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Caltech News Release
Embargoed for Release at 11 a.m. PST, Thursday, February 13, 2003
Images available at
 http://www.gps.caltech.edu/~shane/swiss_press.html

The Martian polar caps are almost entirely
water ice, Caltech research shows

Contact: Robert Tindol
         (626) 395-3631
         tindol{at}caltech.edu

PASADENA, Calif.-For future Martian astronauts, finding a plentiful 
water supply may be as simple as grabbing an ice pick and getting to 
work.  California Institute of Technology planetary scientists 
studying new satellite imagery think that the Martian polar ice caps 
are made almost entirely of water ice-with just a smattering of frozen 
carbon dioxide, or "dry ice," at the surface.

Reporting in the February 14 issue of the journal Science, Caltech 
planetary science professor Andy Ingersoll and his graduate student, 
Shane Byrne, present evidence that the decades-old model of the polar 
caps being made of dry ice is in error.  The model dates back to 1966, 
when the first Mars spacecraft determined that the Martian atmosphere 
was largely carbon dioxide.

Scientists at the time argued that the ice caps themselves were solid 
dry ice and that the caps regulate the atmospheric pressure by 
evaporation and condensation.  Later observations by the Viking 
spacecraft showed that the north polar cap contained water ice 
underneath its dry ice covering, but experts continued to believe that 
the south polar cap was made of dry ice.

However, recent high-resolution and thermal images from the Mars 
Global Surveyor and Mars Odyssey, respectively, show that the old 
model could not be accurate.  The high-resolution images show 
flat-floored, circular pits eight meters deep and 200 to 1,000 meters 
in diameter at the south polar cap, and an outward growth rate of 
about one to three meters per year.  Further, new infrared 
measurements from the newly arrived Mars Odyssey show that the lower 
material heats up, as water ice is expected to do in the Martian 
summer, and that the polar cap is too warm to be dry ice.

Based on this evidence, Byrne (the lead author) and Ingersoll conclude 
that the pitted layer is dry ice, but the material below, which makes 
up the floors of the pits and the bulk of the polar cap, is water ice.

This shows that the south polar cap is actually similar to the north 
pole, which was determined, on the basis of Viking data, to lose its 
one-meter covering of dry ice each summer, exposing the water ice 
underneath.  The new results show that the difference between the two 
poles is that the south pole dry-ice cover is slightly thicker-about 
eight meters-and does not disappear entirely during the summertime.

Although the results show that future astronauts may not be obliged to 
haul their own water to the Red Planet, the news is paradoxically 
negative for the visionary plans often voiced for "terraforming" Mars 
in the distant future, Ingersoll says.

"Mars has all these flood and river channels, so one theory is that 
the planet was once warm and wet," Ingersoll says, explaining that a 
large amount of carbon dioxide in the atmosphere is thought to be the 
logical way to have a "greenhouse effect" that captures enough solar 
energy for liquid water to exist.

"If you wanted to make Mars warm and wet again, you'd need carbon 
dioxide, but there isn't nearly enough if the polar caps are made of 
water," Ingersoll adds.  "Of course, terraforming Mars is wild stuff 
and is way in the future; but even then, there's the question of 
whether you'd have more than a tiny fraction of the carbon dioxide 
you'd need."

This is because the total mass of dry ice is only a few percent of the 
atmosphere's mass and thus is a poor regulator of atmospheric 
pressure, since it gets "used up" during warmer climates.  For 
example, when Mars's spin axis is tipped closer to its orbit plane, 
which is analogous to a warm interglacial period on Earth, the dry ice 
evaporates entirely, but the atmospheric pressure remains almost 
unchanged.

The findings present a new scientific mystery to those who thought 
they had a good idea of how the atmospheres of the inner planets 
compared to each other.  Planetary scientists have assumed that Earth, 
Venus, and Mars are similar in the total  carbon dioxide content, with 
Earth having most of its carbon dioxide locked up in marine carbonates 
and Venus's carbon dioxide being in the atmosphere and causing the 
runaway greenhouse effect.  By contrast, the eight-meter layer on the 
south polar ice cap on Mars means the planet has only a small fraction 
of the carbon dioxide found on Earth and Venus.

The new findings further pose the question of how Mars could have been 
warm and wet to begin with.  Working backward, one would assume that 
there was once a sufficient amount of carbon dioxide in the atmosphere 
to trap enough solar energy to warm the planet, but there's simply not 
enough carbon dioxide for this to clearly have been the case.

"There could be other explanations," Byrne says.  "It could be that 
Mars was a cold, wet planet; or it could be that the subterranean 
plumbing would allow for liquid water to be sealed off underneath the 
surface."

In one such scenario, perhaps the water flowed underneath a layer of 
ice and formed the channels and other erosion features.  Then, 
perhaps, the ice sublimated away, to be eventually redeposited at the 
poles.

At any rate, Ingersoll and Byrne say that finding the missing carbon 
dioxide, or accounting for its absence, is now a major goal of Mars 
research.

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