Today's Image of Mars shows part of the boundary between Melas Chasma (on the left) and the surrounding terrain. Melas Chasma is part of the Valles Marineris rift valley, which spans more than 4000km across the surface of Mars.
Melas Chasma sinks 9km below the surface of the surrounding terrain, giving it the distinction of being one of the lowest depressions on Mars. This image only covers 20,000 sq km, which the ESA mentions is about the size of Slovenia.
This image helps scientists see different layered deposits along the boundary wall, which gives them insight into the conditions on Mars in the distant past when the deposits were made. This is most helpful for the area surrounding Melas Chasma because there is abundant evidence that the Valles Marineris region was once had an abundance of flowing water, as evidenced by water cut channels and lighter-colored sulfates likely deposited by a former lake.
This image was taken by the Mars Express High Resolution Stereo Camera (HRSC) on July 1 2006. Clicking on the image will take you to the hi-res version from the ESA. [See the original article from the ESA on Melas Chasma]