NASA dropped big news this week: Billions of years ago, Mars was covered with lakes and streams, and had a water cycle not unlike the one on Earth today.

It’s not the first time we’ve heard news of liquid water on Mars. There have been major discoveries, including last month’s big news that water still flows on the Red Planet’s surface today.

But this week’s announcement is different. What it shows is that Mars was warm and wet for long, uninterrupted stretches of time, perhaps millions of years. Here on Earth, water is fundamental to life, and this raises the big question of whether similar conditions on Mars could have allowed lifeforms to evolve there, too.

Inverse reached Ashwin Vasavada, a project scientist with NASA’s Mars Science Laboratory, by email. He explains how we know what we know regarding water on Mars, and how his team is getting closer to evidence of Martian life.

Ashwin Vasavada

How do we know there were ancient lakes on Mars? How do we explain their existence?

The Curiosity rover has been exploring Gale Crater for over three years. As it crossed the plains toward a three-mile-high mountain in the center of the crater, images of the landscape began to reveal a pattern. Near the start of its traverse, the rover saw coarse gravel that would have been carried by flowing streams. Then we saw tilted beds of sandstone, like those within river deltas on Earth, that form when rivers and streams meet bodies of standing water. Finally, we saw layers of fine-grained rock that could form as sediment settles out within lakes. So we were able to see the entire “life cycle” of water on Mars, from the headwaters to the lakes, based on the sediments left behind.

What’s our best guess as to what Mars looked like 3.5 billion years ago?

Early Mars remains a mystery, but our results provide a very important clue. We have been debating for decades whether early Mars was cold and dry like today, but with occasional brief wet episodes, or whether early Mars was continuously warm and wet. The abundant evidence for water in the landscapes that date from early Mars were ambiguous; they could have formed either way. But now Curiosity has found evidence that requires a humid planet for perhaps millions of years. This means the atmosphere and climate were quite different from today, but we don’t yet understand how that happened.

Why doesn’t Mars look like that now?

One thing we’re quite sure of these days is that Mars once had a thicker atmosphere. This would have made it warmer, perhaps enough to sustain the ancient lakes that Curiosity discovered. But it didn’t last. Being a smaller planet, Mars cooled more quickly than Earth. At some point early on, it cooled so much that its interior no longer generated a magnetic field. Once that protective shield was gone, its atmosphere was slowly stripped away by the sun. The current, thin atmosphere on Mars is both too cold and too thin to support much liquid water. The NASA MAVEN mission is studying this same process as it occurs today on Mars.

OK, so what does this tell us about the possibility of past or present life on Mars? What does it tell us about the possibility of life elsewhere in our galaxy and universe?

In the past few decades, we’ve found oceans of liquid water all over the solar system. With water required for all life as we know it, the possibilities of life in the past and present have certainly increased. Most of these places have been underground on icy moons of Jupiter and Saturn. But with these discoveries on Mars, we now know that there was a rocky planet in the solar system other than Earth where abundant liquid water was at the surface for millions of years, a time long enough for life to potentially take hold.

What’s the next big Mars discovery going to be about, if you had to guess?

Ongoing exploration of Mars will continue to reveal how diverse and Earth-like the planet is, especially in its past. Who knows what big discoveries lie ahead! We are driven by the ultimate question of whether life ever took hold, but that is a very challenging question to answer.

A view from the Kimberley formation on Mars, taken by NASA's Curiosity rover.

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