If you were an astronaut on the surface of Mars, and you happened to spill your bottle of drinking water, a very strange thing would happen. The water would instantly freeze and boil away at the same time. On Earth we think of freezing and boiling as two completely different processes, and they are. But Mars is literally an alien world. For one, it’s a lot colder than Earth, with an average surface temperature of minus 81 degrees Fahrenheit, plenty cold enough to instantly turn your spilled Evian to ice. The Martian atmosphere is also very thin—equivalent to the air outside an airplane flying at 111,500 feet here on Earth. At that reduced air pressure, water would boil even at 81 below.
Like this oddity of Martian physics, the recent Mike Malin and his colleagues is both remarkable and hard to believe. It’s remarkable in that we now have the capability to monitor geologic processes on Mars in much the same way as we do on Earth. Malin and company presented evidence for 20 different impact craters formed over the last few years, as well as for two recently active gullies. These kinds of "quick response" observations allow us to estimate how geologic processes operate on Mars today, not just millions of years ago. Thanks largely to the success of the Mars Global Surveyor, which took the pictures, we have a much better understanding of what a dynamic place Mars still is.
But the idea that one of these modern-day processes involves liquid water is, quite frankly, unbelievable.
In their paper, Malin and company briefly summarize the different mechanisms scientists have proposed to explain the gullies seen by their camera. One model, proposed by Allan Treiman of the Lunar and Planetary Institute, suggests that water has nothing to do with it. Rather, he proposes that loose, wind-blown dust and sand pile up against an obstacle like a crater wall, then suddenly collapse, resulting in a dry, granular flow.
Malin and his team favor an alternative scenario. Jennifer Heldmann of NASA’s Ames Research Center and her colleagues suggest that if liquid water is released from the ground fast enough, it can overcome the freezing/boiling problem and actually flow across the surface. The conditions have to be just right, however, or the resulting flow will either be too long or too short to explain the gullies. The water has to come gushing out to the surface at 30 cubic meters per second, and the atmospheric pressure and temperature have to be such that water can exist as a liquid, solid, and gas—the so-called triple point.
Unfortunately, 30 cubic meters per second is pretty darn fast. To give you an idea of just how fast, water shooting from a fire hose only travels at eight hundredths of a cubic meter per second. Water from the Midway Geyser Basin in Yellowstone National Park flows at a slightly faster one-quarter of a cubic meter per second. And a geyser is about as good as it gets for explaining liquid water on Mars. It is possible to get flows as fast as 30 cubic meters per second in certain geologic settings, but the water has to be released in a catastrophic flood from very large areas, and we don't see evidence for that in the images.
The other problem with the new claim is that the two gullies seen by Malin’s group are located on nearly opposite ends of the planet, and are offset by more than three miles in elevation. The differences between local pressures and temperatures at these two sites would be huge, essentially invalidating the Heldmann model.
My colleague, Ross Irwin, was on his way to discuss the gullies with a reporter when he poked his head into my office. "What do you think?" he asked.
"Well, I think they’re the result of dry granular flow," I said. "You know, Occam’s Razor and the simple solution."
Ross started laughing. ‘Well, I’ve seen enough to know that natural processes aren’t always simple."