“Follow the salt”: A new strategy for finding life on Mars

For more than 20 years, NASA’s search for life on Mars has hinged on a single, simple strategy: Follow the water. It makes sense. Every living thing on Earth — or every known life form, at least — needs water to survive, so tracing the course of past Martian rivers and lakes could ultimately lead us to the treasure we seek.

But life is infinitely resourceful, and not every organism uses water in the same form, or in the same way. Our failure to fully understand this may have resulted in an ironic tragedy nearly 50 years ago: We may have accidentally killed the only life we ever found on Mars. 

I make that case in a new paper published in Nature Astronomy, in which I also argue that our current exploration strategy has us looking for Martian life in the wrong places, or at least not enough of the right places.

Hardy survivors

What drives me to that conclusion? In trying to assess whether life might have arisen on other worlds, we have to consider their natural history. More than four billion years ago, Mars was still a water-rich planet, with lakes and probably oceans on its surface. Since then, it’s been extremely dry. By studying Mars-analog environments, such as the hyperarid regions of the Atacama Desert, we see a predictable sequence of ecological transitions that occur in such places.

As conditions become increasingly arid, bodies of liquid water dry up, leaving behind desert ecosystems with patchy areas of life clinging underneath rocks or in sheltered habitats. As the climate becomes even more extreme, with almost no rain whatsoever, microbial life retreats inside salt rocks. These hardy survivors are hygroscopic, meaning that they get their meager amounts of life-sustaining water from moisture in the atmosphere, without any need for surface water or rain. 

Hygroscopicity is the ability of certain substances to draw water directly from the air. Have you ever wondered why people put grains of rice inside salt shakers? It’s because rice is even more hygroscopic than salt. If you leave either one out in humid air, they become clumpy and moist.    

Research in the Atacama has shown how indigenous microbes use this property to adapt to excessive dryness. These microbes have adapted so well that what we consider normal amounts of water can be deadly to them. Armando Azua-Bustos and his colleagues from the Centro de Astrobiologia in Madrid, Spain, reported that during an unprecedented rainfall event in the Atacama, 75% to 87% of the indigenous microbial species went extinct. The sudden influx of water overwhelmed the organisms, causing hyperhydration — essentially drowning them.

Now consider that Mars is quite a bit drier than even the most arid part of the Atacama. Martian organisms — if they also are hygroscopic —might suffer the same fate if someone poured water on them.

That’s exactly what may have happened after the twin Viking landers touched down on Mars in 1976. Knowledge about life in hyperarid settings wasn’t as advanced when the Viking life detection experiments were designed. At the time, scientists didn’t know the Martian environment as well as we do now, and it was assumed that liquid water is as essential for life there as it is on Earth. There couldn’t be too much of a good thing, could there?

Well, maybe there was. The inconsistent and confusing results from the Labelled Released Experiment, which was supposed to show microbial metabolism, can be explained by the application of too much water. Intriguingly, the Pyrolytic Release Experiment, which was supposed to show organic synthesis reactions attributable to life, was strongly positive in the first attempt under dry conditions. But when repeated after adding water, it returned the lowest measured value of all seven attempts.

Did we already find life on Mars?

At the time of the Viking experiments, these contradictory results were interpreted to be inconclusive or negative for life, mostly based on the lack of organic compounds found. How could there be life without organics? But later missions in the 21st century — Phoenix, Curiosity, and Perseverance — did find organic material. With hindsight, trace organics found by Viking, which were interpreted at the time to be contamination, were also mostly likely organic compounds indigenous to Mars.

Assuming there is life on the Red Planet, how does it make a living? The main argument against the possibility of Martian life is the sparse amount of water. The technical term is water activity, which ranges from 0 to 1 (0 being no available water; 1 being 100% available water). If the water activity is below a certain threshold, which differs depending on the microbial species, these organisms cannot survive. The minimum water activity for the hardiest life on Earth is currently thought to be between 0.54 and 0.59.  

There’s a clever way around this dilemma, though. Life can temporarily become inactive or dormant when the water activity dips too low, only to resume its activities once it is sufficiently high again. When scientists measure water activity, they usually measure it on a macroscopic scale. Look closer, though, and the picture changes. In one study I participated in, the overall water activity of a liquid asphalt lake was 0.49, meaning no life would be expected. But there were tiny water droplets dispersed within the asphalt matrix, and those droplets contained a whole ecosystem of bacteria where water activity was much higher. What counted for these microbes was the microscopic water availability. To them, a single drop of water was like an ocean.

And as we’ve seen from hygroscopic effects on Earth, the atmosphere itself is a source of water. Many types of salts have this property — some even attract so much water that they dissolve into the water they absorb, which we call deliquescence. The most common of these is common table salt, but others include calcium chloride and sodium perchlorate.

The sweet spot of water activity

That brings us to the adaptive trick that I believe microbes on Mars could use. Bear with me. The “deliquescence relative humidity” is the humidity at which a specific salt attracts water from the atmosphere to be dissolved in. Meanwhile, the “efflorescence relative humidity” is the humidity level at which a salt gives back all its moisture to the atmosphere. These values are different because of the additional energy required to crystallize salt minerals. For example, the deliquescence relative humidity of sodium perchlorate is between 38% and 64%, the level at which moisture from the atmosphere starts to dissolve the salt. But the efflorescence relative humidity of sodium perchlorate is only 13%. That’s the point at which all the water from the salt goes back into the atmosphere.

In the sweet spot between those two values, there is a window for higher water activity on a microscopic level, which may last for several hours. Higher water activity also results when atmospheric moisture exceeds the deliquescence relative humidity. It sounds strange, but Mars sometimes has 100% relative humidity, as evidenced by the Viking landers’ observations of frost on the surface. 

Whether these periods of increased water activity last long enough — with levels high enough for microbial life to make a living — is unclear. It would be challenging for terrestrial life forms. But Martian organisms, if they exist, would have had billions of years to adapt to these harsh conditions. 

Either way, it seems clear from studies of the dryest places on Earth that we should follow the salt on Mars if we want to find sources of life-sustaining water. The strategy for the next generation of Mars missions should be to search for hygroscopic salt deposits of the kind seen in Eastern Margaritifer Terra or the planet’s Southern Highlands. I believe that would give us the best chance of discovering indigenous Martian life if it exists.