40th Lunar and Planetary Science Conference (2009) 2234.pdf PIT-FLOOR CRATERS ON MERCURY: CHARACTERISTICS AND MODES OF FORMATION Jeffrey J. Gillis-Davis1 David T. Blewett2, Brett W. Denevi3, Mark S. Robinson3, Sean C. Solomon4, Robert G. Strom5, and the MESSENGER Team 1Hawaii Institute of Geophysics and Planetology, University of Hawaii, Hono- lulu, HI 96822,
[email protected]; 2Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, Tempe, AZ 85287; 3School of Earth and Space Exploration, Arizona State University; 4Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015; 5Lunar and Planetary Laboratory, Univer- sity of Arizona, Tucson, AZ 85721. Introduction: Assessment of volcanic landforms low floors. on Mercury’s surface is important for understanding The juxtaposition of pyroclastic material and pit the planet's thermal history as well as Mercury's place craters suggests that their formation is the result of within the known geologic histories of the other terres- explosive venting of volatiles from degassing magma trial planets [1, 2]. From images obtained by the too deep to propagate to the near surface. A similar MESSENGER spacecraft during its first and second mechanism of formation was suggested for Rima Hy- flyby of Mercury, we have identified numerous pit ginus on the Moon [10]. Calculations of the volatile craters on the floors of impact craters. Pit craters are abundance reveal that magmas on Mercury would re- rimless depressions that are interpreted to have formed quire more than twice the amount of carbon monoxide by endogenic processes. Impact craters hosting pit cra- to emplace pyroclastic deposits a given distance than ters we term pit-floor craters; those identified to date on the Moon [6].