Did water flow across the surface of an icy Mars?

New research suggests that about 3.6 billion years ago, Mars featured vast water flows and a lake rivaling the size of the Mediterranean. 

This ancient hydrology, scientists say, was preserved under protective ice layers, contrary to the previous notion that warmer climates were necessary for liquid water on Mars. 

The study, led by Peter Buhler, a researcher at the Planetary Science Institute, explores how a process driven by Mars’ carbon dioxide cycles could have created conditions allowing water to flow across the Martian surface.

Layer of frozen carbon dioxide

Buhler’s model proposes that ancient Martian rivers and lakes were enabled not by warmth but by carbon dioxide freezing out of the atmosphere and forming a thick layer atop an ice sheet at the Martian poles. 

This layer of frozen carbon dioxide functioned like an insulating blanket, trapping heat from Mars’ interior, which increased pressure on the underlying ice and melted nearly half of Mars’ water reserves, allowing rivers to form.

“This model describes the origins of major landscape features on Mars – like the biggest lake, the biggest valleys and the biggest esker system (remnants of rivers that once flowed beneath an ice sheet) – in a self-consistent way,” Buhler explained.

He emphasized that the findings don’t depend on a hypothetical warming event but rather on a process observed on Mars even today.

Carbon dioxide cycle on Mars

Decades of research have established that a significant amount of Mars’ carbon dioxide is absorbed in a single-molecule-thick layer around the regolith grains (i.e. the layer of unconsolidated solid material covering the bedrock of a planet). 

By including this regolith component, Buhler found that the Martian atmosphere “is mostly just along for the ride,” with the carbon dioxide cycle playing a primary role. 

Mars’ rotational tilt – shifting every 100,000 Martian years – impacts this cycle. When Mars tilts upright, sunlight bakes the equator while the poles cool, causing carbon dioxide to escape from the regolith into the atmosphere before freezing atop the polar ice. 

As Mars’ tilt becomes more extreme, this pattern reverses, subliming the ice back into gas and causing the regolith to absorb it like a sponge.

The findings demonstrate that this modern-day carbon dioxide exchange mechanism could explain Mars’ ancient landscape features, which had previously baffled scientists. 

In testing his model for the period 3.6 billion years ago, Buhler pinpointed the early phases of Mars’ current carbon dioxide cycle, a time associated with the formation of river valley networks. 

This cycle, he theorized, was responsible for melting large volumes of Martian water without significant warming events.

Water trapped beneath the icy surface 

In Buhler’s model, Mars’ carbon dioxide ice sheet could have been around 0.4 miles thick, lying atop a layer of water ice around 2.5 miles thick. 

This layered arrangement, Buhler suggested, acted as a powerful insulator, trapping geothermal heat below the ice cap and adding substantial weight and pressure, which melted the underlying water ice. 

The meltwater then pooled beneath the ice sheet, saturating the surrounding Martian crust and eventually freezing as permafrost on the edges.

Buhler explained that this setup caused water to flow in only one direction – through the narrow interface between the ice sheet and the bedrock. 

“The only way left for the water to go is through the interface between the ice sheet and the rock underneath it,” he said, pointing out that similar subglacial rivers emerge from glaciers on Earth. 

These Martian rivers left behind eskers – long ridges of gravel, remnants of ancient rivers that once flowed under glaciers. These eskers, commonly observed near Mars’ south pole, align well with Buhler’s subglacial river predictions.

Explaining water on ancient Mars

Buhler’s model provides an alternative explanation for Mars’ eskers without invoking hypothetical warming. “Eskers are evidence that at some point there was subglacial melt on Mars, and that’s a big mystery,” Buhler said. 

Instead of requiring a global warming event, Buhler’s model relies on carbon dioxide insulation to facilitate subglacial melting. 

His findings offer a more consistent explanation, as warmer temperatures would have likely burned off any exposed feathers, fur, or other features.

According to Buhler, the subglacial rivers emerging from beneath the ice formed slow-moving, ice-covered flows. Over time, these flows would gather enough pressure to form larger rivers under a thick ice cover. 

Buhler estimates that these ice-topped rivers could reach depths of several feet and stretch for thousands of miles.

Water flowing on Mars 

As the subglacial rivers reached the edge of the ice sheet, they encountered Mars’ cold atmosphere and initially created oozing, lava-like flows covered by frozen skins. 

Eventually, these rivers poured into the Argyre Basin, a vast basin comparable in volume to the Mediterranean Sea, filling it with meltwater over tens of thousands of years. Once filled, the water would overflow and travel nearly 5,000 miles to the northern plains of Mars. 

Buhler’s model also suggests that this process occurred multiple times across millions of years during a one-hundred-million-year period, making it the first to demonstrate that enough water was present to overtop the Argyre Basin.

“This is the first model that produces enough water to overtop Argyre, consistent with decades-old geologic observations,” Buhler explained, suggesting that this subglacial melting and overflow may have driven a cycle of water sublimation and condensation, creating a pole-to-equator hydrologic cycle.

Redefining Mars’ water cycle 

Moving forward, Buhler plans to continue testing his model to further solidify its findings. Should future results continue to support his work, it may prompt a substantial reevaluation of Mars’ ancient water cycle. 

By demonstrating how ancient Mars could sustain liquid water without requiring atmospheric warming, Buhler’s model reshapes scientific understanding of how Mars’ rivers and lakes formed and persisted billions of years ago.

Ultimately, the research suggests that Mars may have once harbored conditions even more dynamic and water-rich than previously believed.

The study is published in the Journal of Geophysical Research: Planets.

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