This area, on the western edge of Milankovic Crater on Mars, has a thick deposit of sediment that covers a layer rich in ice. The ice is not obvious unless you look in color.
In the red-green-blue images that are close to what the human eye would see, the ice looks bright white while the surroundings are a rusty red. The ice stands out even more clearly in the infrared-red-blue images where it has a striking bluish-purple tone while the surroundings have a yellowish-grey color.
The ice-rich material is most visible when the cliff is oriented east-west and is shielded from the sun as it arcs through the sky to the south.
NASA/JPL/UArizona larger image
