Few people have inspected the canyons and craters of Mars like Alfred McEwen, a planetary geologist at the University of Arizona in Tucson. He’s the principal investigator for the high-resolution camera on the Mars Reconnaissance Orbiter (MRO), which entered orbit in 2006, revealing features as small as a loaf of bread. With 55,000 observations so far, the craft has exposed about 2% of the planet to spy-camera scrutiny.
One of McEwen’s most important finds came in 2011, with the discovery of recurring slope lineae (RSL), thousands of temporary streaks along steep slopes, mostly near the equator, that gradually grow and darken as spring turns to summer, as if fed by seeps of water. They soon became cited as the best evidence for liquid water on the surface of Mars today—and also one of the best places to search for microbial life.
But McEwen has now doused some of the excitement ignited by his initial finding. In a study published online this month in Nature Geoscience, he and his colleagues analyzed 151 of the streaks, finding that they only occur on slopes steeper than 27° and always peter out when the angle drops below that. The researchers interpret this as a sign that the RSL are not formed by water—which would flow down shallower slopes—but rather are dry flows of sand and dust seeking their natural angle of repose.
Science recently caught up with McEwen to chat about his latest findings on Mars, and whether we’ll ever discover life on the Red Planet. This interview has been edited for brevity and clarity.
Q: You’ve actually been cautious from the beginning. Did you come up with this strange name—RSL—so as to avoid presuming any origin?
A: Yes. We wanted a name that was purely descriptive, even though it’s a horrible name from a press outreach perspective.
Q: Are you frustrated when you see loud "water on Mars" headlines? What would your headline be?
A: I’m bemused by it. Ninty-nine point nine nine percent of science results are incremental. You read the news media and you think it’s all breakthroughs. In our first Science paper, the title was, ”Seasonal flows on warm martian slopes.” They are still seasonal flows on warm martian slopes—that has not gone away.
Q: You already had reasons to doubt that RSL are watery seeps.
A: We’ve always stated it’s difficult to explain where this water comes from in the current martian environment. The atmosphere is so dry—you’d have to create a bizarre mechanism that sucks water out and concentrates it in these areas. There may be deep pockets of groundwater in Mars’s crust today, I don’t doubt that. But how do you get that to the surface? It would be expected to come out in specific locations: near a fault at the bottom of Valles Marineris, for instance. Instead, we see these coming out of all the slopes, even at the top of ridges and isolated peaks. That really does not make sense at all for groundwater.
Q: In 2015, scientists used a different instrument on the MRO to find evidence that the streaks contained hydrated salts. Isn’t that definitive evidence that water is involved somehow?
A: That stands as a significant result with water playing some role here. It doesn’t mean flowing water. Which, unfortunately, is the way the NASA press release played it. It was just before the movie The Martian came out.
One hypothesis would be that there is water being trapped from the atmosphere into a thin subsurface layer that is destabilizing the overlying grains. Then when they slip, you expose this damp soil, which is dark. Once it’s actively exposed, it dries and fades. Whether that works in detail hasn’t been explained. Overall, these things are still mysterious.
Q: So, you’re still leaving open the possibility of thin films of water?
A: That’s correct. They’re not only possible, but likely somewhere on Mars. Highly deliquescent [water-absorbing] salts are known to be abundant on Mars. It’s known that a small amount of water is sufficient to darken the surface. But it’s not enough water to flow downhill. It’s not enough to fill the pore space.
Q: How good of a microbial environment would these thin films be?
A: It’s very salty water. That makes it challenging for life as we know it. Honey is a good example of that. Honey is a liquid—it contains water and yet it never spoils.
Q: What will resolve the mystery of the RSL?
A: We’re continuing to learn from monitoring in orbit. If this is related to deliquescence from the atmosphere, we should see a difference in brightness and darkness, with time of day. To really understand these, you’d probably need to go there and really examine it up close. There’s been a lot of talk about this, but there’s no definite plan. These are difficult places to access.
Q: But aren’t these areas off limits to exploration because of the possibility of contamination with earthly microbes?
A: They’re not off limits to exploration, but you have to satisfy planetary protection protocols, which is sometimes called sterilization even though that’s impossible. You reduce the bioburden [of microbes] to some arbitrary limit that they judge to be possible. It’s a good concept in general, but for Mars they have this concept of “special regions.” That’s walking a tightrope—there isn’t any place that we know is habitable to known life and we can’t rule out the possibility that any place isn’t habitable. It’s either all special or none of it is.
Q: What is your favorite thing you’ve seen on Mars?
A: I hate that question. I like everything so much. In general, the surprises have been the activity of modern Mars. The RSL—and dunes that are active. New impact craters are more interesting than we expected—some of them expose remarkably clean ice.
Q: How have your views of the planet changed over the years?
A: My views of the current geological activity have changed. Gullies [channellike incisions, larger than the RSL and discovered in 2000 by the Mars Global Surveyor]—pretty much everyone thought they were ancient and caused by water. What we’ve learned is they’re forming today, and they’re forming in the winter, when there’s [frozen carbon dioxide (CO2)] on the ground. It’s not water at all, it’s this CO2 fluidization process. Basically these are dry debris flows charged with CO2.
Q: Do you think of Mars as a hospitable place?
A: It’s inconceivable to me that there aren’t places where there’s liquid water today within Mars. If there was ever life on Mars—that originated somewhere—why wouldn’t there still be life today in these underground pockets? The surface, on the other hand, is a very harsh environment for life.
Q Should the search for life be focused on Mars? Or should NASA and other agencies be exploring the ocean worlds that orbit Jupiter and Saturn?
A: I’m heavily involved in one mission—the Europa Clipper. Extant life there is much more likely today than on the surface of Mars. On the other hand, they’re further away, and Europa in particular is in a harsh radiation environment. I think we should do both.
*Correction, 28 November, 3:46 p.m.: A previous version of this story overestimated the water content of honey.