Once upon a time on Mars, there was a crater that had a massive lake that may have hosted life. Now researchers are saying that a whole variety of organisms could have flourished there. Sure, that life was probably just microbial, but this is another exciting step toward understanding just how habitable Mars may have been around 3.5 billion years ago.

Petrified mud that was once at the bottom of the lake suggests that, at the time, the lake had different chemical environments that could have hosted different types of microbes. The rocks also show that the Red Planet’s climate may have been more dynamic than we thought, going from cold and dry to warm and wet, before eventually drying out.

We still don’t know whether life once existed on Mars when the planet was warmer and had liquid water. But today’s findings, published in Science, give a much more nuanced and detailed picture of what this area of Mars could have looked like through time. The paper’s results will have to be tested by analyzing more rocks in the crater. But it could have big implications on how we think about the Red Planet’s climate and how habitable Mars was.

“The data and interpretations presented here provide necessary clues to answer the big open question: ‘What happened to the water on Mars?’” Isaac Smith, a postdoctoral researcher at the Planetary Science Institute, who was not involved in the research, writes in an email to The Verge. Smith called the findings "impressive."

Since landing inside the 93-mile-wide Gale Crater in 2012, NASA’s Curiosity rover has been drilling and analyzing rock samples. The rover has discovered the chemical ingredients — like carbon, nitrogen, oxygen, phosphorus, and sulfur — needed for life as we know it on Earth. “The lake had all the right stuff for microbial life to live in,” says study co-author Joel Hurowitz, a geochemist and planetary scientist at Stony Brook University.

Curiosity is currently climbing up a three-mile-high layered mountain inside the crater. This mountain, called Mount Sharp, is believed to be the result of mud and sediments carried through time by rivers into the lake, where it eventually petrified into mudstone. Once Mars dried out, wind carved those layers of mud into the shape of a mountain, says study co-author Ashwin Vasavada, a Mars Science Laboratory project scientist at NASA’s Jet Propulsion Laboratory.

The authors of today’s study analyzed the chemical and mineral composition of the mudstones that were drilled by Curiosity, to see what they could reveal about the lake and the ancient Martian climate. Certain minerals in certain rocks were shown to have interacted with oxygen, while others much less so. This is because some sediments were in shallow waters, where there was more oxygen, while others were in deep waters, where there was very little oxygen, according to the authors. The same happens in deep lakes on Earth, Vasavada says — deep down, there’s not much oxygen.

“This is a new level of detail in terms of our understanding of the chemical environment in this lake on Mars,” says Hurowitz. “It gives us a much more complete picture of the habitability of this lake.”  

The findings basically show that the lake in Gale Crater had different areas — some rich in oxygen, some not — that could have supported different types of microbes. Some microbes thrive in low-oxygen environments, while others require more oxygen to survive. Reactions between oxygen and other chemicals also provide forms of energy certain microbes can feed off of. It’s further proof that, billions of years ago, the lake could have hosted life.

“Liquid water is important for habitability, but providing an energy source is also important,” Janice Bishop, a chemist and planetary scientist at the SETI institute, who was not involved in the research, writes in an email to The Verge.

By analyzing the mudstones, the researchers were also able to figure out the types of climate in which the rocks formed before being transported to the lake. The chemical composition of the older sediments was altered less than in the younger sediments. That means that the older sediments were formed at a time when the climate was cold and dry, while the more recent ones were formed when the climate was warmer and wetter. This suggests that Mars’ climate — at least in this region of the planet — was changing over the years, going from cold and dry to warm and wet.

That seems counterintuitive, since the current idea we have of Mars’ climate is that it went from being warm, with liquid water, to cold and arid, as it is today. The findings suggest the story is more complicated than that, with wet and dry periods that possibly fluctuated through thousands or even millions of years. “This is going a step further, saying that yes... the climate transitions are more complex than we thought,” says Kristen Bennett, a postdoctoral research associate at Northern Arizona University who studies Martian sedimentary geology, who was not involved in the research.

The study has some limitations. It’s only about roughly 330 feet of sediment layers in one corner of Gale Crater, says Bennett. It also represents one interpretation of what could have happened to the sediments, but it’s definitely not the final word. As the study points out, rocks and sediments are altered by many things — like where they’re from, how they interact with the river water that’s transporting them, and how acidic the lake water is, says Bethany Ehlmann, a research scientist at NASA’s Jet Propulsion Laboratory, who did not take part in the research. So there are many ways we can interpret, say, why certain minerals in the sediments show interaction with oxygen. Maybe it doesn’t have to do with the lake water, but with groundwater infiltrating the rocks, Ehlmann says.

So more research will need to happen to prove that the lake had different chemical environments and that Mars’ climate fluctuated as widely as today’s findings suggest. But as Curiosity keeps going up Mount Sharp, it is collecting more sediment samples. Several tens of meters of layers will be analyzed in the new few months by the rover, so within a year, “we’ll have a chance to test the conclusions of this paper,” Ehlmann says.

Still, if confirmed, today’s findings could change how we interpret the planet’s climate and just how habitable ancient Mars was. In fact, one suggestion from the paper — that Mars had warmer and wetter climates that permitted liquid water in lakes as recently as about 3 billion years ago — is pretty significant, since that’s later than some scientists thought. “It pushes the existence of liquid water closer to the present,” Ehlmann says. And the closer we get to understanding what ancient Mars was like, the closer we come to answering the big question: whether the Red Planet actually hosted life.