Early Mars Climate was Complex, with Streams Flowing Intermittently for Millions of Years

There’s overwhelming evidence that Mars was once wet and warm. Rivers flowed across its surface and carved intricate channel systems revealed by our orbiters. Expansive oceans even larger than Earth’s may have covered a third of its surface. Then something happened: Mars lost its atmosphere, cooled down, and surface water disappeared.

But as our observations of Mars become more detailed, it’s looking like Mars didn’t lose its water in one cataclysmic episode. Deepening evidence shows that it lost its water gradually. The planet may have had recurring episodes of surface water that persisted intermittently over a longer period of time. If that’s true, it has implications for potential life on Mars.

New research published in Earth and Planetary Science Letters is bolstering the idea that Mars may have taken a long time to lose its water. The research letter is titled “New Maximum Constraints on the Era of Martian Valley Network Formation.” The author is Alexander Morgan, a research scientist at the Planetary Science Institute who studies geomorphology.

“Mars today is a global desert, but its surface preserves extensive evidence of past flowing water, including what appear to be river valleys,” Morgan says. “The timescale over which these valleys formed has big implications for early Mars’ habitability, as long eras with stable liquid water would be more conducive to life,” he said.

The extensive evidence of ancient rivers on Mars is highlighted by the Perseverance Rover and the area it is exploring. It’s called Jezero Crater, and it’s the site of an ancient impact crater. At some time in the past, the crater was flooded with water, creating a massive sedimentary basin. Orbital images of Jezero show ancient river channels flooding into the gigantic crater.

As Perseverance’s landing site shows, impact craters and rivers mingle with one another across the Martian surface. That fact is the key to Morgan’s research. By dating craters near river channels, he placed temporal constraints on when the rivers that created the channels were flowing.

“In this study, I used craters that predate and postdate valley systems to place maximum bounds of hundreds of millions of years on the era over which these systems formed,” Morgan said. “Previous work had only determined minimum timescales, so these new results provide an upper bound on the timescale over which Martian valleys were active. Given what we know about erosion rates on early Mars, longer timescales imply that conditions permitting rivers were highly intermittent, with long arid periods interspersed with brief episodes of fluvial activity.”

Mars’ river valleys formed over three billion years ago. They’re the strongest evidence that the planet had surface water. Research shows that it takes tens of thousands of years for flowing water to carve a valley into the surface, but nobody has figured out how many different flow events there were and how much total time it took for these valleys to form. Until now.

Our understanding of Mars has grown considerably in recent years and will keep growing. Our understanding of its climate history is undergoing a revolution. Previously, there were two opposing versions of Mars’s ancient past. One says that it was warm and wet and potentially habitable; the other says it was a frigid planet covered in ice sheets.

But things in Nature are seldom so simple, even if we’d like them to be. Growing evidence, including this work, shows that there’s more complexity to the story than either “warm and wet” or “cold and dry” can encapsulate.

“Over the past decade or so, we’ve come to realize that these descriptors are far too general, and it doesn’t really make sense to try to condense hundreds of millions of years of climate history into a two-word description,” Morgan said.

As we’ve studied Earth, we’ve come to realize that the climate oscillated wildly during its long history. During some periods, the Earth was covered with extensive glaciers several kilometres thick. At other times, the glaciers retreated to their mountain redoubts. Why wouldn’t other planets have equally as varied histories?

“Like Earth, early Mars was complex, and the conditions permitting surface water likely varied considerably. Earth has undergone massive climatic changes throughout its history – for example, 20,000 years ago, the area that is now Chicago was beneath half a mile of ice – and surface conditions permitting rivers on early Mars likewise probably waxed and waned.”

That waxing and waning means it took a long time for the rivers to erode the landscape and form channels and valleys. One possible explanation is that large boulders in the riverbeds inhibited further erosion. Another is that the rivers flowed infrequently, possibly as little as 0.001 % of the time. If that’s the case, it could be because of what we call Milankovitch cycles here on Earth.

Milankovitch cycles are changes in the Earth’s relative position and orientation to the Sun. Things like axial tilt, orbital eccentricity, and precession create changes in our planet’s climate. Earth’s axial tilt varies by about 3.5 degrees every 40,000 years or so. Mars has an even more pronounced axial tilt variation that undergoes substantial changes in hundreds of thousands or millions of years.

“Over short timescales, river flow is controlled by rainfall or upstream snow melt. Over longer timescales, Earth’s rivers are affected by climatic changes,” Morgan said. “For example, 20,000 years ago, there were large lakes and larger rivers across what is now Nevada. Martian rivers would have operated in a similar way, with short-term variability due to storms or snowmelt, and longer-term variability due to changes in the planet’s spin and orbit around the Sun.”

Or powerful volcanic activity could’ve periodically warmed the planet, melting ice sheets and spawning rivers that carved telltale channels into the planet’s surface. The Tharsis Montes region shows that volcanoes played a role in Mars’ history. Tharsis Montes is home to three massive shield volcanoes that dwarf Earth’s volcanoes. Another volcano, Olympus Mons, is just northwest of Tharsis Montes and is the largest volcano in the Solar System.

We don’t really know what happened on Mars. Is Mars just a standard example of marginally habitable planets that become uninhabitable? Or is it a striking example of a planet that stubbornly held onto its water through multiple climatic episodes? Did simple life get started on Mars before it was snuffed out, and is that just the way things work? Or is surface water on any planet for any period of time extremely rare?

For now, we don’t have any clear answers to those big questions. Planets are big, complicated, long-lived, and dynamic objects. Understanding what happened billions of years ago on a planet is a daunting task.