At http://phys.org/print326092712.html … we learn that a distinct coma surrounds comet 67P (otherwise known as Comet Churyumov-Gerasimenko).
the hazy circular structure on the right and the centre of the coma are artifacts due overexposure of the nucleus. The image was taken by the Osiris camera onboard the Rosetta spacecraft on July 25th (see also http://tallbloke.wordpress.com/2014/08/02/image-of-the-day-comet-67p-tak…
The coma is 150km acrss but it may reach even further as the image is limited by the view capability of the Osiris camera. Hence, it might just be the inner part of the coma where particles are dense. The brightness of the nucleus is responsible for the artifact on the right – a reflection.
At http://phys.org/print326111431.html … we learn that Rosetta has measured the temperature of Churyumov-Gerasimenko and it is too hot for ice. The comet, at the time of measurement, was some 555 million km from the Sun – three times the distance of the Earth from the Sun. This suggests much of the surface is dusty – but they still think they will find the odd bit of surface that is icy.