Nature: Observations of the atmospheres of comets from Earth and from the Hubble Space Telescope revealed the presence of carbon, hydrogen, and oxygen atoms. It was assumed that UV photons from the Sun were breaking up carbon dioxide and water molecules, but now the European Space Agency's Rosetta has proven otherwise. In orbit around comet 67P/Churyumov-Gerasimenko, the spacecraft's onboard UV camera has found no evidence of UV-triggered molecule destruction. Instead, the shorter wavelengths of light the camera detected suggested that the atoms had been knocked free by high-energy electrons. It is likely that solar UV radiation was still the root cause because the electrons were probably released by the impact of high-energy photons.
