European Space Agency

Eau de Comet isn't, we now know, the most seductive scent floating around in our galaxy. The Rosetta probe's Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has been using its two mass spectrometers to detect the "smell" of 67P/Churyumov-Gerasimenko.

From its position in orbit around the comet, ROSINA was able to detect the chemical makeup of 67P/C-G's coma -- the halo of material surrounding the comet, which increases in intensity as the comet nears the sun and heats up, causing parts of it to sublimate.

At 400 million kilometres (250 million miles) from the sun, the Rosetta team thought the coma would only contain the comet's most volatile molecules -- carbon dioxide and carbon monoxide -- but it is much richer than previously thought.

As of September 11, the ROSINA team knew that the coma contained (in gas form) water, carbon monoxide, carbon dioxide, ammonia, methane and methanol.

The new measurements have detected the presence of formaldehyde, hydrogen sulphide, hydrogen cyanide, sulphur dioxide and carbon disulphide -- albeit in relatively low density.

This heady melange -- aside from being quit toxic to humans -- would smell quite vile.

"The perfume of 67P/C-G is quite strong, 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," said ROSINA principal investigator Kathrin Altwegg.

"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."

As 67P/C-G draws closer to the sun, it's expected that it will begin releasing more molecules. These -- and the changes in the comet's coma -- will allow the scientists to determine the composition of the comet itself. This, in turn, will allow comparison with other comets -- such as Siding Spring, which recently flew past Mars.

The 67P/C-G hails from the Kuiper Belt, within our solar system, and Siding Spring is from the Oort Cloud -- over 1,000 times further away from the sun than the Kuiper Belt. Comparing the two comets could help determine the composition of the nebula that gave birth to the sun and solar system.