Lunar and Planetary Science XXXI 1276.pdf

THE FATE OF SALT ON MARS : GLOBAL EVIDENCE FOR A NORTHWARD OPEN SALT CYCLE R. D. Lorenz and R. A. Beyer, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0092, USA ([email protected]).

Introduction: The general topographic gradient be- ducting a single deposition-dessication cycle, it is more tween the south and north, and the likelihood that likely that the water were cycled through the rocks of oceans on the Martian surface would have been ice- the south several times, leaching out soluble comp o- covered, suggests that the ancient Martian hydrologi- nents each time. Thus the 60m does not indicate an cal cycle had a largely noncycled involatile component upper limit. – salt. It seems likely that massive salt deposits may It is of course likely, especially in such a flood- exist in the low-lying regions of the north and in the ing/dessication cycle, that minerals would be deposited floor of Valles Marineris. in less extensive, but thicker, deposits corresponding Paradigm: Clifford and Parker [1,2] have con- to topographic lows in the surface/groundwater sys- structed a model of Martian hydrology. In particular, tem. they suggest that as the planet cooled, the cryosphere Evidence: Remote sensing evidence exists for would propagate downwards, sealing the aquifer in the evaporite deposits in general (e.g. the ‘White Rock’ [7]) south and freezing over any ocean. The ice-covered and more recently for possible salt domes themselves ocean (for which abundant – albeit largely circumstan- in the floor of Valles Marineris (Beyer et al., this con- tial - evidence now exists [3]) would slowly dry out, the ference) Here we discuss various effects that might be ice being sublimed and deposited on the ice caps. Basal observed on large scales. melting in the south polar cap would replenish Control of Subsurface Properties. Massive salt groundwater there, which would tend to flow north- deposits on Mars might exhibit mechanical properties ward owing to the topographic gradient [4], and could different from regular terrain; and these properties occasionally burst through the cryosphere, emptying might be manifested in impact crater morphology, for the aquifer in catastrophic outflows. example. Garvin et al. [8] report that a change in crater Groundwater would tend to leach soluble minerals morphology, corresponding to a change in target me- from the rocks of the south, and deposit them in the chanical properties, occurs below contact II. Although north. The situation may be compared with Joly’s [5] this may be due to ground ice, cementation by model of the Earth and his attempt to estimate the age evaporite minerals is another possible factor. of the Earth by reconciling the salt flux in rivers with Thermal Inertia. Thermal inertia measurements the salt inventory in the ocean. That attempt failed be- (e.g. [9]) indicate particularly high values in the north- cause of two recycling processes – advection of sea- ern plains, and indeed in the floor of Vallis Marineris salt aerosol, and subduction by plate tectonics. How- and Acidalia Planitia into which it ‘drains’. Part of the ever, on Mars, these two processes may not have been story here may be larger particle size, or rock abun- active – first aerosol production would be suppressed dances, and in part that the low altitude itself modifies on a frozen ocean, and there is no evidence for plate thermal inertia via the gas pressure effect on thermal tectonics on Mars (magentic stripes notwithstanding). conductivity. However, enhanced cementation due to Thus salt transport would have been unidirectional, the deposition of soluble minerals may be an alternative and the north may bear massive salt and brine deposits. possibility. Higher spatial resolution thermal inertia There may be some global-scale signature of this proc- measurements are presently underway: correlation of ess that is available to remote sensing – in part by de- high inertia regions with features described elsewhere pletion of the south, but largely by deposition in the here would be consistent with the salt hypothesis. north. We use the term ‘salt’, although note that the Gravity. Massive salt deposits on Mars would, as dominant components are likely to be sodium and mag- on earth, be associated with negative gravity anoma- nesium sulfates [6]. lies. It is of considerable interest that the floor of Vallis Deposit Volume: Typical seas on Earth contain Marineris, and several ‘streams’ flowing northward about 3% salt by mass, although subterranean brines from it across Vastitis Borealis, show negative gravity can have a mineral content several times larger – e.g. a anomalies [M. T. Zuber, presentation at 1999 Fall AGU]. New Mexico brine cited by Clark and Van Hart [6] has One explanation offered is that these features are asso- nearly 30% salts by mass. If the ocean had a volume of ciated with ice or loose rubble : we suggest here that 1.5x107 km3, (filling to the Parker ‘Contact II’ surface, salt deposits instead are an equally likely scenario. with a mean depth of 560m) then the salt deposits could Consolidated evaporite mineral deposits would have a be ~60m thick, on average. However – rather than con- density of about 1.6 kgm-3 – still allowing a substantial Lunar and Planetary Science XXXI 1276.pdf

