Mars scientists may have many theories over exactly how Valles Marineris, the planet's great canyon, may have formed but one thing that certain is the feature's vast scale.

While the canyon stretches more than 4,000 kilometers across the surface of Mars, it only at the region shown here that Valles Marineris is at its widest. The canyon's Melas, Candor and Ophir chasms appear here in a 300 kilometer by 600 kilometer area imaged by the European Space Agency's Mars Express.

Researchers are unsure if Valles Marineris formed through a tectonic process called 'rifting' - in which a bulge due to subsurface volcanic activity slowly fractures the surface - or if tensions in Mars' upper crust cracked the region's highlands and allowed blocks of surface material to slide between the fissures.

The melting of subsurface water ice is another explanation for the canyon's origins. The theory entails that thermal activity melted underground ice which left voids as it ran off. Those voids could have finally given way once empty of water.

But regardless of its origin, Valles Marineris is certainly a sight. Mars Express took this image with its High Resolution Stereo Camera during two orbital passes over the region. Evidence of erosion is apparent in the sheer cliffs with prominent edges, and several landslides appear below the northern scarp. There, material has slid up to 70 kilometers from its original position.