Mystery of Mars polar spirals unraveled
By Robert Roy Britt
SPACE.com
(SPACE.com) -- Odd spiraling gorges etched deep into the polar ice caps of Mars have stumped scientists for decades. The huge arcing troughs radiate outward like arms of a pinwheel, creating an overall shape that visually and mathematically resembles hurricanes, spiral galaxies and even some seashells.
Now there is an apparent solution to the mystery, put forth by Jon Pelletier of the University of Arizona in Tucson.
The tilted planet causes ice on one side of a crack to heat and vaporize, deepening and widening the crack. Then the water vapor hits the shady, colder side of the growing canyon and refreezes.
Eventually, chasms more than a half-mile (1 kilometer) deep developed, and they now cover hundreds of miles of the polar regions. But only on Mars, it seems.
Characteristics unique to the Red Planet -- its thin atmosphere, chilly climate and specific planetary tilt -- make it the only known place in the solar system where the ice spirals occur. They don't exist at Earth's poles, in part because temperatures are regulated somewhat by global ocean and air currents.
"On Mars, the surface temperature is strongly determined by the angle with respect to the sun, because very little heat transport occurs in the thin atmosphere," Pelletier said.
"The troughs form by an instability in which areas on the ice cap that are slightly steeper towards the sun begin to melt while nearby areas remain frozen," he said. "The steep areas get steeper, face even more directly towards the sun, and further melt in a positive feedback."
It's no accident that the martian spirals take on a familiar look.
Their ever-expanding arms follow a pattern known as a logarithmic spiral, arms tight at the center and growing rapidly farther apart as they splay out. It is a common pattern in nature, repeated in hurricanes (when viewed from above), the seashells of the chambered nautilus, and even in the descending flight pattern of a bird of prey.
"The logarithmic pattern occurs because the troughs migrate at a slower rate near the poles because it is colder there and melting takes place more slowly," Pelletier said. "This causes them to 'bunch up' nearer towards the poles."
Pelletier, an assistant professor of geosciences, applied mathematical formulas involving the spiral patterns formed by slime molds to solve the puzzle on Mars.
Details of the computer simulations are in the April issue of the journal Geology.
The simulations do not include wind, which some previous studies had suggested might contribute to the spirals. Pelletier says the computer model generates patterns that match the scope and spacing of what is seen on the Red Planet, even including imperfections in the spirals.
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