To understand Mars we need to understand its on-again off-again tango with liquid water. It's not just the search for past or present life on Mars that hinges on this, but the search for a complete chemical, geophysical, and climatological history of the red planet. Water is such a potent agent for topographical and mineralogical alteration that its presence leaves layer upon layer of complex clues - the challenge is to pick these apart.
When Curiosity recently discovered clear evidence for a dried up stream-bed in the alluvial fan of a crater this was one example of how Mars' history is littered with episodes where liquid water gushed, seeped, and likely pooled on the planetary surface.
Now a remarkable new study has not only found evidence of liquid water on Mars and its chemical impurities, but also the actual temperature at which it existed. The clues come from one of the best resources we have for studying Mars without actually going there - martian meteorites. The 'nakhlites' are a class of igneous rock (frozen magma) that formed on Mars about 1.3 billion years ago, were blasted from the surface in a giant impact event about 11 million years ago, and have been falling onto the Earth during in the past 10,000 years (including in 1911.)
Based on the ages at which these rocks crystallized and the chronology of martian craters we can even place their origin somewhere on the volcanic plains of northern Mars, either Tharsis or Syrtis Major. Studying the detailed structure and chemical 'veins' in these meteorites provides a unique probe of the conditions in which they once existed on Mars. Publishing in the December issue of Earth and Planetary Sciences Letters, the authors Bridges and Schwenzer present an investigation of the mineralogical structures in one of the nakhlite rocks using electron microscopy.
What they find is clear evidence for a progression of minerals deposited in the rock veins that could only occur in the presence of hot liquid water. First is iron carbonate - which crystallizes out from carbon-dioxide rich water at about 150 Centigrade (super heated), followed by clay-like minerals that form as the water cools down to about 50 Centigrade. All of this stuff has been enriched by sodium and potassium salts - in a slightly alkaline mix. In other words, these rocks were bathed in the equivalent of a hot briny spring.
So what could have driven this sort of environment millions of years ago on Mars? The authors speculate that an earlier asteroid impact on Mars could have melted carbon-dioxide rich water ice and forced it under pressure into the surrounding terrain - like a great hand smacking into a wet sponge. This hot water would have flowed and percolated through the subsurface of the volcanic plains, gradually cooling down and depositing all the minerals we see.
Although the nakhlite rocks might have only been seeped in these hydrothermal waters for a few months there is evidence that the deeper rocks they once sat on could be part of a much longer lived wet and chemically rich environment. And this particular type of environment is a direct analog to hydrothermal systems here on Earth, with hot water and the kind of low-acidity, salt-rich chemistry that we know is very much habitable for all manner of microbial organisms.
All of which I think suggests once again that Mars really has, and used to have, a strong potential for life - but the fundamental nature of the Martian habitable zone is different from that of Earth. It appears to have been episodic, and reliant on mechanisms such as impact events in ways that may be unique to this dusty red world.