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CO2-H2O Phase Equilibria: Residual Ice Layers and Basal Melting of the Martian Polar Ice Caps
| Content Provider | Semantic Scholar |
|---|---|
| Author | Longhi, J. L. |
| Copyright Year | 2004 |
| Abstract | Fig. 1 is an update of the P-T projection of the CO2H2O phase diagram [1] showing the approximate location of the solid-state breakdown of CO2 clathrate (H) to water ice (I) and solid CO2 (S). Because of extremely limited solubility of H2O in CO2 liquid and gas at low temperatures, binary equilibria 1 and 5 are virtually equivalent to the sublimation and boiling curves for pure CO2, respectively. Few solubility data are available with which to draw temperature composition diagrams at the low temperatures and pressures appropriate to Mars’ surface, but it is possible to combine the temperatures and stoichiometry of univariant equilibria in Fig. 1 with some observational data to construct approximate temperature-composition diagrams. For example, it is widely noted that ground ice is stable poleward of ~ 40° latitude and that night frosts occur even where no ice is present [2]. This implies that the martian atmosphere is on average saturated with water-ice (or close to it), and that the atmospheric composition (0.031 mole% H2O) and frost point (195° K) [2] can be employed to construct a T-X diagram for the average martian surface pressure of 5.6 mbar [3] as illustrated in Fig 2 . The diagram predicts that water ice will be the first phase to precipitate upon cooling, that another 40° cooling is needed before the precipitate changes to clathrate (hydrate), and that an additional 10° cooling is needed before solid-CO2 and clathrate precipitate together. Two other curves are also shown for the extremes of surface pressure (10 mb for the north polar regions and 2 mb for the south polar region and the higher portions of Tharsis). These curves are repeated in Fig. 3 where the ranges of measured atmospheric water contents [4] are also shown. For north polar conditions the atmospheric composition is almost always more H2O-rich than the gas-eutectic (heavy arrow). This means that pure water-ice is first to precipitate, but also last to sublimate (relative to clathrate and eutectic mixtures). At the south pole, however, the atmospheric composition lies to the CO2-rich side of the gas eutectic for much of the year. This means that pure solid-CO2 precipitates first and sublimates last. Thus water-ice will tend to form a residual layer in the north, whereas solid-CO2 is more likely in the south Basal melting has been suggested for both ice caps [5,6,7]. Fig 4 illustrates a schematic polybaric section along the temperature gradients calculated by [6]. Depths are given in km. Because ice is a much better conductor than solid-CO2 or clathrate [6], greater thicknesses are possible for water-rich ice caps before basal melting ensues. It is believed that in periods of low obliquity the polar ice caps tend to thicken with the addition of solid-CO2 [8]. Because the solid-CO2 and clathrate are much better insulators than water ice, the thermal gradient would have increased as a CO2-rich ice cap thickened until melting began. The diagram predicts that mixtures of solid-CO2 and clathrate melt at much lower temperature than water ice + clathrate. Therefore, generation of CO2-rich melts is to be expected during low obliquity cycles, and because liquid CO2 is denser than water or water ice, it is likely to percolate into the crust Temperature increases more rapidly along calculated temperaturedepth profiles [5] than along the melting curve, so liquid CO2 will be stable at depth. This process, which sequesters CO2 in the crust, may be a major agent of atmospheric evolution. Migrating subsurface liquid-CO2 will flow below and equilibrate with subsurface water, and on those occasions when water rises to the surface, CO2 rising behind the water will change to gas first and act as a propellant. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.lpi.usra.edu/meetings/lpsc2004/pdf/1857.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
