The Mars HiRISE team at the University of Arizona have released an amazing image of an 800-meter-high vertical cliff of ice at the North Pole of Mars. I was immediately reminded of 'The Wall' from George R.R. Martin's A Song of Ice and Fire, guarded by the Night's Watch. Would be amazing to see a shot of this structure from ground level:
There is an ice sheet at the North Pole of Mars that is a few miles thick at its center. At some places (like in this image) it ends in steep cliffs that can be about 800 meters (2600 feet) high.
The slopes of these cliffs are almost vertical which causes slab-like blocks of ice to break off and crash down to the surrounding plains. Dense networks of cracks cover these icy cliff faces making it easier for these blocks to break free. We've seen new debris at the base of many of these cliffs appearing between successive HiRISE images, so we regularly monitor sites like this to check for new blocks that have fallen. Understanding how these cliffs are formed helps us understand the climatic record stored in the ice sheet itself.
You can view the image in larger sizes at the HiRISE website.