September 16, 2007

New Theory Explains Ice on Mars

Wobbles on Mars cause ice ages that are much more dramatic than those on Earth, says astronomer Norbert Schörghofer of the University of Hawaii.

Thanks to our large, stabilizing Moon, Earth's rotation axis is always tilted by about 23 degrees. The tilt of Mars, however, can wobble by as much as10 degrees from its current 25 degrees. Wobbles cause big changes in the amount of sunlight reaching different parts of Mars, so vast amounts of ice shift between the poles and the rest of the planet every 120,000 years.

Schörghofer's new theory appears in the September 13 issue of the journal Nature.

"We expect to see two types of ground ice when the Phoenix Lander spacecraft arrives at Mars in 2008," says Schörghofer, "ice that formed on the surface and was then buried, and ice hidden in porous soil."

During the 19th century, scientists discovered that Earth experienced ice ages. In the past few years, spacecraft have discovered that ice ages also occurred on Mars, but scientists have been puzzled because more ice than expected has survived far from the polar caps. What is left is now thought to be a combination of old ice from the last major glaciation and younger ice that formed later and in a way entirely different from the way ice formed on Earth.

The new theory sheds light on the history of vast ice-rich areas, which once covered most of Mars. Around 4 to 5 million years ago, ice accumulated from extensive snowfall outside the martian polar caps. The new theory describes what happened to this ice as the rotation axis of Mars continued to wobble over the last few million years.

Surface temperature and atmospheric humidity changed because of varying sunlight. When the climate was dry, the ice receded to a greater depth or disappeared entirely except at the highest latitudes. Dust contained in retreating ice eventually covered the ice, making it no longer visible on the surface.

So much of this subsurface ice has been detected that its only plausible origin was thought to be massive snowfall. However, Schörghofer's theory suggests that a lot of that snowfall ice has since been lost to the atmosphere. It has been replaced by a new layer of ice, formed not from snowfall, since the climate had meanwhile turned less humid, but by diffusion of water vapor into the soil. Atmospheric vapor can freeze inside the soil and form "pore-ice," which is mainly soil with some ice in pore spaces.

As the planet's tilt toward the sun went back and forth, the climate kept changing between dry and humid, causing many cycles of ice retreat and formation. Today we are left with two kinds of ground ice: the old massive ice sheet and very recent pore-ice.

Schörghofer is part of the multidisciplinary UH Astrobiology Institute, which is sponsored by NASA and managed through the Institute for Astronomy. Its research focuses on water as the habitat of, and chemical enabler for, life.


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