Are we sure it’s ice on Mars?

Pretty sure, yes. Some people have asked whether NASA is jumping the gun, since solid CO2 – dry ice – also exists in abundance on Mars, and it can sublimate (turn into gas) as well. Maybe it’s that.

As this recent Q&A posted on Wired explains, it can’t be CO2 because it’s currently ‘way too warm there (they landed during Martian summer) for CO2 to exist in solid form at all. It’s still cool enough for it to be solid H2O under the soil that gradually sublimates when exposed to heat from the sun. That’s why they think it’s ice.

That Wired page also has a link to a nifty weather report from Mars on the Canadian Space Agency’s website.