been unambiguously identified on Titan so far, he asserted that Titan is a geologically dead world, whose surface is shaped only by impact cratering, fluvial and eolian erosion, mass wasting and other exogenic processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan's cold and stiff interior. Ganesa Macula may be an eroded impact crater with a dark dune in the center. The mountainous ridges observed in some regions can be explained as heavily degraded scarps of large multi-ring impact structures or as a result of the global contraction due to the slow cooling of the interior. Even in this case, Titan may still have an internal ocean made of the eutectic water–ammonia mixture with a temperature of , which is low enough to be explained by the decay of radioactive elements in the core. The bright Xanadu terrain may be a degraded heavily cratered terrain similar to that observed on the surface of Callisto. Indeed, were it not for its lack of an atmosphere, Callisto could serve as a model for Titan's geology in this scenario. Jeffrey Moore even called Titan Callisto with weather. In March 2009, structures resembling lava flows were announced in a region of Titan called Hotei Arcus, which appears to fluctuate in brightness over several months. Though many phenomena were suggested to explain this fluctuation, the lava flows were found to rise above Titan's surface, consistent with it having been erupted from beneath the surface. In December 2010, the Cassini mission team announced the most compelling possible cryovolcano yet found. Named Sotra Patera, it is one in a chain of at least three mountains, each between 1000 and 1500 m in height, several of which are topped by large craters. The ground around their bases appears to be overlaid by frozen lava flows. Crater-like landforms possibly formed via explosive, maar-like or caldera-forming cryovolcanic eruptions have been identified in Titan's polar regions. These formations are sometimes nested or overlapping and have features suggestive of explosions and collapses, such as elevated rims, halos, and internal hills or mountains. The polar location of these features and their colocalization with Titan's lakes and seas suggests volatiles such as methane may help power them. Some of these features appear quite fresh, suggesting that such volcanic activity continues to the present. Most of Titan's highest peaks occur near its equator in so-called "ridge belts".