Valles Marineris, Mars

3. Mariner Valley, Mars
Three: The Grandest Canyon
Just to the east of the great volcanoes on Mars is a system of canyons called Valles Marineris, "Mariner Valleys", named after the Mariner 9 spacecraft that discovered it. The system begins as a complex of intertwining canyons and valleys called Noctis Labyrinthus—"Labyrinth of the Night". East of this, the system merges into the greatest canyon known in the solar system. While Earth’s Grand Canyon stretches across the northwestern corner of Arizona for 280 miles (450 km), Valles Marineris would stretch across the entire United States from the Atlantic Ocean to the Pacific Ocean. With a depth of up to 3 to 4 miles (5-7 km) it is about four times deeper than the Grand Canyon. Valles Marineris is so long that when one end has moved well into night, the other end is still in daylight. The difference in temperature this would cause might result in powerful winds tearing down the length of the canyon. The canyon is so wide in most places that if you were to stand on one rim of it you would not be able to see opposite side.
It is not really fair, however, to compare Valles Marineris to the Grand Canyon. It has not been carved out by water, as the Grand Canyon was on Earth, but is the result of enormous blocks of the Martian crust moving apart. Many millions of years ago the opposite walls of the canyon were touching. It would be much more accurate to compare it to features on Earth such as the Red Sea, the Great Rift Valley of Africa or—perhaps best of all—the Atlantic Ocean. These are all features that are being created by the crust of our planet pulling apart. The Atlantic Ocean is in reality an enormous crack in Earth’s crust, running in a long S-curve nearly from pole to pole. In fact, Valles Marineris may look as the Atlantic Ocean did over 100 million years ago, when the Americas first began to pull away from Europe and Africa. Like the Atlantic Ocean, which has the Midatlantic Ridge running down its center, Valles Marineris also has a central ridge. This is caused by the upwelling of fresh material from the mantle as the crust is pulled apart. Much of the present-day Valles Marineris, however, has been shaped by forces other than movement of the crust. Since it was originally formed, erosion—by wind and perhaps even by water—and landslides have sculpted a great deal of the great canyon.
The entirety of the 3000-mile-long canyon system is revealed in this NASA spacecraft image.