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In many ways, Mars is the planet that is most similar to the Earth. The red world has polar ice caps, a nearly 24-hour rotation period (about 24 hours and 37 minutes), mountains, plains, dust storms, volcanoes, a population of robots, many of which are old and no longer work, and even a Grand Canyon of sorts. The "Grand Canyon" on Mars is actually far grander than any Arizonan gorge. Valles Marineris dwarfs the Grand Canyon of the southwestern U.S., spanning 4,000 km in length (the distance between L.A. and New York City), and dives 7 kilometers into the Martian crust (compared to a measly 2 km of depth seen in the Grand Canyon). Newly released photos from the High-Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter (MRO) reveal a stunning look at eroding cliff faces in Candor Chasma, a gigantic canyon that comprises a portion of the Valles Marineris system.

canyon, Valles Mariners was not formed by surface river erosion.

Which geological process could be behind the formation of the largest canyon known to humankind? Is it a dry process like the depression of a chunk of crust along parallel faults known as a graben? Another possible explanation for the formation of some features is the dissolution of rocks by subsurface water in what geologists call karst.

The watery past of Mars is a big part of what makes it such a fascinating planet to study. The Mars Perseverance Rover, already on the way and slated to land in mid-February 2021, will be landing on what is thought to be shore of an ancient Martian ocean. Large bodies of liquid water are very rare outside of the Earth, and are thought to be a necessary ingredient for the rise of living things. It goes without saying that finding evidence of life on another planet, even if it has since gone extinct, would be as impactful a scientific discovery as is possible.