Martian Oceans: Where did all the water go?
BY: Dr. Michael Carr, USGS —
Abstract:Several lines of evidence suggest that there were oceans on early Mars. The youngest and best preserved shoreline, roughly 3.5 b.y. old and traceable for 3000 km at a constant elevation of -3650 m, encloses a volume equivalent to a global layer (GEL) 115 m deep. Estimates of the present near-surface inventory range from 20-35 m GEL, so 70-95 m is unaccounted for in addition to any water that outgassed since the ocean was present. The water is unlikely to have infiltrated into the surface in the last 3.5 b.y. because of the presence of a km-thick cryosphere, and because low heat flows would have the prevent previously suggested mechanism of polar basal melting. D/H measurements (1 x SMOW at 4.5 b.y., 3X at 3.5 b.y. and 6X at present day) suggest most of the water was lost to space from the upper atmosphere. Yet at present loss rates only 3-15 m would have been lost over the life of the planet. Loss of 70-95 m would require not only higher loss rates than at present but a significantly higher fractionation factor. Some estimates of the present fractionation factor are as low as 0.012. Consistency with the present near-surface inventory, the ocean volume, and the D/H enrichment would require a much larger fractionation factor of 0.45. Estimates in the literature of losses of oxygen to space to maintain 2:1 stochiometry range from 5-80 m. The high loss rates of both H and O and the high D/H fractionation factor are attributed to high EUV output of the Sun at the estimated time of formation of the youngest shoreline, roughly 3.5 b.y. ago. If the ocean was already enriched to 3X SMOW 3.5 b.y. ago, as is suggested by the D/H at Gale Crater then as much as 1 km could have been present and interacting with the atmosphere at the start of the Noachian 4.1 b.y. ago.
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