With all the recent things hitting Jupiter in full sight of our telescopes, such as Shoemaker-Levy 9, the possible asteroid of 2009 and a bright flash earlier this year, the appearance of measurable aftereffects has allowed astronomers to start searching for clues to past events in the lives of other planets. SL9 gave the most obvious immediate aftermath, with bruises of upwelling material and the ability to measure chemicals delivered to Jupiter, the evolution of this trail in the atmosphere and relate it back to an object observed before it struck.
Among the first science highlights of the Herchel Space Telescope was data on the atmosphere of Neptune taken by the Photodetector Array Camera and Spectrometer. This revealed an imbalance in the concentration of carbon monoxide – there was more above the tropopause than below it. This was significant as the region in which the imbalance was seen is known as the homosphere – a region where relative concentrations remain the same (homogenous) with altitude. CO is long lived enough to become well mixed even if produced in one bit of the homosphere. The only way for the additional carbon monoxide to be around is if something put it there. It doesn’t quite fit the profile of stuff delivered by dust slowly drifting from space into the atmosphere, but it could be a slowly spreading cloud of debris from a comet. Working back, the comet must have struck the planet two centuries ago to produce this distribution.