Long ago on Mars, water carved deep riverbeds into the planet’s surface–but we still don’t know what kind of weather fed them. Scientists aren’t sure, because their understanding of the Martian climate billions of years ago remains incomplete.
A new study by University of Chicago scientists cataloged these rivers to conclude that significant river runoff persisted on Mars later into its history than previously thought. According to the study, published March 27 in Science Advances, the runoff was intense–rivers on Mars were wider than those on Earth today–and occurred at hundreds of locations on the red planet.
This complicates the picture for scientists trying to model the ancient Martian climate, said lead study author Edwin Kite, assistant professor of geophysical sciences and an expert in both the history of Mars and climates of other worlds. “It’s already hard to explain rivers or lakes based on the information we have,” he said. “This makes a difficult problem even more difficult.”
A photo of a preserved river channel on Mars, taken by an orbiting satellite, with color overlaid to show different elevations (blue is low, yellow is high).
But, he said, the constraints could be useful in winnowing the many theories researchers have proposed to explain the climate.
Mars is crisscrossed with the distinctive tracks of long-dead rivers. NASA’s spacecraft have taken photos of hundreds of these rivers from orbit, 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.
It’s a puzzle why ancient Mars had liquid water. Mars 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,” Kite said.
Their analysis shows clear evidence for persistent, strong runoff that occurred well into the last stage of the wet climate, Kite said.
The results provide guidance for those trying to reconstruct the Martian climate, Kite said. 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,” Kite said. 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, Kite speculated, which 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?” he said
UChicago Planetary GIS/Data Specialist David Mayer, now at the United States Geologic Survey Astrogeology Program, and then-visiting student Gaia Stucky de Quay from Imperial College London, co-authored the study, as well as scientists with the Smithsonian Institution, the Natural History Museum in London and the Centre National de la Recherche Scientifique in Paris. The study used University of Chicago Research Computing Center resources.
Contacts and sources:
University of Chicago
Citation: “Persistence of intense, climate-driven runoff late in Mars history.” Kite, et al, Science Advances, March 27, 2019. Doi: 10.1126/sciadv.aav7710
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