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The Mars Sample Return (MSR) Campaign must meet a series of scientific and technical achievements to be successful. While the respective engineering responsibilities to retrieve the samples have been formalized through a Memorandum of Understanding between ESA and NASA, the roles and responsibilities of the scientific elements have yet to be fully defined. In April 2020, ESA and NASA jointly chartered the MSR Science Planning Group 2 (MSPG2) to build upon previous planning efforts in defining 1) an end-to-end MSR Science Program and 2) needed functionalities and design requirements for an MSR Sample Receiving Facility (SRF). The challenges for the first samples brought from another planet include not only maintaining and providing samples in pristine condition for study, but also maintaining biological containment until the samples meet sample safety criteria for distribution outside of biocontainment. The MSPG2 produced six reports outlining 66 findings. Abbreviated versions of the five additional high-level MSPG2 summary findings are: Summary-1. A long-term NASA/ESA MSR Science Program, along with the necessary funding and human resources, will be required to accomplish the end-to-end scientific objectives of MSR. Summary-2. MSR curation would need to be done concurrently with Biosafety Level-4 containment. This would lead to complex first-of-a-kind curation implementations and require further technology development. Summary-3. Most aspects of MSR sample science could, and should, be performed on samples deemed safe in laboratories outside of the SRF. However, other aspects of MSR sample science are both time-sensitive and sterilization-sensitive and would need to be carried out in the SRF. Summary-4. To meet the unique science, curation, and planetary protection needs of MSR, substantial analytical and sample management capabilities would be required in an SRF. Summary-5. Because of the long lead-time for SRF design, construction, and certification, it is important that preparations begin immediately, even if there is delay in the return of samples.

NASA Lunar Science Institute Cooperative Agreement [NNA09DB33A]; NASA Solar System Exploration Research Virtual Institute Cooperative Agreement [NNA14AB07A]

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Geochronology, or determination of absolute ages for geologic events, underpins many inquiries into the formation and evolution of planets and our Solar System. Absolute ages of ancient and recent magmatic products provide strong constraints on the dynamics of magma oceans and crustal formation, as well as the longevity and evolution of interior heat engines and distinct mantle/crustal source regions. Absolute dating also relates habitability markers to the timescale of evolution of life on Earth. However, the number of geochronologically-significant terrains across the inner Solar System far exceeds our ability to conduct sample return from all of them. In preparation for the upcoming Decadal Survey, our team formulated a set of medium-class (New Frontiers) mission concepts to three different locations (the Moon, Mars, and Vesta) where sites that record Solar System bombardment, magmatism, and/or habitability are uniquely preserved and accessible. We developed a notional payload to directly date planetary surfaces, consisting of two instruments capable of measuring radiometric ages in situ, an imaging spectrometer, optical cameras to provide site geologic context and sample characterization, a trace element analyzer to augment sample contextualization, and a sample acquisition and handling system. Landers carrying this payload to the Moon, Mars, and Vesta would likely fit into the New Frontiers cost cap in our study (~$1B). A mission of this type would provide crucial constraints on planetary history while also enabling a broad suite of investigations such as basic geologic characterization, geomorphologic analysis, ground truth for remote sensing analyses, analyses of major, minor, trace, and volatile elements, atmospheric and other long-lived monitoring, organic molecule analyses, and soil and geotechnical properties., Comment: Submitted to the Planetary Science Journal, October 2020

The NASA/ESA Mars Sample Return (MSR) Campaign seeks to establish whether life on Mars existed where and when environmental conditions allowed. Laboratory measurements on the returned samples are useful if what is measured is evidence of phenomena on Mars rather than of the effects of sterilization conditions. This report establishes that there are categories of measurements that can be fruitful despite sample sterilization and other categories that cannot. Sterilization kills living microorganisms and inactivates complex biological structures by breaking chemical bonds. Sterilization has similar effects on chemical bonds in non-biological compounds, including abiotic or pre-biotic reduced carbon compounds, hydrous minerals, and hydrous amorphous solids. We considered the sterilization effects of applying dry heat under two specific temperature-time regimes and the effects of γ-irradiation. Many measurements of volatile-rich materials are sterilization sensitive—they will be compromised by either dehydration or radiolysis upon sterilization. Dry-heat sterilization and γ-irradiation differ somewhat in their effects but affect the same chemical elements. Sterilization-sensitive measurements include the abundances and oxidation-reduction (redox) states of redox-sensitive elements, and isotope abundances and ratios of most of them. All organic molecules, and most minerals and naturally occurring amorphous materials that formed under habitable conditions, contain at least one redox-sensitive element. Thus, sterilization-sensitive evidence about ancient life on Mars and its relationship to its ancient environment will be severely compromised if the samples collected by Mars 2020 rover Perseverance cannot be analyzed in an unsterilized condition. To ensure that sterilization-sensitive measurements can be made even on samples deemed unsafe for unsterilized release from containment, contingency instruments in addition to those required for curation, time-sensitive science, and the Sample Safety Assessment Protocol would need to be added to the Sample Receiving Facility (SRF). Targeted investigations using analogs of MSR Campaign-relevant returned-sample types should be undertaken to fill knowledge gaps about sterilization effects on important scientific measurements, especially if the sterilization regimens eventually chosen are different from those considered in this report., Astrobiology, 22 (S1), ISSN:1531-1074, ISSN:1557-8070

