PASADENA, CA — 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?
Then Opportunity knocked (sorry, I just had to say it). The second rover's landing site had been chosen in a novel way: instead of looking for evidence of water in the shape of landforms, scientists chose Opportunity's site by its chemistry.
Meridiani Planum looks pretty boring from orbit; it's one of the flattest parts of the entire Martian surface. But in 1998, the Mars Global Surveyor orbiting spacecraft made an intriguing discovery from above: using its infrared spectrometer instrument, the MGS spotted a substantial deposit of gray hematite in the plane.
Hematite is just oxidized iron, and it comes in two forms, red and gray. The red kind is what we on Earth usually think of as "rust", and it's also what gives the Martian soil its distinctive red coloration. That sort of hematite is present all over Mars; nothing so special about that.
But gray hematite is a little bit different. What separates the two forms of hematite is the size of the crystals. Gray hematite has larger, coarser crystals, which are harder to form than the tiny, fine crystals in the red form. The two kinds of hematite have exactly the same chemistry (if you ground up the larger gray crystals into a fine powder, you'd have red hematite), but the larger crystalline form is harder to make. On Earth, gray hematite usually forms around hot springs, or other places where iron has a chance to oxidize in water. Finding gray hematite doesn't guarantee that water once flowed on Mars (it is possible to form gray hematite from some kinds of volcanic activity), but it upped the odds quite a bit.
So down dropped Opportunity, ready to search for gray hematite and other chemical evidence of water. As I said before, Meridiani Planum is almost completely flat, so it was just a stroke of good fortune that sent the airbag-cushioned lander bouncing along the plane, right into the mouth of a shallow crater, rolling around it a few times, golf ball-like, before coming to rest. "Even Tiger Woods couldn't have done better," I heard one of the Mars geologists say.
As the airbags deflated and Opportunity took a first look around the crater, the Mars scientists couldn't believe their eyes. Right in front of the lander was an exposed piece of Martian bedrock, the first such formation ever seen by human eyes. The intriguing rocks, christened the "Dumb Luck Formation" (unofficially) were unlike anything they had seen on Mars before. Instead of the random boulders and fine sand the scientists had become accustomed to, the bedrock was finely stratified, each layer thrown into relief by erosion from the Martian wind. It's possible that such finely layered rock could have been laid down by volcanic ash, but the rocks looked an awful lot like sedimentary structures.
The scientists controlling Opportunity could hardly wait to get a closer look at the bedrock, and when Opportunity rolled up a few days later, they were in for another major shock. Embedded in the rock, and scattered all around the surface of the soil, were tiny, regular spheres about the size of bb's.
Once again, none of the previous landers or rovers had seen anything like these small spheres, called "blueberries" by the scientists. Closer inspection of the bedrock showed the blueberries to be buried in all layers of the rock, neatly tucked into the layered strata. As wind eroded the rock away, the blueberries would drop out and scatter on the ground. The blueberries were hematite-enriched, which started everyone thinking about similar structures on Earth, called "concretions" that form inside wet rocks.
Concretions are formed when mineral-rich water flows through rock strata. When the minerals begin to precipitate out (usually around an irregularity in the rock that slows the flow of water), they can build up into small spheres. Was the presence of these spheres on Mars enough to guarantee that water had once flowed through this bedrock?
Not quite. It is possible to form iron-rich spheres from the condensation of a volcanic cloud, or the debris from a meteor impact. But if either of those had been the case, the blueberries should have been deposited in clear layers, not all throughout the rock strata. Opportunity started drilling into the rock, and sure enough, no discrete layers of blueberries could be seen. In fact, the blueberries didn't seem to distort the layers of rock around them at all, and some of the blueberries even seemed to have lines going through them that matched the layers in the rock around them. They must have formed in the rock itself, and not been deposited later.
Then the evidence for water began to pour in. Other parts of the bedrock had penny-sized holes, where something had either dropped out of the rock or dissolved away. On Earth, such holes are usually caused by the growth of crystals, which subsequently dissolve away or drop out of the rock. After sniffing around the bedrock, Opportunity's instruments detected a substantial amount of jarosite, a mineral that forms in the presence of iron, sulfur and trapped water.
And that was the smoking gun. There's really no way to make jarosite without water. Finally the scientists had no choice but to come to the conclusion that they had hoped for all along: the surface of Mars was once wet.
At the moment, Opportunity is about to amble over to another part of the bedrock, looking for evidence not only that the rock was exposed to water, but that it actually formed in it. Some preliminary images seem to show odd ripples and angles in the layers, as if the sediment was pushed around in flowing water, or maybe ancient waves on a Martian Ocean. After that, the rover will pull itself up out of the crater, and start rolling over the surrounding plane.
Everyone at NASA can breathe a well-earned sigh of relief. The main goal of the rovers was to detect the past-presence of water, and this has been accomplished. Now the rovers can have some serious fun, rolling along the sandy surface of Mars, taking whatever direction whim leads them. The next question to be answered is what actually happened to the Martian seas and rivers, and what caused an entire planet to dry up and die. Or maybe something managed to survive that planetary catastrophe and is still waiting to be found.