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.