Mars holds her secrets close, planetary-science experts have noted. The Red Planet is crisscrossed with the distinctive tracks of big, long-dead rivers, 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 according to a ground-breaking 2019 study from the University of Chicago.
“The River was Huge”
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— sculpted at the bottom of a river. A marked photo of a preserved river channel, taken by NASA’s Mars Reconnaissance Orbiter, is shown below with color overlaid to indicate elevation (blue is low, yellow is high.) The range of elevation in the scene is approximately 35 meters.
In 2013, planetary scientists at the European Space Agency released 3D images shown above of the “striking upper part of the Reull Vallis region of Mars,” which revealed a 932-mile-long (1500 kilometer) river running from the Promethei Terra Highlands to the vast Hellas basin.
ESA’s Mars Express Spacecraft Evidence
This river was huge. The image data from ESA’s Mars Express spacecraft shows that, at some points, the riverbed is 4.3 miles (7 kilometers) wide and 984 feet (300 meters) deep. The stereo cameras onboard the satellite also revealed “numerous tributaries” that fed the gigantic river.
The 2019 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, 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.”
But, the constraints could be useful in winnowing the many theories researchers have proposed to explain the climate, Kite noted.
The Enigma of Liquid Water
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.
Modeled 200 Ancient Martian Riverbeds
Seeking a better understanding of Martian precipitation, Kite and his colleagues analyzed photographs and elevation models for more than 200 ancient Martian riverbeds spanning over a billion years. These riverbeds are a rich source of clues about the water running through them and the climate that produced it. For example, the width and steepness of the riverbeds and the size of the gravel tell scientists about the force of the water flow, and the quantity of the gravel constrains the volume of water coming through.
Their analysis shows clear evidence for persistent, strong runoff that occurred well into the last stage of the wet climate, Kite said.
The Rivers Flowed Continuously
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.”
An “On/Off” Switch?
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.
The University of Chicago study anticipated a 2020 paper by a team led by geologist Francesco Salese, a Marie Curie Fellow at Utrecht University in the Netherlands. Using the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter, The Utrecht team was able to decipher the ‘story’ in the rocks through a closeup view of what was once an ancient river system in Hellas Planitia, an impact basin in Mars’ southern hemisphere.
“OK, it is not like reading a newspaper,” Salese said in a statement from Utrecht University, “but the extremely high resolution imagery allowed us to ‘read’ the rocks as if you are standing very close to the cliff. “Unfortunately, we don’t have the ability to climb, to look at the finer-scale details, but the striking similarities to sedimentary rocks on Earth leaves very little to the imagination.”
The Last Word
“Mars is important because it’s the only planet we know of that had the ability to support life—and then lost it,” Kite said in a recent 2021 paper on warm early Mars surface enabled by high-altitude water ice clouds that suggests that Mars could have had a thin layer of icy, high-altitude clouds that caused a greenhouse effect. “Earth’s long-term climate stability is remarkable. We want to understand all the ways in which a planet’s long-term climate stability can break down—and all of the ways (not just Earth’s way) that it can be maintained. This quest defines the new field of comparative planetary habitability.”
Image credits: NASA/JPL/Univ. Arizona/UChicago