Yesterday NASA held a press conference to announce new evidence for the presence of liquid water on Mars—not millions of years ago, but now—based on a paper published by Lujendra Ojha and co-authors in the journal Nature Geoscience.
The confirmation that liquid water exists on today’s Mars was done based on spectral analysis of dark streaks observed on the slopes of Martian craters, called Recurring Slope Lineae (RSL), which grow larger during warmer seasons when water gets close to or exceeds its freezing point. Using data from NASA’s Mars Reconnaissance Orbiter, the researchers determined that these streaks are composed of hydrated salts, particularly magnesium perchlorate, magnesium chlorate, and sodium perchlorate. All of these salts lower the melting point of water by huge amounts, up to 80 degrees Celsius.
The detected salts are also hygroscopic, meaning they can attract water directly from the atmosphere. Thus, the water within the RSL salts likely derives from the atmosphere. Alternatively, the dark streaks may also be seasonally fed by groundwater, though the distribution and pattern of the RSL seems to be more consistent with an atmospheric source.
Does that mean life exists within the dark streaks? That’s a difficult call. Laboratory results and field studies (for example, from the Don Juan Pond in Antarctica) show that microbes have a difficult time living in brines, particularly brines enriched in calcium and magnesium. Perchlorate solutions, which remain liquid down to really low temperatures, would even be more difficult for microbes to handle (how difficult is a current subject of various laboratory investigations). On the other hand, research from the Atacama Desert shows that the driest habitats on Earth are salt crusts, in which salt-loving bacteria take up water directly from the atmosphere. Still, Mars is quite a bit drier than the driest desert on Earth, probably right at the limit of survivability for microorganisms. Only further research will clarify whether the RSL are habitable.
Unfortunately, it’s unlikely that a space probe will land near these dark streaks to resolve the question any time soon. First, it would be a challenge to land on such a steep slope, and secondly, there are worries about contaminating these unique sites with microbes from Earth. According to planetary protection rules, any spacecraft going near the RSL would have to be thoroughly sterilized first.
As with most discoveries, more questions pop up than answers. However, this result is significant: We now have (indirect) evidence that liquid water does exist on present-day Mars, and the planet seems a bit more life-friendly than it did just a few days ago.