The Voyager mission of the 1990s have provided incredibly detailed images of Mars. After comparing landscape features on Mars with those found on Earth, planetary geologists have identified huge slump scars on Mars (see image on the left). These scars, located along the edge of Olympus Mons (a large shield volcano) and along the edge of Valles Marineris (a large valley), probably formed during times when the permafrost on Mars had melted and liquid water flowed on the planet's surface. As happen on Earth, the moving water undercut cliffs, which then collapsed in huge mass-movement events.?