Evolution of water on Mars and Earth
Encyclopedia
The evolution of water
(H2O) on either planet needs be understood in the context of the other terrestrial planetary bodies and their current water status.
Water (H2O) Inventory of 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 1.6x106 km3 of water at the southern polar region with Water Equivalent to a Global layer (WEG) 11 meters deep. 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. Geomorphic evidence favors significantly larger quantities of surface water
over geologic history, with WEG as deep as 500 m. The current atmospheric reservoir of water, though important as a conduit, is insignificant in volume with the WEG no more than 10 µm. Since the typical surface pressure
of the current atmosphere (~6 hPa ) 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.
) of 1.6x10-4, 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. 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.
s 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. Therefore, the bulk inventory of H2O on Earth can be conservatively estimated as 0.04% of Earth's mass (~6x1024 kg).
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 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:
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.
Early Noachian (4.6 to 4.1 Ga) "phyllosian" era
Atmospheric 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. 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, 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 pressure
s 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 ~3.5Ga )
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 discharge 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, by the action of H2O2 that forms photochemically in the Martian atmosphere, 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. Furthermore, the Lafayette Martian meteorite shows evidence of aqueous alteration as recently as 650 Ma.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
(H2O) on either planet needs be understood in the context of the other terrestrial planetary bodies and their current water status.
Water (H2O) Inventory of MarsMarsMars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
A significant amount of surface hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
has been observed globally by the Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
Odyssey GRS. Stoichiometrically estimated water mass
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
fractions indicate that - when free of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
- the near surface at the poles consists almost entirely of water covered by a thin veneer of fine material. This is reinforced by MARSIS
MARSIS
MARSIS is a low frequency, pulse-limited radar sounder and altimeter used on the ESA Mars Express mission...
observations, with an estimated 1.6x106 km3 of water at the southern polar region with Water Equivalent to a Global layer (WEG) 11 meters deep. 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. Geomorphic evidence favors significantly larger quantities of surface water
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
over geologic history, with WEG as deep as 500 m. The current atmospheric reservoir of water, though important as a conduit, is insignificant in volume with the WEG no more than 10 µm. Since the typical surface pressure
Surface pressure
Surface pressure is the atmospheric pressure at a location on Earth's surface. It is directly proportional to the mass of air over that location....
of the current atmosphere (~6 hPa ) 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
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
of most brines. For comparison, the highest diurnal surface temperatures at the two MER sites have been ~290 K.
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 (VSMOWVSMOW
Vienna Standard Mean Ocean Water is a water standard defining the isotopic composition of water. It was promulgated by the International Atomic Energy Agency in 1968....
) of 1.6x10-4, 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. 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.
H2O Inventories of Mercury, Moon, and Earth
Recent observation made by a number of spacecrafts 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 ~1.46x1021 kg of H2O and sedimentary rockSedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s 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. 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 beltAsteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
beyond 2.5 AU. Mars's original D/H ratio, as estimated by deconvolving the atmospheric and magmatic D/H components in Martian meteorites
Mars meteorite
A martian meteorite is a rock that formed on the planet Mars, was ejected from Mars by the impact of an asteroid or comet, and landed on the Earth. Of over 53000 meteorites that have been found on Earth, 99 are martian...
(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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
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
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
. This would be consistent with the thermodynamic estimate of ~2 earth masses of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
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" era
Atmospheric 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. 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, particularly given the lack of evidence for carbonate deposits
Carbonates on Mars
Evidence for carbonates on Mars has remained elusive until recently. For example, most remote sensing instruments such as OMEGA and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, had not suggested the presence of carbonate outcrops at 100 m or coarser spatial...
. An additional complication is that the ~25% lower brightness of the young Sun would have required an ancient atmosphere with a significant greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
to raise surface temperatures to sustain liquid water. Higher CO2 content alone would have been insufficient, as CO2 precipitates at partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
s exceeding 1.5 atm, reducing its effectiveness as a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
.
Middle to late Noachian (4.1 to 3.8 Ga)
Potential formation of a secondary atmosphere
Secondary atmosphere
A secondary atmosphere is an atmosphere of a planet that did not form by accretion during the formation of the planet's sun. A secondary atmosphere instead forms from internal volcanic activity, or by accumulation of material from comet impacts...
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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
and a spike in meteoritic bombardment. The cessation of the internal magnetic field and subsequent weakening of any local magnetic fields
Magnetic field (disambiguation)
A magnetic field is the physical phenomenon produced by moving electric charges and exhibited by ferrous materials.Magnetic field or magnetic fields may also refer to:-Science:* Earth's magnetic field* A stellar magnetic field...
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 ~3.5Ga )
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 discharge 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, by the action of H2O2 that forms photochemically in the Martian atmosphere, 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. Furthermore, the Lafayette Martian meteorite shows evidence of aqueous alteration as recently as 650 Ma.