Interesting article online in Nature News:

Published online: 27 July 2004
The search for life on Mars
"Several groups have now reported seeing water vapour coming from
certain parts of Mars, such as Arabia Terra and Elysium Planitia.
Other researchers believe that water ice lies under the surface at
these locations.
"The presence of water vapour implies that ice under the martian soil
in these locations is being melted by some heat source and evaporating
into the air. The most likely heat source is volcanic, and, as we have
learned from studies on Earth, where there's heat, there's life. Many
extremophile bacteria live on the chemical energy found in minerals
spewed out by volcanic vents under the sea."
http://www.nature.com/news/2004/0407.../040726-3.html


This article also reports Mars Express found high water vapor over
Arabia:

Probe maps water vapour on Mars.
By Paul Rincon
BBC News Online science staff, in Paris
Mars Express has detected an area of high water vapour over a region
of the Red Planet called Arabia Terra.
http://news.bbc.co.uk/2/hi/science/nature/3927041.stm

Most of the reports I've seen attribute high hydrogen rates seen by
the GRS instrument on Mars Odyssey on Mars at Arabia to chemically
bound water, since this is an equatorial site.
The Nature News article seems to suggest the independent high water
vapor measurements at Arabia are interpreted by some scientists as due
to free water there. Such water deposits could be within a meter or
less of the surface judging by the GRS hydrogen measurements and could
melt to liquid water at Summer, daytime temperatures.
Another near equatorial location showing high hydrogen by GRS is
Memnonia. This is interesting since Viking orbiter measurements also
showed Memnonia to be a region of frequent low lying fogs:

"Early Morning Surface Fog. The presence of morning fogs in some
crater and channel bottoms is a Viking discovery with possible
implications for the future biological exploration of Mars. These
early morning views of the Memnonia region were taken one-half hour
apart using a violet filter to enhance the contrast of the
condensates. The areas marked by arrows are noticeably brighter in the
later picture. The fogs indicate specific spots where water is
exchanged, probably on a daily cycle, between the surface and the
atmosphere. The surface and lower air layers in this region become
unusually cold at night because of the thermal properties of the
surface. When the surface warms in the morning, it seems that a small
amount of water vapor-estimated to be about one-millionth of a meter
thick if liquefied is driven off; this vapor recondenses in the
atmosphere, which warms more slowly, to form a ground fog of ice
particles. [P17487; 13°S,147° W]"
http://history.nasa.gov/SP-441/ch12.htm

Such areas with high hydrogen GRS readings and frequent low-lying
fogs would be my best bet for future lander missions to seek *active*
life on Mars.


Bob Clark


cf.:

From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Latest Mars Odyssey results suggest free water at equator on
Mars.
Newsgroups: sci.astro, alt.sci.planetary, sci.physics,
sci.geo.geology, sci.geo.mineralogy
Date: 2003-07-19 01:43:17 PST
http://groups.google.com/groups?selm...ing.google.com

Martian Clouds
http://planetscapes.com/solar/eng/marscld.htm

Click on the link for the Noctis Labyrinthis image. It has downloadable links for large GIF and TIFF files.

This page has several different examples of the types of clouds seen on
Mars. I found interesting this passage:

"Fog
Fog often appears in low-lying areas. It typically occurs in the
southern hemisphere especially in the Argyre and Hellas basins. It
forms frequently in craters. Occasionally, it occurs in higher
regions such as Sinus Sabaeus and Solis Planum. (Courtesy Calvin J.
Hamilton)
"Clouds in Noctis Labyrinthis
This image shows early morning fog in the Noctis Labyrinthis, at the
westernmost end of Valles Marineris. This fog, which is probably
composed of water ice, is confined primarily to the low-lying
troughs, but occasionally extends over the adjacent plateau. The
region shown is about 300 kilometers (186 miles) across. (Courtesy
NASA/LPI)"

Another low latitude location more clearly gives the indication of
low lying fogs:

Cross multiple parallel bands on eastern Sinai Planum.
http://ida.wr.usgs.gov/html/m08078/m0807848.html

I'm wondering if these low level fogs are ice crystals instead of
liquid water droplets why are they occurring in low level locations?
A low altitude site would make sense for *liquid* water on Mars since
that would provide a higher atmospheric pressure. But you would think
that ice crystals could form at any atmospheric pressure.
Perhaps though sufficient water vapor to form the crystals only
occurs at low altitudes. For example there were some Pathfinder
observations that suggest most of the water vapor at the Pathfinder
site was restricted to the bottom 1 to 3 kms above the surface.
On the other hand this can't be the entire explanation because we
know Martian clouds sometimes formed of H2O can form at high
altitudes.
Another key consideration is the temperature at which these ice
crystal fogs can occur. On Earth fogs are most commonly seen as
liquid water droplets, but there are cases where they occur as ice
crystals. Interestingly, the air temperature for this to occur has to
be not only below freezing but actually below around -30 to -40
degrees centigrade.
Above this temperature fogs will form but usually be still in the
form of supercooled liquid droplets. The temperature at which the
transition from supercooled liquid water droplets to ice crystals
occurs is not exact because the supercooled water will nucleate to
ice at a higher temperature in dusty conditions.
See these sites on ice fogs and supercooled water:

Ice Fog a Product of Temperature, Topography, Dogs.
http://www.gi.alaska.edu/ScienceForum/ASF13/1319.html

Winter Rainbows And Supercooled Water.
http://www.gi.alaska.edu/ScienceForum/ASF7/748.html

Cloud, fog & precipitation.
http://www.auf.asn.au/meteorology/section3.html [link may be inactive, see google cached link:
http://216.239.41.104/search?q=cache...-8&strip=1 ]


An image showing atmospheric temperature readings from MGS and the Opportunity rover shows the -40 C cut-off temperature for supercooled liquid water droplets may also apply to low lying clouds up to within 2km, 6000 ft:

http://www.planetary.org/news/2004/i...st-profile.jpg

Image from:

MARS EXPLORATION ROVERS UPDATE
Spirit Arrives at Bonneville Crater; Opportunity Moves to Berry Bowl; and Both Rovers Turn 'Eyes' to the Skies
12 March 2004
http://www.planetary.org/news/2004/m..._04-03-12.html


Key questions here are how is this temperature of -40C for which
atmospheric water can still be supercooled effected by a CO2
atmosphere and how is it effected by low atmospheric pressure?
It is likely that atmospheric water can be supercooled even under
Martian conditions. Then even if the sites at which the observed fogs
occur are a few degrees below zero the water may still be in
supercooled liquid form.



Bob Clark

Unfortunately, I don't have time to read these articles right now, so my apologies if I'm repeating what's in them.

I don't think CO2 makes any difference to state changes of water, other than to provide additional atmospheric pressure. The added pressure of non-water gases would raise the saturation vapor pressure for water, lowering the relative humidity - and, by correlation, requiring more water vapor availability to reach saturation. At Mars' atmospheric pressures, water should hit saturation very easily, compared to the earth.

Supercooled water droplets occur because of crystalization effects of water that I'm not intimately familiar with. What I do know is that different condensation nuclei have different temperatures at which water begins to crystalize. On earth, the most effective crystalization temperatures (where snow crystals grow best) are somewhere around -10C (I forget the exact numbers right now). From 0 to about -8 C, little or no ice crystal growth occurs with the most common nuclei. That discovery helped us realize why we could have surface-to-upper atmosphere temperatures entirely below 0 C and still get freezing drizzle at the surface.

__________________
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One nice thing about being a meteorologist who also likes astronomy is that the sky is always interesting!