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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