Mercury's Caloris Basin is seen in an enhanced color mosaic. The massive crater, formed some 4 billion years ago, is orange. Smaller craters made subsequently are rendered in blue, while orange splotches around the perimeter may be volcanic vents.
(Side note: Props to the punster who titled the photo "All About that Basin" on the Jet Propulsion Laboratory's Photojournal page.)
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington