COASTAL GEOMORPHOLOGY OF THE MARTIAN NORTHERN PLAINS

The northern lowland plains comprise approximately one third Mars' sur face area. Most outflow channels and many valley networks debouch into the lowlands, yet there is little or no morphologic evidence to sugge st that channel cutting continued far into the plains, despite a conti nued basinward regional topographic gradient. The immediate fate of th e water discharged from these channels was dependent on the prevailing paleoclimate at the time of its emplacement. Though current models of the martian paleoclimate suggest that mean annual temperatures were l ikely below freezing throughout most of martian history, geomorphic ev idence suggests that coastal erosion on a scale comparable to that of well-known terrestrial paleolakes occurred. These landforms can be tra ced to nearly complete closure of the northern plains and appear to re quire at least two, and perhaps several, highstands of a sea or ocean with temperatures above freezing at least for geologically brief perio ds of time. The latest highstand may have been as recent as Early Amaz onian time. The elevations and areal extent of these landforms provide independent estimates of the martian water budget that can be compare d to prevailing models of martian volatile evolution. Estimated volume s of water and sediment discharged by the major channel systems periph eral to the northern plains can be compared to the volume of the basin based on the available topography. Values for the circum-Chryse outfl ow channels alone are sufficient to have produced large bodies of stan ding water within the basin. These estimates may be comparable to the basin volume contained within the younger, least extensive highstand i dentified. The earlier, more extensive highstand delineates a basin wi th a much larger implied volume that may require the presence of a sem i-permanent, possibly ice-covered ocean in the northern plains prior t o major channeling events. The northern plains today probably consist of water-lain sediments interbedded to considerable depths with flood lavas from the major volcanic centers, with sediment comprising most o f the present surface.