Mars is dry today, but numerous ancient rivers are found across the planet’s surface and their existence is a challenge to models of planetary climate evolution. A new study, published in the journal Science Advances, catalogued these rivers to conclude that the runoff production persisted until less than three billion years ago and probably one billion years ago; it was intense — for a given area, the rivers were wider than those on Earth today — and occurred at hundreds of locations on the planet.
Mars is crisscrossed with the distinctive tracks of ancient rivers.
NASA’s and ESA’s Mars orbiters captured photos of hundreds of these rivers, and when the Mars rover Curiosity landed in 2012, it sent back images of pebbles that were rounded — tumbled for a long time in the bottom of a river.
But it’s a puzzle why ancient Mars had liquid water. The planet has an extremely thin atmosphere today, and early in the planet’s history, it was also only receiving a third of the sunlight of present-day Earth, which shouldn’t be enough heat to maintain liquid water.
“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,” said Dr. Edwin Kite, a researcher at the University of Chicago.
Seeking a better understanding of Martian precipitation, Dr. Kite and his colleagues analyzed image data of more than 200 well-preserved river channels, alluvial fans and deltas across Mars, and calculated the intensity of river runoff using multiple methods, including an analysis of the size of the river channels.
In the river basins for which there are the most data, Martian rivers were about two times wider than those on Earth.
Between 3.6 and 1 billion years ago, and likely after 1 billion years ago, there was intense runoff in these channels, amounting to 3 to 20 kg/m2 each day.
“The analysis shows clear evidence for persistent, strong runoff that occurred well into the last stage of the wet climate,” Dr. Kite said.
“The results provide guidance for those trying to reconstruct the Martian climate. For example, the size of the rivers implies the water was flowing continuously, not just at high noon, so climate modelers need to account for a strong greenhouse effect to keep the planet warm enough for average daytime temperatures above the freezing point of water.”
“The rivers also show strong flow up to the last geological minute before the wet climate dries up.”
“You would expect them to wane gradually over time, but that’s not what we see. The rivers get shorter — hundreds of kilometers rather than thousands — but discharge is still strong. The wettest day of the year is still very wet.”
“It’s possible the climate had a sort of ‘on/off’ switch, which tipped back and forth between dry and wet cycles,” he said.
“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 the chronology of the inner Solar System?”
Edwin S. Kite et al. 2019. Persistence of intense, climate-driven runoff late in Mars history. Science Advances 5 (3): eaav7710; doi: 10.1126/sciadv.aav7710