Gullies rake across the raw landscape of Mars in an image released this week by the Mars Reconnaissance Orbiter's HiRISE camera. Some four billion years ago, a comet or asteroid splashed onto the southern highlands of Mars, creating the Argyre Impact Basin just south of the gullies. After the impact, water or lava flow into the basin may have carved these gullies, which are more than 3.6 miles (6 kilometers) long.