Comet found to have unique aurora in first-ever discovery on a celestial object other than a planet or a moon
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 aurora
Aurora, also known as the northern or southern lights, is not unique to our planet. Elsewhere in the solar system, Jupiter and some of its moons, as well as Saturn, Neptune, Uranus, and even Mars, have all exhibited their own version of these spectacular light displays. But scientists have now discovered that a comet too has its own aurora. The European Space Agency or ESA’s Rosetta mission has revealed a unique kind of aurora sparked by the solar wind in the gas surrounding Comet 67P/Churyumov-Gerasimenko (67P/C-G). It confirms that ultraviolet auroras can occur at comets, and brings insight into how these exciting light shows form at different solar system objects.
“ESA’s Rosetta spacecraft has helped reveal auroral emissions in the far ultraviolet around a comet. It is the first time such electromagnetic emissions in the far-ultraviolet have been documented on a celestial object other than a planet or moon. These emissions are created as charged particles stream towards the comet from the Sun – a flow known as the solar wind – and interact with the gas surrounding the comet’s icy, dusty nucleus,” write authors in the study published in Nature Astronomy.
On Earth, auroras are formed when electrically charged particles speeding from the Sun hit the upper atmosphere to create colorful shimmers of green, white, and red. The charged particles follow the planet’s magnetic field lines to the north and south poles. “I’ve been studying the Earth’s auroras for five decades. Finding auroras around 67P, which lacks a magnetic field, is surprising and fascinating,” says study author Dr Jim Burch, vice president of Southwest Research Institute.
The ultraviolet emissions Rosetta observed at comet 67P/C-G have shown up before, but were thought to be ‘dayglow,’ a process caused by solar light particles (photons) interacting with the envelope of gas – known as the coma – that radiates from, and surrounds the comet’s nucleus. However, this study shows that these emissions are aurora instead: they are driven not by photons, but by electrons in the solar wind that have been accelerated in the comet’s nearby environment.
“The glow surrounding 67P/C-G is one of a kind. By digging into data from numerous instruments on Rosetta and linking them together, we’ve discovered that this glow is auroral in nature: it’s caused by a mix of processes, some seen at Jupiter’s moons Ganymede and Europa and others at Earth and Mars,” explains the lead author of the study, Professor Marina Galand from Imperial College London, UK. According to Galand, these processes define how the envelope of gas around 67P/C-G becomes excited – that is, made to glow – and how the particles causing this excitation are given a boost of energy and sped up.
Explaining the process, scientists say that electrons streaming out in the solar wind, that is the stream of charged particles flowing out from the Sun, interact with the gas in the comet’s coma, breaking apart water and other molecules. The resulting atoms give off a distinctive far-ultraviolet light. Invisible to the naked eye, far-ultraviolet has the shortest wavelengths of radiation in the ultraviolet spectrum.
“The electrons interact with molecules in the coma to produce the auroral glow. The process by which the electrons are accelerated is similar to some of the processes that drive auroras at Earth and Mars, despite 67P/C-G lacking an intrinsic magnetic field. In fact, the magnetic environments of moons, planets, and comets are all very different, so it’s exciting and intriguing that we see auroras at all of them,” emphasizes Galand.
Exploring the emission at 67P/C-G will enable scientists to assess how the particles in the solar wind change over time, something that is crucial for understanding space weather throughout the solar system. According to scientists, by providing better information on how the Sun’s radiation affects the space environment they must travel through, such information could ultimately help protect satellites and spacecraft, as well as astronauts traveling to the Moon and Mars.
The team used a physics-based model to integrate measurements made by various instruments aboard Rosetta. Data from NASA instruments and Southwest Research Institute-led instruments (the Alice far-ultraviolet or FUV spectrograph and the Ion and Electron Sensor or IES) helped in detecting these novel phenomena at the comet. Additional instruments contributing to this research were Rosetta’s Langmuir Probe (LAP), the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), the Microwave Instrument for the Rosetta Orbiter (MIRO), and the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS).
“By doing this, we didn’t have to rely upon just a single dataset from one instrument. Instead, we could draw together a large, multi-instrument dataset to get a better picture of what was going on. This enabled us to unambiguously identify how 67P/C-G’s ultraviolet atomic emissions form, and to reveal their auroral nature,” concludes Galand.