I'm not trying to help on this, but I imagine that "your" frost, Emily, could be blown by the wind and become to look like "real" snow for a short period of time.
Welcome back to brain (snow) storm UMSF

The MCS guys would be the first port of call I would have thought - I'm not sure what they're measuring, how they measure it and what they already know about it- but I've heard mention of them looking for snow like things in the polar regions at night.

Doug

Well I dont have any real insight, but I have always thought that the 'snow' imagined by Viking actually was hoarfrost.

Hoarfrost implies saturated air at the surface. Clouds imply saturated air at altitude. Opponents of 'snow' seem to be saying there must always be an unsaturated layer in between where descending ice particles would sublime. I don't see why.

Yes - that assumption presumes that any ice particle that forms will automatically sublimate in unsaturated air, yet if it's cold enough, it will not.

Doug

Here's a link that provides one viewpoint.

http://www-mgcm.arc.nasa.gov/mgcm/HTML/FAQS/rain.html

If the current vapor pressure was equal to the saturation vapor pressure throughout the atmosphere you'd have very thick clouds (somthing like nimbostratus on Earth, at least you'd have more clouds than are observed at lower altitudes). And there aren't any, that is what we see. Thus, the water vapor pressure is lower than the saturation vapor pressure below the martian clouds. Therefore, any ice crystals that fall down would sublime long before they'd reach the surface.

On Earth there are situations where falling precipitation (snow) does not reach the surface, too, because the air layer below is very dry. As the falling snow evaporates/sublimes, the underlying air gets enriched with water vapor until the saturation vapor pressure is reached. So, consecutively, the snow can reach lower altitudes until the whole airmass below the clouds is very moist and the snow reaches the surface. That only works well if the dewpoint is below 0°C between the clouds and the surface (otherwise the air mass does not get cooled below 0°C and the snow melts). That just happened in Denver, CO. Yesterday they had 17°C, but the dewpoint was very low (<0°C) below the clouds. And today you have snow in Denver and temperatures around 0°C. That resulted from the process described above.
Such a process could not occur on Mars under current atmospheric conditions as you need *a lot of snow* = thick clouds to moisten the whole layer of air below the snow producing clouds. There simply isn't enough water vapor.

Michael

Thanks for this link -- just in time for me to cannibalize it for my Planetary Radio Q and A

--Emily

Sure, that's what we see in the daytime. But in the middle of a polar winter night?

Yes, that is certainly true.

I remember now that I read a publication by some french scientists who modelled clouds induced by orography over the north polar regions (observed through MOLA?). These clouds were made of dry ice particles (CO2). I think they wrote something about possible small amounts of precipitation from these clouds, but CO2-ice crystals, not water. Can't find that publication any more, unfortunately.

Michael

From the link provided by MarkL I borrowed the folloing quote: "But theoretical calculations seem to suggest that generally such snow would not tend to reach the surface."

And I tend to agree, look at this image from February 1979 and I think you will agree with me that this look quite more like frost than a result of actual snowfall. The white frost/snow dont cover the entire scene as after a snowfall.

Yet on the page where the image are found the word 'snow' are used again. So the word seems to have been repeated quite often t describe whats seen on those images.

As a sidenote, on that page we see that the martian sky might display colours quite different from the rusty red that seem to dominate the ones made by the excellent image manipulators of this forum.

That's a very early morning image, with the sun off to the right, and the image has been stretched quite a bit to get the detail of the ground to show well, so the sky has been saturated.

Some superb results from the PDS released Viking camera data - http://www.astrosurf.com/nunes/explor/explor_vik.htm

Particularly - http://www.astrosurf.com/nunes/explor/viki...03calstrech.jpg

Doug

Clouds and Snow from MOLA

Corrosive snow

Here is a serie of 2 VL2 images showing the gradual condensation of water frost on Mars and then its full cover on the surface. The 1st image was taken in summer. The 4th image was taken early spring and shows its sublimation.
Enjoy !