Aug. 21, 2006 — Spring arrives at the frigid polar caps of Mars and, just like on Earth, there is a melting of frozen ground cover. Unlike Earth, it is not a gentle affair, however.
Scientists believe the rising sun shoots straight through the thick, clear carbon dioxide ice layer, which froze directly from the atmosphere onto the ground during the winter, and strikes underlying particles of dark sand and dust.
As the darkened areas warm, the carbon dioxide just above vaporizes, shifting from a solid to gaseous state in a process known as sublimation. It is trapped, however, by the solid frozen layer of ice. The pressure builds until a geyser erupts, spewing dark sand and other particles on top of the frozen ground.
That's the picture painted by a team of Mars scientists who have racked their brains for six years trying to explain what was causing mysterious dark spots, fan-like markings, and spider-shaped features on the south pole ice cap.
The dark spots, which range from 50 to 150 feet wide and separated by several hundred feet, appear every spring as the sun rises over the southern pole. They vanish after three or four months, but then reappear, often in the same locations, after winter.
Initially, scientists thought the spots were just bare patches in the ice. Infrared images, which reveal temperatures, showed the dark spots were just as cold as the rest of the cap, leading researchers to the conclusion that the spots were lying on top of the frozen carbon dioxide.
"The closest thing we can say to explain this is these are geysers of carbon dioxide gas that's exploding out through vents and carrying sand up into the air with it. That then falls back down on the ice so that you get this dark sand on top of the ice," Phil Christensen, with Arizona State University in Tempe, said in an interview with Nature, which published the research in this week's issue.
"If you were there, you'd be standing on a slab of carbon-dioxide ice. All around you, roaring jets of carbon dioxide gas are throwing sand and dust a couple hundred feet into the air," he said.
The theory is based on computer models, but Christensen and his colleagues are combing through Mars pictures trying to find evidence of a geyser in action.
"There is nowhere you could go on Earth and find this," Christensen said.
Because 40 percent of the Martian atmosphere is frozen onto the south or north pole at any one time, understanding how the ice caps grow and recede is critical to forming a comprehensive view of the planet's global climate, Yves Langevin, with the University of Paris in France, added in a related research paper, which also appears in Nature.
