Mars Australis

By Phil Plait

SLATE

Posted Monday, Sept. 2, 2013, at 8:00 AM

Mars is many things: Fascinating, scientifically interesting, historically interesting, dry, frozen, weird, inhospitable.

And as much as I like images of the red planet, one adjectival phrase I wouldn’t have immediately thought to match with Mars is “jaw-droppingly artistically gorgeous”. I’ll change that opinion right here and now:

The south pole of Mars, like the cream on a latte. Click to enaresenate.
Photo by ESA / G. Neukum (Freie Universitaet, Berlin) / Bill Dunford

That is the south pole of Mars, as seen by the European Space Agency’s Mars Express orbiter. It’s a combination of blue, green, and infrared images (put together into that stunning picture by Riding with Robots creator Bill Dunford). This exaggerates the ruddy ochre hue of the planet, but magnifies the overall impact of the picture. It’s surreal; it looks a lot like the top of the mug of coffee I make myself every morning.

Where you see white is a vast region of permanently frozen water ice, many kilometers thick, covered in winter by a few-meter-deep veneer of frozen carbon dioxide, commonly called dry ice. In the Martian summer, the temperature at the pole gets high enough to turn the dry ice into a gas, but the water ice stays frozen. Not all the dry ice disappears, but even in winter the underlying water ice cap is far thicker than the dry ice above it.

Amazingly, the atmosphere of Mars—which is primarily carbon dioxide—noticeably thickens locally in the summer when the ice cap melts (for whichever hemisphere is experiencing summer at that time). On Earth we have water ice, which melts into a liquid. But carbon dioxide doesn’t melt, it sublimates, turning directly from a solid into a gas. That goes into the tenuous Martian air, thickening it.

Not that it’s any great shakes then either: Air pressure on Mars is only about 1 percent or less of Earth’s. You wouldn’t want to be there without wearing a spacesuit.

I imagine though that the view standing on the pole would be quite spectacular. Someday, perhaps, we’ll get a chance to see for ourselves. Until then, I’m actually rather happy to let our robot proxies do it for us.