WATER ON EARLY MARS - POSSIBLE SUBAQUEOUS SEDIMENTARY DEPOSITS COVERING ANCIENT CRATERED TERRAIN IN WESTERN ARABIA AND SINUS-MERIDIANI

Western Arabia and northern Sinus Meridiani (30 degrees N-10 degrees S ; 40 degrees W-40 degrees E) are almost entirely covered by wind-erode d, horizontally-bedded, sedimentary layers that lap against local topo graphic features. This portion of Mars' ancient cratered terrain is re latively low (< 1 km elevation) and flat (sloping westward 0.7 m/km ov er 3,000 km). The region lacks valley network channels. We propose tha t at some time in early Martian history, this region was under water. The water hypothesis is strengthened by the presence of (1) a shore-li ke contact between smooth-surfaced deposits and ancient cratered terra in in central Sinus Meridiani, (2) polygonal structures in northern Si nus Meridiani, and (3) sand (which cannot be from airfall), possibly r eworked by wind from the underlying sedimentary units. The September 1 997 arrival of Mars Global Surveyor's thermal emission spectrometer of fers an opportunity to begin to test this hypothesis by searching for carbonate and evaporite minerals among the sediment covering the regio n.