[X] My Assistant

[ElkGroveDan]

Dan, I think you've mistaken a part of sunlit soil in Phil's image as white stuff.

Agreed.

[ugordan]

Here's a flicker between sol 21 and 24 showing change (or, rather, lack of):


Ignore the color of the brightest part of the white stuff, it's overexposed. The small white chunk in sol 21 image appears to disappear in sol 24 (inset).

[elakdawalla]

Too low pressure and high temperature for CO2 ice. We know the temperature at the landing site -- on sol 22, for instance, it was a high of -22 and a low of -80. And the pressure is very low, on the order of 8 millibars. If you look at a phase diagram for carbon dioxide, you'll see it's always a gas under all these conditions -- it has to get really, really cold to make carbon dioxide ice, and you don't get that cold at the low elevations near Mars' north pole during the summer.

I took this phase diagram from an interesting article, "Why You Can't Have a Snowball Fight on Mars"

This may make you wonder, okay, if you read that diagram it looks like water should be stable as a solid under these conditions, so why did the chunks vaporize? The problem is that the diagrams assume equilibrium conditions. But Mars is a big system and there's heating during the day and cooling at night and dry or wet air masses moving in and wind doing stuff, so at any moment things aren't in equilibrium. I understand that there could be some conditions prevailing at the Phoenix landing site that might even lead to net deposition of water ice, but they've always seemed fairly sure that excavated chunks would sublimate during the day. (I wonder if any water ice deposition happens in cold traps overnight.) Large bodies of ice like what's exposed under the lander might show little change, sublimating a tiny bit during the day and getting deposited on a bit overnight. But if you sublimate much of a chunk during the day, it's gone for good; it's not like a chunk is going to reappear from nothing overnight, so chunks that are sufficiently small should go away as a result of cyclic sublimation and deposition.

--Emily