of ³He, ⁴He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of ⁴He into cosmogenic and radiogenic components was performed using the lowest ⁴He/³He ratio we measured on mineral separates (⁴He/³He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic ⁴He. Systematics of cosmogenic ³He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic ⁴He loss, ranging between ?20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant ⁴He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of ⁴He diffusion in the main U, Th carrier phase apatite showed that post‐shock temperatures of ?300 °C are necessary to explain observed losses. This temperature corresponds to the post‐shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for ⁴He loss.     

The chlorine isotopic composition of Martian meteorites 1: Chlorine isotope composition of Martian mantle and crustal reservoirs and their interactions

The Martian meteorites record a wide diversity of environments, processes, and ages. Much work has been done to decipher potential mantle sources for Martian magmas and their interactions with crustal and surface environments. Chlorine isotopes provide a unique opportunity to assess interactions between Martian mantle‐derived magmas and the crust. We have measured the Cl‐isotopic composition of 17 samples that span the range of known ages, Martian environments, and mantle reservoirs. The ³⁷Cl of the Martian mantle, as represented by the olivine‐phyric shergottites, NWA 2737 (chassignite), and Shergotty (basaltic shergottite), has a low value of approximately ?3.8‰. This value is lower than that of all other planetary bodies measured thus far. The Martian crust, as represented by regolith breccia NWA 7034, is variably enriched in the heavy isotope of Cl. This enrichment is reflective of preferential loss of ³⁵Cl to space. Most basaltic shergottites (less Shergotty), nakhlites, Chassigny, and Allan Hills 84001 lie on a continuum between the Martian mantle and crust. This intermediate range is explained by mechanical mixing through impact, fluid interaction, and assimilation‐fractional crystallization.     

New observations of Warrego Valles, Mars: Evidence for precipitation and surface runoff

Most valley networks have been identified primarily in the heavily cratered uplands which are Noachian in age (>3.5 Gyr). A striking exception to this general observation is Warrego Valles located on the southeastern part of the Tharsis bulge. Recent data obtained by the Mars Orbiter Laser Altimeter, the Thermal Emission Imaging System (THEMIS) spectrometer and the Mars Orbiter Camera give new insight into the formation of valley networks and the early Mars climate. We focus our study on the southern Thaumasia region especially on Warrego Valles and determine the organisation of valleys in relation to regional topography and structural geology. Warrego Valles is the most mature valley network that incised the southern side of Thaumasia highlands. It developed in a rectangular-shaped, concave-up drainage basin. Four times more valleys are identified in THEMIS infrared images than in Viking images. Valleys exist on both sides of the main tributary contrary to what was visible in Viking images. Their distribution is highly controlled by topographic slope, e.g. there is a parallel pattern on the sides and dendritic pattern on the central part of Warrego Valles. We quantitatively analyse valley morphology and morphometry to determine the processes responsible for valley network formation. Warrego Valles displays morphometric properties similar to those of a terrestrial fluvial valley network. This valley network is characterised by seven Strahler's orders, a bifurcation ratio of 3, a length ratio of 1.7, a drainage density of 0.53 km⁻¹ and a ruggedness number of 3.3. The hypsometric curve and integral (0.46) indicate that Warrego Valles reached the mature Davis’ stage. Valleys have undergone external degradation since their incision, which masks their main morphological characteristics. Our study supports the assertion that valley networks formed by fluvial processes controlled by an atmospheric water cycle. Further, they seem to develop by successive stages of erosion that occurred during Noachian through the late Hesperian.     

Pole wandering of Mars: Evidence from paleomagnetic poles

We use the mapping-phase high-altitude magnetic measurements provided by Mars Global Surveyor (MGS) between March 1999 and April 2003 to model nine relatively isolated magnetic anomalies of Mars. Each anomaly is modeled with an elliptical prism. Each component of the observed magnetic field is modeled independently using an elliptical prism in order to assess the reliability of the results and suppress non-crustal and nearby crustal source contaminations. The paleomagnetic pole positions are obtained from the magnetization vectors of the model source bodies. We clean the data by removing the bad tracks and then divide the entire data into two sets that are measured at different times. Applying covariance analysis in the Fourier domain to two maps of the same magnetic component that are derived from the two sets provides a means to extract the most common features of the maps. The quality of a model is evaluated and only good models are used in the final geophysical interpretation. Most poles that come from good models cluster in the Tharsis region, suggesting that Mars experienced polar motion since the magnetic source bodies were magnetized.     

Genesis of the IIICD iron meteorites: Evidence from silicate-bearing inclusions

Abstract— Our studies of the silicate-bearing inclusions in the IIICD iron meteorites Maltahöhe, Carlton and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in the host meteorites. An inclusion in the low-Ni Maltahöhe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, plagioclase, graphite and troilite. With increasing Ni concentration of the metal, silicate inclusions become poorer in graphite, richer in phosphates, and the phosphate and silicate assemblages become more complex. Dayton contains pyroxene, plagioclase, SiO₂, brianite, panethite and whitlockite, without graphite. In addition, mafic silicates become more FeO-rich with increasing Ni concentration of the hosts. In contrast, silicates in IAB irons show no such correlation with host Ni concentration, nor do they have the complex mineral assemblages of Dayton. These trends in inclusion composition and mineralogy in IIICD iron meteorites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generation of melt pools, we prefer core crystallization. However, the absence of relationships between silicate inclusion mineralogy and metal compositions among IAB irons analogous to those that we have discovered in IIICD irons suggests that the IAB and IIICD cores/metallic magmas evolved in rather different ways. We suggest that the solidification of the IIICD core may have been very complex, involving fractional crystallization, nucleation effects and, possibly, liquid immiscibility.