Recent Galileo images from the Io flyby are interpreted by NASA scientists in terms of a Sulphur Dioxide cycle involving liquid, solid, and gaseous SO2, just like the CO2 cycle I propose for Mars.
 
 

"Also of great interest is the flat-topped mesa on the right. The scalloped margins are typical of a process geologists call "sapping," which occurs when erosion is caused by a fluid escaping from the base of a cliff. On Earth, such sapping features are caused by springs of groundwater. Similar features on Mars are one of the key pieces of evidence for past water on the Martian surface. However, on Io, the liquid is presumed to be pressurized sulfur dioxide. The liquid sulfur dioxide should change to a gas almost instantaneously upon reaching the near-vacuum of Io's surface, blasting away material at the base of the cliff. The sulfur dioxide gas eventually freezes out on the surface of Io in the form of a frost. As the frost is buried by later deposits, it can be heated and pressurized until it becomes a liquid. This liquid then flows out of the ground, completing Io's version of the 'water cycle.' "

 Now isn't this interesting? On Io, NASA are happy to accept that the sapping is the result of SO2 (which is stable at those temperatures and pressures). On Mars, it has to be water! I think the evidence is far stronger for CO2 as the main erosive agent on modern and ancient Mars, due to the intensely cold temperatures prevailing there. Without the blinkered vision that insists that there must be water on Mars, we can proceed to study much more interesting phenomena in our solar system. The "geology" caused by Carbon Dioxide on Mars will be as fascinating as, yet different to, the geology caused by water on Earth. Unfortunately, all of NASA's effort is being applied (perhaps wastefully) in a search for liquid water on present or ancient Mars, rather than actually understanding what's going on.

There is a strong parallel with Lowell and his canals. With hindsight we know he was wrong, yet at the time he honestly believed he was seeing water-carrying artificial canals. Today, most planetary scientists are making a similar mistake about the outburst "flood" channels. There's no question about artificiality, of course. We all know they're natural, but to ascribe them to water erosion is to raise the spectre of life on Mars. Lowell's reputation was ultimately eroded by his blind insistence on the canals of Mars. Modern scientists have to learn from this lesson and move on from the present fixation with water on Mars.
 
 

Long run-out avalanches
These are analogues for the Chaos zones, and show how a cryoclastic flow would have begun. Earth lacks the extensive deposits of volatile CO2 ice, so does not show the transition to a density flow. More detail here.

Pyroclastic Flows
The best Terrestrial analogue. Clouds of boulders and ash are transported downhill supported by gas, eroding channels and destroying everything in their path, before burying under layers of debris, with boulders left sitting on the surface. More detail here.

Submarine Turbidity Flows
Another example of density flows on Earth. These are totally submarine and exhibit a range of morphologies from straight to anastamosing channels to meandering streams - the same range seen on Mars. Although subaqueous (unlike cryoclastic flows on Mars), they do demonstrate that you don't need two different materials (like air and water) to make a flow. These are density flows at the bottom of the ocean. On Mars, the cryoclastic flows are density flows at the base of the atmosphere. More detail here.

Diagrams of Surface features of Cryoclastic Density Flows on Mars compared to Pyroclastic and TurbiditeDensity flows on Earth.
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au