After some clever detective work, astronomers think Neptune was hit by a large comet… two centuries ago.

Over the past 16 years, astronomers have been treated to three impact events in Jupiter’s atmosphere, which stands them in good stead to spot evidence of a similar event on Neptune. The first was the famous 1994 Shoemaker-Levy 9 break-up and impact, the second was last year’s possible asteroid impact, and last month was the bright flash probably caused by a meteor streaking through the upper Jovian atmosphere.

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In short, astronomers have a good idea about what an impact event looks like and they have done extensive postmortems to identify what the impactors were made of. By analyzing the ‘bruise’ (or lack thereof) in Jupiter’s atmosphere, we can gain a lot of information about the size and composition of the offending asteroid or comet.

For example, in another cosmic case of ‘whodunit’, the 2009 impact event observed by amateur astronomer Anthony Westley was originally believed to have been caused by a rogue comet. However, after analysis by the Hubble Space Telescope, it was deduced that there was a lack of small particles surrounding the impact site. This suggested that the 2009 impactor was made of stronger stuff than a comet. This provided circumstantial evidence for an asteroid, not a comet, striking the Jovian atmosphere.

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Comets also leave behind water, carbon dioxide, carbon monoxide, hydrocyanic acid, and carbon sulfide after impact; these molecules were detected in the bruises of Jupiter’s atmosphere after the Shoemaker-Levy 9 event.

So, knowing what a comet impact looks like, scientists from the French observatory LESIA in Paris and Max Planck Institute have turned up evidence for an ancient impact event on Neptune.

It’s hardly surprising that the gas giants suffer frequent bombardments — indeed, if it wasn’t for Jupiter’s massive gravity vacuuming up chunks of space stuff, life on Earth may not have evolved into its current form — but it amazes me that we have the capability to analyze the conditions of Neptune’s atmosphere and deduce that it was struck centuries ago.

Using the Photodetector Array Camera and Spectrometer (PACS) instrument on board the European Space Agency’s Herschel space observatory, an unusual distribution of carbon monoxide has been detected in Neptune’s upper atmosphere, the stratosphere. There’s more carbon monoxide above the gas giant’s troposphere, something that shouldn’t be possible unless the molecule came from an external source.

What external source could deliver large quantities of carbon monoxide into the upper atmosphere of a planet? A comet, of course!

(There is another theory that the carbon monoxide was delivered by interplanetary dust, but that mechanism doesn’t tally with the measurements Herschel took.)

“Normally, the concentrations of carbon monoxide in troposphere and stratosphere should be the same or decrease with increasing height,” said Paul Hartogh, principle investigator of the Herschel science program.

“From the distribution of carbon monoxide we can therefore derive the approximate time, when the impact took place”, explains Thibault Cavalié, also a member of the Herschel team at the Max Planck Institute.

If this was a cosmic game of Cluedo, it would appear Neptune was shot in the outer solar system, by a comet. However, this crime is a rather extreme ‘cold case’ where only the advanced optics of a space telescope and the ingenuity of modern science could unravel the 200-year-old evidence.

Image credit: NASA