Today NASA held a press conference announcing the discovery of, well, this:

What you’re seeing here is a series of HiRISE images of a crater wall on Mars. Starting in the spring, hundreds of dark streaks form and make their way downhill, and then they fade in winter. They have now been observed in several different craters in the southern highlands of Mars, and these streaks tend to occur on equator-facing slopes. Using infrared imaging, the scientists involved are able to say that these slopes are far too warm for CO2 frost to be involved, but in some cases are too cold for pure water to be a liquid.

So what could they be? The main hypothesis put forward at the press conference is that these are concentrated brines. By dissolving lots of salt in water, you can keep it liquid far below its normal freezing point. A saturated brine would also evaporate slower than pure water, which is important with Mars’ thin atmosphere. The problem with this theory is that the CRISM instrument on MRO hasn’t been able to see any evidence of water at these locations, even as thousands of these streaks are flowing down the slopes.

An alternative that was downplayed at the press conference is that these streaks are formed by dust flows. They tend to form on slopes that are near the angle of repose (the steepest a slope of loose material can be before it fails) and which are receiving direct sunlight. I wonder if maybe what we’re seeing is caused by a tiny bit of frost evaporating and causing a dry cascade.

This isn’t the first time that streaks have been observed on Mars, nor the first time that they have been attributed to liquid water. Mars orbital camera observations discovered a bright streak that appeared between 2001 and 2005. Prior to that, dark slope streaks were thought to be caused by water, but now are generally attributed to dust avalanches.

Bottom line, these are certainly a spectacular discovery but I think it’s premature to assume that they are formed by flowing water, given the track record of previous announcements like this. As the scientists in today’s press conference said, these things really need further study, both in the form of new observations at Mars, and experiments in labs here on Earth.

Finally, on twitter today, people were asking about the potential for sending a rover to features like this. Unfortunately, NASA’s planetary protection guidelines prohibit landing at “special regions” where there are potentially habitable environments, because bugs from earth might contaminate the planet. I believe that if a rover is properly sterilized it would be allowed to land at a location with water on the surface, but in general that sterilization is prohibitively expensive, so it isn’t done. For more on planetary protection considerations, check out this powerpoint from one of the MSL landing site workshop meetings.