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We seek to compile a uniform set of basic capabilities and needs to maximize the yield of Solar System science with future Astrophysics assets while allowing those assets to achieve their Astrophysics priorities. Within considerations of cost and complexity, inclusion of capabilities that make a particular platform useable to planetary science provide a critical advantage over platforms lacking such capabilities., Small Bodies Assessment Group Committee on Planetary Science with Astrophysical Assets, submitted to The National Academies of Science, Engineering, and Medicine\'s Call to the Astronomy and Astrophysics Community for Science White Papers

The world's meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) together is a major challenge for planetary science. Directly probing asteroids achieves this at a high cost. Observing meteorite falls and calculating their pre-atmospheric orbit on the other hand, is a cheaper way to approach the problem. The Global Fireball Observatory (GFO) collaboration was established in 2017 and brings together multiple institutions (from Australia, USA, Canada, Morocco, Saudi Arabia, the UK, and Argentina) to maximise the area for fireball observation time and therefore meteorite recoveries. The members have a choice to operate independently, but they can also choose to work in a fully collaborative manner with other GFO partners. This efficient approach leverages the experience gained from the Desert Fireball Network (DFN) pathfinder project in Australia. The state-of-the art technology (DFN camera systems and data reduction) and experience of the support teams is shared between all partners, freeing up time for science investigations and meteorite searching. With all networks combined together, the GFO collaboration already covers 0.6% of the Earth's surface for meteorite recovery as of mid-2019, and aims to reach 2% in the early 2020s. We estimate that after 5 years of operation, the GFO will have observed a fireball from virtually every meteorite type. This combined effort will bring new, fresh, extra-terrestrial material to the labs, yielding new insights about the formation of the Solar System., Accepted in PSS. 19 pages, 9 figures

We use cosmic-ray exposure (CRE) ages of ureilites, combined with magnesium numbers of olivine, and oxygen isotopes, to search for evidence of specific source events initiating exposure for groups of ureilites. This technique can also be used to investigate the heterogeneity of the body from which the samples were derived. There are a total of 39 ureilites included in our work, which represents the largest collection of ureilite CRE age data used to date. Although we find some evidence of possible clusters, it is clear that most ureilites did not originate in one or two events on a homogeneous parent body.

Noble gases have been instrumental in establishing a link between the SNC(O) meteorites and planet Mars. A superior (compared to the previous Viking measurements) analysis of the Martian atmosphere (MA) by the SAM instrument aboard the Mars Science Laboratory (MSL) has recently become available, which only strengthens this link. Nevertheless the results obtained on Martian meteorites still provide the most complete picture of noble gases on Mars, not just atmospheric Martian noble gases but also gases that can be confidently described as being of “interior” origin. Overall three major components of trapped noble gases have been clearly identified in Martian meteorites: (1) ~recent MA; (2) fractionated and possibly ancient MA; (3) interior (mantle?) noble gases. An additional, perhaps minor, component is only observed during crushing of shergottites. While the fractionated atmospheric component may tell us about processes like weathering, the difference between the atmosphere and the interior reservoir on Mars is unlike the difference observed on Earth and points to significant differences in the volatile history of the two planets. The nonradiogenic gases in the atmospheres of the two planets—apart from the ~2 orders of magnitude difference in absolute abundance—however, share similarities in their element abundance patterns, which sets them apart from solar wind or primordial noble gases in primitive meteorites and may indicate similar acquisition/loss processes. In addition, the atmospheric Xe isotopic composition for these two planetary bodies is quite unique (and remarkably similar), which points to a similar fractionation process, or set of processes, leading to the observed composition. A crucial point to be settled is whether the isotopic composition of the (assumed) atmospheric Xe in the unique, >4 Ga old, ALH 84001 differs from that of modern Martian atmospheric Xe. The indication that it may have a solar wind–like composition instead poses a problem for currently popular models for the early evolution of the MA (as well as that of the terrestrial atmosphere).

