It has a mere dribble of water compared to Earth, yet over the past few million years Mars has experienced periodic ice ages that have shaped layers of ice lurking beneath its dusty surface.
The ice cycle arises from periodic fluctuations in the tilt of its axis similar to those that cause Earth鈥檚 climate to vary from ice age to interglacial. There are important differences, though, because with no large moon to stabilise it, the Red Planet鈥檚 tilt has varied far more widely than Earth鈥檚.
As recently as 5 million years ago, Mars鈥檚 axis wobbled between 25 and 45 degrees from the perpendicular to the plane of the solar system 鈥� enough to evaporate polar ice caps and precipitate snow on the equator. It then shifted to a low-tilt mode, tipping back and forth every 125,000 years in a range from 15 to 35 degrees.
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To see how this affected Mars鈥檚 ice, Norbert Sch枚rghofer of the University of Hawaii modelled its sublimation and diffusion, starting 5 million years ago with an ice sheet covered by a layer of 15 per cent dust and 85 per cent ice. While warm spells sublime the ice, each of the 40 cold intervals caused diffusion of moisture into soil, where it freezes.
Conflict resolution
The model results match the distribution of ice at mid-latitudes, suggesting that it diffused into the soil within the past half-million years. Ice within about 15 degrees of the poles is much older, Sch枚rghofer says.
The new study is 鈥渧ery exciting鈥� because it explains the location and stability of Martian ice, says James Head of Brown University in Providence, Rhode Island, US.
But Jacques Laskar of the Paris Observatory warns that the transport of Martian moisture depends 鈥渟trongly on the parameters of the models鈥�.
His group published results in June showing that the northern ice cap vanished when Mars was strongly tilted, and didn鈥檛 reappear until about a million years ago (Journal of Geophysical Research, vol 112, E06012). However, that would require that more than one layer was deposited during each Martian ice-age cycle.
A resolution of the conflict may come next year when NASA鈥檚 Mars Phoenix Lander touches down in the northern Martian Plains, where Sch枚rghofer expects to find pore ice.
Journal reference: (vol 449, p 192)
