Latest Rosetta news is from an article in Nature (July 2nd, 2015) – see http://phys.org/print354967160.html … where we are told there are sink holes on comet Churyumov-Gerasimenko, almost circular pits. The surface of the comet is undergoing rapid structural change, it seems, and the theory (expressed in the Nature paper), is they form when ice beneath the surface sublimates (or turns directly into gases).
Sink holes occur on Earth when the subsurface erodes enough to remove a large amount of material beneath the surface, creating a cavern. Eventually the roof of the cavern will collapse – leaving behind the sink hole. Jets of gas and dust stream from the sides of the pits on the comet – so it is not really like a terrestrial sink hole. It is eroding – but the erosion does not create a cavern – it creates a hole. The Nature piece describes how the authors used a model to work out the process going on. ASs in all models it is built on assumptions. In this instance it begins with a source of heat beneath the surface that causes ices (water and gases) to sublimate – but what is the source of the heat?
At http://phys.org/print354948968.html … comet Churyumov-Gerasimenko will reach its nearest point to the Sun on 13th August, 2015. It will be roughly at a point 185 million km away. Hence, it is not a sun grazer by a long shot. It will have an orbit at perihelion that is far enough away to perhaps avoid any major fireworks, although expectations are high. In spite of this, activity is increasing.