Billions of years ago, Mars was speckled with murky lakes that may have been home to microbial life, raising the tantalizing possibility that Martian fossils might be buried in the dessicated remains of these ancient waters, which are known as “paleolakes.” Scientists even speculate that briny liquid lakes may still flow under the red planet’s ice caps, perhaps providing a final refuge for microbial Martians—though the odds of extant life on Mars are hotly debated.
The prospect that Martian paleolakes could contain signs of life, or “biomarkers,” has galvanized scientists to scrutinize these dried-out formations from space with orbiters, and to send missions, like NASA’s Perseverance rover, to search for biomarkers on the surface of Mars. Some 500 paleolakes have been identified on Mars, but scientists believe that hundreds or thousands more are waiting to be discovered in this “new era of Martian limnology,” meaning the study of freshwater ecosystems, according to a study published on Thursday in Nature Astronomy. Scientists led by Joseph Michalski, an associate professor of Earth sciences and deputy director of the Laboratory for Space Research at the University of Hong Kong, outlined how lifeforms might have thrived billions of years ago on Mars, and emphasized the importance of Martian lakes in the broader field of astrobiology, which is the study of life in the universe. “For this important topic, I think by doing a meta-analysis of all the data that are out there, and some of our own analyses, we are able to shine a light on some fundamentally new aspects of lakes on Mars,” Michalski told Motherboard in a call. “Lakes are considered by most of the space community to be the astrobiological hotbed for exploration,” he continued. “That's just because on Earth, lakes are generally teeming with life. They have water, a lot of nutrients, and organic materials for building life, and access to the Sun, for photosynthesis, or chemical energy. On Earth, they are full of life, so on Mars it seems like the right place for life to exist, in many peoples’ view. So, we felt it was time to have a fresh take on lakes.”
For instance, the study said that orbiters have mapped out hundreds of large lakes on Mars, and provided insights about their potential habitability in the past, but adds that an estimated 70 percent of Martian paleolakes remain unknown because they are too small to be spotted from space at this time.Most paleolakes discovered on Mars existed in craters formed by ancient impacts, but there’s very little observational evidence of other types of lakes, such as those etched out by ice on mountaintops or left in the wakes of glacial flows. On Earth, lakes formed by tectonic activity can be especially large and long-lived, providing a sedimentary record that can stretch many millions of years into the past, whereas on Mars, this class of tectonically formed lake is virtually unknown.As a result, we are “dramatically underestimating the numbers,” and diversity, of Martian paleolakes, Michalski said. “If we were able to send rovers to an ‘everyday’ part of Mars and just drive around, you’d probably find small lakes if you drive far enough.” “It's just that we don't send rovers to everyday Mars, we send them to the most unusual places and we make a lot of conclusions about everyday Mars from the unusual places,” he added. “That’s just a peculiarity of exploration.”
What we know, and still don’t know, about these bygone Martian lakes is crucial for understanding whether they bore life billions of years ago. While these aquatic environments contained freshwater and nutrients, making them somewhat similar to lakes on Earth, they were also alien environments shaped by Mars’ unique properties. For instance, Mars has only about 38 percent of the surface gravity of Earth, which means that sediments that might sink to lakebeds on our planet would have remained suspended in Martian waters, intermixing with the water column and creating a much murkier environment. Mars is also farther from the Sun than Earth, and the Sun was dimmer in the past, which means that any photosynthetic microbes that arose on Mars would have to make do with just a third of the solar radiation that is available to sunlight-eaters on our planet.Martian lakes were also short-lived compared to lakes on Earth, with most lasting no more than tens of thousands of years. These disappearing lakes put a time limit on microbial communities that might have occupied them.Even with these constraints, Michalski and his colleagues outlined many different forms of life that could have thrived in Mars’ lakes, from radiation-resistant organisms, to microbes that feed on chemical energy, to ultra-efficient photosynthesizers. If briny lakes do still exist deep under the ice of modern Mars—which is a very big assumption—it’s unlikely that new life would emerge there, Michalski said, though microbial survivors from past eras could potentially take shelter in these subterranean habitats.In this way, the new study pulls together what we know about Martian paleolakes, and charts a course to a future of new discoveries that could reveal whether or not life ever existed on this neighboring world. The implications of finding clear biomarkers on Mars cannot be overstated, as the presence of life on two worlds in our own solar system would imply that aliens might be common throughout the universe. Michalski said he wouldn’t be surprised if life existed on Mars in the past, or perhaps even now, but noted that the question should transcend a simple “yes or no” answer.“Our agenda should be to try to understand how life forms, not to look for life,” he said. “Looking for life is a binary ‘’win or lose’ thing, at least in the public eye.” “What I would personally be doing is trying to think about origin-of-life environments, and trying to target origin-of-life environments for Mars, and look at the chemistry and the geobiology in those settings,” Michalski concluded. “If there's no progress toward life, and all the conditions were the same, then that's pretty interesting and might mean you need some other variable that we know about to catalyze or stimulate the process.”