The Grand Canyon, in Arizona, formed by a river that slowly cut through the soil for millions of years. The Valles Marineris canyon on Mars is a tectonic feature. That means it was created by forces within the planet pulling sections of the crust apart. The same process formed the Great Rift Valley in Africa and the Gulf of California. The Valles Marineris might have been the beginning of the formation of an ocean on Mars. That's what happened on Earth 100 million years ago, when the continents broke apart and the Atlantic Ocean came into being. If there ever was an ocean on Mars, it dried up millions of years ago.
