The Curiosity rover team announced yesterday that they'd found the mission's first potentially habitable environment on Mars: an ancient river bed. But why do they think flowing water created the formation?
What exactly did the rover find?
Curiosity's telephoto camera snapped shots of three rocky outcrops not far from the rover's landing site inside Gale Crater. One of them, called "Goulburn", had been excavated by the rover's own landing gear. The other two were natural outcrops dubbed "Link" and "Hottah". All three, and Hottah in particular, were made of thin layers of rock that had been cemented together.
When the rover zoomed in, it saw rounded pebbles in the conglomerates and in surrounding gravel that were fairly large ? up to a few centimetres in diameter. On Earth, roundness is a tell-tale sign that rocks have been transported a long way, since their angular edges got smoothed out as they tumbled. The Mars rocks are too big to have been blown by wind, so the team concluded it must have been flowing water. This dovetails with orbital images hinting that the rover landed in an alluvial fan, a feature that is formed on Earth by water flows.
Let's not be H2O-centric here. Could the liquid have been something other than water?
The chemical evidence for hydrated minerals at Gale Crater and elsewhere means water is definitely the top contender ? although one team member likes to joke that, for all we know, the liquid could have been beer.
Some Mars scientists have suggested quickly evaporating carbon dioxide ice could have triggered rockslides, forming gullies elsewhere on the planet, but that's a less popular theory, says team member Sanjeev Gupta of Imperial College London. Also, such bursts of CO2 vapour probably couldn't have transported these rocks all the way from the rim of Gale Crater, where the river seems to have begun, adds Bill Dietrich of the University of California, Berkeley.
It's possible the water could have been quite salty or briny, which would make it more viscous. It could have been slushy or icy as well. "Whether pure water or salty water, it would all behave similarly in terms of sediment transport," says team member Rebecca Williams of the Planetary Science Institute in Tucson, Arizona.
How long ago was the water there?
On Earth, the most reliable way to measure the ages of alluvial fans is by radiocarbon dating ? but that requires organic carbon, which we haven't yet found on Mars. And even if it found some, Curiosity's on-board chemistry lab isn't quite up to the task. The best Mars scientists can do is estimate the age of the surrounding surface based on counting craters. On a large scale, the older an area is, the more craters it likely accumulates over time.
"We think what we're looking at is several billion years old," Dietrich says of the region around Gale Crater. "How to get better than that, I don't know. This is a common discussion point."
How deep was the stream? How fast was it flowing?
Dietrich calculated that it was probably between ankle and hip deep, and flowed at about a metre per second.
How can he possibly know that?
The key was knowing the size of the rocks: "Up until now, no one knew what the size of the material in the bed was," Dietrich says. "That affects when grains start to move and the velocity. It propagates through all the calculations."
The equations for calculating river depth on Earth apply just as well on Mars, despite Mars' weaker gravity. The amount of water needed to push a rock down a slope depends on the weight of the rock and the steepness of the slope. The team knew the slope's angle, at least roughly, based on measurements of Gale Crater taken from orbit. Curiosity's new images gave size estimates for the rocks, which translated to a range of depths. Then Dietrich could plug the depth into equations for flow speed, which do depend on gravity. The river flows more slowly on Mars than it would on Earth, he says.
Its speed could change depending on how cold the water was ? cold water is about twice as viscous as warm water. Another uncertainty is how dirty the water was, since carrying a lot of small particles would slow down the stream. With the data we have now, though, a metre per second is a pretty safe estimate, Dietrich says. "Based on a mixture of hydraulic calculations and field experience, this is typically what we would find."
What does this mean for life?
The team's project scientist, John Grotzinger of Caltech, called the river bed the rover's first potentially habitable environment.
"But these fluvial environments aren't the best habitable environments," Gupta says. "They're not the best at preserving evidence of life." That's part of why the rover has already left the outcrops behind and is now heading first toward a spot called Glenelg, where three different rock types come together, and then full speed towards the mountain in the middle of the crater, alternately called Aeolis Mons or Mount Sharp. Orbital images show tantalising evidence of clays in the mountain's layers, and clays are known to better preserve organics.
"The longer-term aim is to get to Mount Sharp," Gupta says. "That's always on our mind. We can't get too distracted."
But if they run across something else that's more exciting on the way, might they change course?
"Absolutely!" says Williams. "This mission is responsive to discoveries."
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