Ancient Martian waterways carved beneath icy caps

Ancient Martian waterways carved beneath icy caps
by Clarence Oxford
Los Angeles CA (SPX) Nov 02, 2024

On a cold, ancient Mars, thick ice layers protected vast lakes and river systems, providing clues to how water flowed across the Red Planet some 3.6 billion years ago. According to new findings by the Planetary Science Institute, carbon dioxide frozen out of Mars' atmosphere helped create conditions that allowed substantial water flow even without a warmer climate.

Peter Buhler, a researcher at the Planetary Science Institute, led the study published in the *Journal of Geophysical Research: Planets*. His work revealed that as carbon dioxide froze over an extensive water ice sheet at the Martian poles, it effectively insulated the surface, trapping internal heat and elevating pressure. "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 said.

Scientists have known for decades that Mars' carbon dioxide has been largely bound within the regolith. Buhler's research extended his carbon dioxide modeling to include interactions with the Martian regolith, thus offering a more complete picture of the Red Planet's carbon cycle. He found that the atmosphere plays only a minor role, while the main action lies in carbon dioxide exchanges between the regolith and the southern polar ice cap, influenced by shifts in Mars' tilt over thousands of Martian years.

Under these changing conditions, carbon dioxide would leave the regolith, accumulate atop polar ice caps, and serve as an insulator, eventually causing the base of the ice to melt. This meltwater moved outward, but instead of penetrating deep into the crust, it froze as permafrost along the ice sheet's sides. Buhler explained, "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."

The meltwater formed subglacial rivers that left behind gravel ridges called eskers. These eskers, observed near Mars' south pole, match the characteristics of subglacial rivers predicted by Buhler's model.

"Eskers are evidence that at some point there was subglacial melt on Mars, and that's a big mystery," he noted. "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."

As these rivers reached the edges of ice sheets, they emerged as slow-flowing, ice-covered streams, eventually filling basins such as Argyre. Over tens of thousands of years, a lake comparable in size to the Mediterranean swelled in Argyre Basin before overflowing to the northern plains. This process, Buhler suggests, likely repeated over millions of years, maintaining a polar-to-equator water cycle without the need for a warming climate.

Buhler believes that ongoing tests of his model could lead to a major reevaluation of Mars' ancient hydrology. "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."