Go home Comet Wirtanen, you're drunk. Comets consist of some surprisingly complex molecules, often including alcohol. However, Comet 46P/Wirtanen released an unusually large amount of CH3OH as it flew past Earth in 2018. Perhaps it's not surprising then that it also showed some other strange and as yet unexplained behavior.
Comet Wirtanen is a short-period comet, with an orbit of just 5.4 years, not a visitor from the depths of space like Comet Borisov or the recently discovered megacomet Bernardinelli-Bernstein. Indeed, so typical does it seem it was once planned to be the target for the Rosetta mission, which ended up going to comet 67P/Churyumov-Gerasimenko instead.
When it made one of the 10 closest approaches to Earth by a comet (around 12 million kilometers) in the last 70 years telescopes worldwide were trained on it. On previous passes, Wirtanen had been dubbed “hyperactive” because it seemed to shed material unusually fast. Results from the Keck telescope, using its NIRSPEC spectrograph upgraded just before the visit, have now been published in the Planetary Science Journal, including some eyebrow-raising features.
"46P/Wirtanen has one of the highest alcohol-to-aldehyde ratios measured in any comet to date," said Dr Neil Dello Russo of Johns Hopkins University in a statement."This tells us information about how carbon, oxygen, and hydrogen molecules were distributed in the early Solar System where Wirtanen formed."
That's not the only odd thing about Wirtanen they found. Comets are "dirty snowballs", icy balls of frozen gases, rocks, and dust. The heat of sunlight falling on a comet's nucleus as it passes the Sun heats the ice. In the absence of an atmosphere, the liquid phase is impossible, so once a particular type of ice melts it sublimates, going directly to a gas, producing a coma around the comet.
The coma is expected to cool down the further away it gets from the nucleus, just as air cools as it expands, but that barely happened for Wirtanen. “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," co-author Professor Erika Gibb of the University of Missouri-St Louis said.
The authors are not certain what was heating up the incredibly diffuse molecules within the coma, but offer two possibilities for further exploration. “There may be solid chunks of ice flying off 46P/Wirtanen. We've seen this in some comets visited by spacecraft,” Gibb said. We know Witanen underwent a spectacular explosion at the time. “Those ice chunks tumble away from the nucleus and sublimate [turn to gas], releasing energy further out into the coma.”
Alternatively, ionization of molecules in the coma by sunlight may release high-energy electrons that warm up other molecules when they collide with them.
Sadly for anyone hoping to make Wirtanen the ultimate space party destination, the alcohol detected was methanol not ethanol, so it would send you blind, and not in a good way. Other molecules detected in its coma include formaldehyde and hydrogen cyanide, so it is just as well they are millions of times too diffuse to do us any damage.
If a hot, drunk comet traveling around the inner Solar System and behaving erratically sounds alarming the good news is that Wirtanen never crosses Earth's orbit. Its closest approach to the Sun is 5 percent further out, and its elongated orbit takes it out as far as Jupiter.
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