Surface features on Titan, combined infrared and optical Cassini image of surface features on Titan, the largest moon of Saturn. The dark circular feature near centre is thought to be an old impact basin. A mountain range is at lower centre. Titan, the largest moon of Saturn, has a diameter of 5150 kilometres. These images were obtained in two flybys at different distances in October 2006. Cassini used its Visual and Infrared Mapping Spectrometer, an instrument that allows high resolution viewing down to objects 400 metres across. Titan has a thick atmosphere of nitrogen, making it difficult to see surface features, so mapping of Titan is being carried out using infrared and radar sensors.