Researchers believe that deep in its past, Mars wasn’t the dusty red planet we know today. Instead, 3.8 billion years ago it was warmer and wetter, with water flowing over its surface and even an ocean of liquid water covering 20 percent of its surface. As the sun grew brighter and solar winds stripped away its atmosphere, liquid water could no longer exist, and Mars transformed into a cold, dry place.
But new research suggests that large rivers of liquid water continued to flow on the Martian surface much longer than researchers thought possible, and that the planet may have experienced several wet periods, rather than just one, according to a study in the journal Science Advances.
To understand the history of water on Mars, geophysicist Edwin Kite of the University of Chicago and his colleagues pored over high resolution images of 200 ancient river systems identified on Mars, reports Mike Wall at Space.com. By closely examining things like the width, steepness and size of gravel in the river channels, they were able to make estimates of how much water moved through the system and for how long. The age of surrounding terrain also helped them date the riverbeds.
The results show that many of the rivers—most larger and wider than rivers found on Earth—still had a strong flow 3 billion years ago, well into the period when the planet had begun to dry up and even as recently as 1 billion years ago. The phenomenon wasn’t just restricted to one region; these rivers were found all over the surface of the planet.
The rivers show signs of shortening over time, but they still had strong flows until the tail end of the wet period, when the climate completely dried up. “You would expect them to wane gradually over time, but that's not what we see,” Kite says in a press release. “The wettest day of the year is still very wet.”
Wall reports that the longevity of the rivers suggests that the history of the Martian climate is more complex than we realize. “We can start to see that Mars didn’t just have one wet period early in its history and then dried out,” Kite tells Wall. “It’s more complicated than that; there were multiple wet periods.”
The study actually complicates what we know about the early climate of Mars. Maya Wei-Haas at National Geographic reports scientists had thought that even when Mars had a more substantial atmosphere, liquid water was still a tough proposition on the planet. That’s because the sun was 25 to 30 percent less bright than it is today, keeping the planet relatively cool. “Things were always kind of right at the edge of being able to have water flowing across the surface,” Alan Howard of the Planetary Science Institute in Tucson, not involved in the study, says.
It’s possible that either volcanism, a strong magnetic field that protected the atmosphere or the unique composition of its atmosphere allowed Mars to support liquid H2O longer than hypothesized. Whatever the case, the new study suggest that one of our assumptions about early Mars is wrong.
“Our work answers some existing questions but raises a new one,” Kite says in the press release. “Which is wrong: the climate models, the atmosphere evolution models or our basic understanding of inner solar system chronology?”
We may soon get a few new clues about Mars’ watery past. Wall at Space.com reports that NASA’s Curiosity rover is currently exploring a mountain in the Gale Crater, which may include rock layers showing climate shifts in the planet’s past. And when the as-yet-unnamed Mars 2020 rover scheduled to launch in July reaches the planet, one of its first missions will be to scour an ancient river delta in the Jezero Crater, which it’s believed was a lake during a Martian wet period.