GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 246-7
Presentation Time: 9:35 AM


GRANT, John A.1, WARNER, Nicholas H.2, WEITZ, Catherine M.3, GOLOMBEK, Matthew P.4, WILSON, Sharon A.1, HAUBER, Ernst5, ANSAN, Veronique6, CHARALAMBOUS, Constantinos7, WILLIAMS, Nathan R.8, CALEF, Fred4, PIKE, W.T.7, DEMOTT, Alyssa9, KOPP, Megan A.9, LETHCOE-WILSON, Heather A.4 and BANKS, Maria E.10, (1)Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Independence Ave at 6th St. SW, Washington, DC 20560, (2)Department of Geological Sciences, SUNY Geneseo, 1 College Circle, Geneseo, NY 14454, (3)Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, (4)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, (5)DLR Deutsches Zentrum für Luft- und Raumfahrt, Berlin, Germany, (6)Laboratoire de Planetologie et Geodynamique, University of Nantes, France, Nantes, 44322, France, (7)Department of Electrical and Electronic Engineering, Imperial College, London, United Kingdom, (8)Jet Propulsion Laboratory, California Institute of Technology, M/S 183-301, 4800 Oak Grove Dr., Pasadena, CA 91109, (9)Geological Sciences, SUNY-Geneseo, 1 College Circle, Geneseo, NY 14454, (10)NASA Goddard Space Flight Center, Greenbelt, MD 20771

The Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission landed at 4.50° N, 135.62° E in what is likely a highly degraded ~20 m-diameter impact crater in Elysium Planitia dubbed “Homestead hollow”. The hollow is one of many small craters in the area, is up to ~0.6 m deep, and the interior surface shows mostly sand to pebble-sized fines variably punctuated by mostly gravel/pebbles and cobbles. The floor is quite flat down to the cm-scale and slopes <3° to the SE. The hollow margin lacks a significant elevated rim, but does show an abrupt increase in cobble to boulder size rocks. If the hollow is a degraded impact, it probably formed ~500 million years ago into regolith derived from an underlying Hesperian-aged basaltic plain and had an initial depth of ~2-4 m that was bounded by a ~1 m high rim and fairly steep walls. Perched or largely exposed rocks represent ~70% those seen on the rim. By contrast, embedded and buried rocks comprise 60% of rocks mapped inside the hollow. There is an overall higher density of rocks exposed in the western quadrant relative to the rest of the interior.

Impact formation ejected debris resulting in an initial landform whose surface was out of equilibrium with local geomorphic thresholds. Early eolian stripping of the rim and associated partial infilling likely dominated, leading to more perched or exposed rocks on the rim relative to embedded/buried rocks in the interior. Early gravity-driven infilling along the wall decreased with time as slopes were reduced and the crater was filled. Impacts continue to play a triple role in hollow degradation: 1) direct modification during crater formation; 2) short pulses of infilling occurring during emplacement of ejecta from nearby impacts; and 3) generation of fines for additional infilling by the wind. It is possible that the increased rock density in the western quadrant is impact ejecta, though there is no obvious source crater. Otherwise, ongoing degradation occurs at a greatly diminished rate over time limited by very slow weathering and breakdown of resistant basaltic rim blocks. The juxtaposition of attributes of Homestead hollow are comparable to those observed around small craters formed into basaltic rubble in Gusev crater as is their inferred degradation sequence.