SALT ON MARS : R. D. Lorenz and R. A. Beyer

contrast against even fractured bedrock. A large-scale preserved in evaporite layers, as would possible micro- negative gravity anomaly beneath the northern plains fossils. as a whole is also consistent with the hypothesis of The impermeability of massive salt deposits leads to massive salt beds. several applications on Earth. One is the storage of Surface Expressions. The buoyancy of buried salt hydrocarbon fuels. Another application is the storage layers leads to the formation of diapirs and salt domes of air at modest overpressure for off-peak energy stor- (and, indeed, where diapirs breach the surface under age: air is pumped in at night when demand for energy the right conditions, salt ‘glaciers’ [10].) Although the is low, and is used to drive turbines when demand low temperatures and the scarcity of liquid water may peaks. In the long term, the existence of salt deposits conspire to reduce the mobility of salt material on Mars, on Mars – and the availability of large, easily sealed such features may nevertheless be present on the Mar- volumes which could be filled with locally- tian surface (see the companion paper by Beyer et al.), manufactured atmospheric or propellant gasses - might especially if mobilized by ‘high’ water abundances offer important opportunities in the industrialization and/or geothermal heat flow. and colonization of Mars. Additionally, the manufac- We may also consider other features, often taken to ture of optically-active gases such as chlorofluorocar- be indicative of ground ice, as possibly representing bons for planetary engineering [12] would probably evaporites instead – dessication polygons and pat- require access to economic concentrations of chloride terned ground are examples. As discussed in [3], these minerals. We suggest that the whorl features seen by features appear to be associated with the depressions Beyer et al. in Candor Chasma, and the gravity and in Acidalia and Utopia, where evaporite minerals would thermal inertia data to hand, suggest that such eco- be expected to collect.. nomic concentrations may exist. Elemental Abundances. No significant differences References: between elemental abundances is evident in the data [1] Clifford, S. M., J. Geophys. Res. 98, 10973 (1999) [2] available from the three landers. However, these span Clifford S. M. & Parker, T. J., 5th Mars Conference, only a modest latitude range and accessed only the top Pasadena, July 1999. [3] Head, J. W. III et al., Science few millimeters of soil, which is likely to be windblown 286, 2134, (1999). [4] Smith, D. E. et al, Science, 284, 1495 and thus more-or-less homogenous. The Gamma-Ray [5] Joly, J. Trans. Roy. Dublin. Soc., 1899 [6] Clark, B. C. Spectrometer on the 2001 orbiter mission will be sensi- and Van Hart, D. C., Icarus, 45, 370 (1981) [7] Williams, tive down to depths of ~50cm, and so may be able to S. H. and Zimbelman, J. R. Geology, 22, 107 (1994) detect concentrations of elements (Na, Mg, Cl, S) asso- Garvin, J. et al., EOS (80,suppl) F610 (1999) [9] Chris- ciated with soluble minerals, assuming aeolian proc- tensen, P. H. and Moore, H. J., in Mars, UA Press esses have not homogenized the surface to this depth. (1992) [10] Talbot, C. J. and Jarvis, R. J., J. Structural Correlation of concentrations of these minerals with Geology, 6, 521 (1984) [11] Lorenz, Icarus, in press Vallis Marineris, the negative gravity anomalies (2000)[12] McKay et al., Nature, 352, 489 (1991) emerging from it, and/or the basin floors of Acidalia and Utopia, would be consistent with our salt deposit hypothesis. Clearly, unlike the gravity and morphologi- cal data, such observations cannot be explained by ground ice. Ground-Penetrating Radar on future missions might shed light on subsurface salt concentrations, although interpretation is likely to be difficult given the unknown subsurface moisture abundance and the widespread existence of magnetic minerals which would be radar-absorbing . Implications The confirmation of massive salt deposits in the lo- cations discussed in this paper would lend support to the hydrological cycle proposed by Clifford and others. Study of the evaporite layers by drilling would be a valuable tool in understanding the hydrological and climatological history of Mars : interbedded betwen evaporite layers might be volcanic and aeolian material, as well as microtektites [11], which would be well-