Sand-laden jets shoot into the polar sky in this view by noted space artist Ron Miller. It shows the Martian south polar icecap as southern spring begins.

Credit: Arizona State University/Ron Miller

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SYDNEY, 23 August 2006: Scientists have determined that the spider-like markings and dark spots that appear every Martian spring on the southern polar icecap are formed when particles erupt through melted ice and grow as the wind carries them, and not because they are alive.

"If you were there," says Phil Christensen of Arizona State University, "you'd be standing on a slab of carbon-dioxide ice." Looking down, you would see dark ground below the 3-foot-thick ice layer. "All around you, roaring jets of CO2 gas are throwing sand and dust a couple hundred feet into the air."

"The key to figuring out the spiders and the spots," says Christensen, "was thinking through a physical model for what was happening."

The dark spots, typically 50 to 150 feet wide and spaced several hundred feet apart, appear every southern spring as the Sun rises over the icecap. They last for three or four months and then vanish - only to reappear the next year, after winter's cold has deposited a fresh layer of ice on the cap.

According to Christensen, the whole process begins during Mars' frigid Antarctic winter, when temperatures drop to -129 degrees Celsius. That's so cold that the Martian air - which is 95 percent carbon-dioxide - freezes directly onto the surface of the permanent polar icecap, which is made of water ice covered with layers of dust and sand.

"Originally, scientists thought the spots were patches of warm, bare ground exposed as the ice disappeared," notes Christensen.

So are the dark spots signs of life?

Seeking to answer this question, Christensen developed the Thermal Emission Imaging System (THEMIS), a multi-wavelength camera. "But observations made with THEMIS on NASA's Mars Odyssey orbiter told us the spots were nearly as cold as the CO2 ice, which is at -198 degrees Fahrenheit [-128 degrees Celsius]."

That finding suggested the spots were just a thin layer of dark material lying on top of the ice and kept chilled by it.

This new theory downplays a previous hypothesis that the spots are colonies of photosynthetic Martian microorganisms that photosynthesise in early spring, according to a report from the European Space Agency.

"A few places remained spot-free for more than 100 days," notes Christensen. "Then they developed a large number in a week."

"Once a 'spider' becomes established," says Christensen, "it affects the surface so that a vent will form in the same place the following year."

The largest and heaviest particles fall closest to the vent, piling up around it to make the spots. As lighter sand grains tossed out by the jet blow downwind, they create the fans, which can extend tens to hundreds of yards. The lightest particles, meanwhile, drift away on the wind to form a thin layer of dust.

The growth of the Martian spots, says Christensen, "is unlike anything that occurs on Earth."