Mars: Formation and fate of a frozen Hesperian ocean

Abstract

Late Hesperian-aged, circum-Tharsis floods interpreted to have formed by catastrophic release of groundwater cut large channels and debouched significant quantities of water into the northern lowlands of Mars. The floods are thought by many to have formed an ocean of significant volume and depth, encircled by contacts that have been interpreted as shorelines. Models of catastrophic groundwater release require a thick and continuous cryosphere with mean annual temperatures well below freezing much like those today. In this environment, the bodies of liquid formed by individual outflow events would have been very short lived, undergoing rapid freezing. We investigate the case where floods repeatedly flowed into the northern lowlands under climatic conditions that resemble those of the present day; the water from each flood froze in a geologically short period of time to form an ice layer. Successive ice layers accumulated to form an ocean-sized body of ice that filled the basin up to the -3650 contour, thereby enclosing a 110 m global equivalent layer (GEL) of water. Subsequent to the filling of the basin the ice slowly sublimated into the atmosphere to be lost to space or to accumulate in various surface and near-surface cold traps, such as the polar layered deposits. Where is this excess water today? The presence of the thick global cryosphere meant that only minor amounts of water were lost from the surface back into the global groundwater system. Approximately 20-30 m GEL of water is estimated to be at or near the surface today and exchanging with the atmosphere on geologic time scales (this includes the polar layered deposits and deposits elsewhere at depths less than approximately 80 m). Below the exchangeable reservoir is a non-exchangeable reservoir of unknown capacity. Present day loss rates to space fall far short of those needed to eliminate the mid-Hesperian ice ocean and those needed to cause the observed doubling of the D/H of the exchangeable reservoir since the mid-Hesperian. The discrepancy implies earlier loss rates must have been higher. Assuming a linear increase in loss rates because of the higher early EUV output of the Sun, we estimate that for a present inventory of 30 m GEL, 42 m GEL remains to be accounted for. Possibilities include greater dependence of losses of EUV than assumed, and enhanced losses during periods of high obliquity. In addition, large volumes of ice may be present near the surface at high latitudes outside the polar layered deposits as indicated by recent discoveries of ice layers in cliffs.