See those weird, black, spidery things dotting the dunes in this colorized photo taken by the Mars Reconnaissance Orbiter in 2010? Yeah. Nobody knows what the hell those things are.
What we do know about them just underlines how incredibly unfamiliar Mars really is to us. First spotted by humans in 1998, these splotches pop up every Martian spring, and disappear in winter. Usually, they appear in the same places as the previous year, and they tend to congregate on the sunny sides of sand dunes — all but shunning flat ground. There's nothing on Earth that looks like this that we can compare them to. It's a for real-real mystery, writes Robert Krulwich at NPR. But there are theories:
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Scientists from the U.S. Geological Survey, from Hungary, from the European Space Agency have all proposed explanations; the leading one is so weird, it's transformed my idea of what it's like to be on Mars. For 20 years, I've thought the planet to be magnificently desolate, a dead zone, painted rouge. But imagine this: Every spring, the sun beats down on a southern region of Mars, morning light melts the surface, warms up the ground below, and a thin, underground layer of frozen CO2 turns suddenly into a roaring gas, expands, and carrying rock and ice, rushes up through breaks in the rock, exploding into the Martian air. Geysers shoot up in odd places. It feels random, like being surprise attacked by an monstrous, underground fountain.
"If you were there," says Phil Christensen of Arizona State University, "you'd be standing on a slab of carbon dioxide ice.