Variations in the onset diameter for Martian layered ejecta morphologies and their implications for subsurface volatile reservoirs

We investigated regional variations in the onset diameter of craters displa ying a single layer ejecta morphology within +/- 30 degrees latitude using Viking imagery. Our results generally agree with those of previous studies which show onset diameters of 5 to 6 km in the equatorial region, but we ha ve identified localized regions with unusually small onset diameters. The l argest region is located in Solis and Thaumasia Planae. The 3-5 km onset di ameter range in this area indicates a near-surface ice-rich reservoir (dept h similar to 110 m). This unusual concentration of near-surface ice may hav e resulted from magmatic-driven uplifts associated with the Tharsis rise, w hich modified parts of a regional aquifer/drainage basin system and resulte d in the transfer and concentration of subsurface volatiles in this region.