DETAILED GEOMORPHIC MAPPING OF THE ODIN FORMATION, CALORIS BASIN, MERCURY

The Caloris Basin provides a natural borehole that exposes layers of the Mercurian crust, differing stress regimes and evolving volcanic units. Mariner 10 data revealed a unit of closely-spaced knobs separated by smooth, plains-like material near the eastern rim of the basin. Although this knobby unit and smooth plains differ fundamentally in texture, early workers labeled both rock types as the Odin formation, and speculate that the knobby unit is Caloris ejecta. However, crater counts based on MESSENGER data suggest that the smooth intra-ejecta plains are younger than the smooth plains found in the interior of Caloris. Similarly, the interior smooth plains are much brighter than the intra-ejecta plains. Thus the smooth Odin material may be a younger volcanic flow that embayed Caloris ejecta blocks, or the plains may be related to the initial Caloris impact. To better understand the geologic processes that created the smooth plains of the Odin formation, we use high resolution imaging data from the MDIS instrument. We use a principal component color map to distinguish subtle color variations to more precisely differentiate rock types. We use a crater classification scheme based on the degradation state of the crater and whether the crater is embayed or superposed on the surface to confirm relative age dates. Consistent with previous observations, we observe two spectrally distinct units in the Odin formation. Northwest of the basin rim, the darker sub-unit has a higher concentration of knobs, contains both bright and dark knobs, and hosts craters that are embayed and superposed. The brighter sub-unit has less knobs, mostly bright knobs and fresh and/or superposed craters. We suggest that the brighter unit is younger than the darker unit. In this contribution, we analyze the spatial distribution of knobs to the east of the crater rim to better understand the genetic origin of the Odin formation.