The Gao-Guenie H5 chondrite that fell on Burkina Faso (March 1960) has portions that were impact-melted on an H chondrite asteroid at ~300 Ma and, through later impact events in space, sent into an Earth-crossing orbit. This article presents a petrographic and electron microprobe analysis of a representative sample of the Gao-Guenie impact melt breccia consisting of a chondritic clast domain, quenched melt in contact with chondritic clasts, and an igneous-textured impact melt domain. Olivine is predominantly Fo80–82. The clast domain contains low-Ca pyroxene. Impact melt-grown pyroxene is commonly zoned from low-Ca pyroxene in cores to pigeonite and augite in rims. Metal–troilite orbs in the impact melt domain measure up to ~2 mm across. The cores of metal orbs in the impact melt domain contain ~7.9 wt% of Ni and are typically surrounded by taenite and Ni-rich troilite. The metallography of metal–troilite droplets suggest a stage I cooling rate of order 10 °C s−1 for the superheated impact melt. The subsolidus stage II cooling rate for the impact melt breccia could not be determined directly, but was presumably fast. An analogy between the Ni rim gradients in metal of the Gao-Guenie impact melt breccia and the impact-melted H6 chondrite Orvinio suggests similar cooling rates, probably on the order of ~5000–40,000 °C yr−1. A simple model of conductive heat transfer shows that the Gao-Guenie impact melt breccia may have formed in a melt injection dike ~0.5–5 m in width, generated during a sizeable impact event on the H chondrite parent asteroid.

The 40 Ar/ 39 Ar technique applied to impact glass has been used to date both terrestrial and lunar impact events. The ability to utilize the 40 Ar/ 39 Ar technique rests on the assumption that impact glasses are closed to the loss of daughter product, 40 Ar ∗ , after formation. Diffusion experiments were performed on three Apollo 16 lunar impact glasses and yielded activation energies for 39 Ar of ∼17 to 20 kcal mol −1 and log 10 ( D 0 /a 2 ) values of −5.2 to −6.0 s −1 . The resulting diffusion coefficients are interpreted as minimum values and the Apollo 16 glass is probably some of the least retentive of lunar glasses, as the degree of non-bridging oxygen is at one end of the range in lunar glasses. At temperatures below the glass transition temperature (i.e., ∼660 °C), the data can be explained by volume diffusion from a single diffusion domain. Modeling shows that Apollo 16 composition glass could lose significant quantities of radiogenic argon ( 40 Ar ∗ ) (∼90–100% over 20–40 Myr assuming a diffusion domain size ( a ) of 75 μm) due to diurnal temperature variations on the lunar surface, although 40 Ar ∗ loss is highly sensitive to exposure duration and effective diffusion domain size. Modeling shows that loss from transient thermal events (e.g., heating to ∼200 °C for 10 2 yr duration) can also cause partial resetting of apparent 40 Ar/ 39 Ar ages. In small ( a = 75 μm) glasses a maximum of 50–60% of 40 Ar ∗ is lost over 4 Ga when buried to depths corresponding to temperatures of −15 °C. Results indicate that caution should be exercised in interpreting lunar impact glass 40 Ar/ 39 Ar ages, as the assumption of closed system behavior may have been violated, particularly in glasses with low fractions of non-bridging oxygen.

