Melbourne - Oct 23, 2001
The main evidence for liquid water on Mars is the past development of giant flood channels that have been seen for the last 25 years as proof of the escape of water from the subsurface. All the models for modern and ancient Mars are based on the premise that there was once liquid water available, and perhaps even an ocean.
On the basis of this, the search for past life on Mars includes the possibility of surface life and lakebeds and ocean floors are a prime target for fossil indicators of life and water activity. Indeed, the theme of the exploration of Mars is "Follow the Water".
However, an alternative "White Mars" model for the evolution of volatiles on Mars approaches the situation from another angle. Developed in part by myself, I was given the opportunity at GeoMars conference to present this model.
In short the White Mars model shows how outbursts of liquid carbon dioxide can mimic the erosion patterns of water floods, by generating an erosive gas and debris cloud like that seen in explosive volcanic eruptions - pyroclastic flows and surges.
In detail, the analogues supporting this theory come in various forms including from the seabed of Earth where giant meandering channels are carved by suspended clouds of mud and sand.
In my presentation, I included a wide variety of detailed images of the seabed and volcanic terrains that shook many of the audience's preconceptions about channel-forming processes on Earth and Mars.
In short, meanders and channels are not clear evidence for rivers, and flat "lakebeds" may simply be sediment traps for the giant gas and dust clouds. Even the boulders at the Pathfinder landing site are eerily similar to those moved and deposited by volcanic gas clouds.
Smaller modern channels on Mars can also be explained by carbon dioxide-based flows, not water flows, making Mars very unlike Earth.
CO2 Answers to Watery Puzzles
When Mars' history is viewed from this perspective, suddenly a lot of things make sense that are otherwise puzzling. Mars has always been ultra cold and dry - even colder in the past than it is now.
The puzzling failure to detect carbonate rock by infra-red methods or surface chemistry is explained by never having liquid water at the surface (or almost never, anyway).
Carbon dioxide activity on Mars fits much better with observed modern temperatures, and predicted past ones, and avoids the need to postulate powerful greenhouse atmospheres in the past of Mars.
Mars clearly has both CO2 and H2O in its atmosphere, surface, and subsurface. Till now, Mars scientists have focussed on the H2O component and showed that at some depth below the surface it must melt and be available for rock chemistry and perhaps biology.
I showed that the CO2 component must also exist as liquid below the surface of ancient and modern Mars, and do so at much shallower depths than liquid water.
Indeed, I predicted that attempts to drill for liquid water on Mars will be problematical because the geothermal gradient on Mars is weaker than most early estimates suggested, and liquid water will not be found until great depths (7-10 km). Meanwhile, explosive pockets of liquid CO2 will exist as shallow as a few hundred metres.
CO2 can also form a joint ice comprising ~70% H2O and 30% CO2 which has very interesting properties. This CO2 clathrate locks up water at higher temperatures than pure water ice, and prevents brines from melting ice at low temperatures.
Studies of the polar regions particularly need to focus on this clathrate, which has interesting consequences for the thermal structure and stability of the polecaps.
Water or CO2?
In a spirited question and answer panel session involving myself and Michael Carr (who literally wrote the book on water on Mars), the audience found that the water model for Mars was less secure than they had believed, while the CO2 model was at least as viable and could explain any existing observational data they proposed.
Although at the start of the conference, only one scientist supported CO2 models for the "fluvial" channels on Mars and 80 favoured water, by the end of the first day the situation was a draw.
We simply do not yet have the definitive observational evidence to decide one way or the other, and indeed some sort of mixed model may eventually arise, but certainly a model of 100% water for the surface features of Mars took a major sidestep that week.
It became a theme of the conference that many of the speakers would begin with the title of their talk and would revise it in the light of the new understanding of volatiles -e.g. "Using subsurface radar to detect liquid water AND carbon dioxide on Mars"
Although the majority of Mars scientists continue to adhere to the conventional watery model for Mars, there is a growing realisation across the community that CO2 has been ignored for too long and that its complex physics and chemistry also needs to be addressed in studies of the Red Planet. If a paradigm shift is on the way for Mars, then the shift began in earnest in Houston at the GeoMars conference.
Dr Nick Hoffman is a lecturer with the Victorian Basin Studies Centre, VIEPS Department of Earth Science, La Trobe University, Victoria Australia. He may be contacted via firstname.lastname@example.org
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Geophysical Detection of Subsurface H2O and CO2 On Mars
Melbourne - Oct 10, 2001
In early August some eighty international scientists from around the world met in August in Houston, Texas, at the NASA-sponsored GeoMars conference to discuss current and future missions to the Red Planet and instruments, data and observations relevant to the subsurface distribution of volatiles.
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