Lunar and Planetary Science XXXV (2004) 1857.pdf

CO2-H2O PHASE EQUILIBRIA: RESIDUAL ICE LAYERS AND BASAL MELTING OF THE MARTIAN POLAR ICE CAPS. J. Longhi , Lamont-Doherty Earth Observatory, Palisades, NY 10964 ([email protected])

Shifts in the gas+ice saturation surfaces with pressure CO2-rich side of the gas eutectic for much of the favor formation of residual layers of solid CO2 at the year. This means that pure solid-CO2 precipitates martian south pole and water-ice at the north pole. first and sublimates last. Thus water-ice will tend to Preferential melting of CO2 ice and clathrate layers form a residual layer in the north, whereas solid-CO2 within polar ice caps during periods of low obliquity is more likely in the south may lead to sequestration of liquid CO2 in the Basal melting has been suggested for both Martian crust. ice caps [5,6,7]. Fig 4 illustrates a schematic polybaric section along the temperature gradients Fig. 1 is an update of the P-T projection of the CO2- calculated by [6]. Depths are given in km. Because H2O phase diagram [1] showing the approximate ice is a much better conductor than solid-CO2 or location of the solid-state breakdown of CO2 clathrate [6], greater thicknesses are possible for clathrate (H) to water ice (I) and solid CO2 (S). water-rich ice caps before basal melting ensues. It is Because of extremely limited solubility of H2O in believed that in periods of low obliquity the polar ice CO2 liquid and gas at low temperatures, binary caps tend to thicken with the addition of solid-CO2 equilibria 1 and 5 are virtually equivalent to the [8]. Because the solid-CO2 and clathrate are much sublimation and boiling curves for pure CO2, better insulators than water ice, the thermal gradient respectively. would have increased as a CO2-rich ice cap Few solubility data are available with which thickened until melting began. The diagram predicts to draw temperature composition diagrams at the low that mixtures of solid-CO2 and clathrate melt at temperatures and pressures appropriate to Mars’ much lower temperature than water ice + clathrate. surface, but it is possible to combine the temperatures Therefore, generation of CO2-rich melts is to be and stoichiometry of univariant equilibria in Fig. 1 expected during low obliquity cycles, and because with some observational data to construct liquid CO2 is denser than water or water ice, it is approximate temperature-composition diagrams. likely to percolate into the crust Temperature For example, it is widely noted that ground increases more rapidly along calculated temperature- ice is stable poleward of ~ 40° latitude and that night depth profiles [5] than along the melting curve, so frosts occur even where no ice is present [2]. This liquid CO2 will be stable at depth. This process, implies that the martian atmosphere is on average which sequesters CO2 in the crust, may be a major saturated with water-ice (or close to it), and that the agent of atmospheric evolution. Migrating subsurface atmospheric composition (0.031 mole% H2O) and liquid-CO2 will flow below and equilibrate with frost point (195° K) [2] can be employed to construct subsurface water, and on those occasions when water a T-X diagram for the average martian surface rises to the surface, CO2 rising behind the water will pressure of 5.6 mbar [3] as illustrated in Fig 2 . The change to gas first and act as a propellant. diagram predicts that water ice will be the first phase to precipitate upon cooling, that another 40° cooling REFERENCES: [1] Longhi J. (2004) submitted to is needed before the precipitate changes to clathrate Geochim. Cosmochim. Acta. [2] Carr M.H. (1996) (hydrate), and that an additional 10° cooling is Water on Mars. Oxford Univ. Press, New York, 229 needed before solid-CO2 and clathrate precipitate pp. [3] Zureck R.W., Barnes J.R., Haeberle R.M., together. Two other curves are also shown for the Pollack J.B., Tillman J.E., and Leovy C.B. (1992) in extremes of surface pressure (10 mb for the north MARS,H. H. Kieffer, B. M. Jakosky, C. W. Snyder, polar regions and 2 mb for the south polar region and and M. S. Matthews eds., pp. 835-933, Univ. of the higher portions of Tharsis). These curves are Arizona Press, Tuscon. [4] Jakowsky B.M and repeated in Fig. 3 where the ranges of measured Haberle R.M. (1992) in MARS, pp. 969-1016. [5] atmospheric water contents [4] are also shown. For Clifford S.M. (1993) J. Geophys. Res. 98, 10,973- north polar conditions the atmospheric composition is 11,016. [6] Mellon M.T. (1996) Icarus 124, 268-279. almost always more H2O-rich than the gas-eutectic [7] Kargel J.S. and Lunine J.I. (1998) in Solar System (heavy arrow). This means that pure water-ice is first Ices, B. Schmitt et al. (eds), Kluwer Academic to precipitate, but also last to sublimate (relative to Publishers. [8] Kieffer H.H. and Zent A.P. in MARS, clathrate and eutectic mixtures). At the south pole, pp. 1180-1218. however, the atmospheric composition lies to the Lunar and Planetary Science XXXV (2004) 1857.pdf

400 South Pole 10000 a 350 ice II (winter) (summer)

? 1000 ? avg. Mars Atm W 300 S = Solid CO2 H Mars Surface gas L = Liq - CO 2 S L I ? T °K H LW avg. P = 5.6 mbar 100 W = Liq - H2O H solid L I = Ice liq L c.e.p. 250 H = clathrate GW {SI} South Pole G = gas H CO GH GW P bars 2 {SHI} P ~ 2 mbar 10 {SL} 200 {WI} gas t.p. gas + ice ? hydrate + ice ? 1 S H G W 150 gas + hydrate G gas I H solid CO2 + gas GI solid CO2 + ice S 0.1 I H water 0.00001 0.0001 0.001 0.01 0.1 1.0 H O ice CO2 2 mol fraction H O CO2•5.75H2O 0.01 t.p. solid CO2 + hydrate 2 H2O gas 400 North Pole {LW} 0.001 S I 100 150 200 250 300 350 350 (winter) (summer) G T °K ! avg. 300 Mars Atm North Pole Fig. 1.Low-temperature portion of CO2- P ~ 10 mbar T °K gas H2O phase diagram after [1]. Proposed 250 binary invariant points indicated by open Mars Surface avg. P = 5.6 mbar 200 gas + ice circles. Position of low-T stability limit of gas gas + hydrate hydrate + ice clathrate (H = S + Ice) is calculated. 150 solid CO2 + hydrate solid CO2 + gas solid CO2 + ice

0.00001 0.0001 0.001 0.01 0.1 1.0 CO H2O 2 CO •5.75H O 400 mol fraction H2O 2 2 Mars Surface P = 5.6 mbar Fig. 3 T-X sections at pressures appropriate 350 G = gas I = ice (H2O) H = hydrate S = ice (CO2) to south and north polar regions.

300 Mars Atm P (North Pole) T °K G

250 P (South Pole) L Mars Polar Profile Frost Point 200 40 (south/north) G G + I W G + H H + I L + W 150 90 W 2 / 10 km S + I S + H T, P L + H + W I 0.001 0.01 0.1 1 10 100 L + S CO CO •6H O H O 2 wt % H2O 2 2 2 L + H 1 / 6 km H Fig. 2 Schematic isobaric sections based on + I frost point and atmospheric composition. S + G S + H

G G + H 0.01 0.1 1 10 100 CO CO •6H O H O 2 % H2O 2 2 2

Fig. 4 Schematic profile along polar P-T profiles after [6]. Numbers are depths in km along thermal profiles calculated by [6]: S = solid-CO2.