The problem with that theory is that such features can be reproduced in the lab by letting small particles slide away, says granular materials researcher Troy Shinbrot of Rutgers University. "You find that every single thing that geologists say, 'Ah, that means there's water,' you can duplicate," says Shinbrot, who described his research on sliding grains at this week's meeting of the American Physical Society here.
To understand problems in pharmaceutical manufacturing, Shinbrot and his colleagues worked with hollow glass grains so small and light that they slosh back and forth like milk. The setup was simple: they put the grains in a box and tilted it. But by varying the angle of the box and the time the grains have to settle, "we saw these guys that looked in every way like what was reported on Mars," Shinbrot says. They created wide, branching shapes similar to the deltalike Martian patterns interpreted as signs of ancient liquid water as well as elongated ones that mimic the recently observed tracks.
Because the grains are so light, the mini-slides kick up clouds of grains that gradually settle like snow-globe snow into various patterns. The shapes are much smaller than those seen on Mars but Shinbrot says the effect should be the same as long as the grains take more time to settle than to slide, as they should in Martian gravity, because it is about one third the strength of Earth's.
Shinbrot reported at the meeting that they also attempted to reproduce sharp, centimeters-tall ridges of Martian material called razorbacks, which have been interpreted as deposits left by liquid water, too. They were able to create small razorbacks by applying an electric field to their sliding grains, Shinbrot says. Because Mars is so dry, he notes, sands could build up static electricity while sliding over each other. He says the electric fields needed are so high that they should be detectable on Mars.
Geologist Allan Treiman of Houston's Lunar and Planetary Institute agrees that the recent tracks seen in Martian gullies could have a dry origin. The planet's low temperature and atmospheric pressure would rapidly vaporize water, he notes, and he has found signs of similar patterns in avalanches of earthly snow, which is essentially dry. The nonliquid explanation, though, is a minority view, he says—at least for now. "The idea of it being liquid water was a very reasonable hypothesis to start with," he notes. "From my standpoint liquid water hasn't been proved at all."