MAVEN captured this ultraviolet image of hydrogen surrounding comet C/2013 A1 Siding Spring on October 17, 2014, two days before the comet's closest approach to Mars, When the comet was 8.5 million kilometers away. The image shows sunlight that has been scattered by atomic hydrogen. Comets are surrounded by a huge cloud of atomic hydrogen because water vaporizes from the icy nucleus, and solar ultraviolet light breaks it apart into hydrogen and oxygen. Hydrogen atoms scatter solar ultraviolet light, and it was this light that was imaged by the Imaging Ultraviolet Spectrograph.
The bulk of the scattered sunlight shows a cloud that was about a half degree across on the "sky" background, comparable in size to the Earth's Moon as seen from Earth. Hydrogen was detected to as far as 150,000 kilometers away from the comet's nucleus. The distance is comparable to the distance of the comet from Mars at its closest approach. Gas from the comet is likely to have hit Mars, and would have done so at a speed of 56 kilometers per second. This gas may have disturbed Mars' atmosphere.