HOW AND WHEN DID THE NOACHIAN PERIOD ON MARS END?

The Noachian represents the earliest period on Mars, when high impact cratering rates formed densely cratered terrains still preserved on 40% of the planet. These terrains also were heavily degraded during the Noachian. We have been investigating the density of craters with recognizable ejecta assumed to be post-Noachian in the 5 to 16 km diameter range on Noachian units as a function of latitude, elevation, and geologic unit to better characterize the timing of when major resurfacing on Noachian terrains ceased. We used MOLA elevation data, the Viking-based global geologic map, and the crater inventories of Barlow and Kuzmin. Preliminary results indicate that densities of craters with ejecta in Noachian terrains vary by a factor of 2 to 4 depending on geologic unit, elevation, and latitude. For all units, crater densities show some decrease in latitudes above 20°N and 50°S. Most of the Noachian surface shows increasing preservation of crater ejecta blankets with increasing elevation in the range of -3 to 3 km. The spatially diverse Noachian geology and topography indicates that significant local to perhaps global processes erased crater ejecta, including impacts, fluvial dissection, mass wasting, aeolian deposition and erosion, and upland basin sedimentation and volcanic infilling. Dissected terrain covers a third or more of the Noachian surface between 20°N and 30°S and much less at higher latitudes, indicating that valley-network resurfacing was chiefly a regional equatorial phenomenon that could have been related to precipitation of rain and/or snow leading to runoff and/or sapping processes. Also, local valley systems may have resulted from or been enhanced by magmatic- or impact-heated hydrothermal areas. Some of the processes may have been longer lived and/or more intense at higher latitudes, including mass wasting and aeolian deposition. The apparent diversity of resurfacing processes and histories that have operated on Noachian units suggests that the termination of modification as defined by crater degradation has been complex in space and time and may have extended in places into the Hesperian and Early Amazonian. Improved geomorphic analyses using new Mars Global Surveyor and Mars Odyssey data sets will help elucidate that story.