Blueberries on Mars
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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.
