New Theory Explains Ice Dynamics on Mars
by Norbert Schörghofer
Wobbles on Mars cause ice ages that are much more dramatic than those on Earth. 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 as 10 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.
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.
My 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 theory describes what happened to this ice as the rotation axis of Mars continued to wobble over the last few million years.
The history of subsurface ice layers on Mars over the last few million years. The tilt of the rotation axis changes with time, and the planet periodically experiences dry and humid climates.
The surface temperature and atmospheric humidity on Mars 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, my 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 It has been replaced by a new layer of ice, formed not from snowfall, since the climate had meanwhile turned less humid, but by diffusion of water vapor into the soil.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," where the voids between soil grains are filled with ice.
The two types of ice found on Mars, by latitude. Dry soil covers both types of ice.
As the planets 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.
The theory will be tested very soon. When the Phoenix Lander touches down on Mars in 2008, I expect to find pore-ice instead of a massive ice sheet.
Article by Norbert Schörghofer: "Dynamics of Ice Ages on Mars," Nature, 449, 192-194 (13 September 2007)
This comic strip appeared in September, shortly after Schörghofer's theory was published in the journal Nature. According to Scott Stantis, the author of the strip, Winslow, the coyote, was supposed to have said, "you humans" instead of "you people." a reference to those who blame humans for all of the ills large and small here on Earth. PRICKLY CITY © 2007 Scott Stantis. All rights reserved. Reprinted with permission of UNIVERSAL PRESS SYNDICATE.