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Titan was a mostly unknown world prior to the Cassini spacecraft's arrival in July 2004. We review the major scientific advances made by Cassini's Titan Radar Mapper (RADAR) during 13 years of Cassini's exploration of Saturn and its moons. RADAR measurements revealed Titan's surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surfaceatmosphere interactions. Titan has also joined the ranks of "ocean worlds" along with Enceladus and Europa, which are prime targets for astrobiological research.

Cassini's third and fourth radar flybys, T7 and T8, traversed diverse terrains in the high southern and equatorial latitudes, respectively. The T7 synthetic aperture radar (SAR) swath is somewhat more straightforward to understand in terms of a progressive poleward descent from a high, dissected, and partly hilly terrain down to a low flat plain with embayments and deposits suggestive of the past or even current presence of hydrocarbon liquids. The T8 swath is dominated by dunes of what are likely organic solids, but also contain somewhat enigmatic, probably tectonic, features that may be partly buried or degraded by erosion or relaxation in a thin crust. The dark areas in T7 show no dune morphology, unlike the dark areas in T8, but are composed of a similar material as suggested by a relationship between radar dark/radiometrically warm like that seen in the dunes. The Huygens landing site lies on the edge of the T8 swath; correlation of the radar and Huygens DISR images indicates that to the north of the landing site sit two large longitudinal dunes. Indeed, had the Huygens probe trajectory been just 10 kilometers north of where it actually was, images of large sand dunes would have been returned in place of the fluvially-dissected terrain actually seen-illustrating the strong diversity of Titan's landscapes.4