At http://phys.org/print362294586.html … which begins by telling us that comets are composed of dust and ices which they periodically shed as they swing round the Sun. Sunlight heats up the frozen nucleus of the comet, we are told, and water and gases are released – all standard stuff.
The Rosetta mission scientists say they have identified a region on the comet's surface where water-ice appears and disappears in sync with its rotation period. As the comet rotates, taking around 12 hours for a full revolution, regions undergo different illumination -and different levels of heating. Hence the idea of a cyclical behaviour of water-ice during each comet revolution.
However, the tale is slightly different at http://phys.org/print362390462.html … where it says the big surprise of the Rosetta mission is that the Churyumov-Gerasimenko comet was completely unlike a dirty snowball – at last the truth is being squeezed out. The comet is unmistakably rocky and almost completely covered in a layer of organic compounds – which was not expected.
Where is the water ice? The researchers have come up with an explanation – they day and night cycle of ice sublimation (where solid turns to gas without first becoming a liquid). This differs from the rotation explanation but is somewhat similar. When in shadow (the night side) ice forms, and when the sun shines on the comet (the day side) the ice melts. The effects, we are told, penetrate a few centimetres down, which they think may be deep enough to get at the water (which they still thinks is an important component of the comet) – never let a mainstream theory die without attempting to revive the corpse.
At the moment Rosetta is having a rest and is travelling in order to get into position to observe the bow of the comet, the region where the solar wind hits the comet as it moves away from the Sun. The bow shock of a comet has never been observed before, although other missions have flown past or through the bow region they have never lingered. Studying the interface between the solar wind and the plasma region around another body (in this case a comet) is important because it will help science understand the physics of the interactions taking place between the particles (charged particles in the solar wind) and the outflow on the comet. Currents in the solar wind are modified at the bow shock leading to changes in space weather (above earth) so what does it do to a comet?
it looks like a lot of rock.