Billions of years ago, it rained on Mars. The water collected in valleys and rivers, filled and spilled over the rims of craters, and was funneled into canyons, perhaps even making its way to a large Martian ocean.
On Earth, the areas around large river systems are among the most ecologically diverse regions on the planet — think of the Amazon River basin with its tens of thousands of known species. Researchers think that similar systems on Mars could have been potential cradles for life when the water was flowing.
A new study published in PNAS from researchers at The University of Texas at Austin is the first to define large river drainage systems on the red planet. They outlined 16 large-scale river basins where life would have been most likely to thrive on the neighboring planet.
"We've known for a long time that there were rivers on Mars," said co-author Timothy A. Goudge, an assistant professor in the Department of Earth and Planetary Sciences at the UT Jackson School of Geosciences. "But we really didn't know the extent to which the rivers were organized in large drainage systems at the global scale."
Goudge and postdoctoral fellow Abdallah S. Zaki brought together previously published individual datasets of Mars' valley networks, lakes and rivers, then outlined the combined drainage systems to determine their total area. They identified 19 big clusters of valley networks, streams, lakes, canyons and sediment deposits, 16 of which connected together into watersheds of 100,000 square kilometers or larger. This is the threshold for what is considered a large drainage basin on Earth. Their work is the first time a systematic, planet-wide identification of large river basins has been conducted on Mars.
"We did the simplest thing that could be done. We just mapped them and pieced them together," said Zaki, who led the research.
On Earth, large watersheds spanning at least 100,000 square kilometers are much more common than on Mars; there are 91. The Amazon River basin system, the largest on the planet, is about 6.2 million square kilometers. Texas' Colorado River basin system just barely qualifies as large at 103,300 square kilometers.
Where these large river basins sit, life follows. As a general rule, the larger the river, the more nutrients are transported throughout the system. That's why some of the most diverse ecosystems on the planet exist in the largest drainage basins. The most sprawling of these watersheds, such as the Indus River basin, are often considered the cradles of human civilization.
On Earth, tectonic activity has built mountains, valleys and other diverse topography that directs water where to flow and connects it to other systems. This varying topography is part of what makes a large drainage system. Because Mars lacks tectonic activity, it has fewer large drainage systems, the researchers said.
Nevertheless, although the large drainage systems only make up 5% of the planet's ancient terrain, the researchers found that they represent about 42% of the total material eroded by rivers on Mars.
Since sediment contains nutrients, these are the best spots to look for signs of past life, Zaki said; although more work needs to be done to pinpoint exactly where the sediment ended up.
"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," he said.
By and large, Mars is covered by what the researchers describe as a mosaic of smaller drainage systems. While each one represented a potentially habitable environment, the researchers said that the 16 large drainage areas could be the most worthwhile areas of future study for Mars' habitability.
"It's a really important thing to think about for future missions and where you might go to look for life," Goudge said.
Department Chair Danny Stockli said this research is another example of the impactful work coming out of the Jackson School.
"Tim Goudge and his team continue to be leaders in the field, making groundbreaking contributions to the understanding of Mars' planetary surface and hydrologic processes," he said.
This study was also co-authored by David Mohrig, professor in the Department of Earth and Planetary Sciences.
