MISSING MARS: CLUES FROM RELICT CRATERS (Invited Presentation)

Broad areas on Mars are gone. Evidence includes: small craters perched on relict terrains (pedestal craters) at high latitudes, inverted crater floors, relict mesas and plateaus, and ghost craters consistent with ice-welded air-fall deposits [Schultz & Lutz, 1988; Tanaka, 2000]. Missing deposit thicknesses range from 100 m at high latitudes (responding to obliquity cycles) to much greater thicknesses (up to 3 km) in localized deposits near the equator (e.g., Medusa Fossae Formation, Arabia). While obliquity cycles account for the missing high-latitude deposits [e.g., Schultz & Lutz, 1988; Thomson & Schultz, 2007], extreme obliquity shifts [e.g., Touma & Wisdom, 1993] could have transferred polar deposits to equatorial locations, thereby leaving behind deposits 50-100 m thick [e.g., Fassett & Head, 2007]. The brief durations of such extreme shifts, however, limit both deposit thicknesses and interaction times with the cratering flux and are inconsistent with clustered large pedestal craters (> 40 km in diameter) and missing deposits over 2 km thick, even within closed systems (basins and craters).

New estimates based on relict craters and terrains indicate the disappearance of more than ~15 million km³, which greatly exceeds the inventory of both north and south layered terrains (~3 million km³). The re-distribution and/or loss of such large volumes of unconformable deposits is problematic unless once-frozen volatiles once comprised a significant fraction.

Impacts into a layer 3 km thick significantly reduce shock effects in the substrate [Stickle & Schultz, 2013] and once gone, can erase craters as large as 30 km [Schultz, 2007]. Relict features include inverted crater floors and ghost craters, but muted terrains (buried craters) also implicate significant residual deposits. Removal of such deposits constrains exhumation (rather than formation) ages based on crater statistics [e.g., Schultz & Lutz, 1988; Grant and Schultz, 2010]. Because a low-impedance layer is as an ideal capture medium, its disappearance should leave behind relict antecedent drainage systems and lag deposits (composed of accumulated debris including ejecta fragments, impact glass, meteorites, and diagenetic nodules). If such lag surfaces are not evident in exhumed terrains, then they must be masked by ongoing reworking or later burial.