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Martian mystery solved?

Ron Miller / ASU
Sand-laden jets shoot into the south polar sky in this artist's conception.

Newly published findings suggest a solution to the mystery of the Martian trees – those dark, bristly spots on aerial photography of the Red Planet that some have compared to fans or forests. Even Arthur C. Clarke, the author of "2001: A Space Odyssey" and other science-fiction classics, has wondered whether Mars' seemingly branching "banyan trees" represent signs of biological activity.

But now researchers propose that the spots are of geological origin: They say the marks are left behind every spring when gas and dark sand blast through rumbling fissures in the ice. "If I was ever going to go to Mars, I'd want to observe this," said Arizona State University's Phil Christensen, one of the authors of the research, which appears in Thursday's issue of Nature.

The key observations behind the latest claims were made with the Thermal Emission Imaging System, or THEMIS, an infrared camera aboard NASA's Mars Odyssey orbiter.

Previous imagery, gathered by another orbiter called Mars Global Surveyor, provided ample documentation of the mystery spots, particularly in Mars' south polar region. That imagery led scientists to suggest the spots were the result of a defrosting process that exposed the darker ground beneath the carbon dioxide ice.

However, thermal readings from THEMIS indicated that the dark spots were about the same temperature as the ice. That led Christensen and his colleagues - Hugh Kieffer and Timothy Titus of the U.S. Geological Survey - to conclude that the dark material was actually sitting on top of the ice layer, rather than exposed below the ice.

Dark spots (left image) and fans (right image) cover
the icy landscape near Mars' south pole in two
images taken by Mars Global Surveyor during spring.
Each image is about 2 miles (3.2 kilometers) wide.
Click on the image to see a larger version.

So how did the stuff get up there? Here's the scenario sketched out in this week's research paper, based on about 200 days' worth of THEMIS surveillance:

The process starts during the Martian winter with the buildup of carbon dioxide ice over a layer of dark sand and dust. That dark material is thus sandwiched between a couple of feet of CO2 ice on top, and the permanent polar cap of water ice below.

As spring approaches, sunlight shines through the CO2 ice and warms the dirt enough to make the ice just above it sublimate - that is, turn directly from a solid into a gas. Pressure builds up beneath the remaining CO2 ice, eroding the dirt layer in the process. Eventually, that pressure becomes so great that a blast of gas, sand and dust breaks through fissures in the ice - spewing out at speeds of 100 mph (160 kilometers per hour) or more.

The activity leaves behind a dark burst of dirt, surrounding the vent on the ice sheet. Wind may blow the dust into a fanlike pattern. But as the CO2 ice fades to nothingness, so does the burst pattern. All that's left is a spidery pattern of erosion carved into the underlying water ice. Those "spiders" provide a template for the process to begin all over again during the following winter.

Spiders trace a pattern on top of the residual polar
cap after the seasonal CO2 ice slab has disappeared.
This image is about 2 miles (3.2 kilometers) wide.
Click on the image to see a larger version.

"Once a spider becomes established, it affects the surface so that a vent will form in the same place the following year," Christensen said in today's ASU news release.

Christensen told me that "it was that day-to-day-to-day imaging that really allowed us to unravel what's going on." A companion paper, yet to be published, will go into the detailed physics behind the phenomenon, he said.

"There isn't anything like it on Earth," Christensen said. "On Earth, this doesn't happen."

However, some researchers have suggested that a similar process may be at work on Triton, a moon of Neptune that also is speckled with mysterious dark spots.

As for those Martian banyan trees, Christensen said the phenomenon probably has its roots in a process similar to the one he and his colleagues have sketched out.

"It's a geologically sound explanation," he said.

Do you agree? Feel free to weigh in with your comments below.