GSA Connects 2021 in Portland, Oregon

Paper No. 14-10
Presentation Time: 10:30 AM


CALEF III, Fred, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, ALWMARK, Sanna, Lund University, Department of Geology, Sölvegatan 12, Lund, 22362, Sweden, AMUNDSEN, Hans E.F., CENSSS, Institute for Technology Systems, Univ. of Oslo, Kjeller, Norway, BECHTOLD, Andreas, Department of Lithospheric Research, University Vienna, UZA 2, Althanstrasse 14, Vienna, 1020, Austria, BELL III, James, School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, QUANTIN, Cathy, Universite de Lyon, Lyon, France, DYPVIK, Henning, CENSSS, Institute for Technology Systems Univ. of Oslo, Kjeller, Norway, HAMRAN, Svein-Erik, University of Oslo, Kjeller, Norway, SCHMITZ, Nicole, German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany, SIMON, Justin I., NASA Johnson Space Center, Houston, TX 77058 and STACK, Kathryn M., Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109

Fresh craters provide an opportunity for close examination into the subsurface for landed missions. Adziilii crater is one of many fresh craters with extant ejecta within Jezero crater, the field site for the Mars 2020 Perseverance rover, formed in the unit termed Crater Floor- Fractured Rough (CF-Fr) which comprises much of the Jezero crater floor. This ~80x70 m elliptical crater has a depth/diameter ratio of 0.05 consistent with a low-angle secondary impact. Considering two similar appearing elongated impact craters lying to the southwest and southeast of the landing site, Adziilii crater is probably part of a secondary crater cluster. Meter scale-sized blocks line the Adziilii crater rim out to one crater radii to the north and south. Such asymmetric ejecta distribution is also related to shallow impact angles. Several sharp-rimmed kilometer diameter craters, for example the 2 km diameter Dacono crater east of Adziilii, are close enough to have been the source for ejecta blocks traveling at sub-hypervelocity. In the case of Dacono, a mere 320-350 m/s initial velocity at an ejection angle ranging from 30-45 degrees can loft an ejecta block the ~28 km distance to form Adziilii crater. Excavation depths of a crater this size are approximately 3-5 meters.

Some larger ejecta blocks seen in Mastcam-Z images exhibit unique vesicular, sometimes ropey, textures different from the surrounding dusty low-lying rocks nearby and appear rougher than similar dark toned, smooth textured blocks examined by the Perseverance rover. Observations from the rover’s ground penetrating radar system (RIMFAX) reveal at least one higher density subsurface transition at about 3-5 m depth. Given the unique textures and excavation depths, there’s potential these blocks represent a unique buried surface. While the ejecta block textures are consistent with a geologic unit with a volcanic origin, as are several subsurface structures, the pattern is also consistent with aeolian erosion as seen in Gale Crater rocks at Rocknest. It is also possible the unique lithologic characteristics hint at impactor fragment survivability, more likely in a low velocity impact. However, if the source crater is local, the differences in lithology may be small or unobservable.

Future observations of fresh impact craters along the rover's traverse should help elucidate more subsurface stratigraphy of the crater floor and other buried units examined by Perseverance at the surface.