It happens every spring. Just as the sun peeks above the horizon at the Martian south polar icecap, powerful jets of carbon-dioxide (CO2) gas erupt through the icecap's topmost layer. The jets climb high into the thin, cold air. Those jets carry fine, dark sand and spray it for hundreds of feet around.
This dramatic scene emerges from new research by a team of Mars scientists that includes Arizona State University's Phil Christensen.
Christensen is a Regents' Professor of Geological Sciences at ASU’s new School of Earth and Space Exploration in the College of Liberal Arts and Sciences. He designed a multi-wavelength camera called THEMIS, for Thermal Emission Imaging System. Scientists are using THEMIS to solve many long-standing Martian mysteries.
The team began its work in an attempt to explain what caused mysterious dark spots, fan-like markings, and spider-shaped features on the icecap at the Martian south pole. The dark spots are typically 50 to 150 feet wide and spaced several hundred feet apart. They appear every southern spring as the sun rises over the icecap.
The mysterious markings 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. Most spots even seem to recur at the same locations.
Christensen says this mechanism is unlike anything that occurs on Earth. “If you were there,” he explains, “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 and dust a couple hundred feet into the air.”
You'd also feel vibration through your spacesuit boots, he says. “The ice slab you're standing on is levitated above the ground by the pressure of gas at the base of the ice.”
“Once a spider becomes established,” says Christensen, “it affects the surface so that a vent will form in the same place the following year.”
As they erupt, the jets carry loose sand and particles high in the air. 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.
“It's like separating wheat and chaff,” he says. “The finest-grained materials are carried off by the wind, while coarser grains are sifted again and again, year after year.”
The vents and jets continue to erupt until the ice slab completely sublimates and vanishes.
The full research report, co-authored with Hugh Kieffer (U.S. Geological Survey, retired) and Timothy Titus (USGS), appeared in the Aug. 17, 2006, issue of the scientific journal Nature. High-resolution images of the spots and spiders are available at: http://themis.asu.edu/news-polarjets. For more information about other projects, visit the Mars Education Program at http://marsed.asu.edu.