spiritsilica: The bright spot that Spirit's broken wheel excavated is nearly pure silica, the best evidence yet of Mars's watery past Photo by NASA/JPL/Cornell

Day 1,464 of the Mars rovers' 90-day mission to Mars (for those of you keeping track), and Steve Squires, the head of science operations for the Spirit and Opportunity rovers is getting us up to date on their latest findings. Most important: serendipity in action. The Spirit rover's right front wheel has broken, so engineers turn the rover around, drive it in reverse, and drag the wheel behind the rover. As it slogs across the planet, it carves a trench. And my, what a trench it carves.

The upturned white dust in the above image is 90 percent pure silica. Now, there's only two ways to get that purity. The first is to have very high temperature water bubble through the rocks (like in a geyser), dissolving the silica and drawing it to the surface. The other is to have some sort of acid eat through the other minerals in rock, leaving silica behind. We don't know which of these conditions once existed on Mars, but we do know that similar environments on Earth are teeming with life. The Mars Science Laboratory launches in August 2009 to search for more clues.

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