NASA’s Curiosity rover has uncovered evidence of what looks like an ancient Martian lake that might have supported life.

Close examination of sedimentary rock at a site called Yellowknife Bay, a 16-foot-deep trough in the 93-mile-wide Gale Crater near the Martian equator, shows signs that the area once harbored a lake. The watery reservoir may have lasted for tens or even thousands of years, according to the geological traces it left behind after drying up. Further analysis of the rocks also suggests the ancient lake would have been freshwater, and relatively calm, and would have contained key biological elements.

"It is important to note that we have not found signs of ancient life on Mars,” Imperial College London researcher and Curiosity mission member Sanjeev Gupta said in a statement. “What we have found is that Gale Crater was able to sustain a lake on its surface at least once in its ancient past that may have been favorable for microbial life, billions of years ago. This is a huge positive step for the exploration of Mars.”

All of those ingredients add up to an environment that could have been home to simple microbes that feasted on minerals inside Mars rocks. Earth has such creatures, in caves and near hydrothermal vents under the sea; they’re called chemolithoautotrophs.

Some of the key evidence for a lake on Mars comes from mudstones, which are a kind of rock that can be formed from clay or mud. Curiosity drilled into a particular formation of this sediment known as the Sheepbed mudstone. In six papers published in the journal Science, researchers from across the globe dissected the geological and chemical evidence locked in the mudstones. One interesting insight is that, based on an analysis of decaying radioactive isotopes in the mudstone, the rocks of Yellowknife are about 4.2 billion years old, and have only been exposed on the surface for about 78 million years.

“The surprising result is that… Yellowknife Bay may not only preserve evidence of a habitable environment, but one that is relatively young by Martian standards,” Caltech researcher John Grotzinger and colleagues wrote in their paper for Science.

Since it landed on Mars in August 2012, Curiosity has been working tirelessly (it has punched more than 100,000 tiny holes in the Martian surface with its laser). In March, Curiosity unearthed the first signs that the Red Planet contains the basic building blocks for life, including the elements carbon, sulfur, nitrogen and hydrogen. But questions remain: could life be still lurking somewhere on Mars, perhaps near its icy poles? Curiosity has been sniffing the Martian winds for signs of methane, one of the clearest signatures left by biological life, but so far has come up empty. But even if true Martians died out eons ago, we still may be able to find some sign that they once thrived on the Red Planet.

"It is exciting to think that billions of years ago, ancient microbial life may have existed in the lake's calm waters, converting a rich array of elements into energy,” Gupta said. “The next phase of the mission, where we will be exploring more rocky outcrops on the crater's surface, could hold the key whether life did exist on the Red Planet."

SOURCES: Grotzinger et al. “A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars”; Vaniman et al. “Mineralogy of a Mudstone at Yellowknife Bay, Gale Crater, Mars”; McLennan et al. “Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars”; Hassler et al., “Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover”; Ming et al. “Volatile and Organic Compositions of Sedimentary Rocks in Yellowknife Bay, Gale crater, Mars”; Farley et al. “In Situ Radiometric and Exposure Age Dating of the Martian Surface,” Science published online 9 December 2013.