George Cooper^a, Friedrich Horz^b, Alanna Spees^c and Sherwood Chang^a

^aSpace Science Division, NASA Ames Research Center, Moffett Field, CA 94035, USA
^bAstromaterials Research and Exploration Science, NASA–Johnson Space Center, Houston, TX 77058, USA
^cDepartment of Medical Microbiology and Immunology, University of California, Davis, CA 95616, United States

Multiple missions to search for water-soluble organic compounds on the surfaces of Solar System bodies are either current or planned and, if such compounds were found, it would be desirable to determine their origin(s). Asteroid or comet material is likely to have been components of all surface environments throughout Solar System history. To simulate the survival of meteoritic compounds both during impacts with planetary surfaces and under subsequent (possibly) harsh ambient conditions, we subjected known meteoritic compounds to comparatively high impact–shock pressures (>30 GPa) and/or to extremely oxidizing/corrosive acid solution. Consistent with past impact experiments, α-amino acids survived only at trace levels above ~18 GPa. Polyaromatic hydrocarbons (PAHs) survived at levels of 4–8% at a shock pressure of 36 GPa. Lower molecular weight sulfonic and phosphonic acids (S&P) had the highest degree of impact survival of all tested compounds at higher pressures. Oxidation of compounds was done with a 3:1 mixture of HCl:HNO₃, a solution that generates additional strong oxidants such as Cl₂ and NOCl. Upon oxidation, keto acids and α-amino acids were the most labile compounds with proline as a significant exception. Some fraction of the other compounds, including non-α amino acids and dicarboxylic acids, were stable during 16–18 hours of oxidation. However, S&P quantitatively survived several months (at least) under the same conditions. Such results begin to build a profile of the more robust meteoritic compounds: those that may have survived, i.e., may be found in, the more hostile Solar System environments. In the search for organic compounds, one current mission, NASAʼs Mars Science Laboratory (MSL), will use analytical procedures similar to those of this study and those employed previously on Earth to identify many of the compounds described in this work. The current results may thus prove to be directly relevant to potential findings of MSL and other missions designed for extraterrestrial organic analysis.

Reference
Cooper G, Horz F, Spees A and Chang S (in press) Highly stable meteoritic organic compounds as markers of asteroidal delivery. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2013.10.021]
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Megumi Matsumoto^a, Kazushige Tomeoka^a, Yusuke Seto^a, Akira Miyake^b, Mitsuhiro Sugita^a

^aDepartment of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe 657-8501, Japan
^bDepartment of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan

Ningqiang is an ungrouped carbonaceous chondrite that chemically and petrologically resembles CV3 chondrites. The matrix of Ningqiang shows much higher abundances of Na, K, and Al by factors of 4.4, 2.7, and 1.6, respectively, than in CV3 chondrites. Our scanning and transmission electron microscope observations and synchrotron radiation X-ray diffraction measurements reveal that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Rietveld refinement of X-ray diffraction data shows that the feldspathoids constitute 7.7 vol.% of all crystalline phases in the matrix. Nepheline and sodalite occur mostly as discrete, equidimensional grains 2–5 μm in diameter that are dispersed homogeneously in the matrix. Most of the grains contain inclusions of Fe-rich olivine and minor Ca pyroxene, magnetite, troilite, and pentlandite.
Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those of the CV group (e.g., Rubin et al., 1988). This means that the chondrules, which constitute a major proportion of the volume other than the matrix in Ningqiang, are depleted in Na, K, and Al. In fact, our analyses and observations show that the chondrules in Ningqiang overall contain very small amounts of these elements. Our interpretation of these findings suggests that nepheline and sodalite in the Ningqiang matrix were originally formed by Na-metasomatism of the chondrules and Ca–Al-rich inclusions in the meteorite parent body. Afterward, they were likely disaggregated and scattered into the matrix. However, it is difficult to envisage that the disaggregation and scattering occurred in situ in the present setting of the meteorite. Hence, we suggest that the Ningqiang meteorite underwent these processes before final lithification.

Reference
Matsumoto M, Tomeoka K, Seto Y, Miyake A and Sugita M (in press) Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.11.016]
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Rebecca J. Thomas^a, David A. Rothery^a, Susan J. Conway^a, Mahesh Anand^a,b

^aDepartment of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K
^bDepartment of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, U.K

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking. In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles.

Reference
Thomas RJ, Rothery DA, Conway SJ and Anand M (in press) Hollows on Mercury: materials and mechanisms involved in their formation. Icarus
[doi:10.1016/j.icarus.2013.11.018]
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