An aurora has been spotted surrounding a comet orbiting Jupiter - the first time the glowing emissions have been seen around a cosmic snowball.
Astronomers from the Southwest Research Institute (SwRI) identified the shimmering phenomenon using instruments on the European Space Agency (ESA) Rosetta spacecraft.
Image details: Instruments on the European Space Agency (ESA) Rosetta spacecraft helped Southwest Research Institute (SwRI) astronomers spot the aurora that shines in ultraviolet light
Rosetta was the first space mission to place a lander on the surface of a comet, the first to orbit a comet nucleus and the first to follow it into the inner solar system.
On the Earth, auroras are formed when charged particles from the Sun follow the planet's magnetic field lines to the north and south poles where they strike atoms in the atmosphere and create a shimmering curtain of colourful lights in the sky.
Similar phenomena have been seen on planets and moons in our solar system and even around a distant star - this is the first time it has been spotted around a comet.
It was caused by charged particles coming on solar winds and interacting with the coma - gas and dust - surrounding the nucleus of the comet, known as 67P/Churyumov-Gerasimenko.
The team were amazed to find that the UV emissions in the aurora were driven by electrons (inward arrows) in the solar wind that break apart water and other molecules in the coma
In the case of the comet the 'lights' are only visible in ultraviolet so couldn't be seen by the naked eye - unlike the aurora at high latitudes on Earth.
A number of instruments onboard Rosetta were involved in the discovery including a far-ultraviolet spectrograph and ion and electron sensors.
The Ion and electron sensor (IES) detected the electrons that caused the aurora surrounding the comet, according to Jim Burch from SwRI.
'Charged particles from the Sun streaming towards the comet in the solar wind interact with the gas surrounding the comet's icy, dusty nucleus and create the auroras,' said Burch who leads the IES team.
The envelope of gas around 67P/C-G, called the 'coma,' becomes excited by the solar particles and glows in ultraviolet light - not visible to the naked eye.
'Initially, we thought the ultraviolet emissions at comet 67P were phenomena known as 'dayglow,' a process caused by solar photons interacting with cometary gas,' said SwRI's Dr Joel Parker who leads the Alice spectrograph project.
The team were amazed to find that the UV emissions in the aurora were driven by electrons in the solar wind that break apart water and other molecules in the coma.
They had 'been accelerated in the comet's nearby environment. The resulting excited atoms make this distinctive light,' said Parker.
Dr Marina Galand of Imperial College London led a team that used a physics-based model to integrate measurements made by various instruments aboard Rosetta.
'By doing this, we didn't have to rely upon just a single dataset from one instrument,' said Galand, the lead author of the paper outlining the results.
'Instead, we could draw together a large, multi-instrument dataset to get a better picture of what was going on,' Galand explained.
'This enabled us to unambiguously identify how 67P/C-G's ultraviolet atomic emissions form, and to reveal their auroral nature.'
'I've been studying the Earth's auroras for five decades,' Burch said. 'Finding auroras around 67P, which lacks a magnetic field, is surprising and fascinating.'
Following its rendezvous with 67P/C-G in 2014 through 2016, Rosetta has provided a wealth of data revealing how the Sun and solar wind interact with comets.
In addition to discovering these cometary auroras, the spacecraft was the first to orbit a comet's nucleus, the first to fly alongside a comet as it travelled into the inner Solar System and the first to send a lander to a comet's surface.
The findings have been published in the journal Nature Astronomy.