Geochronology is a fundamental measurement for planetary samples, providing global and solar system context for the conditions prevailing on the planet at the time of major geological events. The potassium (K)-Argon (Ar) laser experiment (KArLE) will make in situ noble gas geochronology measurements aboard planetary robotic missions such as rovers and landers. Laser-induced breakdown spectroscopy (LIBS) is used to measure the K abundance in a sample and to release its noble gases; the evolved Ar is measured by mass spectrometry, and relative K content is related to absolute Ar abundance by sample mass, determined by optical measurement of the ablated volume. This approach allows K and Ar to be measured on identical volumes multiple times to create an isochron, which improves the age determination and reveals irregularities in the rock if they exist. The KArLE technique measures a whole-rock K-Ar age with 10% uncertainty or better for rocks 2 Ga or older, sufficient to resolve the absolute age of many planetary samples. The LIBS–mass spectrometry approach is attractive because the analytical components have been flight-proven, do not require further technical development and provide essential measurements (complete elemental abundance, evolved volatile analysis, micro-imaging) as well as in situ geochronology. La mesure geochronologique est fondamentale pour les echantillons planetaires, fournissant le contexte a la fois global et du systeme solaire pour les conditions qui prevalent sur la planete au moment des grands evenements geologiques. Le systeme experimental laser Potassium (K) -Argon (Ar) (KArLE) fera des mesures geochronologiques in situ de gaz rares a bord de missions robotiques planetaires tels que les « rovers » et « landers ». La spectroscopie sur plasma induit par laser (LIBS) est utilisee pour mesurer l'abondance du K dans un echantillon et pour liberer les gaz rares qu'il contient; l'Ar evolue est mesuree par spectrometrie de masse, et la teneur relative en K est liee a l'abondance absolue de l'Ar par la masse de l'echantillon, determinee par la mesure optique du volume ablate. Cette approche permet de mesurer K et Ar pour des volumes identiques et ceci a plusieurs reprises afin d'obtenir une isochrone, ce qui ameliore la determination de l'âge et revele des heterogeneites dans la roche, si elles existent. La technique KArLE mesure un âge K-Ar sur roche totale avec 10% d'incertitude voire mieux pour des roches vieilles de 2 Ga ou plus, ce qui est suffisant pour contraindre l'âge absolu de nombreux echantillons planetaires. L'approche spectrometrie de masse-LIBS est attrayante parce que les composants analytiques ont ete eprouve en vol, ne necessitent pas de nouveaux developpements techniques et fourniront les donnees essentielles (abondance elementaire complete, analyse des volatils evolues, micro-imagerie) ainsi que les mesures geochronologiques in situ.

Traces of collisions within collisions The Moon is widely accepted to have been created by the collision of a Mars-sized body with Earth. However, information about exactly when this event occurred is still welcome. To find out more, Bottke et al. compared models and the meteorite record with estimates of impact heating. When ejecta was thrown off during the main collision, high-velocity kilometer-sized fragments hit and heated main-belt asteroids. Evidence of such collisions emerges when pieces of those asteroids turn up as meteorites on Earth. The model and empirical record converge on 4.48 billion years ago, confirming previous estimates reached by different approaches. Science , this issue p. 321

Argon–argon dating (a variation of potassium–argon dating) of ordinary chondrites is being used to reconstruct the collisional impact history of their parent bodies. However, due to the fine-grained, multi-mineral, highly shocked nature of chondrites, the sources of potassium (K) in these meteorites have not been fully identified. By locating and isolating the different sources prior to analysis, better ages can be obtained. To distinguish between possible sources, we have analyzed Chico and Northwest Africa 091 (both L6 chondrites) via K mass balance, Raman spectroscopy, and argon (Ar) diffusion studies. In accordance with previous studies on other ordinary chondrites, the Ar in these two chondrites is nearly equally split between two releases, and the lower temperature release is identified as sodium-rich feldspar. Various scenarios for the higher temperature release are investigated, but no scenario meets all the required criteria. The Ar activation energy of the higher temperature release is the same as pyroxene, but the pyroxene has no detectable K. The K mass balance shows feldspar can account for all the K in the chondrite; hence feldspar must be the ultimate source of the higher temperature release. Raman spectroscopy rules out a high-pressure phase of feldspar. Neither melt veins, nor feldspar inclusions in pyroxene, are abundant enough to account for the higher temperature release in these meteorites.

We analyzed the argon diffusion parameters of albite and pyroxene as a function of shock, as well as an olivine of unknown shock pressure. In albite, shock pressures of ∼30–60 GPa all have an equal effect on the diffusion parameters, lowering the activation energy (E) from ∼45 to ∼25 kcal/mol, and the log D0/a2 from ∼4 to ∼1 s−1. In a mixture of low-Ca and high-Ca pyroxene, similar shock pressures also have an equal effect, lowering the E from ∼130 to ∼80 kcal/mol, and the log D0/a2 from ∼14 to ∼7 s−1. However, given that some shocked and unshocked pyroxene splits have similar release patterns, making a clear distinction between the two is difficult. Additionally, splits of pure unshocked high-Ca pyroxene have an E of ∼100 kcal/mol and a log D0/a2 of ∼10 s−1, intermediate to the values of mixed pyroxene when shocked and unshocked. The olivine (Fa18) has an E of ∼45 kcal/mol and a log D0/a2 of ∼1 s−1, somewhat similar to that of feldspar. These results will be useful in interpreting complex argon release patterns from shocked multi-mineral samples too fine-grained for easy mineral separation (such as chondrites).

