I would have guessed this wasn't possible. The idea, I guess, is that space dust mixed with frozen gases and when the frozen gases sublimate a structure of space dust is left behind. As I write this I realize there's a lot I don't know here. What is the principle constituent of the coma that we can see from earth? Is the material spewed out of the comet mostly in a liquid state? If it is mostly in a gaseous state it seems surprising that it is opaque enough to reflect light.
The volatiles that come out are gaseous. The bright coma that you see is created by reflection from gas and dust, which is shot into space by the escaping gas.
Although 67P/CG passes by the sun quite often (every 6 years) there is still enough ice inside to make it active. At first determination it was about 2 glasses of water per second, at its highest, during the last passage, the production rate was about 350 10
23 s
-1. With that rate taken and say 1 year of outgassing gives a upper mass loss of 10
7 kg with a total mass of the comet of 10
13 kg.
The principle constituent at the moment is actually CO2, but that will be overtaken by H2O (or maybe has already been).
I've looked around for a discussion of this kind of thing and I haven't found it, but it seems like one of the things that people would have been doing calculations and maybe simulations of. Does it exist and I haven't found it or has it just not been released yet?
Yes, there have been all kinds of simulations and observations. I think the two papers best used as a starting point are:
Snodgrass, C., Tubiana, C., Bramich, D. N., Meech, K., Boehnhardt, H., and Barrera, L.: Beginning of activity in 67P/Churyumov-Gerasimenko and predictions for 2014–2015, Astron. Astrophys., 557, 15 pp., oi:10.1051/0004-
6361/201322020, 2013.
Rubin, M.; Koenders, C.; Altwegg, K.; Combi, M. R.; Glassmeier, K.-H.; Gombosi, T. I.; Hansen, K. C.; Motschmann, U.; Richter, I.; Tenishev, V. M.; Tóth, G, Plasma environment of a weak comet - Predictions for Comet 67P/Churyumov-Gerasimenko from multifluid-MHD and Hybrid models, Icarus, Volume 242, p. 38-49. 2014.
