Hydrated states of MgSO4 at equatorial latitudes on Mars



[1] The stability of water ice, epsomite, and hexahydrite to loss of H2O molecules to the atmosphere at equatorial latitudes of Mars was studied to determine their potential contributions to the measured abundance of water-equivalent hydrogen (WEH). Calculation of the relative humidity based on estimates of yearly averages of water-vapor pressures and temperatures at the Martian surface was used for this purpose. Water ice was found to be sufficiently unstable everywhere within 45° of the equator that if the observed WEH is due to water ice, it requires a low-permeability cover layer near the surface to isolate the water ice below from the atmosphere above. In contrast, epsomite or hexahydrite may be stable in many near-equatorial locations where significant amounts of WEH are observed.

1. Introduction

[2] Significant deposits of hydrogen have been observed within large areas near the equator of Mars [Feldman et al., 2004]. The molecular associations of this hydrogen are not known. Several possibilities have been suggested: 1) buried water ice [Clifford, 1998], 2) water molecules adsorbed on the surfaces of soil grains [Tokano, 2003; Möhlmann, 2004], and 3) structural OH and interlayer H2O in clays, 4) water associated with extra-framework cations in zeolites, 5) structural OH and H2O in salt hydrates [Clark, 1978; Zolotov, 1989; Basilevsky et al., 2003; Bish et al., 2003a, 2003b; Fialips et al., 2004; Vaniman et al., 2004].

[3] In order to sort through these possibilities we examine the stability of various hydrated MgSO4 phases. MgSO4 was chosen because the typical mass percent of anhydrous MgSO4 in Martian soil at the Viking and Pathfinder landing sites is estimated to be about 10% [Wänke et al., 2001] and because MgSO4 is predicted to be a weathering product on Mars [Goodling et al., 1992]. In addition, MgSO4 and other potentially water-bearing salts have been identified in abundances up to 30% by mass recently at the MER-B landing site in Meridiani Planum [JPL, 2004 (available at http://www.jpl.nasa.gov/mer2004/rover-images/mar-02-2004/images-3-2-04.html)]. On Earth, MgSO4 has three common hydration states; kieserite (MgSO4.1H2O), hexahydrite (MgSO4.6H2O) and epsomite (MgSO4.7H2O). Where these minerals are stable, the water mass fraction in a soil with 10% by mass anhydrous MgSO4 concentration will be about 1.5% for kieserite, 8.3% for hexahydrite, and 9.5% for epsomite, comparable to the range of water-equivalent-hydrogen (WEH) abundances that have been observed near the equator of Mars [Feldman et al., 2004].

[4] Our analysis builds on the physical structure of surface soils assembled from mid-latitude maps of: 1) albedo and thermal inertia determined from visible and IR images measured using the Infrared Thermal Mapper [Paige and Keegan, 1994; Mellon et al., 2002] and the Thermal Emission Spectrometer [Christensen et al., 2001; Mellon et al., 2002], 2) surface elevation measured using the Mars Orbiter Laser Altimeter (MOLA) experiment [Smith et al., 1999], 3) mean annual surface temperature determined primarily from an energy balance between insolation, energy loss to thermal radiation, and subsurface conduction, calculated using maps of thermal inertia and albedo [Mellon et al., 2004], 4) water vapor density near the soil surface calculated using the MOLA-determined topography and under the assumption of a mean annual column mass vapor density of 20 precipitable microns (pr μm) at zero-km altitude [Mellon et al., 2004], and 5) the WEH abundance modeled from epithermal neutron data measured using the Mars Odyssey Neutron Spectrometer [Feldman et al., 2004; Prettyman et al., 2004].