Antarctic meteorite LAP 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments. Its petrography, oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between H and L chondrites. Petrographic indicators suggest LAP 031047 suffered a shock metamorphic overprint below ∼10 GPa, which did not destroy its unusually high porosity of ∼27 vol%. Metallographic textures in LAP 031047 indicate heating above ∼700 °C and subsequent cooling, which caused massive transformation of taenite to kamacite. The depletion of thermally labile trace elements, the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition, and the occurrence of coarsely crystallized chondrule fragments is further evidence for post-metamorphic heating to ∼700–750 °C. However, this heating event had a transient character because olivine and low-Ca pyroxene did not equilibrate. Nearly complete degassing up to very high temperatures is indicated by the thorough resetting of LAP 031047’s Ar–Ar reservoir ∼100 ± 55 Ma ago. A noble gas cosmic-ray exposure age indicates it was reduced to a meter-size fragment at

– LaPaz Icefield (LAP) 04581 is a shock-stage S2 LL5 chondrite that initially consisted of unrecrystallized LL3 material with a moderately abundant fine-grained porous matrix (on the order of 15 vol%). A rare oblique impact created shearing stresses that produced a petrofabric in the rock, induced frictional melting of troilite (thereby forming a large troilite vein), and caused chondrule flattening. The latter process was facilitated by impact-induced collapse of matrix pores. Chondrule flattening could not have occurred if the rock had been impacted after it had been metamorphosed to type 5 levels because the fine-grained matrix would have previously recrystallized and developed low porosity. Ar-Ar dating of LAP 04581 yields an age of 4175 Ma. This date is long after 26Al had decayed away and most likely reflects the timing of a second impact event that shocked the rock to S4–S5 levels. The troilite vein became polycrystalline at this time and the whole rock was annealed to petrologic type 5, perhaps by being buried beneath hot ejecta of low thermal diffusivity. After annealing, the rock was weakly shocked to S2 levels. LAP 04581 serves as an example of impact-induced heating being a viable mechanism for chondrite metamorphism.

– Miller Range (MIL) 05029 is a slowly cooled melt rock with metal/sulfide depletion and an Ar-Ar age of 4517 ± 11 Ma. Oxygen isotopes and mineral composition indicate that it is an L chondrite impact melt, and a well-equilibrated igneous rock texture with a lack of clasts favors a melt pool over a melt dike as its probable depositional setting. A metallographic cooling rate of approximately 14 °C Ma−1 indicates that the impact occurred at least approximately 20 Ma before the Ar-Ar closure age of 4517 Ma, possibly even shortly after accretion of its parent body. A metal grain with a Widmanstatten-like pattern further substantiates slow cooling. The formation age of MIL 05029 is at least as old as the Ar-Ar age of unshocked L and H chondrites, indicating that endogenous metamorphism on the parent asteroid was still ongoing at the time of impact. Its metallographic cooling rate of approximately 14 °C Ma−1 is similar to that typical for L6 chondrites, suggesting a collisional event on the L chondrite asteroid that produced impact melt at a minimum depth of 5–12 km. The inferred minimum crater diameter of 25–60 km may have shattered the 100–200 km diameter L chondrite asteroid. Therefore, MIL 05029 could record the timing and petrogenetic setting for the observed lack of correlation of cooling rates with metamorphic grades in many L chondrites.

We present the Volatile Analysis by Pyrolysis of Regolith (VAPoR) instrument design and demonstrate the validity of an in situ pyrolysis mass spectrometer for evolved gas analyses of lunar and planetary regolith samples. In situ evolved gas analyses of the lunar regolith have not yet been carried out and no atmospheric or evolved gas measurements have been made at the lunar poles. VAPoR is designed to do both kinds of measurements, is currently under development at NASA's Goddard Space Flight Center, and will be able to heat powdered regolith samples or rock drill fines up to 1400 C in vacuo. To validate the instrument concept, evolved gas species released from different planetary analogs were determined as a function of temperature using a laboratory breadboard. Evolved gas measurements of an Apollo 16 regolith sample and a fragment of the carbonaceous meteorite Murchison were made by VAPoR and our results compared with existing data. The results imply that in situ evolved gas measurements of the lunar regolith at the polar regions by VAPoR will be a very powerful tool for identifying water and other volatile signatures of lunar or exogenous origin as potential resources for future human exploration.

