Mars is home to the largest known canyon system in our solar system, and while the jury is still out on what exactly formed these canyons - be it water or lava - we do finally know what helped form the unusually hilly geological formations that can be found in their depths.

That's at least according to experts at NASA and the United States Geological Survey (USGS), who recently worked together to craft the most detailed map ever seen of Mars' canyon floors.

The incredibly massive gorge of Valles Marineris on Mars is more than 2,500 miles (4,000 kilometers) long. And unlike many canyons back on Earth, which are constantly altered by rushing waters or changing weather, the floor of this gorge is dotted with mysterious wave-like hills frozen in time.

How exactly these hills formed has been an issue of debate for decades. Possibilities proposed have included accumulation in lakebeds, volcanic eruptions under glaciers within the canyons, or even the simple accumulation of wind-blown sand and dust.

However, the USGS map of western Candor Chasma canyon within the Red Planet's Valles Marineris is favoring a combination of these options, and supports the idea that Mars was once dotted with lakes.

The map, derived from extensive and detailed images take by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter, shows that these "hills" are in fact impressively built-up piles of pebbles and sand.

"At the time these sediments were deposited, a part of west Candor Chasma, specifically Condor Colles, contained numerous shallow, spring-fed lakes," map author Chris Okubo of the USGS Astrogeology Science Center, said in a NASA release. "These lakes helped to trap wind-blown sand and dust, which accumulated over time and formed the extensive sedimentary deposits we see today."

NASA and the USGS team suggest that these sediments could have initially been relatively flat, like silty lakebeds, when wet. However, after experiencing a shocking series of "marsquakes," they became frozen into the hilly formations that baffled experts for all these years.

[Credit:  NASA/JPL-Caltech/Univ. of Arizona/USGS]

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