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For the first time, scientists have mapped vast, continent-scale river drainage systems on Mars — ancient networks that may also be among the most promising places to search for signs of past life.

Billions of years ago, before the Red Planet became the frigid desert it is today, water sculpted its surface on a massive scale. For decades, Mars has tempted scientists with whispered clues of that watery past, long-dry rivers that carved valleys and spilled through crater rims into deep canyons, hinting at a world that once looked far more like Earth. But although scientists had cataloged thousands of these ancient waterways, they didn't know how they fit together, or whether Mars once hosted large, integrated river systems similar to those that support some of Earth's most biodiverse environments.

"We've known for a long time that there were rivers on Mars," Timothy Goudge, an assistant professor in the department of geological sciences at the University of Texas at Austin, said in a statement. "But we really didn't know the extent to which the rivers were organized in large drainage systems at the global scale."

Stitching together Mars' watery past

In a new study, Goudge and his colleagues have now compiled decades of orbital observations and previously published maps of valleys, lakebeds and outlet canyons, drawing on datasets from NASA's Mars Odyssey spacecraft, which has mapped more than 90% of the planet. The team then traced how these features once connected, revealing which belonged to cohesive, basin-spanning drainage networks.

"We did the simplest thing that could be done — we just mapped them and pieced them together," Abdallah Zaki, a postdoctoral fellow in the department of Earth and planetary sciences at the University of Austin, who led the new study, said in the same statement.

In regions where impact craters or billions of years of erosion had distorted the ancient landscape, the researchers inferred how rivers once flowed by examining topography and the orientations of surrounding valleys, the study notes.

Their results suggest that early Mars was a patchwork of isolated watersheds, but that a small number of mega-basins acted as planetary conveyor belts, transporting nutrients and potential biosignatures across immense distances.

The team identified 16 major drainage basins, each spanning at least 38,610 square miles (100,000 square kilometers), the same minimum size used to define large river basins on Earth. Together, these networks once covered about 1.5 million square miles (4 million square kilometers), or roughly 5% of Mars' ancient terrain. That fraction is also likely conservative, the researchers note, as impact events and wind erosion have erased much of the planet's original fluvial landscape.

On Earth, tectonics build mountain ranges and deep lowlands that guide and connect river systems. Without that process, Mars ended up with just 16 major basins compared with Earth's roughly 91.

Despite their small footprint, those few Martian basins may be among the most scientifically valuable places yet to explore, scientists say. When the researchers estimated how much sediment ancient rivers carried, they found that the 16 large basins transported nearly half of all river-eroded sediment on Mars, suggesting they had an outsized influence on Mars' geologic evolution. One basin alone, feeding into one of the largest canyons on Mars called Ma'adim Vallis, accounted for roughly 15 percent of the total.

On Earth, large-scale river systems are biodiversity hotspots, where water flows through diverse rock types and creates long-lived, chemically rich environments. Mars' mega-basins may have played a similar role when liquid water was abundant. And if life ever gained a foothold on the Red Planet, the team says these ancient river highways — which once carried nearly half the sediment Mars' rivers ever moved — may be the places where evidence of it still lingers.

"The longer the distance, the more you have water interacting with rocks, so there's a higher chance of chemical reactions that could be translated into signs of life," Zaki said in the statement.

The new megabasin map could thus serve as a powerful roadmap for future Mars missions, especially those searching for chemical traces of life or planning sample-return campaigns, the researchers say.

"It's a really important thing to think about for future missions and where you might go to look for life," Goudge said in the statement.

This research is described in a paper published Nov. 24 in the journal Proceedings of the National Academy of Sciences.