Mars may seem inhospitably dry now, but NASA's rovers have shown that water was once abundant on the Red Planet. Photos gathered during NASA's missions orbiting Mars have revealed that the planet was once home to raging rivers -- and that those rivers existed more recently than previously thought, according to a new study.
The rivers were two times wider than any on Earth, according to the deep riverbeds they left carved into the surface, and they could be found at hundreds of locations across the planet. NASA's photos showed well-preserved river channels, deltas and fan-shaped masses of material called alluvium that is carried and deposited by water.
The width, steepness and size of the gravel in the riverbeds helped researchers determine the force of the water flow.
Researchers were able to analyze 200 river channels and determine that between 1 and 3.6 billion years ago, intense runoff occurred: 3 to 20 kilograms per meter squared each day. Their study was published Wednesday in the journal Science Advances.
The runoff happened across the planet, so it wasn't localized or occurring over a short period of time.
The time frame changes things for scientists who have modeled what the climate of ancient Mars was like. If their dates are correct, this runoff happened later in Martian history than thought possible.
Around the time as the runoff, which may have been driven by heavy-precipitation events, the climate model for ancient Mars suggests that it was drying out and losing its atmosphere to become the dry planet we know today, with a thin whisper of an atmosphere.
"It's already hard to explain rivers or lakes based on the information we have," said Edwin Kite, lead study author and University of Chicago assistant professor of geophysical sciences, in a statement. "This makes a difficult problem even more difficult."
But it's already a bit of a mystery as to why liquid water was possible on Mars in the first place. Even in its earliest history billions of years ago, Mars was only receiving about a third of the sunlight we get on Earth. That's not enough heat to maintain water in a liquid state on a cold planet. Today, it's too cold to support the liquid water that was once on its surface.
"Indeed, even on ancient Mars, when it was wet enough for rivers some of the time, the rest of the data looks like Mars was extremely cold and dry most of the time," Kite said.
The analysis of the riverbeds also revealed that the flow of water was continuous, not just at points when it would have been warm enough during the day. That means a strong greenhouse effect may have kept Mars warm enough to keep water above freezing. And the intense flow of the rivers subsisted until the last second before the wet climate shifted to a dry one.
"You would expect them to wane gradually over time, but that's not what we see," Kite said. "The wettest day of the year is still very wet."
The rivers shortened from thousands of kilometers to hundreds, but their powerful force remained. The researchers think it's possible that the Martian climate tipped back and forth between dry and wet cycles.
"Our work answers some existing questions but raises a new one. Which is wrong: the climate models, the atmosphere evolution models, or our basic understanding of inner solar system chronology?" Kite said.
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