Ceres appears to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30 percent ice. It probably no longer has an internal ocean of liquid water, but there is brine that can flow through the outer mantle and reach the surface. The surface is a mixture of water ice and various hydrated minerals such as carbonates and clay. Cryovolcanoes such as Ahuna Mons form at the rate of about one every fifty million years. In January 2014, emissions of water vapor were detected from several regions of Ceres. This was unexpected because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets. The atmosphere, however, is transient and of the minimal kind known as an exosphere.