Wobbles inthe rotation of Mars swung the planet into about 40 extreme ice ages in thepast 5 million years and allowed thick ice layers to remain far away from thepoles, an astronomer says.
Variousspacecraft have revealed evidence for ice ages on Mars. Around 4 million to 5million years ago, precipitation events sent piles of snow and ice thataccumulated around the ice caps. Nowadays, the only visible iceon Mars is the pair of polar caps. But in recent years, orbiting probeshave found solid evidence for vast sheets of underground ice near the redplanet's equator, at what scientists call mid-latitudes.
How iceended up so far from the poles has remained a mystery.
The answercould be in the wobble of Mars, concludes Norbert Sch?rghofer of the University of Hawaii's Astrobiology Institute.
Earth'srotation axis is tilted by about 23.5 degrees, a slant that is pretty muchfixed due to the gravitational influenceof our moon. Mostly due to Mars' lack of a stabilizing moon, its tilt canwobble as much as 10 degrees from the current 25-degree angle.
Usingcomputer simulations, Sch?rghofer found that while not topsy-turvy, the wobbleschange the amount of sunlight reaching Mars' surface and can cause vast amountsof ice to shift between the poles and the rest of the planet every 120,000years.
Here's howit works: When the planet's axis swayed one way, sun rays vanished fromsome areas and beamed down on others. Regions beneath sunlight became dry withwarmer temperatures, causing the ice to recede or disappear entirely except atthe highest latitudes.
Thereceding ice, lost to evaporation, provided plentiful water vapor for new iceto form once the sun retracted and humidity soared. In the humid climate, Sch?rghofersaid, the water vapor diffused into the Martian soil and froze to form"pore-ice," which is mainly soil with bits of ice that plug porespaces.
The wobblewould leave twotypes of ice on or near the surface of Mars: the very old massive ice sheetand the recent pore-ice, less than half a million years old. Dry soil coversboth types of ice.
"Today,this gives rise to pore-ice at mid-latitudes and a three-layered depthdistribution in the high latitudes," Sch?rghofer writes in the Sept. 13issue of the journal Nature. At high latitudes, the original ice sheetis covered by a layer of pore-ice, which is veiled by a layer of dry sediment.
When the PhoenixLander spacecraft arrives at Mars, which is expected in 2008 and will landin the Martian north polar region, Sch?rghofer expects it will "see"the abundant and layered ice forms.
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