User:Racineswick/Water on terrestrial planets of the Solar System

Mars
Main article: Water on Mars

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).

Mercury
Due to the close proximity to the Sun and lack of visible water on its surface, the planet Mercury had been though of as a non-volatile planet. Data retrieved from the Mariner 10 mission found evidence of H, He, and O in Mercury's exosphere. Volatile's had also been found near the polar regions. MESSENGER, however, sent back data from multiple on-board instruments that lead scientists to the conclusion that Mercury was volatile rich. Mercury is rich in K, which has been suggested as a proxy for volatile depletion on the planetary body. This leads to assumption that Mercury could have accreted water on its surface, relative to that of Earth if its proximity had not been so near that of the Sun.

Moon
Main article: Lunar water

Recent observation made by a number of spacecraft confirmed significant amounts of Lunar water. The Secondary Ion Mass Spectrometer (SIMS) measured H2O as well as other possible volatiles in Lunar volcanic glass bubbles. In these volcanic glasses, 4-46 ppm wt of H2O was found and then modeled to have been 260-745 ppm wt prior to the lunar volcanic eruptions. SIMS also found Lunar water in the Apollo astronauts rock samples returned to Earth. These rock samples were tests in three different ways and all came to the same conclusion that the Moon contains lunar water.

There are three main data sets for water abundance on the Lunar surface: Highland samples, KREEP samples, and Pyroclastic Glass Samples. Highlands samples were estimated for the Lunar magma ocean at 1320-5000 ppm wt of H2O in the beginning. The urKREEP sample estimates a 130-240 ppm wt of H2O, which is similar to the findings in the current Highland samples (before modeling). Pyroclastic Glass sample beads were used to estimate the water content in the mantle source and the bulk silicate Moon. The mantle source was estimated at 110 ppm wt of H2O and the bulk silicate Moon contained 100-300 ppm wt of H2O.

See also: Water Distribution on Earth
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)).

See also: Atmosphere of Venus
The current Venusian atmosphere has only ~200 mg/kg H2O(g) in its atmosphere and the pressure and temperature regime makes water unstable on its surface. Nevertheless, assuming that early Venus's H2O had a ratio between deuterium (heavy hydrogen, 2H) and hydrogen (1H) similar to Earth's Vienna Standard Mean Ocean Water (VSMOW) of 1.6×10−4, the current D/H ratio in the Venusian atmosphere of 1.9×10−2, at nearly ×120 of Earth's, may indicate that Venus had a much larger H2O inventory. While the large disparity between terrestrial and Venusian D/H ratios makes any estimation of Venus's geologically ancient water budget difficult, its mass may have been at least 0.3% of Earth's hydrosphere. Estimates based on Venus's levels of deuterium suggest that the planet has lost anywhere from 4 metres (13 ft) of surface water up to "an Earth's ocean's worth".