Valles Marineris: The Grand Canyon of Mars July 3, 2014 By Observing Space Share this:Valles Marineris: The Grand Canyon of Mars http://t.co/G2gqXSVAzV #apod #mars #viking #usgs #nasa pic.twitter.com/quzAiAebEy — Observing Space (@ObservingSpace) May 11, 2014 Share this: