When John Grant was only 16, the Viking landers were sent to Mars. Today, Grant  helps lead the operation groups controlling two Mars rovers,  Opportunity and Curiosity, as a geologist at the National Air and Space Museum’s Center for Earth and Planetary Studies. Recent data collected by Curiosity and published in Science describes an ancient lake environment located at Gale Crater—an environment Grant, a coauthor of the article, believes holds further clues to whether there was ever life on the Red Planet.

By analyzing the chemical evolution of the lake, scientists working with Curiosity were able to determine what conditions on Mars were like, “before there was significant oxygen in the Earth’s atmosphere,” Grant said. “It tells us that those conditions were much more Earth-like than perhaps even Earth was several billion years ago.”

At the time of the Viking mission in 1976, scientists knew little about Mars. Since then, adjustments have been made as to how rovers explore Martian land. You can’t just go to Mars and expect to find evidence of life, Grant mentioned. You need to know what you’re looking for and where to go, and you need to have the right tools to do so.

In fact, Curiosity was designed for the sole purpose of finding evidence of habitability on our neighboring planet. And for Grant, that means finding water.

“I’m interested in how geomorphic processes that shape the landscape have changed in terms of their number and intensity over time on Mars,” Grant said. “It all gets back to this question of the role of water.”

So far, Curiosity has made a number of major discoveries including: clays, which form in the presence of chemically-stable water, and stratified layers of soil — from the bottom portion  of Mount Sharp — that reveal the area was once wetter compared to the dry, acidic conditions of present day Mars. Curiosity is currently making its way higher up that mountain, where Grant assumes the soil will become drier and less habitable.

But because the environmental conditions recorded in  the rocks on Mars suggest similarities with what is observed in some lakes on Earth, Grant hypothesizes that if you were to put someone on Mars nearly 3.5 billion years ago, when the lake existed, they would have been able to drink the water. 

Finding formerly habitable locations like Gale Crater means that finding life on Mars is a possibility. In 2020, NASA’s next rover — as a part of the Mars Exploration Program — will have the capability to excavate and look for biosignatures, or the tiny signatures of past microbial life. It’s one reason why studying Martian fine-grained mudstones was so important.

Not only do mudstones tell scientists how the landscape on Mars has changed over time, they can also potentially hide tiny microfossils. A future rover's tools would be able to, “get down at that small scale and start looking at the details,” Grant said. “If there was ever life on Mars, in those ancient rocks is where you want to go and look, probably not in the stuff that’s formed today.”

Getting to those ancient rocks can be difficult, especially since most are covered with dust or sand or have been exposed to radiation for too long. Grant is looking to place the next rover where it can more easily access those older pieces, such as at a relatively recent meteorite impact site that may have created a deeper opening.

The new rover, though, won’t touch down at Gale Crater—an expansive 177-kilomter (110-mile) hole—at all. It will instead be placed in an alternative location because if there’s one lake, there’s bound to be another.

It’s a realization that excites Grant.

“You can’t have a lake in isolation while the rest of Mars is doing something else,” Grant said.

Much like Earth, Mars has huge canyons and giant volcanoes. Grant’s specific interests are rooted in dissecting how water has carved out its surface.

“My thinking about Mars as a place where water once flowed … has changed dramatically over the span of my career from ‘oh, there must be dribs and drabs of water here and there’ to, ‘oh my Gosh, there were lakes here that actually existed for long periods of time,’” Grant explained. “There were actually places on Mars that I didn’t think existed.”

And the observation of these places has only improved over time. Before Curiosity, Spirit, and Opportunity, pictures were taken of Mars from above and could only reveal so much. Grant could see there was the possibility of water flow, yet there was still the questions of how much and how it happened that needed to be answered. “Briney little pools?” Grant once wondered. “Or was it in long, large, lakes?”

Curiosity just came across some insight: lakes.

From the report, scientists will have a greater understanding of what ancient Martian lakes look like from all angles due to their similarities to water bodies on Earth. As of right now, Grant is helping the team determine where the next landing spot will be and they’re down to just three final locations.

After getting a greater understanding of the rocks from the other rovers, the next machine will be able to focus on identifying biosignatures. Still, Curiosity’s work was no small feat, especially considering it wasn’t guaranteed to last for more than one full Martian year (about two years on Earth).

It’s now been nearly five Earth years since Curiosity landed, and an additional eight since Opportunity  first started exploring the Red Planet. By functioning longer than anticipated and travelling over longer distances than expected, more doors of discovery have been opened, casting Mars in a fresh light.

“It’s almost like Opportunity’s been a whole new mission in the past four or five years,” says Grant.

So for Grant, the idea of finding life on Mars isn’t simple, but it’s certainly possible.

“How many worlds are out there now or in the past that might have had life on them?” Grant posed. “Personally, I think the odds are in favor of that, but it would be nice to actually prove it I think the big question is more of a philosophical one: are we really alone?”