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This series of images of Comet 67P/Churyumov-Gerasimenko was captured by the Rosetta spacecraft's OSIRIS narrow-angle camera on Aug. 12, 2015, a few hours before the comet reached perihelion, or the closest point to the sun along its 6.5-year orbit.
The rubber ducky comet is glowing. But you can't see it.
*That's because this cometary aurora shines in the far-ultraviolet range, a part of the electromagnetic spectrum with a higher frequency than any light human eyes can detect. Researchers discovered this first cometary aurora in data from comet 67P/Churyumov-Gerasimenko, which the European Space Agency (ESA) visited with the probe Rosetta from 2014 to 2016. Digging through Rosetta's recordings of light around the duck-shaped comet, they uncovered the ultraviolet glow. They showed that glow came from charged particles from the sun hitting gas particles around the comet, the same effect that produces shimmering auroras around the poles on Earth.
And the glow is more intense the closer you get to the rocky, icy heart of the comet, known as the nucleus.
"The effect gets more pronounced closer to the nucleus as also the density of water molecules increases," said Martin Rubin, a co-author of the study and an astrophysicist at the University of Bern in Switzerland.
The nucleus of the comet spits out dust and gas as the comet approaches the sun, forming a tail. Bern and his coauthors showed that electrons — negatively charged particles from the sun — excite the water particles in the comet’s tail to produce an aurora just like they do the particles in Earth's atmosphere.
When an electron strikes a molecule of water vapor, that molecule gets excited to a higher energy state, Rubin told Live Science. Then the energy state drops down again, and the excess energy blasts off of the particle in the form of an ultraviolet photon, or light particle.
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On Earth and other planets, this same process produces a visible-light effect high in the atmosphere over the polar latitudes. Earth's powerful magnetic field deflects most of the sun's charged particles — its solar wind — away from the surface. The particles follow the magnetic field lines around the planet and mostly have no effect at all.
This aurora over Earth was captured on camera from the International Space Station on June of 2017.
(Image credit: NASA)
"However, at the poles the magnetic field passes through the terrestrial atmosphere," Rubin said. "and charged solar wind particles moving along these field lines strike atoms and molecules of the upper atmosphere."
The interaction of charged particles and Earth's complex atmosphere produces ghostly, towering lights in the sky.
Comets don't have magnetic fields though. So on comet 67p the charged particles can penetrate all the way through the tail to the comet's surface. The ultraviolet glow is everywhere — except maybe on the nighttime side of the comet, where the ice and rock block the charged particles, and there is little gas coming off the comet there anyway.
The comet's aurora is most intense in the densest parts of the tail around the nucleus, he said, and fainter toward the low-density parts of the tail. The aurora extends up to about 60 miles (100 kilometers) from the nucleus.
The paper was published Sept. 21 in the journal Nature Astronomy.
Rafi joined Live Science in 2017. He has a bachelor's degree in journalism from Northwestern University’s Medill School of journalism. You can find his past science reporting at Inverse, Business Insider and Popular Science, and his past photojournalism on the Flash90 wire service and in the pages of The Courier Post of southern New Jersey.
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