You frequently hear said Mars is much drier than any place on Earth as an indication of the difficulty of having liquid water on Mars, as in this news release:
Deep down, Mars harbors a lot of ice
Frozen water may even be drinkable, scientists say.
"Mars is extremely dry, drier than any (place) we have on Earth," said Gerhard Neukum, a German scientist who has analyzed stereo images of the Martian surface recorded by the European Space Agency's Mars Express satellite that began orbiting the fourth planet from the Sun in late 2003."
http://www.sfgate.com/cgi-bin/article.cgi?...L&feed=rss.news
This terminology "drier" refers to the amount of water vapor in its atmosphere.
However, during southern Winter, Antarctica can actually have less atmospheric water vapor than the highest concentrations of water vapor on Mars:
South Pole Transmissivity Plots.
"Because of its high altitude, low water vapor column, and low temperatures, Antarctica may contain some of the driest and thus best sites for infrared, submillimeter, and millimeter astronomy [Bally, 1989; Harper, 1989; Chamberlin and Bally, 1996]. Potential sites on the Antarctic plateau vary in elevation from nearly 3000~m to over 4000~m. The center of the plateau is in a permanent high-pressure zone where air is descending from high altitudes. Temperatures at the south pole range from 200~K to 260~K [Chamberlin and Bally, 1994]. Measurements of the precipitable water vapor column [e.g., Smythe and Jackson, 1977; Burova et al., 1986] show that the water column can be as low as 50 microns in the austral winter, and is rarely above 1~mm."
http://casa.colorado.edu/~bally/AT/cara.html
The term "precipitable microns" means the thickness of liquid water you would have if the entire water vapor content in a column were condensed to liquid.
Mars can have water vapor content above 100 precipitable microns over the northern pole in Summer, though the measurements close to the equator can be a as low as 10 precipitable microns.
However, I believe there are some locations even near the equator on Mars that are far above the 10 micron level. For instance Noctis Labyrinthus frequently shows dense low lying clouds/fogs that give the appearance of precipitation carrying clouds on Earth:
Noctis Labyrinthus
An even better image of these dense clouds/fogs is given in this report on Mars Express:
Adsorption water driven processes on Mars.
D. Möhlmann
http://sci.esa.int/science-e/www/object/do...fobjectid=36779 [pdf file]
The mistaken idea that Mars' atmosphere always contains many times less water vapor than *anywhere* on Earth is a big reason why many Mars scientists discount the possibility of liquid water on the surface of Mars. Therefore I think it is important that the actual state of affairs is more well known among Mars scientists.
Key also then is that there are isolated spots in Antarctica that still remain liquid even during the period of Antarctic winter when the atmospheric water vapor reaches these extremely low values. One such spot is Don Juan Pond:
"H. This is a picture of Don Juan Pond, the only unfrozen body of water on the entire continent of Antarctica. It's located in the Wright Valley of the Dry Valleys and the "water" in the pond is an almost-saturated solution of calcium chloride. Don Juan Pond is so salty that it doesn't ever freeze, even in the dead of the Antarctic winter."
Other photos of Don Juan Pond located here:
SIUC Microbiologists in Antarctica!
http://www.science.siu.edu/microbiology/An.../Antarctic.html
Note that such spots in Antarctica are so isolated that they wouldn't have been known from orbital imagery alone. Don Juan Pond was spotted from helicopter surveys and subsequentely investigated on foot.
It may take such similar very low altitude investigation of Mars, perhaps only hundreds of feet altitude, before they are discovered there as well.
My suggestion for where such surveys should start would be Noctis Labyrinthus.
Bob Clark