NASA's Mars Announcement: Present-day transient flows of briny water on steep slopes
NASA held a press briefing today to publicize a cool incremental result in the story of present-day liquid water on Mars. It concerns a paper published today in Nature Geoscience by Luju Ojha and several coauthors: "Spectral evidence for hydrated salts in recurring slope lineae on Mars." How big a deal is this story? Was all the pre-announcement hype justified? Is this just NASA discovering water on Mars for the zillionth time? What does this mean for things many space fans care about: life on Mars or future human exploration?
Past work on slope lineae, with the HiRISE camera on Mars Reconnaissance Orbiter, showed that they form in warm seasons when temperatures reach 250-300 kelvins, which strongly suggested that a volatile species like water was responsible.
The newly published work involves data from the CRISM spectrometer on Mars Reconnaissance Orbiter, and shows spectral evidence for hydrated salts (minerals containing molecular water in their structures) during the times when the slope lineae recur.
The best mineral matches to the spectral data are magnesium perchlorate, magnesium chlorate, and sodium perchlorate.
The presence of perchlorate salts could lower the melting temperature of water at Martian conditions by 40 kelvins, making it much easier for water to melt.
This work is considered very strong evidence that at widespread locations on present-day Mars, conditions sometimes arise for brief flows of briny liquid water -- probably not rivulets, just spreading wetness in the soil.
The widely varying locations and geologic settings where slope lineae have been observed to form and recur make it difficult to identify a single mechanism for replenishing liquid water to drive the recurrent activity.
The science team including Ojha and Alfred McEwen favor deliquescence as the source: perchlorate salts adsorb water vapor from the atmosphere until enough water is available to form a liquid and dissolve the salts.
Recurring slope lineae (RSL) in Newton Crater, Mars
Recurring slope lineae are narrow (0.5-5 m wide), relatively dark-toned features that form on steep (25-40˚), southern-hemisphere slopes, and that appear in early spring, grow longer in the downslope direction during spring and summer, and fade during autumn and winter.
Over the weekend, the Internet whipped itself into a frenzy anticipating this story and what it might mean for life on Mars. At the briefing, science mission directorate associate administrator John Grunsfeld fed that, saying: "The discovery we're talking about today is most exciting because it suggests it would be possible for life to be there today." Personally, I don't think extant life on Mars is any more likely because of today's announcement than it was before. An incredibly salty, corrosive, transient water environment is not a very good place to look for life. I think a much more habitable environment is available in the thin films of water that Phoenix observed in the soil at its near-polar landing site. A less-accessible, but also less-radiation-fried and more-continuously-habitable place would be deep underground, where Mars' internal heat could keep groundwater liquid for very long periods of time.
So this research doesn't have a whole lot to tell us about life on Mars, I don't think. What this research is telling us is that we are beginning to understand Mars, and that's no small thing. What we have here is a solid incremental result. Scientists made an observation: huh, there are fresh-looking dark streaks on Martian slopes. They followed up that observation by looking for more of them in similar environments, and found them. They followed up those observations by looking again at the same spots, and found that some of them faded over time, and there were new ones. What do you know, active geology on Mars! They kept looking at the same spots over and over, and found a pattern: they grew during the warm months, and faded during cold months. That suggested a hypothesis: maybe you need warmth because you are melting a volatile component, specifically water. That suggested a test: look for chemical evidence of water. The present work is the result of that test, and the result was consistent with the hypothesis.
It's really a lovely example of the scientific process working, and it doesn't happen very often in planetary science. Our instruments are so coarse, our understanding so generally poor, that it's a rare thing to enjoy this cycle of observation, hypothesis, test, confirmation. Look at what's happening on Pluto this year: the photos are absolutely amazing but, for the most part, scientists have no idea how to explain what we see.
Of course, NASA didn't lead the story by saying "Cool New Incremental Result!" They said: "Mystery Solved!" and "Major Announcement!" Those phrases led to a weekend full of hype. They successfully got the mainstream media talking about science on Mars. As jaded and snarky as many of us are about NASA's frequent rediscovery of water on Mars, you can't deny that this a successful way to get space science stories into mainstream media. And it's no coincidence that today's science story meshes well with the politics of future Mars exploration, with the story NASA is trying to sell of scientific robotic exploration leading to future human exploration.
While I think the science is very cool, our new understanding that there may be extant liquid water appearing transiently all over Mars is actually not entirely good news for the future of Mars exploration. The reason why not is summed up in an article by Lee Billings for Scientific American: "Searching for Life in Martian Water Will Be Very, Very Tricky." The problem is one of planetary protection. We build all our spacecraft on a world that is just disgusting with life. Microbes bred on Earth can survive in every imaginable environment. We do our best to sterilize everything, but our best is not good enough; any spacecraft that departs Earth for Mars will necessarily carry stowaway microbes. How terrible would it be if we discovered microbes living on Mars, and then couldn't be sure that we didn't bring them to Mars from Earth?
NASA recognizes that the potential for contamination is a problem, so there is a Planetary Protection Office that is specifically charged with overseeing how missions avoid contaminating Mars with Earth biota. There are two main approaches. One approach is to sterilize the heck out of anything that will actually be touching Mars. That's why Curiosity's wheels were specially wrapped throughout its final assembly, and why it was such a scandal that the drill bits were handled after sterilization. The other approach is to avoid landing in any location where you might encounter -- or accidentally create, should you crash -- a present-day habitable environment where Earth microbes could thrive. For instance, current rules prohibit NASA from targeting a mission containing a hot radioisotope thermoelectric generator (such as Mars 2020) anywhere near a place where a failed landing might place that generator close enough to subsurface ice that the heat of the decaying plutonium could melt it.
But all bets are off once you send humans to Mars. There is absolutely no way to make a human clean of microbes. We are filthy with microbes, thousands and thousands of different species. We continuously shed them through every pore, every orifice, with every exhalation, and from every surface. True, almost all of our microscopic friends would fail to thrive in the radiation-baked, intensely cold and arid Martian environment. But life is incredibly tenacious. Sooner or later, humans will get to Mars; even if they die in the attempt, some of their microbial passengers will survive even the worst crash. Once we've put humans on the surface, alive or dead, it becomes much, much harder to identify native Martian life.
This is one of many reasons I'm glad that The Planetary Society is advocating an orbit-first approach to human exploration. If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars -- and obfuscation of the answer to the question of whether we're alone in the solar system -- for a little while longer. Maybe just long enough for robots to taste Martian water or discover Martian life.