Thermal Inertia
PEP reads in a global thermal inertia data set gathered by the Mars Global Surveyor. You turn on on by clicking on "Layers" on the main menu bar and selecting "Thermal Inertia".
Thermal inertia is a measure how fast things heat up and cool down. On Mars, this is basically a measure of the size of the particles on the surface. Where there are just small particles on the surface, they can heat up and cool down very quick. This is what happens in deserts on Earth and why they cool down so much at night. Regions that have large rocks that can hold heat tend to have smaller daily temperature extremes.
As with all the data layers, you can obtain cross sections of thermal inertia data and apply any image processing filter.