2. The Observed Distribution of Water-Equivalent Hydrogen on Mars

[5] The WEH maps used here represent a lower limit of the hydrogen abundance in terms of an equivalent amount of water, assuming a homogeneous depth distribution of hydrogen-bearing material within the upper-most meter of Mars. A map in cylindrical projection derived from epithermal neutron data between ±60° latitude [Feldman et al., 2004] is shown in the upper panel of Figure 1. The contours correspond to WEH mass percentages of 3%, 5.5%, and 8%. Separate hydrogen reservoirs are seen to maximize near Arabia Terra (centered at −5°N, +25°E) and Medusae Fossae (centered at −15°N, +180°E) at about 9.5 ± 1.5% WEH by mass. These maxima represent extended equatorial reservoirs that connect to the northern high-latitude reservoir along the western margin of Tharsis and through Elysium Mons [Feldman et al., 2004]. Relative minima amounting to between 2% and 3% WEH by mass occur in isolated patches that overlap Solis Planum, Argyre basin, Hellas basin, Noachus-Hellespontus, Isidis Planitia, Utopia Planitia, northern Acidalia, Echus Chasma, Chryse Planitia, and Cerberus.

Figure 1.

From top to bottom, maps of WEH, and the stability indices of water ice-water vapor, epsomite-kieserite, and epsomite-hexahydrite. The contours in the WEH map correspond to 3% (enclosing the black color), 5.5% and 8% (enclosing the orange color) WEH by mass. These contours are included as white lines in the other three panels. The stability boundaries in these lower three panels are given by the 1.0 contour drawn between the light blue and yellow colors as a thick pink line.

3. Index of Stability

[6] Using the physical properties and conditions of Martian soils as input, we have developed maps of a stability index, SI, given by the ratio of mean-annual water-vapor pressure, Pa, near the Martian surface to the equilibrium pressure, Ps, of water as ice or bound within particular hydrated minerals at the mean annual temperature, Ta [Mellon et al., 2004],

equation image

Where the relevant SI is >1, water ice and/or the hydrated states of MgSO4 are stable with respect to the relative humidity of the atmosphere. Where SI is <1, they are unstable and their water molecules will be lost to the atmosphere. The magnitude of the stability index provides a quantitative measure of the certainty with which we can conclude that water ice and/or the hydrated states of MgSO4 are either stable or unstable, and the qualitative rate at which these deposits either gain or lose their H2O loads, respectively.

[7] There is a current dearth of information regarding the stability of hydrated minerals at temperatures and water-vapor pressures that are relevant to the near-equatorial zone of Mars. Best known is the Clausius-Clapeyron relation giving:

equation image

Here Po(Ice) = 3.47 × 1012 Pa, the change in enthalpy required to vaporize one mole of ice to water vapor, ΔH(Ice-Vapor) = 50.87 kJ/mol [Möhlmann, 2004], and R = 8.3147 J/mol/K, the gas constant.

[8] Similar stability boundaries in the form of equation (2) for the various hydrated states of MgSO4 can be developed from recent laboratory work [Chou and Seal, 2003], although significant extrapolation to Martian conditions is necessary. According to this work, hexahydrite may only be metastable at temperatures below 284 K, and therefore the only equilibrium relation below this temperature would be between epsomite and kieserite. However, preliminary work [Vaniman et al., 2004a] shows that hexahydrite may be the dominant hydration state at Martian conditions even though it may only be metastable. We have therefore generated maps of stability indices based on tabulated or plotted information from Chou and Seal [2003] for both the epsomite-kieserite (E-K) equilibrium (Po(E-K) = 4.38 × 1012 Pa,_ΔH(E-K) = 54.05 kJ/mol), and the epsomite-hexahydrite (E-H) equilibrium (Po(E-H) = 4.85 × 1013 Pa,_ΔH(E-H) = 59.72 kJ/mol) using equation (2). We note that a hydrated MgSO4 is stable relative to anhydrous MgSO4 under all martian surface conditions.