Calculations of the trapping of heavy noble gases within multiple guest clathrates under Mars-like conditions show that a substantial fraction of the martian Xe, perhaps even the vast majority, could be in clathrates. In addition, the Xe/Kr ratio in the clathrates would probably be much higher than in the atmosphere, so the formation or dissociation of a relatively small amount of clathrate could measurably change the atmospheric ratio. Relatively crude (factor of 2) measurements of the seasonal variability in that ratio by in situ spacecraft would be sensitive to �10% of the seasonal atmospheric CO 2 variability being a result of clathrates, rather than pure CO2 frost. In addition, sequestration of Xe in clathrates remains a viable mechanism for explaining the variable Xe/Kr ratios seen in different suites of martian meteorites.

Geochemical and 40Ar/39Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) 40Ar–39Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta, 70, 6016–6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) 40Ar/39Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) 39Ar–40Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093–3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stoffler D., Spettel B., Palme H. and Wanke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43–E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.

40Ar-39Ar analyses of a total of 26 samples from eight shock-darkened impact melt breccias of H-chondrite affinity (Gao-Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao-Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300-400 Ma ago, while two others record impacts 3900-4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the lunar cataclysm and shortly thereafter (3500-4000 Ma ago), one or more impacts ~300 Ma ago, and perhaps an impact ~500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the lunar cataclysm (4400-4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500-1000 Ma), or at the time of final breakup into meteorite-sized bodies (

The major- and minor-element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ~30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non-volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open-system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low-Mg high-K Fra Mauro; lmHKFM) were selected for 40Ar/39Ar dating. The individual fragments of lmHKFM glass all yielded ages of ~3750 ± 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 ± 40 Ma, close to the accepted age for Imbrium.

Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the 40 Ar/ 39 Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. 40 Ar/ 39 Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing.

The age of the surface of Europa is probably tens of Myr or less, but is poorly constrained. Two different geochronology schemes could potentially be applied to near-surface samples to provide far more precise ages. First, the surface salts apparently contain enough potassium to make potassium–argon dating feasible. Second, the bombardment of the surface with both galactic cosmic rays and protons trapped within the jovian magnetosphere will cause nuclear reactions that can lead to measurable buildups of cosmogenic noble gases, which can be used to determine both cosmic-ray exposure ages and erosion, deposition, or mixing rates for surface modification. The major differences between Europa's salt-rich ice and the rocks (meteorites, lunar samples and terrestrial rocks) in which cosmogenic noble gases are normally measured are that the abundance of target elements for nuclear reactions creating neon and argon are lower (because of the high water content), but neutron-capture reactions, particularly 35 Cl( n , γ ) 36 Cl( β − ) 36 Ar, are enhanced because of the thermalizing effects of the water. As well as presenting calculations of noble gas production near the surface of Europa, we also show that the measurements required to determine ages are within reach of technology in the near future, if an instrument can be landed on the surface of Europa.

Currently, the absolute chronology of Martian rocks, deposits and events is based mainly on crater counting and remains highly imprecise with epoch boundary uncertainties in excess of 2 billion years. Answers to key questions concerning the comparative origin and evolution of Mars and Earth will not be forthcoming without a rigid Martian chronology, enabling the construction of a time scale comparable to Earth's. Priorities for exploration include calibration of the cratering rate, dating major volcanic and fluvial events and establishing chronology of the polar layered deposits. If extinct and/or extant life is discovered, the chronology of the biosphere will be of paramount importance. Many radiometric and cosmogenic techniques applicable on Earth and the Moon will apply to Mars after certain baselines (e.g. composition of the atmosphere, trace species, chemical and physical characteristics of Martian dust) are established. The high radiation regime may pose a problem for dosimetry-based techniques (e.g. luminescence). The unique isotopic composition of nitrogen in the Martian atmosphere may permit a Mars-specific chronometer for tracing the time-evolution of the atmosphere and of lithic phases with trapped atmospheric gases. Other Mars-specific chronometers include measurement of gas fluxes and accumulation of platinum group elements (PGE) in the regolith. Putting collected samples into geologic context is deemed essential, as is using multiple techniques on multiple samples. If in situ measurements are restricted to a single technique it must be shown to give consistent results on multiple samples, but in all cases, using two or more techniques (e.g. on the same lander) will reduce error. While there is no question that returned samples will yield the best ages, in situ techniques have the potential to be flown on multiple missions providing a larger data set and broader context in which to place the more accurate dates.

