Just when I thought the Rosetta mission couldn't surprise me any more, it does exactly that. Using multiple instruments on board the spacecraft, scientists found that the comet 67P/Churyumov-Gerasimenko has a glowing aurora! Faint, but it's there, and it's definitely what I would classify as an actual aurora.
That's bizarre, and cool.
An aurora on Earth is a glow in the sky caused by subatomic particles streaming away from the Sun. The Earth has a magnetic field, like a giant bar magnet, and that accelerates the electrons in the solar wind as they approach the Earth. It funnels them to the north and south magnetic poles (which are relatively close to the physical poles) where they slam into our atmosphere. They hit the oxygen and nitrogen and give those electrons energy, sometimes even knocking them right off the atoms and molecules. When the electrons release that energy (or recombine with the atoms), they emit light at a specific wavelength — color. What we see down on the ground is a beautiful and eerie sheet of color; green, red, even pink, and blue, depending on the kind of atom or molecule.
This is why I was surprised at the news about the comet 67P. Comets don't have magnetic fields, certainly not ones strong enough to accelerate electrons. So how do they generate an aurora? And how did the scientists figure it out?
The Rosetta spacecraft arrived at the comet in 2014 after a ten-year journey through space. It orbited 67P for three years, taking incredible images and measurements of the comet and its environment. It was using data from multiple instruments onboard that scientists teased out the information about the cometary glow.
Comets are composed of a solid nucleus made of rock and ice (which are themselves things we think of as gases on Earth: carbon monoxide, carbon dioxide, water, and so on). When it nears the Sun, the heat causes those ices to turn into a gas that surrounds the nucleus (in a cloud called the coma). Ultraviolet light from the Sun can zap those atoms, exciting their electrons and causing them to emit light. But that's not an aurora; it's similar, but it's caused by light, not electrons. That light is called dayglow (we see something similar in Earth's atmosphere as well).
But there is another glow they measured at the comet, ultraviolet light coming from hydrogen and oxygen. This glow was coming from a shadowed part of the nucleus, which meant it wasn't coming from atoms getting hit by sunlight. That means the atoms must be excited by energetic electrons pinging into them.
That glow was seen by an instrument called Alice, a camera that can see in the ultraviolet. The scientists then used other instruments to figure out what else was going on. The Rosetta Plasma Consortium electron spectrometer could measure high-energy electrons and where they came from (the Sun). Molecules of water were detected by several other instruments, and from all this they put together a picture of events.
Electrons from the solar wind approach the comet. There's no magnetic field there to accelerate them, but there is a cloud of ionized atoms and molecules surrounding the comet. These charged particles create an overall electric field, weak and diffuse, but strong enough. When a solar wind electron flows into this field, it still gets accelerated. The electrons slam into the water molecules, breaking them apart into oxygen and hydrogen. These electrons in the atoms are excited (have extra energy), and they release that energy in the form of ultraviolet light.
That UV light is the glow of the aurora. It's diffuse because the cloud around the comet nucleus is diffuse, so not like Earth's where we see the energy shaped into sheets and ribbons by our magnetic field. But the solar wind electrons were indeed accelerated, so I'm comfortable calling that an aurora.
We see aurorae around other astronomical bodies; Jupiter and Saturn have intense ones, Mars has a weak one, and Venus kinda sorta has one, though there's no local magnetic field involved. Some moons have one (like Ganymede), and aurorae have even been detected on rogue planets and other stars! But this is the first time it's ever been seen at a comet.
And this is the first comet that's been examined for an extended period up close, I'll note. Every comet that gets near the Sun generates a coma, so any comet with a cloud of gas around it should make an aurora as well. That's pretty cool! And useful; it allows scientists to study the composition of comets, the behavior of their comae, and even understand better the solar wind. Quite the bargain.
And this happened by putting together many observations from multiple instruments on Rosetta long after the mission ended and the spacecraft was shut down after touching down on 67P's surface. It shows that missions end, but science never does. There's always more to learn, even from older data.