[thedoc (OP)]

Sam asked the Naked Scientists:
   
Precipitation in Earth weather usually requires Pseudomonas syringae bacteria. Shouldn't this be true for Mars, thereby confirming bacteria in Mars atmosphere and on Martian plants where the bacteria originate?

What do you think?

[evan_au]

At the very low pressures on Mars (on average 0.6% of Earth's atmospheric pressure), and very low temperatures, water can exist as a solid and a gas - liquid rain is quite unlikely.

The strong winds on Mars can sweep up dust storms, form water vapour clouds, and water may fall as powdered ice and snow. But the rain with which we are familiar on Earth does not happen on Mars.
http://en.wikipedia.org/wiki/Water_on_Mars

If there are bacteria on the surface of Mars, they too would be swept up by winds - but it gets even colder at high altitudes, and it is subject to extreme levels of ultraviolet.

I think its a bit early to make deductions about life on Mars based on water clouds. But it is certainly one area to look for in future Mars missions.

[chris]

It's not true that precipitation "requires" Pseudomonas syringae. These bacteria do promote precipitation, but anything that acts as a nucleation centre - including dust and even dandruff - can seed clouds and cause rain. Pseudomonas syringae has evolved to take advantage of the Earth's system, deploying a pattern of threonines on the surface of the bug to promote ice crystal formation at above-normal temperatures, so that bacteria blown into clouds come raining back out again more promptly.

[Bored chemist]

It seems reasonable to accept that it rained on earth before there were bacteria.