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.
PHOTOS: The Psychedelic Landscape of Mars
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."