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An Odyssey Of Mars Science: Part 1

Sacramento - Dec 18, 2003
It's been two months since this year's meeting of the Division of Planetary Sciences -- the Solar System-related branch of the American Astronomical Society, which is the primary professional organization of America's professional astronomers -- and one might think that any interesting news has been pretty much exhausted. This reporter, however, attended as many of the talks as was practical, and I now hope to provide a delayed but still interesting overview of the meeting's talks and posters more thorough than has been done up to now -- because a surprising number of seriously interesting items revealed at that conference have been either under-reported or not mentioned by the press at all.

We'll begin with the planet which is first in the hearts of almost everyone: Mars.

Most of the DPS talks and poster devoted to Mars focused on highly abstruse scientific details of its atmosphere and weather patterns, which most non-specialists find thoroughly boring. There were relatively few talks about its surface geology and history or the prospects for Martian life; most of those tend to be delivered instead at the annual Lunar and Planetary Sciences Conference in Houston or other conferences. The DPS is, after all, composed of astronomers, who tend to specialize in the sorts of measurements that can still be made by Earth-based telescopes.

But there were still several very interesting talks delivered -- including three lectures delivered by Chris McKay and by Bill Boynton and Philip Christensen (the chief investigators for the Mars Odyssey spacecraft's two main experiments).

Except for its dramatic revelation soon after entering Mars orbit that Mars' polar regions are completely covered by a concentrated layer of ground ice extending to within centimeters of the surface and stretching all the way down to 30 degrees latitude, Odyssey has produced few of the sensational headline-grabbing findings made by its predecessor, the Mars Global Surveyor ("MGS"). The reason is that its instruments -- the gamma-ray and neutron spectrometers making up its "GRS", and the visible-light and infrared multispectral cameras making up "THEMIS" -- turn out abstruse kinds of data that require much analysis before firm results can be reported from them.

But their findings are now providing us with some important new clues on the most puzzling aspect of Mars: the question (crucial to appraising the chances for life) of just how much liquid water has existed on or near its surface during its history.

Up to now, the evidence has maddeningly pointed in contradictory directions. Mars has resembled the story of the Six Blind Men who felt different parts of an elephant and came to totally different conclusions as to the animal's basic nature.

But Mars Odyssey has made it newly clear that Mars' surface features have actually been dramatically molded by two completely different climate histories: one stretching over the planet's entire 4.5 billion year history, and the other repeating over cycles lasting only about 100,000 years. To discuss these properly, I'll need to digress at some length during this section, to recent theories and papers NOT mentioned at the DPS meeting, because they're important to the issue.

Regarding the longer history of Mars: it's generally accepted that during its earliest, "Noachian" era -- up to roughly 3.8 billion years ago -- it had a much denser carbon dioxide atmosphere, which caused the features we see on the remaining old Noachian surface to be eroded much more than features on the more recent surfaces created during the "Hesperian" and "Amazonian" eras. Indeed, it may have had an air pressure twice or more that of present-day Earth!

Some process removed most of early Mars' air. This probably involved both the splashing of atmosphere permanently into space by the gigantic meteoroid impacts that were still raining down on this low-gravity planet during the Solar System's early days, and the fact that -- ever since Mars' interior cooled and its early magnetic field shut down -- the electrically charged solar wind blowing directly over Mars' upper atmosphere has been slowly but gradually sweeping more of the planet's gases into space.

But while early Mars was vastly more hospitable where air pressure is concerned than the current near-vacuum planet is, it's still uncertain whether it was also warm enough for large amounts of liquid water to exist on its surface -- which is crucial in deciding whether life might have been able to evolve there.

That dense early blanket of CO2 MIGHT have provided enough greenhouse effect to warm Mars' surface above freezing, so that -- after the atmosphere was removed -- the now almost-airless planet first chilled down to the point that its surface water all froze, and then had its air pressure drop further to the point that its surface ice "sublimated" (evaporated directly into water vapor) in Mars' lower latitudes and then drifted to the even colder polar regions where it could refreeze.

(Meanwhile, both that temperature drop and the fading of Mars' early internal geothermal warmth also caused the upper few kilometers of its surface soil and rock to drop below freezing, wrapping the planet in a "cryosphere" -- a thick permafrost-filled layer that has sealed off Mars' remaining underground liquid water, still deeper down, from any further contact with the surface, except through increasingly rare spots of volcanic activity.)

But the Sun was up to 30% fainter during the Solar System's early days, and so scientists are having trouble devising a way in which even the greenhouse effect from a CO2 atmosphere several times as dense as Earth's present-day air could have produced enough greenhouse effect to ever warm Mars above freezing even during its Noachian days. They have to come up with jury-rigged explanations -- such as traces of other efficient greenhouse gases in early Mars' air, or possible additional greenhouse warming from a very thick high-altitude layer of frozen CO2 clouds -- that may well not have really existed. The only way to know whether early Mars was above freezing is to look at the physical evidence itself.

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