Gusev Crater, Mars from the NASA 3D model library. Created by Ames SpaceShop
The original file had a very thin base. I've thickened it to make the model a little more durable.
Download the original file here: http://nasa3d.arc.nasa.gov/detail/SpiritLanding
After printing I glued sand to the flat areas and painted the model with acrylics and inks.