Evolution of water on Mars and Earth

The evolution of H₂O on either planet needs be understood in the context of the other terrestrial planetary bodies and their current H₂O inventories.

H₂O Inventory of Mars

A significant amount of surficial H has been observed globally by the Mars Odyssey GRS.Boynton, W. V. et al. (2007), Concentration of H, Si, Cl, K, Fe, and Th in the low and mid latitude regions of Mars, Journal of Geophysical Research Planets, in press [http://dx.doi.org/10.1029/2007JE002887 doi 10.1029/2007JE002887] ] Stoichiometrically estimated H₂O mass fractions indicate that - when free of CO_2(s) - the near surface at the poles consists almost entirely of H₂O_(s) covered by a thin veneer of fine material. This is reinforced by MARSIS observations, with an estimated 1.6x10⁶ km³ of H₂O(s) at the southern polar region with Water Equivalent to a Global layer (WEG) 11 m deep.Plaut, J. J. et al. (2007), [http://dx.doi.org/10.1126/science.1139672 doi 10.1126/science.1139672] ] Additional observations at both poles suggest the total WEG to be 30 m, while the Mars Odyssey NS observations places the lower bound at ~14 cm depth.Feldman, W. C. et al. (2004), [http://dx.doi.org/10.1029/2003JE002160 doi 10.1029/2003JE002160] ] Geomorphic evidence favors significantly larger quantities of surficial H₂O over geologic history, with WEG as deep as 500 m. The current atmospheric reservoir of H₂O, though important as a conduit, is insignificant in volume with the WEG no more than 10^-5 m. Since the typical surface pressure of the current atmosphere (~6 hPa Jakosky, B. M. and Phillips, R. J. (2001), [http://dx.doi.org/10.1038/35084184 doi 10.1038/35084184] ] ) 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, 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.Spanovich, N. et al. (2006), [http://dx.doi.org/10.1016/j.icarus.2005.09.014 doi 10.1016/j.icarus.2005.09.014] ]

H₂O Inventory 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 D/H ratio similar to Earth's Vienna Standard Mean Ocean Water (VSMOW) of 1.6x10^-4, National Insitute of Standards and Technology (2005), [https://srmors.nist.gov/certificates/8535.pdf?CFID=12001608&CFTOKEN=5c24bc7fc8132ce6-2B2520A5-A7C5-6123-0D31F83B0741932B&jsessionid=b430d26f379de78357e1 Report of Investigation] ] the current D/H isotopic ratio in the Venusian atmosphere of 1.9x10^-2, at nearly x120 of Earth's, may indicate that Venus had a much larger H₂O inventory.Kulikov, Yu. N. et al. (2006), [http://dx.doi.org/10.1016/j.pss.2006.04.021 doi 10.1016/j.pss.2006.04.021] ] While the large disparity between terrestrial and Venusian D/H ratios makes any estimation of Venus's geologically ancient water budget difficult, Drake, M. J. (2005) Origin of water in the terrestrial planets, Meteoritics and Planetary Science 40 (4), 515-656] its mass may have been at least 0.3% of Earth's hydrosphere.

H₂O Inventories of Mercury, Moon, and Earth

Neither Mercury, nor the Moon appears to contain observable quantities of H₂O, presumably due to loss from giant impacts. In contrast, Earth's hydrosphere contains ~1.46x10²¹ kg of H₂O and sedimentary rocks contain ~0.21x10²¹ kg, for a total crustal inventory of ~1.67x10²¹ kg of H₂O. The mantle inventory is poorly constrained in the range of (0.5 - 4)x10²¹ kg. Morbidelli, A. et al. (2000), Source regions and timescales for the delivery of water to the Earth, Meteoritics and Planetary Science, 35, 1309-1320] Therefore, the bulk inventory of H₂O on Earth can be conservatively estimated as 0.04% of Earth's mass (~6x10²⁴ kg).

Accretion of H₂O by Earth and Mars

The D/H isotopic ratio is a primary constraint on the source of H₂O 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 H₂O. The best constraints for accreted H₂O are determined from non-atmospheric H₂O, 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 equilbrium with surface H₂O. Earth's VSMOW D/H ratio of 1.6x10^-4 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.Lunine, J. I. et al. (2003), [http://dx.doi.org/10.1016/S0019-1035(03)00172-6 doi 10.1016/S0019-1035(03)00172-6] ] 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 x(1.9+/-0.25) the VSMOW value. The higher D/H and impact modeling (significantly different than for 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 x1.6 and x1.2 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 0.6 - 2.7 km, consistent with a 50% outgassing efficiency to yield ~500 m WEG of surface water. Comparing the current atmospheric D/H ratio of x5.5 SMOW ratio with the primordial x1.6 SMOW ratio suggests that ~50 m 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:
##The higher D/H ratios in Martian meteorites could be a consequence of biased sampling since Mars may have never had an effective crustal recycling process
##Earth's Primitive Upper Mantle estimate of the ¹⁸⁷Os/¹⁸⁸Os isotopic ratio exceeds 0.129, significantly greater than that of carbonaceous chondrites, but similar to anhydrous ordinary chondrites. This makes it unlikely that planetary embryos compositionally similar to carbonaceous chondrites supplied water to Earth
##Earth's atmospheric content of Ne is significantly higher than would be expected had all the rare gases and H₂O been accreted from planetary embryos with carbonaceous chondritic compositions.

An alternative to the cometary and asteroidal delivery of H₂O 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 ~2 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 H₂O content) per terrestrial planet. Even though much of the nebular H₂O(g) may be lost due to the high temperature environment of the accretionary disk, it is possible for physisorption of H₂O on accreting grains to retain nearly 3 Earth hydrospheres of H₂O at 500 K temperatures. This adsorption model would effectively avoid the ¹⁸⁷Os/¹⁸⁸Os isotopic ratio disparity issue of distally-sourced H₂O. However, the current best estimate of the nebular D/H ratio spectroscopically estimated with Jovian and Saturnian atmospheric CH₄ is only 2.1x10^-5, 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 H₂O accretion for Earth in particular and the terrestrial planets in general.

Evolution of Mars's water inventory

The variation in Mars's surface water content is strongly coupled to the evolution of its atmosphere and may have been marked by several key stages.

Early Noachian (4.6 to 4.1 Ga) "phyllosian"Chaufray, J. Y. et al. (2007), [http://dx.doi.org/10.1029/2007JE002915 doi 10.1029/2007JE002915] ] eraAtmospheric loss to space from heavy meteoritic bombardment and hydrodynamic escape. Ejection by meteorites may have removed ~60% of the early atmosphere. Significant quantities of phyllosilicates may have formed during this period requiring a sufficiently dense to sustain surface water, as the spectrally dominant phyllosilicate group, smectite, suggests moderate water: rock ratios.Chevrier, V. et al. (2007), [http://dx.doi.org/10.1038/nature05961 doi 10.1038/nature05961] ] However, the pH-pCO₂ equilibria between smectite and carbonate show that the precipitation of smectite would constrain pCO₂ to a value not more than 10^-2 atm. As a result, the dominant component of a dense atmosphere on early Mars becomes uncertain if the clays formed in contact with the Martian atmosphere,Catling, D. C. (2007), [http://dx.doi.org/10.1038/448031a doi 10.1038/448031a] ] particularly given the lack of evidence for carbonate deposits. An additional complication is that the ~25% lower brightness of the young Sun would have required an ancient atmosphere with a significant greenhouse effect to raise surface temperatures to sustain liquid water. Higher CO₂ content alone would have been insufficient, as CO₂ precipitates at partial pressures exceeding 1.5 atm, reducing its effectiveness as a greenhouse gas.

Middle to late Noachian (4.1 to 3.8 Ga)Potential formation of a secondary atmosphere by outgassing dominated by the Tharsis volcanoes, including significant quantities of H₂O, CO₂, and SO2. Martian valley networks date to this period, indicating globally widespread and temporally sustained surface water as opposed to catastrophic floods. The end of this period coincides with the termination of the internal magnetic field and a spike in meteoritic bombardment. The cessation of the internal magnetic field and subsequent weakening of any local magnetic fields allowed unimpeded atmospheric stripping by the solar wind. For example, when compared with their terrestrial counterparts, ³⁸Ar/³⁶Ar, ¹⁵N/¹⁴N, and ¹³C/¹²C ratios of the Martian atmosphere are consistent with ~60% loss of Ar, N₂, and CO₂ by solar wind stripping of an upper atmosphere enriched in the lighter isotopes via Rayleigh fractionation. Supplementing the solar wind activity, impacts would have ejected atmospheric components in bulk without isotopic fractionation. Nevertheless, cometary impacts in particular may have contributed volatiles to the planet.

Hesperian to the present (the "theiikian" era from ~3.8 Ga to ~3.5 Ga and the "siderikian" era postdating ~3.5GaBibring, J-P. et al. (2006), [http://dx.doi.org/10.1126/science.1122659 doi 10.1126/science.1122659] ] )Atmospheric enhancement by sporadic outgassing events were countered by solar wind stripping of the atmosphere, albeit less intensely than by the young Sun. Catastrophic floods date to this period, favoring sudden subterranean release of volatiles, as opposed to sustained surface flows. While the earlier portion of this era may have been marked by aqueous acidic environments and Tharsis-centric groundwater dischargeAndrews-Hanna, J. C. et al. (2007), [http://dx.doi.org/10.1038/nature05594 doi 10.1038/nature05594] ] dating to the late Noachian, much of the surface alteration processes during the latter portion is marked by oxidative processes including the formation of Fe3+ oxides that impart a reddish hue to the Martian surface. Such oxidation of primary mineral phases can be achieved by low-pH (and possibly high temperature) processes related to the formation of palagonitic tephra,Morris, R. V. et al. (2001), [http://dx.doi.org/10.1029/2000JE001328 doi 10.1029/2000JE001328] ] by the action of H₂O2 that forms photochemically in the Martian atmosphere,Chevrier, V. et al. (2006), [http://dx.doi.org/10.1016/j.gca.2006.06.1368 doi 10.1016/j.gca.2006.06.1368] ] and by the action of water, none of which require free O2. The action of H₂O2 may have dominated temporally given the drastic reduction in aqueous and igneous activity in this recent era, making the observed Fe3+ oxides volumetrically small, though pervasive and spectrally dominant. Nevertheless, aquifers may have driven sustained but highly localized surface water in recent geologic history, as evident in the geomorphology of craters such as Mojave.McEwen, A. S. et al. (2007), [http://dx.doi.org/10.1126/science.1143987 doi 10.1126/science.1143987] ] Furthermore, the Lafayette Martian meteorite shows evidence of aqueous alteration as recently as 650 Ma.

References