¹Erwin Dehouck, ²Anne Gaudin, ³Vincent Chevrier, ²Nicolas Mangold
¹Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA
²Laboratoire de Planétologie et Géodynamique de Nantes (LPGN), CNRS/Université de Nantes, 44322 Nantes, France
³Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA

Sulfates and Fe-oxides identified on the Martian surface by orbital and in situ missions indicate that oxidizing conditions have existed on early Mars, at least locally and/or episodically. In the context of rock alteration and weathering, redox conditions are especially critical for the behavior of iron, which is soluble in its divalent state but insoluble in its trivalent state. Here, we combine results from a series of laboratory experiments conducted under Mars-like conditions to address the influence of highly-oxidizing compounds such as hydrogen peroxide (H2O2) on the alteration pathways of primary materials. We show that, if early Mars had a dense CO2 atmosphere allowing for relatively “warm and wet” conditions and surface weathering, highly-oxidizing conditions would have strongly inhibited the formation of Fe/Mg-smectite clays from alteration of igneous ferromagnesian minerals, and possibly enhanced the formation of carbonates. But a decade of mineral mapping of the Martian surface show abundant, widespread Fe/Mg-clays and rare carbonates, which we interpret here as a mineralogical record of poorly-oxidizing (or even reducing) conditions during most of the Noachian era. Oxidizing conditions would have occurred later in Martian history as a consequence of a higher rate of H2 escape or of a lower rate of volcanic outgassing, or both.

References
Dehouck E, Gaudin A, Chevrier V, Mangold N (2016) Mineralogical record of the redox conditions on early Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.030]
Copyright Icarus

¹Marie Protin, ¹Pierre-Henri Blard, ¹Yves Marrocchi, ¹François Mathon
¹CRPG, UMR 7358, CNRS-Université de Lorraine, 54500 Vandoeuvre-lès-Nancy, France

This study reports new experimental results that demonstrate that large amounts of atmospheric helium may be adsorbed onto the surfaces of olivine grains. This behavior is surface-area-related in that this contamination preferentially affects grains that are smaller than 125 μm in size. One of the most striking results of our study is that in vacuo heating at 900°C for 15 minutes is not sufficient to completely remove the atmospheric contamination. This suggests that the adsorption of helium may involve high-energy trapping of helium through irreversible anomalous adsorption. This trapping process of helium can thus be compared to a “lobster pot” adsorption: atmospheric helium easily gets in, but hardly gets out. While this type of behavior has previously been reported for heavy noble gases (Ar, Kr, Xe), this is the first time that it has been observed for helium. Adsorption of helium has, until now, generally been considered to be negligible on silicate surfaces. Our findings have significant implications for helium and noble gas analysis of natural silicate samples, such as for cosmic-ray exposure dating or noble gas characterization of extraterrestrial material. Analytical procedures in future studies should be adapted in order to avoid this contamination. The results of this study also allow us to propose an alternative explanation for previously described matrix loss of cosmogenic 3He.

Reference
Protin M, Blard P-H, Marrocchi Y, Mathon F (2016) Irreversible Adsorption of Atmospheric Helium on Olivine: A Lobster Pot Analogy. Geochmica et Cosmochmica (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.032]
Copyright Elsevier