Frans J. M. Rietmeijer^1,*, Joseph A. Nuth², Aurora Pun¹

¹Department of Earth and Planetary Sciences, 1-University of New Mexico, Albuquerque, New Mexico, USA
²Astrochemistry Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase-condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe-source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe³⁺-ferrosilica, amorphous Fe³⁺-Mg, Fe-silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe-silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid-like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley-type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe-silicates detected in the impact-induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.

Reference
Rietmeijer FJM, Nuth JA and Pun A (in press) The formation of Mg,Fe-silicates by reactions between amorphous magnesiosilica smoke particles and metallic iron nanograins with implications for comet silicate origins. Meteoritics & Planetary Science
[doi:10.1111/maps.12194]
Published by arrangement with John Wiley & Sons

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C.S. Edwards^a,*, J.L. Bandfield^b, P.R. Christensen^c and A.D. Rogers^d

^aCalifornia Institute of Technology, Division of Geological and Planetary Sciences, 1200 E. California Blvd., MC 150-21, Pasadena, CA, 91125
^bSpace Science Institute, 4750 Walnut St., Boulder, Co, 80301
^cArizona State University, School of Earth and Space Exploration, Mars Space Flight Facility, PO BOX 876305, Tempe, AZ, 85287-6305
^dDepartment of Geosciences, Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100

Flat-floored craters have long been recognized on Mars with early work hypothesizing a sedimentary origin. More recently, high-resolution thermal inertia measurements show that these craters contain some of the rockiest materials on the planet, inconsistent with poorly consolidated sedimentary materials. In this study, the distribution, physical properties (morphology and thermal inertia), and composition of these craters are thoroughly investigated over the entire planet. The majority of the ~2,800 rocky crater floors identified are concentrated in the low albedo (0.1-0.17), cratered southern highlands. These craters were infilled at ~3.5 Ga and are associated with the highest thermal inertia values and some of the most mafic materials identified on the planet. Although several processes may have led to the formation of the crater floors, the most likely scenario is volcanic infilling through fractures created by the impact event. The primitive magma source directly results from decompression melting of the martian mantle by the removal of the crustal material excavated by the impactor. Volcanic infilling of craters by decompression melting appears to only have occurred in early martian history when the lithosphere was still relatively thin and the thermal gradient was high. This process was widespread and responsible for the eruption of significant volumes of primitive material, inside and likely outside of craters. Impact induced decompression melting of the martian mantle accounts for the unusual infilling of martian craters and is a widespread planetary process that has gone previously undocumented.

Reference
Edwards CS, Bandfield JL, Christensen PR and Rogers AD (in press) The formation of infilled craters on mars: evidence for widespread impact induced decompression of the early martian mantle?. Icarus
[doi:10.1016/j.icarus.2013.10.005]
Copyright Elsevier

Link to Article