^1,2E.Heggy,¹E.M.Palmer,²T.W.Thompson,³B.J.Thomson,⁴G.W.Patterson
Earth and Planetary Science Letters 541, 116274 Link to Article [https://doi.org/10.1016/j.epsl.2020.116274]
¹University of Southern California, Ming Hsieh Department of Electrical and Computer Engineering, 3737 Watt Way, Los Angeles, CA 90089, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Department of Earth and Planetary Sciences, The University of Tennessee Knoxville, 1621 Cumberland Avenue, Knoxville, TN 37996, USA
⁴The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant () of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have ranging from 2.3-to-3, and large ones >5-km have higher values of that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations.

¹Forrest Gilfoy,¹Jie Li
Earth and Planetary Science Letters 541, 116285 Link to Article [https://doi.org/10.1016/j.epsl.2020.116285]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
Copyright Elsevier

A series of multi-anvil experiments have been conducted to define the iron-rich liquidus of the iron-nickel-sulfur (FeNi-S) system at 20 GPa, the estimated pressure of the martian core-mantle boundary (CMB). The liquidus curve of FeNi-S containing about 9 wt.% Ni has a concave up shape, and is as much as 400 K lower than the liquidi previously applied to the martian core with sparse experimental constraints. Unlike existing liquidi of Fe-S and FeNi-S at 23 GPa, which predict a fully molten core for a narrow range of sulfur content between 14 and 15 wt.% S, our results are consistent with a molten state for all proposed core compositions, and establishes a new minimum CMB temperature of 1500 K for 10 wt.% S and 1250 K for 16 wt.% S. Extrapolating our FeNi-S liquidus to high pressures and comparing it to calculated areotherms, we find that three core crystallization regimes are possible. For a martian core with moderate sulfur content (10 to 13 wt.%) or lower, crystallization takes the form of iron snow near the CMB, while for cores with higher sulfur content (15-16 wt.%), solidification occurs near the center of the planet in the form of solid Fe3S. At an intermediate sulfur content of 14 wt.%, Fe3S would precipitate over a broad depth range and may appear fully molten to surface observations.

^1,2Lionel G.Vacher,¹Laurette Piani,¹Thomas Rigaudier,¹Dorian Thomassin,¹Guillaume Florin,¹Maxime Piralla,Yves Marrocchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.007]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²Department of Physics, Washington University, St. Louis, St. Louis, MO, USA
Copyright Elsevier

Hydrogen occurs at the near percent level in the most hydrated chondrites (CI and CM) attesting to the presence of water in the asteroid-forming regions. Their H abundances and isotopic signatures are powerful proxies for deciphering the distribution of H in the protoplanetary disk and the origin of Earth’s water. Here, we report H contents and isotopic compositions for a set of carbonaceous and ordinary chondrites, including previously analyzed and new samples analyzed after the powdered samples were degassed under vacuum at 120°C for 48 hours to remove adsorbed atmospheric water. By comparing our results to literature data, we reveal that the H budgets of both H-poor and H-rich carbonaceous chondrites are largely affected by atmospheric moisture, and that their precise quantification requires a specific pre-degassing procedure to correct for terrestrial contamination. Our results show that indigenous H contents of CI carbonaceous chondrites usually considered the most hydrated meteorites might be almost a factor of two lower than those previously reported, with uncontaminated D/H ratios differing significantly from that of Earth’s oceans. Without pre-degassing, the H concentrations of H-poor samples (e.g., CVs chondrites) are also affected by terrestrial contamination. After correction for contamination, it appears that the amount of water in chondrites is not controlled by the matrix modal abundance, suggesting that the different chondritic parent bodies accreted variable amounts of water-ice grains. Our results also imply that (i) thermal metamorphism play an important role in determining the H content of both CV and ordinary chondrites but without affecting drastically their H isotopic composition since no clear D enrichment is observed with the increase of petrographic type and (ii) the D enrichment of ordinary chondrite organics does not result from the loss of isotopically light H₂ induced by metal oxidation but is rather linked to the persistence of a thermally resistant D-rich component.