Actually, its entirely possible that during the first half billion years of its existence, Mars experienced plate tectonics. This is based upon measurements of a remnant magnetic field on Mars that harkens to a much more tectonically active time deep in Mars' past.

But its very clear that, if Mars did experience plate tectonism, that this effectively shut down around 4 billion years ago.

This would correlate well with the discovery of 3.5 billion year old olivine deposits ubiquitous to the surface of Mars. It paints a picture of a potentially very geologically and climatically active early Mars that all but evaporated between 4 and 3.5 billion years ago.

With the shutdown of plate tectonism and its associated diminution of its magnetic field, the martian atmosphere would have been exposed and unprotected from the solar wind. The loss of atmosphere would result in a precipitous drop in surface temperatures and would have make liquid water at the surface less likely. These cold and dry conditions would allowed allowed for the persistent olivine deposits we observe on Mars today.

One other note, Johannes. Today, the martian atmosphere is 10,000 drier than the atmopshere of earth. That is incredibly dry. By all accounts, it appears to have been this way for a very, very long time.