Gases in the Martian atmosphere obey Dalton’s Law of Partial Pressures, but they are not ideal gases as Dalton defined them: water vapor slowly diffuses through the Martian atmosphere.
While there is abundant evidence that large quantities of liquid water existed and flowed on Mars eons ago, water on the surface of Mars has been seen by orbiting spacecraft, landers and rovers only in solid form, as surface frost, snow and polar ice. However, radar data indicate that subsurface permafrost, with possible subsurface liquid lakes, contain the considerable bulk of water remaining on the planet. Mars is generally reported as bone-dry, with the low atmospheric pressure and sub-freezing temperatures cited as prohibiting liquid water at the planet’s surface.
Water vapor in the Martian atmosphere was measured by the Viking Orbiter Mars Atmospheric Water Detection (MAWD) Experiment. Observations from periapsis altitude of 1,500 km revealed the global distribution of the water vapor content through the full atmospheric column to range between 10 to 100 precipitable m m. Prospects for liquid surface water seemed very bleak.
As the temperature rises under a saturated surface atmosphere, frost will liquefy. This previously unknown reservoir of available calories may replace those lost through evaporation, allowing the frost to reach 0° C and then to melt. The soil may, thus, develop and retain minute quantities of liquid water for biologically significant periods of time. In addition, as fast as liquid water might appear, it would dissolve solutes abundant in the highly hygroscopic Martian soil. This would delay loss of liquid water through evaporation
Together, the Viking Orbiter surface temperature data, the Viking Lander surface temperature, the Viking images of diurnal frost, with even heavier deposits as seen in Figure 1, and the Pathfinder atmospheric data showing rapidly rising temperatures near the surface suggest the following water cycle operating on Mars. The air at all but the surface layer on Mars is too cold to support much humidity. At 45° latitude, the total capacity of the atmosphere is less than 15 precipitable µm.18 Even at the equator, this amount is barely exceeded at dawn. The coldness of the atmosphere above this slim layer of air acts as a sweep, moving any water vapor in the atmospheric column downward toward the surface. Even though convective air may temporarily elevate water vapor above the severe limits imposed by the Martian atmospheric lapse rate, the net flux is downward.
Tthe atmosphere at the surface cools, its water vapor capacity diminishes by two orders of magnitude, reaching 100% humidity. The vapor condenses, then freezes, and, along with any falling ice crystals and upwelling sublimate, deposits on the surface. A very large fraction of the water vapor in the atmospheric column is thus deposited. The ground at this point is very cold, IR irradiation having removed the heat of the day, and retains the fresh, very thin coating of ice. The ground acts as a cold plate, further trapping moisture from the air, thereby establishing a concentration gradient scavenging moisture from the atmosphere.
In the morning as the sun rises, its rays strike the translucent frost ice coating. The frozen water is warmed by partial absorption of the sun’s direct rays and by re-emission in the IR of the sun’s rays which passed through the ice and were absorbed by the underlying surface material. Starting at approximately –50° C, each gram of water must receive 50 calories to achieve 0° C, and another 80 calories to melt. As vaporization increases, the warming atmosphere immediately above the surface becomes saturated. As the temperature rises above 0°C and until it exceeds the Mars liquid water envelope seen in Figure 3, the water vapor pressure exceeds the triple point. The water vapor is restricted from rising by the cold air above the vapor-saturated surface layer, which may be only millimeters or centimeters thick. As the sun continues to rise (Figure 8), the ice heats faster than the vapor can rise into the cold air just above the saturated layer. The saturated layer prevents further evaporation, with its attendant cooling. Thus, the heat of insolation and re-irradiation absorbed by the ice supplies the heat of fusion. The result is water moisture released and trapped in the warming surface soil. Any water that does evaporate will remove 540 calories per gram, which must be replaced from the environment. Otherwise, the water will freeze and sublime at a temperature where the absorbed heat and the heat lost by sublimation come into equilibrium until the ice sublimes completely.19 (The ice would sublime at a slower rate than the water would evaporate because the heat of sublimation is higher than the heat of vaporization.) There are, however, mitigating circumstances: 1) the albedo of the surface is likely significantly lower than was presumed in the cited20; 2) solar heat from the nearby bare soil and rock may contribute absorbed heat to the frosted area; and 3) the sublimation rate given may be overstated when the atmosphere is near saturation, and the rate must be zero at saturation.
As the day progresses, under increased warmth from the sun, the warm layer ascends, perhaps as high as a meter. The growing volume of warmer air just above the surface then accepts additional water vapor from the warming liquid. Within the liquid water temperature-atmospheric pressure envelope prevailing on Mars, as shown in Figure 4, boiling cannot occur. However, as the temperature of the soil exceeds the limit of that envelope, boiling would occur. This entire diurnal cycle would then repeat the following day.
The above mechanism provides for the daily moisturization of surface soil over large areas of Mars.
The rarified atmosphere and low temperatures concentrate most of this water near the planetary surface. If all of the water were driven to the surface nightly, and if the liquid water produced by the above model were retained in the top 1 mm of the soil, perhaps prevented from percolating downward by the frozen ground beneath, this would produce between 1% and 10% moisture by volume. If less water deposits, the resultant moisture percentage is adjusted by that factor. The Viking Lander images of ground frost and snow demonstrate that the percentage deposited is sufficient to be readily visible, which indicates that a substantial fraction of the total water vapor content of the atmosphere must be deposited, or that a substantial amount of vapor must arise from the permafrost, or both.
Extracted from an article by Gilbert V. Levin (Biospherics Incorporated, 12051 Indian Creek Court, Beltsville) and Ron L. Levin (Lincoln Laboratory, Massachusetts Institute of Technology, Lexington,)