Chondrules, silicate spheres typically 0.1 to 1 mm in diameter, are the most abundant constituents in the most common meteorites falling on Earth, the ordinary chondrites. In addition, many primitive meteorites have calcium-aluminum-rich inclusions (CAIs). The question of whether comets have chondrules or CAIs is relevant to understanding what the interior of a comet is like and what a cometary meteorite might be like. In addition, one prominent model for forming chondrules and CAIs, the X-wind model, predicts their presence in comets, while most other models do not. At present, the best way to search for chondrules and CAIs in comets is through meteor showers derived from comets, in particular, the Leonid meteor shower. Evidence potentially could be found in the overall mass distribution of the shower, in chemical analyses of meteors, or in light curves. There is no evidence for a chondrule abundance in the Leonid meteors similar to that found in chondritic meteorites. There is intriguing evidence for chondrule- or CAI-sized objects in a small fraction of the light curves, but further work is required to generate a definitive test.

We have studied the petrography, reflectance spectra, and Ar-Ar systematics of the Orivinio meteorite. Orvinio is an H chondrite not an L chondrite as sometimes reported. The material in the meteorite was involved in several impact events. One impact event produced large swaths of impact melt from H chondrite material surrounding relict clasts of chondrule-bearing material. Not only were portions of a bulk H chondrite planestesimal melted during the impact event, but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra. Both the melt and relict clasts are darker than unshocked H chondrite material, bearing spectral similarities to some C-class asteroids. Such shock metamorphism, which lowers the albedo of an object without increasing its spectral slope, may partially explain some of the variation among S-class asteroids and some of the trends seen on asteroid 433 Eros. Noble gases record the evidence of at least two, and perhaps three, impact events in the meteorite and its predecessor rocks. The most significant evidence is for an event that occurred 600 Ma ago or less, perhaps ~325 Ma ago or less. There is also a signature of 4.2 Ga in the Ar-Ar systematics, which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event.

20-25 mg whole rock samples of the nakhlites Lafayette and Nakhla have been analyzed via the 40Ar-39Ar technique, in part to verify their formation ages, but primarily, in an attempt to determine the timing of aqueous alteration in these martian meteorites. As in previous studies, plateaus in apparent age are observed at about 1300 Ma (1322 ± 10 for Lafayette, 1332 ± 10 and 1323 ± 11 for Nakhla), presumably corresponding to crystallization ages. The plateaus are not entirely flat, perhaps reflecting the effects of recoil during creation of 39Ar in the nuclear irradiation. The first 5-20% of the K-derived Ar released from all three samples give apparent ages

For Mars in situ landed missions, it has become increasingly apparent that significant value may be provided by a shared system that we call a Sample Preparation and Distribution (SPAD) System. A study was conducted to identify the issues and feasibility of such a system for these missions that would provide common functions for: receiving a variety of sample types from multiple sample acquisition systems; conducting preliminary characterization of these samples with non-destructive science instruments and making decisions about what should happen to the samples; performing a variety of sample preparation functions; and, finally, directing the prepared samples to additional science instruments for further analysis. Scientific constraints on the functionality of the system were identified, such as triage, contamination management, and various sample preparation steps, e.g., comminution, splitting, rock surfacing, and sieving. Some simplifying strategies were recommended and an overall science flow was developed. Engineering functional requirements were also investigated and example architectures developed. Preliminary conclusions are that shared SPAD facility systems could indeed add value to future Mars in situ landed missions if they are designed to respond to the particular requirements and constraints of those missions, that such a system appears feasible for consideration, and that certain standards should be developed for key SPAD interfaces.

Northwest Africa 482 (NWA 482) is a crystalline impact-melt breccia from the Moon with highlands affinities. The recrystallized matrix and the clast population are both highly anorthositic. Clasts are all related to the ferroan anorthosite suite, and include isolated plagioclase crystals and lithic anorthosites, troctolites, and spinel troctolites. Potassium-, rare-earth-element-, and phosphorus-bearing (KREEP) and mare lithologies are both absent, constraining the source area ofthis meteorite to a highland terrain with little to no KREEP component, most likely on the far side of the Moon. Glass is present in shock veins cutting through the sample and in several large melt pockets, indicating a second impact event. There are two separate events recorded in the 40Ar-39Ar system: one at -3750 Ma, which completely reset the K-Ar system, and one at :::;2400 Ma, which caused onlypartial degassing. These events could represent, respectively, crystallization ofthe impact-melt breccia and later formation of the glass, or the formation of the glass and a later thermal event. The terrestrial age of the meteorite is 8.6 ± 1.3 ka. This age corresponds well with the modest amount ofweathering in the rock, in the form ofsecondaryphyllosilicates and carbonates. Based on terrestrial age and location, lithology, and chemistry, NWA 482 is unique among known lunar meteorites.

