Dust, water, carbon dioxide, wind, snow, landslides, avalanches, flowing water, take your pick, something has caused all the thousands of fresh gullies on (mostly) crater walls.

Something.

What I want to know is why so many Mars researchers are so intent on determining that one thing and one thing only has caused all these gullies? Or indeed, that one process is responsible for all roughly similar features and formations on Mars.

Allan Treiman is on to something: no one hypothesis for fresh gully formation on Mars satisfies all the evidence we have. Not even his hypothesis does so.

Ergo: could there be more than one process involved in gully formation on Mars? For example, maybe some are formed by water flow beneath a nice cover of dust, snow and ice. Maybe some are formed by percolations of CO2. Could some result from dust flows that act like liquids? Could there be other processes -- some unique to Mars -- that give rise to surface gullying at the present time?

What fun! Lot's of different processes could produce similar results on Mars, thus confounding researchers -- but leaving plenty for them to do!

Whatever the case, the water-flow hypothesis for some gullies on Mars is hardly threatened by the Treiman's new take on the question. On the other hand, the possibility that dry processes have produced many of the gullies on Mars is essentially self-evident.

More study!

Cheers, BRFelix