Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p42c..09b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P42C-09
Other
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The surface of Mars has been exposed to impact cratering processes for billions of years and massive megabreccia deposits have been observed in many craters. However, the materials caught up in the hot plume in the atmosphere above the crater should be present in surficial ejecta, but have not been identified by the martian rovers. These materials can include shocked ejecta, impact melts and accretionary lapilli. The effects of melting of the surface due to impinging airbursts from projectiles that breakup in the atmosphere might also be present (Newsom et al., 2010). The erosional effects of wind and water have probably erased much of this evidence from all but more recent craters, but the evidence from terrestrial craters suggests that these features should exist. The Henbury, Australia crater field (largest crater 157m diameter) contains Ni-bearing impactites (confirmed by analysis of ~30 samples by portable XRF) that are hard to explain by any mechanism other than breakup of the iron-meteorite high in the atmosphere and impingement of this Fe-bearing high-temperature gas onto the surface. The Lonar crater in Maharashtra, India (1.8 km diameter) was emplaced in Deccan basalt. The basaltic nature of Lonar crater and its well preserved ejecta blanket makes it an important Mars analog. We have discovered impact melt particles coated in fine grained material and melt fragments in the ejecta at Lonar that suggests that significant welding of material took place within the initial ejecta plume. Although impact melt spherules have been previously observed in Lonar ejecta, we have recently identified two new classes of sub-millimeter (avg size 900 μm) particles. These particles display a layering of fine-grain material and are similar to lapilli at volcanic as well as at other impact craters. Accretionary lapilli are spheroidal ash aggregates deposited either in discrete layers or enclosed in a matrix of fine-grain ash that exhibit a target nucleus interpreted to be particles of target rock which have been shocked to varying degrees. Armored lapilli are a variety of accretionary lapilli containing rock or mineral fragments covered by fine ash. Their coatings are much finer grained than the surrounding ejecta; both types of particles were found at Lonar. Back scattered electron images show continuous or nearly continuous coatings of fine-grain material on almost all particles; The compositions of rim materials are consistent with Deccan basalt or individual pyroxene, plagioclase, and oxide minerals. Some of the rock particles are scoriacious and appear sintered to the lapilli. Our results suggest that accretionary and armored lapilli may be diagnostic features of impact craters on Earth. It is possible that accretionary and armored lapilli are still present in young craters on Mars due to the small amount of atmosphere, weathering and erosion. They may be recognizable in the regolith in images collected by the MERs Microscopic Imager, Phoenix's arm camera, and the Mars Hand Lens Imager on MSL the same way they were recognizable in the Lonar ejecta.
Beal R.
Newsom Horton E.
Wright Shawn P.
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