In recent years, the evidence for extensive water on the surface of Mars has continued to pile up, and, with it, the interest in the possibility that the red planet once supported life. But finding evidence of life that might have died out a few billion years ago poses a significant challenge, to put it mildly. Now, some researchers have identified an attractive place to look: apparent remnants of hot springs on an extinct volcano. On Earth, similar deposits have preserved indications of bacterial life.
Evidence for liquid water on Mars largely comes from sources like surface features that are best explained by the influence of water, and deposits of minerals that form under aqueous conditions. In the paper that describes the hot springs, the authors note that the Spirit rover had provided evidence of the latter, water-rich silicates that bore the signs of having been produced in a hot geothermal environment. Unfortunately, this was in an area that was heavily eroded, so it was difficult to tell what the original context for the deposits were, or where the hot water had come from.
The new paper neatly avoids this by detecting large silicate deposits associated with an extinct volcano using a spectrometer on the Mars Reconnaissance Orbiter. The deposits are on the volcano's flanks, which suggests that they are preserved where they formed (it's hard to get anything to erode upwards, after all).
Because the deposits were detected from orbit, it's difficult to determine their precise nature, and thus make firm conclusions about how they formed. Nevertheless, the authors make a compelling case for a source that resembled a volcanic hot spring, primarily by a process of elimination. Other options for forming silicates include an acidic fog derived from volcanic activity, but this should have had a widespread impact; instead, the deposits appear to be tightly clustered. A second option, hot ground water, is also unlikely, since there wouldn't be much groundwater near the peak of a volcano.
All of that, in the authors' view, leaves volcanic activity as the most likely cause of these silicates. The authors suggest that water-rich minerals separated within the magma chamber, and the heated, mineral rich water bubbled to the surface, leaving the deposits behind. Based on life on Earth, there are two appealing aspects of this. Many microbes are capable of extracting chemical energy from volcanic vents and hot springs, so the water comes equipped with a possible source of fuel for any ancient microbes. And, again on Earth, similar deposits have actually preserved evidence of microbial life.