Earth is called the "blue planet" by virtue of its thin veneer of blue ocean water. Mars is now red because of its endless vistas of reddish soil and rocks, But most planetary scientists now insist that Mars was once blue, too. It was drained by gushing rivers and supported a vast ocean. The planet's many channels and layered deposits seem to prove Mars was, like Earth, once a water-rich planet. We like to think this way because we so much want to find life on our sister planet, and to us water is life!
Not so fast! says N. Hoffman from La Trobe University in Victoria, Australia.
According to his hypothesis, water has had little or no importance in shaping the Martian landscape. Instead, his 'White Mars' model contends that the gurgling, frothing and sometimes explosive venting of carbon dioxide gas from beneath the surface created the features.
Outrageous as Hoffman's hypothesis is, it is supported by several significant observations that seem to deny a watery Mars.
The Mars Global Surveyor has not detected any carbonate minerals that should have been created when water interacted chemically with the planet's CO2 atmosphere.
Surface photos fail to show the fine networks of tributaries that should have fed the larger drainage features if water were the agent of erosion.
There is no evidence of large lakes or reservoirs at the heads of the so-called "flood channels."
Except for a small amount of water ice at the poles, Mars is dominated by solid and gaseous CO2.
One would not expect that carbon dioxide could mimic erosion features like those carved out by terrestrial water. However, Hoffman points to the devastating erosion caused by terrestrial volcanic explosions with their blasts of gases and ash, their mudflows, and their avalanches that gouge the terrain for many miles.
(O'Hanlon, Larry; "The Outrageous Hypothesis," Nature, 413:664, 2001.)
Comment. See under GEOPHYSICS for a spectacular "dry" erosion event on Earth.