Blueberries on Mars

Yes, it's official: there was once water on Mars, a whole lot of it.

It's worth mentioning that the Mars scientists have been very conservative about making this claim. For the last few weeks, we Caltech astronomers have been wandering over to the geology building to look at the new images of Martian rocks and soil, and for weeks we've been wondering why the geologists haven't claimed success.

The pictures we've been shown looked a whole lot like sedimentary rocks, with concretions (more about that later), and cavities carved out by minerals dissolving away as water flowed through. The data seemed overwhelming to support the presence of water, or maybe it's just that astronomers are used to working with such sparse data, we were the ones that were overwhelmed. At any rate, it's now official. Mars was once wet, and the water sat around for a long time. Whether the water extended to planet-wide oceans, or was more localized like lakes and rivers remains unclear. It's also unclear exactly when the water disappeared and where it went. But water there was.

So what, exactly, is this evidence? What threw the balance so far in favor of water that NASA decided to make this extraordinary claim?

As you might expect, it was more than one single piece of evidence, but a whole story that started to take shape. The most dramatic evidence of water has been found by Opportunity, the second of the rovers to land on Mars. The first rover, Spirit, could hardly be called a disappointment, but the landing site, Gustav Crater, was, at first, a bit disappointing.

From orbit, Gustav Crater looks like it must have been a lake at some point. There's a large, sinuous channel that parts the crater wall to the South, and it looks exactly like a river that might have either fed or drained a long-past lake bounded by the crater walls. There are also drainage patterns to the North, suggesting the crater may have been a pooling region for a small system of channels. If water passed into the crater and sat around for a while, it may have dropped sentiment and laid down a lake bottom, and that's what scientists were hoping Spirit would find.

This may indeed be what happened, but it now appears that something else altered the crater in the more recent past: volcanic activity. When Spirit started sampling the soil, it looked a lot like the Martian soil we had seen in the past: loose, red sand dotted with jagged pebbles, probably the residue of a lava field that was broken up by meteorite impacts. Spirit drilled into a few rocks and analyzed the chemistry, which turned out to be very much like basalt (old lava) rocks on Earth.

But on closer inspection, the lava rocks had tiny cracks and cavities, which seemed to have been filled in with a light-colored mineral. That sort of thing happens when water containing dissolved minerals interacts with lava, either as part of the original lava flow, or later on after the rock has cooled and solidified.

Was this the best evidence Mars could give us of past surface water?

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