Capri Chasma is located in the eastern portion of the Valles Marineris canyon system, the largest known canyon system in the Solar System. Deeply incised canyons such as this are excellent targets for studying the Martian crust, as the walls may reveal many distinct types of bedrock.
This section of the canyon was targeted by HiRISE based on a previous spectral detection of hematite-rich deposits in the area. Hematite, a common iron-oxide mineral, was first identified here by the Mars Global Surveyor Thermal Emission Spectrometer (TES). In this TES image, red pixels indicate higher abundances of hematite, while the blue and green pixels represent different types of volcanic rocks (e.g., basalt). [More at link]