Water on terrestrial planets
The origin and development of water on terrestrial planets, Venus, Earth, Mars, and the closely related Earth's Moon, varies with each planetary body, with the exact origins remaining unclear. Additionally, the terrestrial dwarf planet, Ceres is known to have water ice on its surface.
A significant amount of surface hydrogen has been observed globally by the Mars Odyssey GRS. Stoichiometrically estimated water mass fractions indicate that—when free of carbon dioxide—the near surface at the poles consists almost entirely of water covered by a thin veneer of fine material. This is reinforced by MARSIS observations, with an estimated 1.6×106 km3 (3.8×105 cu mi) of water at the southern polar region with Water Equivalent to a Global layer (WEG) 11 metres (36 ft) deep. Additional observations at both poles suggest the total WEG to be 30 m (98 ft), while the Mars Odyssey NS observations places the lower bound at ~14 cm (5.5 in) depth. Geomorphic evidence favors significantly larger quantities of surface water over geologic history, with WEG as deep as 500 m (1,600 ft). The current atmospheric reservoir of water, though important as a conduit, is insignificant in volume with the WEG no more than 10 µm (0.00039 in). Since the typical surface pressure of the current atmosphere (~6 hPa (0.087 psi)) is less than the triple point of H2O, liquid water is unstable on the surface unless present in sufficiently large volumes. Furthermore, the average global temperature is ~220 K (−53 °C; −64 °F), even below the eutectic freezing point of most brines. For comparison, the highest diurnal surface temperatures at the two MER sites have been ~290 K (17 °C; 62 °F).
Recent observation made by a number of spacecraft confirmed significant amounts of Lunar water. Mercury does not appear to contain observable quantities of H2O, presumably due to loss from giant impacts. In contrast, Earth's hydrosphere contains ~1.46×1021 kg (3.22×1021 lb) of H2O and sedimentary rocks contain ~0.21×1021 kg (4.6×1020 lb), for a total crustal inventory of ~1.67×1021 kg (3.68×1021 lb) of H2O. The mantle inventory is poorly constrained in the range of 0.5×1021–4×1021 kg (1.1×1021–8.8×1021 lb). Therefore, the bulk inventory of H2O on Earth can be conservatively estimated as 0.04% of Earth's mass (~2.3×1021 kg (5.1×1021 lb)).
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