We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface-atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic-ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ~9 atom/cm^2/s, ~4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are "freshest".

Noble gas studies have been done on well-documented samples from two very different solar system bodies, the Earth and Moon. There is one other planet, Mars, from which we have samples, but those samples are nearly 20 (at the start of 2002) Martian meteorites, rocks from unknown locales on Mars. Even from those, though, we have learned enough to realize that Mars is a fascinating compromise between the Earth and Moon. Like the Earth, Mars has a true atmosphere and interior volatile reservoirs, and it has been generating volcanic rocks throughout most of its history. Like the Moon, Mars has a surface that is exposed to enough galactic cosmic rays to generate abundant spallation reactions in surface rocks, and impact is likely one of the most common sources of heating for metamorphism. In reviewing noble gas studies of Mars, we will touch on topics that are covered for the Earth, Moon or more common meteorites in several other chapters. In most cases, Mars provides another place to apply the techniques developed for the Earth and Moon, and gives intriguingly different results. The first measurement of noble gas on Mars came not from a Martian meteorite, but from a spacecraft. The Viking landers carried mass spectrometers that were able to measure the composition of noble gases in the Martian atmosphere well enough to determine that the 129Xe/132Xe and 40Ar/36Ar ratios are considerably higher than in the terrestrial atmosphere (Owen et al. 1977). On the other hand, the relative elemental abundances of the non-radiogenic noble gases are very similar to the terrestrial atmosphere, although the absolute abundances are all roughly a factor of 100 lower than in the Earth’s atmosphere. The 15N/14N ratio is also higher than that in the terrestrial atmosphere. In …

One of the reasons noble gases make such good tracers of processes occurring in rocks is their scarcity. Thus, a process that converts a small fraction of a relatively rare element into a noble gas can have a large effect on the noble gas. Radioactive decay can be such a process. In a meteorite which has formed very early in the solar system’s history and been little altered since, the effect can be even more dramatic. Furthermore, since radioactive decay proceeds at a known rate, radiogenic noble gases, those produced by decay, have been crucial in deciphering the chronology of the solar system. Several “radionuclides,” radioactive isotopes that decay to noble gases, are listed in Table 1⇓. In a radioactive decay, the radioactive isotope is referred to as the “parent” isotope, while the decay product, the noble gas isotope, is referred to as the “daughter” isotope. In some cases listed, there is more than one mode of decay possible. In other cases, a single decay starts a chain that will ultimately produce several noble gas atoms. To take both into account, the yield (the number of noble gas atoms produced for each parent atom) is also given. Finally, radionuclides that fission may produce any of the several different isotopes of Xe in a characteristic spectrum (Table 2⇓). All of the systems listed, with the exception of the decay of U and Th to Xe, have been exploited in extraterrestrial samples at one time or another. View this table: Table 1: Radiogenic noble gas isotopes and their parents View this table: Table 2: Xenon fission spectra of actinides A typical analysis of radiogenic noble gases involves trying to find the ratio of the daughter isotope to some other, stable, isotope of the parent element, for example, the ratio 129Xe*/127I or 40Ar*/40K, where …

Variations in the Ar/Kr/Xe ratios of martian atmosphere trapped in martian meteorites correlate with the time of ejection of these meteorites from Mars. We suggest that these variations reflect actual differences in the Ar/Kr/Xe ratios of the martian atmosphere at the time of ejection of the meteorites. We propose that the variation of the Ar/Kr/Xe ratios of the martian atmosphere is caused by time varying uptake and release of these gases by CO 2 -H 2 O clathrate-hydrate in the polar caps. Nakhlites, martian meteorites representing cumulate magmatic rocks, contain evidence of atmosphere from the time of their ejection from Mars. We postulate that the martian atmosphere at that time had lower Ar/Xe and Kr/Xe ratios compared with the modern atmosphere because of the release of these and other gases from polar clathrate, possibly during a period of high (∼60°) obliquity.

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals — which comprise up to ~1 vol.% of the meteorites — provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 – 150 °C) brines. This is consistent with the simple mineralogy of these assemblages — Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays — and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 “rosettes”. Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event postdating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes.