Water on Mars: we need to be careful how we investigate other worlds

Once again, The Onion gets it right. “Emphasizing [sic] that they only wanted to hear announcements about actual cool stuff in space, millions of impatient Americans flat-out demanded Monday that NASA stop holding all press conferences until they discover some little alien guys”, it reports. The Onion’s archetypal “Indiana resident” is reported as declaring that “Nobody even cares that there’s some water on Mars; we have water on Earth—so you shouldn’t even bother having a stupid press conference unless there’s proof of alien guys with a bunch of eyes or tentacles or something… And none of that microscopic organism crap. It should be real alien dudes with way more arms than humans and that can talk using their minds instead of their mouths.”

Yet I’m guessing that you really do want to know more about water on Mars, which has been making headlines since NASA announced it yesterday. If so, here’s the science you need to know:

We are pretty certain that water once flowed on Mars in its distant past, and that it used to have substantial bodies of water on the surface, albeit probably rather shallow ones. There are plenty of geological features on Mars that can only be plausibly interpreted as dried-up river valleys.

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The idea that water might have flowed relatively recently (which is to say, within a few millennia), or even that it still does, is not new. Scientists have been talking about the evidence for it for several years now.

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All the same, the new results reported by NASA yesterday provide fairly persuasive evidence that the possible flows seen at least nine years ago on the slopes of some valleys near the Martian equator really are water, rather than dust or sand.

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This water seems to be extremely salty—much more so than our oceans. That’s why it can still be liquid at temperatures well below zero centigrade—salt lowers the freezing point.

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Whether any life could possibly survive in such an environment isn’t clear. The excitement about possible martian life (yes, we are talking microbes here, not malevolent purple tentacles) stems not so much from the idea that it subsists within these very shallow, short (a few hundred metres) and seasonally intermittent flows, but that there must be a water reservoir somewhere that is feeding the flow.

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We really have little idea how likely this is. And barring some other big and unforeseen discovery—NASA’s Opportunity or Curiosity rovers stumbling across a patch of mould, say—we aren’t likely to find out for a long time. Maybe a very long time. If we do, it’s more likely to be robots than Matt Damon who tells us.

Whether all this is exciting and novel, or humdrum and incremental, is a matter of taste. I’m settling down in my customary location on the middle of the fence. It definitely adds some spice to the study of Mars, which was arguably in danger of being eclipsed by still smaller yet more exotic worlds such as Pluto (which continues to supply head-scratching marvels but is almost certainly as lifeless as a neutron star), or the moons of the gas giants Jupiter and Saturn, some of which apparently host liquid water oceans beneath their icy crust that could plausibly sustain living forms. I’m just glad that claims of water on our neighbouring worlds no longer seem to elicit suggestions that we will (or should) get over there before Christmas to start building our colonies that will save the race from extinction.

The latest excitement is all about channels apparently carved into Martian slopes by small flash floods, which discharge perhaps a few swimming pools’ worth of water before soaking into the martian “soil” and drying or freezing. Such flows, creating small gullies, were first seen in images of martian craters taken between 2001 and 2005: the later images showed light streaks of what looked like watery flow that were not there in the earlier shots.

There was no definitive evidence that these were indeed made by water rather than by sandslides that exposed lighter-coloured material, though. Last year, streak-like gullies that recur on steep slopes during the martian spring and summer but fade in cooler seasons, called recurring slope lineae (RSL), were reported to be found extensively in the equatorial regions of Mars. What’s more, these features are more active when the slopes on which they occur face the sun, consistent with the idea that they were formed by water from melted ice on or within the surface.

Those images were taken by the Mars Reconnaissance Orbiter, a NASA spacecraft launched in 2005 that reached Mars the following year. Now that same spacecraft has found pretty compelling evidence that the RSL are indeed made by flowing water. The light reflected from these structures contains the distinctive fingerprint of hydrated salts (perchlorates, used on Earth in weed killer, bleach and rocket fuel). “Hydrated” here means that the crystallized salts contain water, implying that the deposits are made by the drying up of transient flows of concentrated brine. These salt deposits have been seen in four equatorial regions containing RSL, particularly in the massive canyon system called Valles Marineris that scars the planet’s midriff for over 4000 km. The findings, say the researchers, “strongly support the hypothesis that recurring slope lineae form as a result of contemporary water activity on Mars.” No one is too sure where the water comes from, but there seems to be some kind of water cycle supporting the flows, which raises the possibility of other wet spots elsewhere—where, just maybe, life might eke out a precarious existence.

The hyper-salty water in the RSL flows doesn’t itself look too promising as a habitat for martian microbes. But one thing life on Earth has taught us is that we can almost never say never: our planet is peppered with microbial “extremophiles” that exist in the most seemingly inclement conditions, including extremes of hot and cold as well as degrees of saltiness that would desiccate our own cells. Actually we don’t have any compelling grounds for supposing that water itself is a sine qua non of life on other worlds. But given that life on Earth seems to have got going almost as soon as liquid water covered most of its surface, it’s reasonable to suppose that liquid water is generally a very inviting medium for it. The deeper question than whether life can exist in very cold and very salty water is whether a biosphere can sustain itself at all in the marginal way that would seem to pertain on Mars. James Lovelock and others have argued a rather convincing case that life on any planet would have to either colonize it totally and transform its environment, as has happened on Earth, or die out.

What are the prospects for getting a closer look? These steep gullies aren’t the kind of place Curiosity and Opportunity can go, but NASA is already apparently working on cliff-climbing robots, and you can feel confident that such machines will be ready for the journey far sooner than we will. But should we visit these places anyway? One of the big concerns about sending explorers to Mars is that we might end up contaminating the planet with terrestrial microbes that could interfere with or eliminate any life forms there. Even robot explorers are potential contamination hazards, since some terrestrial organisms have been found to be stunningly resilient to the rigours of space travel, and so strict sterilization procedures are needed. The international Committee on Space Research (COSPAR), whose protocols for planetary exploration guide the US, Europe, Russia and others, have defined Special Regions on Mars, rather like endangered habitats on Earth, where life might conceivably exist and which should therefore be treated with special caution, if not avoided outright.

It might strike some people as ironic that the very places where life on other worlds would be most probable are the ones we can’t explore more thoroughly—and that the more possible martian life seems, the more concerns about contamination might deter us from investigating it. Should we really be so fastidious over the nascent concept of interplanetary ethics when faced with the chance of such a culturally transformative discovery?

For what it’s worth, I’m on the martians’ side: I think we do have a duty to respect and protect other worlds, not least because of the mess we are making of our own. It’s frustrating for sure, but in any case I suspect The Onion is not so far from the mark anyway in gauging the true public response to any discovery of martian microbes. They would be eclipsed the next day by the latest stew over Jeremy Corbyn.