By now, we’re all aware that Mars used to be a lot wetter than it is now. With the help of NASA’s two operational rovers, Curiosity and Opportunity, we also know that large volumes of water used to flow and the Martian surface is rich in minerals formed in the presence of water.
Unfortunately, because Mars is so small and lacks a hefty global magnetic field to prevent its atmosphere from being ripped into space, the planet lost the majority of its atmosphere and its once-dynamic water cycle froze into its crust. However, to this day, eerie visual hints of the ancient Martian water can be seen from orbit.
With the help of the awesome High-Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter, ancient meandering riverbeds that used to carve into the Martian terrain have been imaged with beautiful clarity. However, these riverbeds are now twisted ridges, created after eons of erosion processes.
“These ridges are thought to be old river channels, but wind erosion has created inverted topography,” writes Alfred McEwen, lead scientist of the HiRISE mission and geologist at the University of Arizona. “What was low (the channel bottoms) was more resistant to erosion, so now it is relatively high.”
When rivers flow, their beds accumulate sediment that becomes compressed and resistant to erosion. When they run dry, an empty trough remains. After millions of years, the surrounding landscape becomes eroded by Mars’ persistent winds, slowly wearing it away. However, the ancient riverbed erodes far more slowly (owed to it being “erosion-hardened” by the ancient flowing water), and the surrounding landscape erodes away deeper than the bottom of the ancient riverbed, creating an inverted ridge.
Interestingly, many of the features we find in terrestrial rivers can be seen in these Martian meanders.
In the image above, some extreme meanders can be seen. On Earth, at the most extreme bends in some meandering rivers, the looping beds can be “cut off” as the river relentlessly erodes its edges, eventually breaking through and leaving an oxbow lake (see the photograph below). It appears similar formations were likely created on Mars.
As noted by McEwen, this particular example of a Martian river was likely formed over a long period of time. “This type of river system forms slowly over time, unlike the catastrophic flood channels seen elsewhere on Mars,” he writes.