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Why does the universe smell so bad?


The sight of the universe inspires wonder. The smell of it, though, might prompt other reactions. Joel F. Hooper explains.


The universe: looks amazing, smells awful.
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Smell is perhaps our most mysterious sense.

It can trigger memories and link us to specific times and places. It’s not surprising that we often wonder what distant and exotic places would smell like, from the frequent mention of odour in Gulliver’s Travels to Professor Farnsworth’s Smell-O-Scope in Futurama.

So, setting aside the practical problems of trying to take a lungful of vacuum, what would it be like to get a whiff of the sparse gases and particles that occupy deep space?

If we turn our nose to Sagittarius B2, a cloud of gas about 390 light years from the centre of the Milky Way, we would encounter a host of olfactory delights. Almost every chemical that has been detected in space can be found there.

Among the smellier components of Sagittarius B2 is hydrogen sulfide (H2S), often described as rotten-egg gas.

This chemical can be detected at around 10 billion molecules per cubic centimetre by the human nose, and can cause death in high concentrations. At its most dense, the Sagittarius cloud contains only about one million molecules per cubic centimetre, about 10,000 times beneath the human threshold.

We might also encounter hydrogen cyanide (HCN), another deadly gas, though this one smells of bitter almonds. Chemists in the early twentieth century used to smoke cigarettes while working with this chemical, because a release of hydrogen cyanide would change the flavour of the tobacco and act as an early warning sign of a leak.

There are also much more agreeable odours in space. Ethyl formate belongs to a class of molecules called esters, which often have sweet and fruity aromas. It is one of the chemicals responsible for the smell of raspberries.

Space is also home to compounds called polyaromatic hydrocarbons, flat molecules made up of rings of carbon atoms. These chemicals were named “aromatic” by early chemists before their structure was known, due to the strong smells they produce.

Their fragrances range from faintly pleasant to the strong smell of coal tar. A study just published in The Astrophysical Journal found they are present in much higher concentrations than previously throught, especially in older galaxies.

The main difference between the gases of space and those in our own atmosphere is the abundance of oxygen on earth, meaning that many smelly chemicals based on sulfur or phosphorus exist here in their milder, oxidised forms. So taking a deep whiff of space gas would probably smell closest to rotting garbage, fish, or flatulence.

Joel Hooper is a senior research fellow at Monash University, in Melbourne, Australia.
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