The crater with ice disc is on the Vastitas Borealis, a broad plain that covers much of Mars’s far northern latitudes. The crater is 35 km wide, with a maximum depth of about 2 km. The image was taken by the High Resolution Stereo Camera on ESA’s Mars Express.
Scientists believe the water-ice is present all year round because the temperature and pressure are not sufficient to allow it to change states and vaporise into the atmosphere. It is not frozen carbon dioxide, because this type of ice had already disappeared from the north polar cap by the time this image was taken.'
From 2005, so I must have missed it when it was first reported. I don't understand why this doesn't sublimate, like the frozen CO2 did. If the crater is 35 km wide, then the ice is about 12 km (7.5 miles) across. Fascinating.
Text and photo from the Flickr photostream of the European Space Agency. More information at the ESA website.
Well to understand why the dry ice sublimates and not the water, you can just look at their phase diagrams:
ReplyDeletehttp://www1.lsbu.ac.uk/water/phase.html
http://scifun.chem.wisc.edu/chemweek/pdf/carbondioxide.pdf
If we consider that the pressure at the surface of Mars is in the order of 1kPa (or 0.01 bar), CO2 sublimates at around -120°C, while water stays solid until around 0°C.
I've looked at the links, Seban, and can't quite comprehend them. But I'll take your word for it.
ReplyDeleteIt looks like the core of a large ice ball that mostly melted from the heat of impact but the native temp stopped the process before all could melt. It seems to have a dome shape like a snow ball melting and flowing out in stop action.
ReplyDelete