The rubber duck comet glows. But you can’t see it.
* This is because this comet’s radiance shines in the far ultraviolet range, part of the electromagnetic spectrum at a higher frequency than any light that the human eye can detect. Researchers discovered this first comet aurora borealis in data from comet 67P / Churyumov-Gerasimenko, which the European Space Agency (ESA) visited with the Rosetta spacecraft from 2014 to 2016. Digging up Rosetta’s recordings from light around a duck-shaped comet, they found ultraviolet light. They showed that the glow came from charged particles from the sun, striking gas particles around the comet, the same effect that produces brilliant auroras around the Earth̵
And the glow is more intense the closer you get to the comet’s rocky, icy heart, known as the nucleus.
“The effect becomes more pronounced closer to the nucleus as the density of water molecules increases,” said Martin Rubin, co-author of the study and astrophysicist at the University of Bern, Switzerland.
The comet’s nucleus spits out dust and gases as the comet approaches the sun, forming a tail. Bern and his co-authors showed that electrons – negatively charged particles from the sun – excite water particles in the comet’s tail to produce a glow, just as they do particles in the Earth’s atmosphere.
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When an electron hits a molecule of water vapor, that molecule is excited to a higher energy state, Rubin told Live Science. Then the energy state falls down again and the excess energy erupts from the particle in the form of an ultraviolet photon or a light particle.
On Earth and other planets, this same process produces the effect of visible light high in the atmosphere above the polar latitudes. The Earth’s powerful magnetic field deflects most of the sun’s charged particles – its solar wind – far from the surface. The particles follow the lines of the magnetic field around the planet and mostly have no effect.
“However, at the poles, the magnetic field passes through the Earth’s atmosphere,” Rubin said. “and the charged particles of the solar wind moving along these field lines hit atoms and molecules of the upper atmosphere.”
The interaction of charged particles and the complex atmosphere of the Earth creates ghostly, soaring lights in the sky.
However, comets do not have magnetic fields. So in comet 67p, the charged particles can penetrate through the tail to the comet’s surface. Ultraviolet glow is everywhere – except perhaps on the night side of the comet, where ice and rock block charged particles and there is little gas coming out of the comet anyway.
The comet’s aurora borealis is most intense in the densest parts of the tail around the nucleus, he said, and less so in the parts of the low-density tail. The aurora borealis extends about 100 miles from the nucleus.
The report was published Sept. 21 in the journal Nature Astronomy.
Originally published in Live Science.