When asked to think up a bad smell, your mind will likely drift toward the putrid stench of rotten eggs. That tell-tail smell is caused by hydrogen sulfide and is produced by bacteria feasting on rotting organic matter.

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It also turns out that, to the surprise of scientists studying data from the European Rosetta mission, Comet 67P/Churyumov-Gerasimenko is venting hydrogen sulfide. But that’s not all. Churyumov-Gerasimenko has gone all-out on the sniff-o-meter — it’s also venting ammonia, methane, hydrogen cyanide and formaldehyde, chemicals that have smells associated with urine, alcohol, bitter almonds and vinegar.

In short, if we were to return a sample of this nasty cometary cocktail to Earth, it would not only bring a tear to the eye if you were unfortunate enough to sniff it, you’d also likely gag to death.

This delightful array of chemicals in the comet’s coma (the comet’s ‘atmosphere’ surrounding the nucleus) were detected by Rosetta’s mass spectrometer, the Rosina-DFMS instrument.

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“The perfume of 67P/C-G is quite strong,” said Rosina’s chief scientist Kathrin Altwegg in a Rosetta blog update. “With the odour of rotten eggs (hydrogen sulphide), horse stable (ammonia), and the pungent, suffocating odour of formaldehyde, this is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide.

“Add some whiff of alcohol (methanol) to this mixture, paired with the vinegar-like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide, and you arrive at the ‘perfume’ of our comet.”

Sounds delightful.

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The detection of these chemicals has surprised scientists; it was assumed that only the most volatile of ices — water and carbon dioxide — would be sublimated into space as the comet approaches the sun. Churyumov-Gerasimenko is currently over 250 million miles from the sun, but as it gets closer, solar energy heating the dusty surface will cause more ices to be released, generating more jets of gas and dust.

In new images sent back to Earth, an uptick in comet activity is being measured by Rosetta (pictured top).

“At this point, we believe that a large fraction of the illuminated comet’s surface is displaying some level of activity,” said OSIRIS scientist Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research (MPS) in Germany.

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“Being able to monitor these emissions from up close for the first time gives us much more detailed insights,” added OSIRIS Principal Investigator Holger Sierks. “But one image alone cannot tell us the whole story; from one image we cannot discern exactly where on the surface a jet arises.”

The mission is continuing its detailed campaign as it circles Comet 67P/Churyumov-Gerasimenko, discovering not only how the comet is evolving as it approaches the sun, but also the cocktail of chemicals it's outgassing in the process.