[9] The map of lower-limit estimates of WEH is compared in the bottom three panels of Figure 1 with the stability-index maps for water ice, and the (E-K) and (E-H) equilibria. We note in the ice panel that water ice is unstable to sublimation everywhere within about 45° of the equator (the boundary between stability and instability is shown in all panels by the pink line threading the light purple color). This result is consistent with that found previously [Farmer and Doms, 1979; Clifford and Hillel, 1983; Fanale et al., 1986; Mellon and Jakosky, 1993]. Comparison of the 3%, 5.5%, and 8% WEH contours with the stability boundary in the ice panel shows a complete lack of correlation. Water ice is therefore an unlikely candidate for contributing to the equatorial reservoir of hydrogen if the water ice is in moderate diffusive contact with the atmosphere, and if the recharge of this reservoir comes only through equilibration with the atmosphere.

[10] Comparison of the same WEH abundance contours with the E-K stability boundary in the next panel down shows a better correlation than seen for water ice. Regions where epsomite is stable relative to kieserite overlap well regions where WEH is high, such as in the northern portions of Arabia Terra and of the east-west lane of high WEH centered at −10°N and +180°E. However, many regions also exist where WEH is relatively high but epsomite is unstable, such as just south of Arabia Terra and in the central and southern portions of the east-west lane at (−10°N, +180°E). In addition there are other regions where epsomite is stable yet WEH is relatively low, such as in the southern portions of Argyre, Hellas, and Amazonis Planitia.

[11] A similar comparison between contours of WEH abundances and the stability boundaries in the E-H stability map shows no net improvement in correlation over that for E-K. Whereas better agreement is obtained in regions where WEH is high and epsomite is stable (or is at least metastable) relative to hexahydrite, poorer agreement is obtained where WEH is low yet epsomite appears to be stable.

4. Discussion and Conclusions

[12] A comparison of the domains of stability of water ice and three of the hydration states of MgSO4 with lower-limit near-equatorial WEH abundances shows that water ice is unlikely to contribute significantly if our understanding of the physical structure and elemental composition of surface soils is correct, and if recharge of the observed WEH reservoir comes through equilibration with the atmosphere. However, our analysis shows that both epsomite and hexahydrite could contribute significantly if MgSO4 is sufficiently abundant. Where MgSO4 is present but epsomite and hexahydrite are not stable, kieserite is stable and could therefore contribute to the WEH inventory. This last possibility is consistent with the fact that a contribution of 10% anhydrous MgSO4 as kieserite to WEH (1.5%), when added to the contribution of adsorbed water (∼0.5% [Möhlmann, 2004]) on regolith grains, can account entirely for the minimum abundance of WEH observed at equatorial latitudes (∼2% [Feldman et al., 2004]). However, this may be fortuitous given that amorphous MgSO4.nH2O (D. T. Vaniman et al., manuscript in preparation, 2004), jarosite [JPL, 2004], gypsum, and other hydrous phases are likely to be present and may account for the same amount of water with or without any contribution from kieserite.

[13] However, the overall poor correlation between the E-H stability boundaries with measured WEH abundances seen in Figure 1 shows that other factors must also contribute. For example, hydrated and hydroxylated states of other minerals such as jarosite and gypsum [JPL, 2004] and clays and zeolites may contribute importantly to the WEH inventory [Clark, 1978; Basilevsky et al., 2003; Tokano, 2003; Bish et al., 2003a; Fialips et al., 2004]. Additionally, the abundance of MgSO4 may vary considerably with geographic location near the equator. It is also possible that most of the MgSO4 present is amorphous, with a poorly defined hydration state between epsomite and kieserite [Bish et al., 2003b]. Another factor is the possible redistribution of water-vapor density at ground level caused by zonal and Hadley cell-driven winds, a factor not included in our estimated stability boundaries. A last possibility is that the mean annual subsurface vapor pressures and temperatures estimated from assumptions of the structure of near surface soils (parameterized by their albedo, thermal inertia, and gaseous permeability [Mellon and Jakosky, 1993, 1995; Clifford, 1998; Mellon et al., 2004]), may not be applicable over the depths (of order 0.5 m) that contain the reservoir of WEH observed at all geographic locations.

