What Does a Comet Smell Like?

It's not Chanel No. 5, But It Is an Important Observation

This image of the "neck" region of Comet 67P/Churyumov-Gerasimenko show increasing jet activity as the comet gets closer to the Sun. Dust and gas are being emitted from nearly the entire surface of the comet, and dust jets are clearly emitting from various parts of the surface. More images will tell a clearer study, and the OSIRIS camera (which took this image) is busily imaging the comet. Courtesy ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/ INTA/UPM/DASP/IDA.

It's not very often that astronomers get to sniff the objects they study. That's because stars and planets and galaxies are just too far away, and besides — who ever thought of what a distant celestial object would smell like?

It turns out that astronomers can determine what a comet smells like because it's made of chemical compounds we know of here on Earth, such as ammonia and formaldehyde, to name a few. So, when the Rosetta mission astronomers built the spacecraft instruments, they included a spectrometer — an instrument that does chemical analysis of materials. After the spacecraft arrived at Comet 67P/Churyumov-Gerasimenko and began orbiting its nucleus, the spectrometer (called the Spectrometer for Ion and Neutral Analysis, or ROSINA, has been getting quite a workout. It works by sampling the materials in the coma of the comet. That's the cloud of gases and dust that exists around the nucleus, and it forms as the comet is warmed by the Sun. The ices sublimate (much as dry ice does if you leave it out) and lift off the surface of Comet Churyuymov-Gerasimenko. This coma-building action actually happens with all comets as they near the Sun. 

So, what does the comet smell like? According to Kathrin Altwegg, one of the spacecraft science team members, the perfume of this comet is quite strong. It smells like a mix of rotten eggs (which comes from hydrogen sulfide), a whiff of horse stable (from ammonia) and the pungent, suffocating odor of formaldehyde (which is familiar to us as embalming fluid). The tincture of comet also contains a little almond-like hint of hydrogen cyanide,plus a little alcohol (in the form of methanol). Top it off with a finish of vinegar-like sulfur dioxide and a hint of the sweet aromatic scent of carbon disulfide and, voila!  You have Essence of Comet 67P!

Kathrin points out that this perfume isn't exactly Chanel No. 5, and wouldn't be a big hit with Earth-based perfume lovers, but it's important to remember that the overall density (the amount of these molecules in a given sample) is very low and the main part of the coma is made up of sparkling water (water and carbon dioxide molecules) mixed with carbon monoxide. That is, if you could stand on the comet and sniff this mix of gases and dust, you probably wouldn't detect much of the odor at all, it's so faint. But, if you were the spectrometer, it would be the scent of a successful mission.

"This all makes a scientifically enormously interesting mixture in order to study the origin of our solar system material, the formation of our Earth and the origin of life," said Altwegg, who works at the Center of Space and Habitability (CSH) of the University of Bern in Switzerland.

One thing astronomers hope to figure out as they study the data about the different materials sizzling off the comet is whether or not there's a chemical difference between comets that originate in a vast region at the edge of our solar system described called the Oort Cloud or in a somewhat closer (but still distant) region that lies just beyond the orbit of Neptune called the Kuiper Belt (named after astronomer Gerard Kuiper). The Kuiper Belt is the birthplace of Comet Churyumov-Gerasimenko and is now being explored by the New Horizons mission.

The Oort Cloud was first described by astronomer Jan Oort, and it stretches out to a quarter of the way to the nearest star. It's the birthplace of Comet C2013 A1 Siding Spring (which just passed by Mars.

If there IS a difference between the chemical makeup of comets from either region, that will give important clues to what conditions were like in different parts of the nebula that gave birth to the Sun and planets some 4.5 billion years ago. 

The Rosetta mission ended on September 30, 2016, when the spacecraft finished its work and made a soft crash-landing on the comet's nucleus. It will ride along on the comet as it orbits the Sun, and the data the spacecraft provided will keep astronomers busy for years.