Paper No. 6
Presentation Time: 9:30 AM
CONSTRAINTS ON RIPPLE MIGRATION AT MERIDIANI PLANUM, MARS FROM FIELD AND ORBITAL OBSERVATIONS OF FRESH CRATERS
GOLOMBEK, M.P.1, ROBINSON, K.
2, MCEWEN, A.
3, BRIDGES, Nathan T.
4, IVANOV, B.
5, TORNEBENE, L.
3 and SULLIVAN, R.
6, (1)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, (2)Binghamton University, Binghamton, NY 13902, (3)Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, (4)Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, (5)Institute for Dynamics of Geosphere, Russian Academy of Sciences, Moscow, 119334, Russia, (6)Department of Astronomy, Cornell University, Ithaca, NY 14853, mgolombek@jpl.nasa.gov
Field observations of two fresh craters, the Resolution cluster and Concepción by the Opportunity rover and comparison to high-resolution orbital images constrain the latest phase of granule ripple migration at Meridiani Planum to have occurred between ~50 ka and ~200 ka. The Resolution craters have uplifted rims and ejecta blocks perched on top of the ripples. Concepción crater displays a fresh blocky interior wall, uplifted rim, and ejecta and rays on top of the ripples. Color images show a dearth of blueberry hematite granules perched on top of the surface adjacent to the Concepción crater or its ejecta, suggesting they have been dispersed or buried by the impact and its ejecta. These observations indicate these craters are superposed on and thus younger than the large north-trending granule ripples of Meridiani Planum. These fresh craters also have small dark pebbles scattered across their surfaces, which are most likely fragments of the impactor, suggesting that the dark pebbles and cobbles observed by Opportunity are a lag of impactor-derived material (either meteoritic or secondary impactors from elsewhere on Mars).
Two nearby larger, fresh-rayed craters in Meridiani Planum observed in 0.25 cm/pixel HiRISE images bracket ripple migration; secondaries from the 2.2 km diameter Ada crater are clearly superposed on, and secondaries from an unnamed 0.84 km diameter crater have been modified and overprinted by the ripples. Three methods were used to estimate the age of these craters and thus when the latest phase of ripple migration occurred. Craters clearly younger than the ripples, and craters superposed on the continuous ejecta of Ada and the unnamed 0.84 km diameter crater with uplifted rims and fresh bowl shapes were counted and compared to model isochrons, and recurrence intervals were calculated for the freshest small, rayed craters in the equatorial region of Mars. The inactivity of the ripples over the past ~50 ka at Meridiani is also consistent with other evidence for the stability of the ripples, the lack of observed eolian bedforms in craters that impacted in the past 20 yr, and little evidence for much dune motion in the past 30 yr on Mars. Observations of crater morphology and their interaction with the ripples allow the development of a general time scale for craters in Meridiani Planum over the past million years.
