Water on terrestrial planets of the Solar System
The presence of water on the terrestrial planets of the Solar System (Mercury, 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. Water inventories. 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 of water at the southern polar region with Water Equivalent to a Global layer (WEG) deep. Additional observations at both poles suggest the total WEG to be , while the Mars Odyssey NS observations places the lower bound at ~ depth. Geomorphic evidence favors significantly larger quantities of surface water over geologic history, with WEG as deep as . The current atmospheric reservoir of water, though important as a conduit, is insignificant in volume with the WEG no more than . Since the typical surface pressure of the  current atmosphere (~) is less than the triple point of HO, liquid water is unstable on the surface unless present in sufficiently large volumes. Furthermore, the average global temperature is ~, even below the eutectic freezing point of most brines. For comparison, the highest diurnal surface temperatures at the two MER sites have been ~. Mercury. Due to its proximity to the Sun and lack of visible water on its surface, the planet Mercury had been thought of as a non-volatile planet. Data retrieved from the Mariner 10 mission found evidence of H, He, and O in Mercury's  exosphere. Volatiles have also been found near the polar regions. MESSENGER, however, sent back data from multiple on-board instruments that led 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. Earth. Earth's hydrosphere contains ~1.46×10 kg (3.22×10 lb) of HO and sedimentary rocks contain ~0.21×10 kg (4.6×10 lb), for  a total crustal inventory of ~1.67×10 kg (3.68×10 lb) of HO. The mantle inventory is poorly constrained in the range of 0.5×10–4×10 kg (1.1×10–8.8×10 lb). Therefore, the bulk inventory of HO on Earth can be conservatively estimated as 0.04% of Earth's mass (~2.3×10 kg (5.1×10 lb)). Earth's Moon. Recent observation made by a number of spacecraft confirmed significant amounts of Lunar water. The Secondary Ion Mass Spectrometer (SIMS) measured HO as well as other possible volatiles in Lunar volcanic glass bubbles. In these volcanic glasses, 4-46 ppm wt of HO 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 tested 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 HO 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 HO and the bulk silicate Moon contained 100-300 ppm wt of HO. Venus. The current Venusian atmosphere has only ~200 mg/kg HO(g) in its atmosphere and the pressure and temperature regime makes water unstable on its surface. Nevertheless, assuming that early Venus's HO 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, the current D/H ratio in the Venusian atmosphere of 1.9×10, at nearly ×120 of Earth's, may indicate that Venus had a much larger HO 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 of surface water up to "an Earth's ocean's worth". Accretion of  water by Earth and Mars. The D/H isotopic ratio is a primary constraint on the source of HO of terrestrial planets. Comparison of the planetary D/H ratios with those of carbonaceous chondrites and comets enables a tentative determination of the source of HO. The best constraints for accreted HO are determined from non-atmospheric HO, as the D/H ratio of the atmospheric component may be subject to rapid alteration by the preferential loss of H unless it is in isotopic equilibrium with surface HO. Earth's VSMOW D/H ratio of 1.6×10 and modeling of impacts suggest  that the cometary contribution to crustal water was less than 10%. However, much of the water could be derived from Mercury-sized planetary embryos that formed in the asteroid belt beyond 2.5 AU. Mars's original D/H ratio as estimated by deconvolving the atmospheric and magmatic D/H components in Martian meteorites (e.g., QUE 94201), is ×(1.9+/-0.25) the VSMOW value. The higher D/H and impact modeling (significantly different from Earth due to Mars's smaller mass) favor a model where Mars accreted a total of 6% to 27% the mass of the current Earth hydrosphere, corresponding respectively to  an original D/H between ×1.6 and ×1.2 the SMOW value. The former enhancement is consistent with roughly equal asteroidal and cometary contributions, while the latter would indicate mostly asteroidal contributions. The corresponding WEG would be , consistent with a 50% outgassing efficiency to yield ~ WEG of surface water. Comparing the current atmospheric D/H ratio of ×5.5 SMOW ratio with the primordial ×1.6 SMOW ratio suggests that ~ of has been lost to space via solar wind stripping. The cometary and asteroidal delivery of water to accreting Earth and Mars has significant caveats, even though it is favored by  D/H isotopic ratios. Key issues include: An alternative to the cometary and asteroidal delivery of HO would be the accretion via physisorption during the formation of the terrestrial planets in the solar nebula. This would be consistent with the thermodynamic estimate of around two Earth masses of water vapor within 3AU of the solar accretionary disk, which would exceed by a factor of 40 the mass of water needed to accrete the equivalent of 50 Earth hydrospheres (the most extreme estimate of Earth's bulk HO content) per terrestrial planet. Even though much of the nebular HO(g) may be  lost due to the high temperature environment of the accretionary disk, it is possible for physisorption of HO on accreting grains to retain nearly three Earth hydrospheres of HO at temperatures. This adsorption model would effectively avoid the Os/Os isotopic ratio disparity issue of distally-sourced HO. However, the current best estimate of the nebular D/H ratio spectroscopically estimated with Jovian and Saturnian atmospheric CH is only 2.1×10, a factor of 8 lower than Earth's VSMOW ratio. It is unclear how such a difference could exist, if physisorption were indeed the dominant form of HO accretion for  Earth in particular and the terrestrial planets in general.