QUOTE (edstrick @ Dec 12 2005, 08:58 AM)
http://ltpwww.gsfc.nasa.gov/tharsis/Mars_t...aphy_from_MOLA/I stand corrected on the topography. This is in the eastern part of the Labrynthus where the large open end of the Valles Marineris is below datum. The larger image (of the labrynthus) is oriented with south up, and the densest appearing clouds (on the right) are over terrain near the zero datum and a bit higher.
Note that because of the low atmosphere pressure, air temperatures are strongly decoupled from surface temperatures. Afternoon atmosphere temperatures a few meters above the surface are something like a few tens of degrees cooler than the surface. Note that that 20 deg C surface is the Martian equivalent of the "Sidewalk you can fry an egg on" and corresponds to dark material in full early afternoon sun.
These images were taken during the morning with oblique illumination providing good relief information, and surface temperatures will be much colder than the maximum.
Also, my impression of the images is that the cloud's optical depth is near one or a bit higher, max of maybe 2 in the densest regions. I'd need the original data well processed to be sure, but my impression is the surface of the valley bottoms is visible in most areas, though at low contrast.
Actually for salt containing brines as for example Don Juan Pond in Antarctica, *anywhere* on Mars would have sufficient surface pressure to maintain the brine in liquid form. This fact is discussed here:
On the possibility of liquid water on present-day Mars.
Haberle, Mckay, Schaeffer, Cabrol, Grin, Zent, and Quinn.
Journal of Geophysical Research, no. E10, p. 23,317-23,326, Oct. 25, 2001
It mentions that not only is the freezing point depressed by the addition of salts but so also is the equilibrium vapor pressure. That is, the required pressure for the water to stay liquid will be reduced:
"We now examine the effect of dissolved salts on the potential for melting. Pure water is unlikely on Mars since salts are believed to be a significant component of the Martian soil [Clark and Van Hart, 1981]. The presence of salts will lower the melting point and reduce the equilibrium vapor pressure of the solution. An example of the effect of a NaCl brine on the potential for melting is shown in Figure 7. In this example, the eutectic point is 251 K and the equilibrium vapor pressure of the solution at that temperature is 1.23 mbar. Clearly, the presence of salts greatly expands the regions where melting could occur and increases the total time such conditions might exist. In this particular example, virtually the entire planet (except the polar regions) experiences conditions favorable for melting at some point during the year, including the Tharsis plateau.
...
"For the NaCl brine mentioned above, boiling would not occur at its eutectic anywhere on Mars since the surface pressure never falls below 2.60 mbar anywhere on the planet during the year."
On the possibility of liquid water on present-day Mars, p. 23,321-23,322.
Bob Clark