We investigated regional variations in the onset diameter of craters displaying a single layer ejecta morphology within ¿30¿ 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 have identified localized regions with unusually small onset diameters. The largest region is located in Solis and Thaumasia Planae. The 3--5 km onset diameter range in this area indicates a near-surface ice-rich reservoir (depth ~110 m). This unusual concentration of near-surface ice may have resulted from magmatic-driven uplifts associated with the Tharsis rise, which modified parts of a regional aquifer/drainage basin system and resulted in the transfer and concentration of subsurface volatiles in this region. ¿ 2001 American Geophysical Union