Valles Marineris, or Mariner Valley, is one of the most prominent features on Mars. It’s often compared to the Grand Canyon, but is about 7 times wider, 4 times deeper, and 9 times longer. If such a feature existed on Earth it would stretch from New York to Los Angeles. But despite their similarities as great canyons, they have radically different histories.
The Grand Canyon likely began to form about 60 – 70 million years ago as different sections began to be carved by water erosion. About 6 million years ago the different regions merged to form the truly grand canyon with the Colorado river flowing through it. Although there is still debate about the exact age and history, it’s clear that the formation of the canyon was driven by water erosion.
That makes sense given that water flow is pretty common on Earth. But Mars is not a river world. While it may have had liquid surface water at some point in its early history, Mars has had its water frozen beneath the surface for much of its history. So it isn’t likely that Valles Marineris is a river canyon. In fact there is plenty of evidence to say that it’s not. What it appears to be is a rift canyon due to geologic activity.
Just to the west of the canyon is a volcanic plateau known as the Tharsis bulge. Driven by volcanic and tectonic processes, the bulge began to form about 3.5 billion years ago. As it swelled, fissures began to form in the valley. These fissures then exposed sub-surface water, which flowed through the canyon, eroding it further. As the valley continued to crack and widen, more water was released, which eventually flooded the region to the north and east of the canyon. In the image here you can see this effect, where the high regions of the canyon have distinct cracks where the crust fractured, while the lower regions of the east show more signs of water erosion.
Obviously things aren’t quite that simple, and there is still much debate over the details, but it’s clear that Valles Marineris shows you don’t need a river world to make a grand canyon.