Water in the Martian Bedrock

The U.S. space agency drew back the curtain March 2 on a new Mars, one oddly familiar to geologists. And NASA's scientists on the Mars mission are, refreshingly, talking like geologists too.

The lander named Opportunity has spent weeks poring over the first actual bedrock outcrop we've seen on Mars. Being near an outcrop was an accident: Mission controllers have been avoiding bedrock for 30 years for fear of putting their spacecraft in a precarious spot. This time geologists got their fond desire in Meridiani Planum.

Several lines of evidence, some stronger than others, point to the presence of liquid water here:

1. The presence of sulfate minerals is the most conclusive. These clearly come from the evaporation of a brine, just as they do on Earth. The specific minerals found—jarosite and kieserite—are not what we typically find on Earth. The first is an iron sulfate, the second a magnesium compound, whereas Earth's most common sulfates are gypsum and anhydrite, which are calcium compounds. But a large field of basalt (which is high in iron and magnesium) might well produce such a calcium-poor mineral suite. So could a change in groundwater chemistry, which is always likely.
2. The coin-shaped holes in the rock are vugs: openings created where mineral grains have been removed by dissolution. The most likely candidate for this mineral is gypsum (although barite and other less common sulfates also form tablet-shaped crystals). Both the formation of the crystals and their dissolution point to liquid water.
3. The rocks show distinct signs of layering. This is a telltale mark of sedimentary rocks, but not necessarily of water. Windblown sediment also forms layers, as do some parts of volcanic ashfalls. The NASA researchers talked about crossbedding (as shown in this hoodoo rock), but the signs are rather faint. The temptation is very great to see crossbeds where they may not exist. It was a pleasure to see the geologists admitting their uncertainty.
4. The spherulites, or "blueberries," have a random occurrence, as if they had not been carried with the sediment in a stream and strewn into layers. Also, the sediment has not slumped into place around them, as clay might slump around a pebble as it is compressed into stone. The NASA geologists argue that these features mean the spherules must have grown in place as concretions, which would indeed indicate groundwater.

But not every sign has to point to the same thing, and I harbor a belief that the blueberries may be tektites, not concretions. For one thing, we still don't know their composition because Opportunity's sensors can't get a clear view of them. Earthly concretions are not this small and clean, nor do they have the glossy surface that the blueberries show. Their random occurrence may mean only that streams or currents never sorted them. And it may be that the rock has not been significantly compacted. So there is still room in the scenarios for volcanic or meteoric droplets to have been part of the sediment from the beginning.

On the other hand, these things are on Mars. No one in NASA has looked at the Martian outcrop and recognized anything on Earth quite like it. Neither have I. So it was a peculiar pleasure to watch the panel of scientists on TV articulating exactly what they knew, what they thought they knew, and what they thought they needed to know. Remember, when geologists don't know something for sure, they wonder, and they create explanations to test against the evidence. This kind of wondering and storytelling is the best part of doing science.