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Accumulating data from Rosetta is now giving scientists an excellent picture of how this comet interacts with the solar wind as it moves in towards its closest approach to the Sun.
They have seen that the number of water ions – molecules of water that have been stripped of one electron – accelerated away from the comet increased hugely as 67P/C-G moved between 3.6AU (about 538 million km) and 2.0AU (about 300 million km) from the Sun. Although the day-to-day acceleration is highly variable, the average 24-hour rate has increased by a factor of 10,000 during the study, which covered the period August 2014 to March 2015.
The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. Once in gaseous form, the collision of extreme ultraviolet light displaces electrons from the molecules, turning them into ions. Colliding particles from the solar wind can do this as well. Once stripped of some of their electrons, the water ions can then be accelerated by the electrical properties of the solar wind.
Not all of the ions are accelerated outwards, some will happen to strike the comet’s surface. Solar wind particles will also find their way through the coma to hit home. When this happens, they cause a process called sputtering, in which they displace atoms from material on the surface – these are then ‘liberated’ into space.
There’s more at the link, including animations and simulations.