Wobbles in
the rotation of Mars swung the planet into about 40 extreme ice ages in the
past 5 million years and allowed thick ice layers to remain far away from the
poles, an astronomer says.
Various
spacecraft have revealed evidence for ice ages on Mars. Around 4 million to 5
million years ago, precipitation events sent piles of snow and ice that
accumulated around the ice caps. Nowadays, the only visible ice
on Mars is the pair of polar caps. But in recent years, orbiting probes
have found solid evidence for vast sheets of underground ice near the red
planet's equator, at what scientists call mid-latitudes.
How ice
ended up so far from the poles has remained a mystery.
The answer
could be in the wobble of Mars, concludes Norbert Schörghofer of the University of Hawaii's Astrobiology Institute.
Moon
mechanics
Earth's
rotation axis is tilted by about 23.5 degrees, a slant that is pretty much
fixed due to the gravitational influence
of our moon. Mostly due to Mars' lack of a stabilizing moon, its tilt can
wobble as much as 10 degrees from the current 25-degree angle.
Using
computer simulations, Schörghofer found that while not topsy-turvy, the wobbles
change the amount of sunlight reaching Mars' surface and can cause vast amounts
of ice to shift between the poles and the rest of the planet every 120,000
years.
Here's how
it works: When the planet's axis swayed one way, sun rays vanished from
some areas and beamed down on others. Regions beneath sunlight became dry with
warmer temperatures, causing the ice to recede or disappear entirely except at
the highest latitudes.
The
receding ice, lost to evaporation, provided plentiful water vapor for new ice
to form once the sun retracted and humidity soared. In the humid climate, Schörghofer
said, the water vapor diffused into the Martian soil and froze to form
"pore-ice," which is mainly soil with bits of ice that plug pore
spaces.
Dual
processes
The wobble
would leave two
types of ice on or near the surface of Mars: the very old massive ice sheet
and the recent pore-ice, less than half a million years old. Dry soil covers
both types of ice.
"Today,
this gives rise to pore-ice at mid-latitudes and a three-layered depth
distribution in the high latitudes," Schörghofer writes in the Sept. 13
issue of the journal Nature. At high latitudes, the original ice sheet
is covered by a layer of pore-ice, which is veiled by a layer of dry sediment.
When the Phoenix
Lander spacecraft arrives at Mars, which is expected in 2008 and will land
in the Martian north polar region, Schörghofer expects it will "see"
the abundant and layered ice forms.
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