Mars’ ancient climate is one of our Solar System’s most perplexing mysteries. The planet was once wet and warm; now it’s dry and cold. Whatever befell the planet, it didn’t happen all at once.
New research shows that on ancient cold Mars, sheets of frozen carbon dioxide allowed rivers to flow and a sea the size of the Mediterranean to exist.
Mars’ climatic change from warm and wet to cold and dry wasn’t abrupt. There was no catastrophic impact or other triggering event. Throughout its gradual shift, there were different climatic episodes.
The planet’s surface is characterized by features that indicate water’s presence. River channels, impact craters, and basins that were once paleolakes illustrate Mars’ complex climatic history. Mars is much different from Earth, but they both follow the same set of natural rules.
In Earth’s frigid climates, rivers can flow underneath thick, protective ice sheets. New research shows that a similar thing happened on Mars. The research is published in JGR Planets and is titled “Massive Ice Sheet Basal Melting Triggered by Atmospheric Collapse on Mars, Leading to Formation of an Overtopped, Ice-Covered Argyre Basin Paleolake Fed by 1,000-km Rivers.” The lead author is Peter Buhler, a Research Scientist at the Planetary Science Institute.
The research examines a period about 3.6 billion years ago when Mars was likely transitioning from the Noachian Period to the Hesperian Period. At that time, most of the surface water was frozen into large ice sheets in Mars’ southern region, according to the research. The planet’s CO2 atmosphere suffered periodic collapses, and sublimated out of the atmosphere. Those collapses formed a layer of CO2 650 meters (0.4 miles) thick that created a massive ice cap over the South Pole. It insulated the 2.5-mile-thick (4 km) layer of frozen water that made up the ice sheets.
Buhler modelled how the CO2 cap acted as a thermal blanket and showed that it released massive amounts of meltwater from the frozen pole. This water flowed down rivers, with the top layers freezing and insulating the liquid water underneath.
“You now have the cap on top, a saturated water table underneath and permafrost on the sides,” Buhler said. “The only way left for the water to go is through the interface between the ice sheet and the rock underneath it. That’s why on Earth you see rivers come out from underneath glaciers instead of just draining into the ground.”
According to Buhler’s work, enough water was liberated to fill the Argyre Basin.
The Argyre Basin is one of the largest impact basins on the planet, measuring roughly 1800 km (1100 mi) in diameter. This massive impact basin was formed billions of years ago by a comet or asteroid striking Mars. It drops about 5.2 km (3.2 mi) below the surrounding plains, making it the second deepest basin on Mars. Scientists have long thought that the basin once held water—as much as the Mediterranean Sea—and Buhler’s work shows how it may have filled.
“Eskers are evidence that at some point there was subglacial melt on Mars, and that’s a big mystery,” Buhler said. Eskers are long stratified ridges of sand and gravel deposited by meltwater streams that flow under glaciers. They’re common on Earth, where glaciers once covered the surface. Mars’ eskers support the idea that the same thing happened on that planet.
The subglacial rivers would have flowed underneath the ice, where they were insulated from the cold. When they exited the glacier, they would have oozed along until a thick enough ice cap formed to insulate them. Buhler says that the ice would’ve grown until it was hundreds of meters thick, and the water flowing under the ice caps would’ve been several feet deep. The water would’ve carved out river channels thousands of miles long, and there are several of those that go from the polar cap to the Argyre Basin.
“People have been trying to discover processes that could make that happen, but nothing really worked,” Buhler said. “The current best hypothesis is that there was some unspecified global warming event, but that was an unsatisfying answer to me, because we don’t know what would have caused that warming. This model explains eskers without invoking climatic warming.”
Argyre Basin is massive and voluminous, and proposed explanations for how it was filled with water were left wanting. It has approximately the same volume as the Mediterranean Sea. Buhler’s model shows that it took about ten thousand years for the basin to fill, and after it filled, the water emptied into plains about 8,000 km (5,000 miles) away.
This process happened repeatedly over a one-hundred-million-year era, with each event separated by millions of years.
“This is the first model that produces enough water to overtop Argyre, consistent with decades-old geologic observations,” Buhler said. “It’s also likely that the meltwater, once downstream, sublimated back into the atmosphere before being returned to the south polar cap, perpetuating a pole-to-equator hydrologic cycle that may have played an important role in Mars’ enigmatic pulse of late-stage hydrologic activity. What’s more, it does not require late-stage warming to explain it.”
Buhler’s work is supported by other research. “Previous literature supports the presence of a ~0.6 bar (atmospheric) CO2 inventory, as utilized in the model, near the Noachian-Hesperian boundary,” he writes in his research. The history of Mars’ atmospheric pressure is backed up by cosmochemistry, mineralogy, atmosphere and meteorite trapped-gas isotopic ratios, geomorphology, and extrapolations of modern-day atmospheric escape.
“Thus, there is strong evidence that Mars had a sufficiently large mobile CO2 reservoir to drive the atmospheric-collapse-driven melting scenario described in this manuscript, with collapse occurring at a time commensurate with Valley Network formation during Mars’ intense, Late Noachian/Early Hesperian terminal pulse of intense fluvial activity,” Buhler writes.
That period of Mars’ history stands out as its own distinct phase of geological activity, whereas changes were more gradual in the earlier Noachian Period. The Late Noachian/Early Hesperian saw intense valley network formation. Many of these valleys are deeply carved into the landscape, often cutting through older geological features. That suggests that the water flow was powerful and erosive. This fluvial activity also created large deposits of sediment, like the ones NASA’s Perseverance Rover is exploring in Jezero Crater.
Buhler’s research is partly based on modern-day observations of Mars’ atmospheric CO2 and its cycles. Much of it is actually frozen and bound to the regolith. Mars’ rotational tilt shifts over a 100,000-year timeline. When it’s closer to straight up and down, the Sun hits the equator, and CO2 is released from the regolith into the atmosphere. It eventually reaches the poles, where it’s frozen into the caps.
When Mars is tilted, the poles are warmed, and the CO2 sublimates and is released into the atmosphere again. It eventually reaches the now-cooler regolith, which absorbs it. “The atmosphere is mostly just along for the ride,” Buhler said. “It acts as a conduit for the real action, which is the exchange between the regolith and the southern polar ice cap, even today.”
Buhler is still working with his model and intends to continue testing it more rigorously. If it successfully withstands more testing, our understanding of Mars will take a big leap forward.