As evidence from NASA’s Curiosity rover and other probes has made clear, the Red Planet was once much bluer, and probably had lakes, rivers, and maybe a vast ocean in its northern hemisphere. But over time, 87 percent of Mars’ water escaped into space, and its surface became increasingly hostile to life. Many researchers think that conditions became unbearable for organisms sometime billions of years in the past.
But as Mars dried out there would have been “huge evolutionary pressure to adapt to drier and drier conditions,” said environmental scientist Dirk Schulze-Makuch of the Technical University Berlin in Germany.
In a recent paper in the journal Astrobiology, Schulze-Makuch and his co-author, astrobiologist Alfonso Davila of NASA’s Ames Research Center, explain how Martian microbes might have crawled out of Martian oceans, evolved ways to deal with decreasing water, and colonized the last hospitable spots on the Martian surface. Referencing analogous biomes on Earth—including Antarctic dry valleys and the Atacama Desert—they have created a timeline for how organisms might have survived such extreme environmental changes. The findings suggest that our exploration efforts should focus on these final outposts in order to have the best chance of discovering life on Mars.
In Schulze-Makuch and Davila’s thinking, microbial organisms could have evolved independently on Mars or been carried there in meteorites from Earth. Either way, it seems possible that there once a teeming biosphere in ancient Martian waters. The shorelines of lakes and ponds on the Red Planet would have been the perfect places for organisms to emerge onto land, just as they did on our own world billions of years ago.
As Mars dried out, conditions would have begun to resemble arid places on Earth such as California’s Mohave Desert; mostly rocks and sand with a few hearty plant species growing in small patches. Complex communities of microbes form what’s known as a biological crust in the top few centimeters of soil in such regions on Earth, surviving on intermittent rains and snows. Some analogous population of species might have lived on Mars during a period known as the Hesperian, which lasted up until around 3 billion years ago.
But at a certain point, there would no longer be enough water to sustain so much biomass, and life would have to find new coping strategies. In hyper-arid areas on Earth, such as the Namib Desert in Southern Africa and the outer edges of the Atacama Desert, microbes cling to the surface and underside of rocks, which protect against wind, radiation, and wild temperature swings, as well as providing areas where fog and dew accumulate and then evaporate slowly. Some species, known as endoliths, have even evolved ways to live inside rocks. Hypothetical Martian life might have held on in this way for another couple billion years, exploiting small rocky niches all over the Red Planet.
Finally, there is the driest of the dry—the most extreme environments on Earth, such as the McMurdo Dry Valleys in Antarctica and the core of the Atacama Desert, where no rain once fell for 400 years. But even in the Atacama, relatively abundant communities of bacteria and archaea can be found inside of porous salt crystals. As anyone who has picked up a salt shaker on a humid day and found nothing but a solid clump can tell you, salt is hygroscopic, meaning it pulls water out of the air. This tiny amount of condensation is enough to allow for slow-growing organisms to eke out an existence. Salt-tolerant microbes on Mars could have survived in a similar setting.
Does that mean life has made it to the modern day? The current aridity on Mars is anywhere from 2 to 50 times drier than the Atacama, said Schulze-Makuch. “That’s right at the edge,” he said. “So it’s a difficult call to say if life is still around or not.” In the coldest and driest of Antarctica’s dry valleys, for instance, many microbial communities appear to be headed toward extinction. “They’d rather be somewhere else,” said Davila. “They’d be happier in the tropics.”
Even if Mars is currently too harsh and parched to sustain organisms, that doesn’t necessarily spell doom. Because it lacks large moons, the Red Planet’s axial tilt is somewhat unstable, leading our near neighbor to occasionally tip over like a spinning top. When this happens, water from the poles evaporates and redistributes all around the planet. Such events occur on hundred-thousand- or million-year timescales, but these wetter periods could allow life to hang on.
Under extreme duress, Earth organisms are known to form what’s known as endospores or microbial cysts, small seed-like structures that carry their genetic information and are extremely resistant to radiation, temperature, and dryness. Some scientists suspect these spores can last for millions of years or more and remain viable. Perhaps life on Mars has also figured out this trick and is just waiting for the next rain to appear so it can bloom.
But such long-term cycles might also mean that the processes of natural selection would have broken down. During the brief wet window, an organism would most likely be focused on collecting enough energy to grow a bit and then safely go dormant again, with little to no time left over for reproduction. “Evolution can give microbes the tools to survive,” said Davila. “But there’s a point where evolution doesn’t work anymore.”
Regardless of whether organisms on Mars have made it to today, Davila thinks that our exploration efforts should focus on these final potential outposts—the interiors of salt crystals and other rocks—in order to have the most recent and least degraded examples of Martian life. “We need to find these last possible habitats because that’s where the biochemistry would be preserved,” he said.