The Grand Canyon is impressive – a multi-colored chasm that’s 275 miles long, up to 18 miles wide, and more than a mile deep. But compared to the largest canyon system on Mars, it’s like something a toddler dug in a sandbox. Valles Marineris extends a quarter of the way around the planet – almost 2500 miles from end to end. At its widest, it spans 120 miles – so far that you couldn’t see from one rim to the other. And it’s more than four miles deep.

Valles Marineris is named for the Mariner 9 orbiter, which discovered the canyon half a century ago. And geologists have been debating its origin ever since. They’ve wondered whether it was carved by water, ice, winds, volcanic activity, or some combination.

Today, the leading view says it began as a crack in the crust. Over millions of years, a giant volcano near the eastern end of the canyon grew larger and larger. It became so heavy that it fractured the crust. The walls of that crack then collapsed, making it wider. The wind scoured out more of the landscape. And parts of the canyon may have been eroded by water or ice – giving Mars one of the grandest canyons in the solar system.

And Mars is in grand view. It passed closest to Earth a few weeks ago, so it’s still pretty close. It looks like a bright orange star, high in the east at nightfall. Castor and Pollux, the twin stars of Gemini, line up to its left and lower left.

Script by Damond Benningfield