Posted to sci.astro:

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This page shows the air temperatures 100 meters above Meridiani:

PIA07103: Seasonal Air Temperatures Above Opportunity
http://photojournal.jpl.nasa.gov/catalog/PIA07103

The graph shows late afternoon air temperatures exceed -40 C even into the current Winter season at Meridiani.
Recent imaging at Meridiani also shows water condensing to ice on the surface which proves the presence of atmospheric water vapor there.
It is well known that liquid water can form in air at temperatures below 0 C. Even down to -40 C some proportion of condensed water will be in liquid form. Down to -20 C the majority of condensed water is in liquid form:

Super-cooled Clouds (1 of 3)
"At temperatures below -20 degs. C, clouds consist mostly of ice particles,
while below -40 degs. C they are composed entirely of ice particles.
However, between 0 degs. C and -20 degs. C, a significant number of clouds may
be primarily composed of water droplets. Water droplets below 0 degs. C are
supercooled, and clouds containing large, supercooled droplets can pose
an extreme hazard to aviation. During day-time, 3.9 um imagery can be used to
infer droplet phase at cloud top because of differences in reflection between
ice particles and water droplets, as discussed earlier in the section on
emission and reflection. By using the 3.9 um imagery to
identify phase, and the 10.7 um imagery to determine cloud top temperature,
cloud tops consisting of supercooled water droplets may be located."
http://www.cira.colostate.edu/ramm/goes39/sprcool1.htm

So the difference in reflectivity at these different infrared wavelengths
could help in distinguishing ice from liquid water. Further links on this page
illustrate this method for distinguishing ice clouds from supercooled
liquid water clouds, which may also work for making this determination for condensed water

close-up with the mini-TES spectrometer on the rover.

Another references for making this distinction:

Cloud Water Phases.
"The deployment of a spectroradiometer covering the spectrum in the 1.0-2.5µm
region makes it possible to identify the three phases of water in the
atmospheric column. Primary water vapor absorption bands are centered
at 0.94, 1.13, 1.37, 1.9 µm, while unsaturated liquid water bands are centered
at 1.0 and 1.2 µm. A strong ice feature is found at 1.5 µm. Because the
absorption features of the three phases of water are offset from one another,
and because there are accurate models for the absorption
features, the irradiance spectra can be simultaneously analyzed for the column
abundance of the different phases.
"Gao and Goetz (1990, 1991) showed that, by using a non-linear least squares
method, total column precipitable water vapor and equivalent liquid water
thickness in vegetation could be obtained simultaneously on a pixel
basis in Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data. The
absolute value of the precipitable water vapor was confirmed to be accurate to
5% by AVIRIS overflights of upward-looking microwave radiometers (Gao et
al,1992)."
http://www.asdi.com/asdi_ap_en_apnts_s_fsp.html

The PanCam imager on the rover can cover at least part of this infrared wavelentgh range,

up to 1100 nanometers, 1.1 microns.

Other references:

DISCRIMINATING BETWEEN WATER AND ICE CLOUDS USING NEAR-INFRARED AVIRIS
MEASUREMENTS.
http://makalu.jpl.nasa.gov/docs/work...s/Knap_web.pdf

MEASURMENT OF THE SPECTRAL ABSORPTION OF LIQUID WATER IN MELTING SNOW WITH AN
IMAGING SPECTROMETER.
http://makalu.jpl.nasa.gov/docs/work...95_docs/26.PDF

The OMEGA infrared spectrometer on the Mars Express orbiter is also
expected to be able to distinguish between the different phases of water on
Mars. See:

DISCRIMINATION BETWEEN SOLID, LIQUID AND GASEOUS H2O, AND
ASSESSMENT OF H2O TEMPERATURE BY SPECTROSCOPY USING THE OMEGA IMAGING
SPECTROMETER (MARS EXPRESS).
Bernard SCHMITT, Sylvain DOUTÉ, Stéphane ERARD, Yves LANGEVIN, Jean-Pierre
BIBRING.
Laboratoire de Planétologie de Grenoble, CNRS - Université J. Fourier, France;
Institut d’Astrophysique Spatiale, CNRS -Université Paris-Sud, Orsay, France.

in:

MARS INFRARED SPECTROSCOPY: FROM THEORY AND THE LABORATORY TO FIELD
OBSERVATIONS
June 4-6, 2002 at the Lunar and Planetary Institute.
LPI Mars Spectral Workshop II (2002).
p. 45.
http://www.lpi.usra.edu/science/kirk..._workshop2.pdf

So visible to near-infrared imaging of Mars may be able to make this
determination.

Hubble has also made some observations of Mars in visible and near-infrared wavelengths:

Martian Colors Provide Clues about Martian Water
http://oposite.stsci.edu/pubinfo/pr/1999/07/
"Accurate colors are needed to determine the composition and mineralogy of
Mars. This can tell how water has influenced the formation of rocks and
minerals found on Mars today, as well as the distribution and abundance
of ice and subsurface liquid water. Confirmation of the presence of certain
oxidized (rusted) minerals (processed by heat or water action) would imply the
possibility of different, perhaps much more Earth-like, past Martian
climate periods. Because the smallest features visible in this image are only
about 14 miles (22 km) across, Hubble can track small-scale variations in the
distribution of minerals that do not follow global trends. The image
was generated from three separate Wide Field and Planetary Camera 2 images
acquired at wavelengths of 410, 502, and 673 nanometers, in March 1997."

Near-Infrared Imaging of Mars from HST: Surface Reflectance, Photometric
Properties, and Implications for MOLA Data.
"We report initial results from Hubble Space Telescope (HST) images of Mars obtained at a

near-IR wavelength of 1042 nm between February 1995 and June 1997. The images provide the

highest spatial resolution global near-IR measurements of Mars obtained to date."
http://marswatch.tn.cornell.edu/icarus_mola.html

Note again that the "Cloud Water Phases" article above discusses making the distinction

between ice and liquid at around the 1 micron (1000 nm) wavelength.


Bob Clark

Robert Clark wrote:
> 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...on3.html&h

l=en&lr=&ie=UTF-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

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I will have to look at this in greater detail but my gut reaction is that Martian atmosphere contains a lot of fine suspended dust particles that would act as nucleation points for ice crystals to form without being supercooled to anything like -30C to - -40C.

Where I live we get 'freezing fog' sometimes, I've lived here about 40 years and as far as I'm aware the temperature has never dropped to -30C even on a clear winter's night. The coldest I've known was about -15C.

Typically round here, freezing fog occurs somewhere between -2C and -5C. We get mist (haar) developing over the North Sea which comes in over the cold land, freezes and the crystals can stay in suspension until they grow in size to precipitate out as frost.

As I say, this is first comment, it's pre Christmas so I'm kind of busy at the moment but I'll try to get back on this one.

__________________
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