Comet 46P/Wirtanen in December 2018. (Martin Stojanovski/Wikimedia Commons/CC BY-SA 4.0)

Visiting Comet 46P/Wirtanen Is 'Abnormally High' in Alcohol

3 JULY 2021

Comets are boozy beasts.

They come in here to the inner Solar System from goodness-knows-where (the outer Solar System), get a little warmth, and start spewing alcoholic compounds into space, willy-nilly.


Comet 46P/Wirtanen, which visited the inner Solar System in 2018, takes the martini. According to an analysis of its atmosphere, or coma, it was giving off what scientists have called an "abnormally high" amount of alcohol.

And this can tell us some really interesting things about the evolution of the Solar System.

"46P/Wirtanen has one of the highest alcohol-to-aldehyde ratios measured in any comet to date," said cometary scientist Neil Dello Russo of Johns Hopkins University Applied Physics Laboratory. "This tells us information about how carbon, oxygen, and hydrogen molecules were distributed in the early Solar System where Wirtanen formed."

Comet 46P/Wirtanen is a fairly regular visitor to the inner Solar System. It swings around the Sun every 5.4 years, occasionally veering so close to Earth that it is visible in the night sky to the naked eye.

On its most recent visit, in December 2018, it came within 11.6 million kilometers (7.2 million miles) of Earth, around 30 times the average distance between Earth and the Moon.

Astronomers took full advantage of this opportunity to study this comet from relatively close quarters, using the Keck Observatory's newly-upgraded Near-Infrared Spectrograph (NIRSPEC).


This instrument can collect data on the sunlight that shines through the comet's coma so that scientists can then analyze it to determine its chemical composition.

Cometary comas can tell us a lot about the outer and early Solar System. Comets differ from asteroids in that they're filled with all sorts of frozen compounds – ices – that got bound up in them when they formed, hence the nickname "dirty snowball".

For most of a comet's orbit, these ices remain frozen, but when the comet draws close enough to the warmth of the Sun, the ices start to sublimate, dislodging dust and creating a dusty, gaseous envelope.

It's this material that forms the comet's gas and dust tails, streaming away from the Sun due to solar wind and radiation pressure.

And, because this material has been sitting locked frozen in a comet from the time the body formed – when the Solar System was a baby – until sublimation, it contains information about the composition of the cloud from which the Solar System itself formed.

In the coma of 46P/Wirtanen, NIRSPEC took just 10 to 20 minutes to detect its composition: acetylene, ammonia, ethane, formaldehyde, hydrogen cyanide (which breaks apart to create cyanogen, the compound that makes the comet glow green), methanol, and water.


The NIRSPEC data can also reveal the temperature of the coma, and here the scientists found something really odd. There was evidence of more heat than could be accounted for by just the Sun.

"We found that the temperature measured for water gas in the coma did not decrease significantly with distance from the nucleus, which implies a heating mechanism," said astronomer Erika Gibb of the University of Missouri-St. Louis.

It's unclear what this heating mechanism could be, but there are multiple possibilities.

One possibility is that solar radiation could have ionized some of the molecules in the coma, close to the cometary nucleus, which would result in the release of energetic electrons. These electrons could collide with other molecules and transfer energy, which is released as heat.

Another is that solid chunks and grains of ice broke off the comet, tumbling away from the nucleus before sublimating and releasing energy via collisions in the cooler cloud at that distance, rather than closer in. The team did find a significantly higher proportion of water in the outer coma compared to other compounds, which is consistent with this model.

This may help explain how water could have been delivered to planets like Earth. Although the water ice sublimates at the comet, it may return to liquid or ice form when it lands on a planet.

Other ingredients for life have also been found on comets, so these dirty snowballs could be vitally important not just for our own existence, but for life elsewhere in the Universe.

"Comet studies like this are exciting because they serve as a launchpad for answering the million-dollar question – are we alone?" said astronomer Greg Doppmann of Keck Observatory.

"The organic compounds on comets tell us what ingredients formed our solar system and served as precursors to life. We can then look for these same prebiotic molecules in other planetary systems, which opens an exciting door to the very real possibility of finding microbial life beyond Earth – not in our kids' lifetimes, but our own lifetime."

The research has been published in The Planetary Science Journal.