Gullies on Mars

Phil Plait has a Slate article about Mars:

 My first two science loves are, in order, astronomy and geology. If you combine the two, your options are limited. You can observe the Earth from space, you can collect meteorites, or you can soak in the beauty of landscapes on other worlds provided by spacecraft.
I have a deep affection for all three, and which one I like best depends on when you catch me. For example, today, I’m fond of that last one, due to this gorgeous picture taken of ridges and gullies on Mars:

This shot shows some of the terrain (or aresain or whatever term you use for Mars) inside Gasa crater, a seven kilometer (four mile) wide crater in the southern mid-latitude of the red planet. Weirdly, Gasa sits inside a larger twenty-kilometer-wide crater; whatever impact formed it happened in the same place as a larger impact a long time before. Gasa is young, probably less than two million years old.
The edge of Gasa is heavily scalloped, clearly caused by the rim eroding away. There’s a lot of carbon dioxide ice in this location, much of it beneath the surface. Every summer the ground warms up, the ice sublimates (turns directly into a gas) and some of the crater rim collapses inward.
Over time this forms gullies, erosional channels. They’re not terribly hard to find in some craters, since they look like, well, gullies: nearly straight channels in the crater wall dug out by material flowing downhill. There are fan-shaped regions surrounding them, and in the top image, which has been color enhanced, the fans are bluish, showing fresh deposits of carbon dioxide.
How fresh? This picture was taken in the austral spring of Mars, and those deposits weren’t there the year before! So the CO2 ice was laid down last winter in the Martian southern hemisphere, just months ago.
Mars is not a dead planet, at least not geologically. It changes year by year, even month by month. You can see how sharp the ridges are. The Sun is shining down from the upper left, and the tops of the ridges look like a knife’s edge. You can also see boulders dotting the landscape as well, casting long shadows. That’s convenient, since the size of the boulders can be directly measured, and the angle of sunlight is known. That means the slope of the ridges can be determined, too. It’s amazing what you can squeeze out of an image like this.

It gets better, too. Here is a wider shot, showing more of the crater rim: 

See how the scalloped region with gullies is on the right? That’s the northern rim of the crater. Now think this through: in the southern hemisphere summer, when it’s warmer, the Sun will be shining more directly into the northern rim of the crater (just like having southern exposure in a house in the northern hemisphere on Earth gets you more sunlight). That’s why there’s more erosion there on the northern rim; the ground gets warmer and frees more carbon dioxide. As you look around the crater rim in both directions (east and west, heading south) the scalloping gets weaker, because the ground doesn’t get as much direct sunlight, and so is somewhat cooler. That’s evidence of seasonal variation in the erosion, just like you’d see on Earth!
Mars is such an odd place. Thick gray basaltic sand is everywhere, covered itself by fine dust (tinged red due to iron oxide: rust). Cratered, battered, barely holding an atmosphere, yet it still has ways of reminding us of home. Comparative planetology is a fascinating field. Huh. I may have to make that Number Three on my list.

Rico says ain't space wonderful?