Ice on Mars

I mentioned the other day that snow had been seen above Mars but was evaporating before it hit the ground. Sweetly Single asked if thats what global warming will lead to and wondered if evaporating snow contradicts the existence of the polar ice caps.

The temperature on Mars can get as high as 80°F on a very rare summer day but rarely breaks 32°F (where water freezes on Earth). During winter at the poles get down to -200°F. This data varies depending on the probe you ask but it's all around that area.

The average air pressure on Mars is 7 millibars. That's about 1% of Earth normal.

You may remember doing experiment in science class where the teacher took a glass of water, put it in a bell jar, and sucked out all the air. In the lower air pressure the water boiled at room temperature. This is also why some cake packages have different oven settings for people who live in the Rockies from those who live on the shore.

So, the same basic thing is happening to the snow. It's a little surprising that the droplets formed so that they could sublimate [def. ice turns to air without melting first].

Similarly, a couple of months ago the same rover that saw the snow dug a trench and found ice under the soil.

Alas, NASA's pictures don't want to animate on my site. Go to http://phoenix.lpl.arizona.edu/images.php?gID=6936&cID=89 to see the sublimating ice trench.
This picture shows images taken on Martian days 20 and 24.

So, the flakes evaporated due to the low pressure. On Earth snow can melt and rain can evaporate before it hits the ground. But that's for totally different reasons. Even in extreme cases of global warming we're still gonna get rain and snow.

The ice caps on Mars. This questions a bit trickier. What I mean by that is that I don't know the answer off the top of my head.

We think of ice caps as places where snow piles up year after year until it packs into ice. The caps then grow and shrink depend on the season. Mars does the same thing. Telescopes have seen them get bigger and smaller over the course of Mars' year (roughly two of our years).

The Martian ice caps are a mix of frozen water and dry ice (frozen carbon dioxide). During the summer the dry ice sublimates to form gas pockets inside the ice caps which then explode violently outwards. So setting a colony ON the cap would be a very bad idea. That's why the probe I keep mentioning was exploring near the edge of the ice caps. To see if there's enough recoverable water for a future colony to use.

Ok, the best I've been able to find about the formation of Martian ice caps is that it does snow there. The one I reported was simply the first seen from the ground. The ice caps appear off centered in pictures from space because different weather systems cause it to form differently. The western equator gets fluffy white snow while the eastern equator gets black ice. I got that from a 2 year old press release. Don't ask me how they know.

If Phoenix remains active long enough to actually get snowed on you can be sure I'll let you know.

I'll leave you with this last picture. On the 104th Martian day of it's mission the Phoenix rover saw 6 dust devils. These were the first indications of that kind of weather pattern on Mars.
Again, the picture doesn't want to animate. Go to http://www.nasa.gov/mission_pages/phoenix/images/press/15996-animated.html to see the dust devil move and read more about it.
City dwellers may not be too familiar with these. But I spent enough time on a tractor as a kid that I saw a great many of them out in the field. It's like an itty bitty puny tornado. My uncle, El Cid used to jump off the tractor and try to stand in the middle of them.