[14] Changes needed in Pa at constant Ta, that are required to attain marginal stability for water ice, epsomite, and hexahydrite, (given by SI = 1) can be derived by partial differentiation of equation (2) with respect to Pa, to give,

equation image

Changes needed in Ta (ΔTa), can be estimated by inverting equation (1) using equation (2) for a fixed Pa. Maps of ΔPa/Pa (for a constant temperature) and ΔTa (for a constant water-vapor pressure) that satisfy these criteria for ice, E-K, and E-H show that to achieve stability in the southern margins of Arabia Terra and Medusae Fossae, where significant amounts of WEH are observed, requires an increase in water-vapor pressure by factors that are higher than 6, and decreases in temperatures by more than 15 K. Both requirements are sufficiently large as to make it unlikely that the deposits of WEH seen near Arabia Terra and Medusae Fossae stem from stable water ice if all assumptions made to create these maps are valid. For example, the minimum factor of 6 for the partial pressure of water vapor in the atmosphere would then require a yearly average of 6 × 20 + 20 = 140 pr μm of water vapor column thickness at zero km altitude. According to Figure 2 of Mellon and Jakosky [1995], this vapor pressure would, in turn, require an obliquity of about 30°, a condition that has not been obtained during the last 0.5 Myr. Our conclusion is reinforced by observation of the top two panels of Figure 1, which show that water ice is most unstable at equatorial locations other than Arabia Terra and Medusae Fossae.

[15] In contrast, epsomite appears to be either stable or close to stability at all locations where WEH is abundant at equatorial latitudes. This fact is especially true for the E-H boundary. Although the Chou and Seal [2003] study suggests that the E-H boundary may be metastable at martian surface temperatures, the location of the three-phase equilibrium among epsomite, hexahydrite, and kieserite has not been well established experimentally. It is also reasonable to believe that hexahydrite may readily form and persist during the decomposition of epsomite toward kieserite through the loss of a single water molecule as suggested by the observations of Vaniman et al. [2004]. Epsomite is stable relative to hexahydrite everywhere in Medusae Fossae and is marginally unstable only in a small longitudinal zone at about −10°S in Arabia Terra. Indeed, calculated values of ΔPa/Pa and ΔTa indicate that it only takes an increase in water-vapor pressure of about 5 pr μm and/or a temperature reduction of about 2°K to achieve stability throughout the high WEH reservoir near Arabia Terra.

[16] Of course, this result does not rule out the very likely possibility that clays, zeolites, and other hydrous salts such as gypsum and jarosite may also contribute to the observed reservoir of WEH. In this regard we note the evidence for Cl associated with S, which greatly broadens the range of possible salt hydrates. We are also investigating the possibility that MgSO4 on Mars may be amorphous with a hydration state intermediate between hexahydrite and kieserite [Bish et al., 2003b]. Preliminary studies of the properties of admixed clays, zeolites, and MgSO4 salts at martian surface conditions indicate both durable cementation and complex interactions with formation of gypsum as well as hydrated MgSO4 [Vaniman et al., 2004]. In addition, our results do not preclude the possibility that deposits of water ice are present more than about 0.5 m below the surface because orbital neutron spectroscopy cannot uniquely quantify the mass percent of WEH at these depths, or if these deposits are not in moderate diffusive contact with the atmosphere, as suggested by Clifford [1998]. If correct, then the recharge mechanism of these deposits must be through an aquifer, as first suggested by Clifford [1993], or from special circumstances associated with previous ice ages at equatorial latitudes that are accompanied by the deposit of a low-permeable cover that isolates the water ice deposit below, from the atmosphere above.


[17] We wish to thank B. Jakosky for useful suggestions regarding interpretation of the association of low WEH abundances with the initiation of regional dust storms. Partial support of this work was provided by both the DOE (through laboratory directed research and development) and by NASA, and was conducted under the auspices of the DOE.