A group of astronomers across Europe have recently announced that Neptune was struck by a comet 230 years ago. While the recent impacts on Jupiter all left obvious visible evidence, the signs all faded away quite rapidly. So how do they know about an impact which occurred 65 years before the planet had even been discovered?
May last year, the Herschel research satellite was placed in solar orbit. It recently returned observations of Neptune which indicated an anomaly in the planet’s atmosphere: Carbon Monoxide (CO) present in the atmosphere should be evenly distributed at al altitudes, but Neptune has unusually high concentrations of the deadly gas in its stratosphere. The only way that this could happen is if extra CO arrived from off the planet. Of the two possible mechanisms which could cause this to happen, space dust gradually filtering down could not produce high enough levels to match what has been seen. This leaves only a comet smashing into the atmosphere and disintegrating while still very high up (Comet’s are quite delicate fragile things). Once this had been established, the team were able to model the rate at which CO would filter down to the lower levels, and could finally calculate that the impact must have happened 230 years ago.
For more information on this impressive piece of scientific detective work, view the original press release here: http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2010/pressRelease201007161/