In 2008 Jeffrey Moore (planetary geologist of Ames Research Center) proposed an alternate view of Titan's geology. Noting that no volcanic features had 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".