Astronomers Spot Comet Chury’s Ultraviolet Aurora
By: Robert Lea
Tags: Space, Science, Astronomy, The Universe, Comet

Astronomers Spot Comet Chury’s Ultraviolet Aurora

The first identification of an ultraviolet aurora around the comet Chury is further evidence that nearly half a decade after the end of the Rosetta mission, the ESA’s historic project is still delivering breakthrough science.

Earth’s aurora provides a stunning visual light show called the Northern Lights which has fascinated observers for centuries. But astronomers have discovered that other bodies such as other planets and moons have auroras too. Now, researchers at the University of Bern have for the first time discovered an ultraviolet wavelength aurora around the comet 67P/Churyumov-Gerasimenko — or Chury for short.

Comet 67P/Churyumov-Gerasimenko or Chury for short. ( © ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0)

The effect — discovered by the team from their analysis of data from the European Space Agency’s (ESA) Rosetta mission — is caused when charged particles from the Sun, carried in what is known as the solar wind, strike Chury’s coma — gas in situ around the comet. The phenomenon is similar in ways to that which causes the Northern Lights above Earth but strikingly different in others.

“Rosetta is the first mission to observe an ultraviolet aurora at a comet,” says Matt Taylor, ESA project scientist. “Auroras are inherently exciting — and this excitement is even greater when we see one somewhere new, or with new characteristics.”

Marina Galand of Imperial College London, lead author of the study, says: “The resulting glow is one of a kind. It’s caused by a mix of processes, some seen at Jupiter’s moons Ganymede and Europa and others at Earth and Mars.”

The study comes almost exactly four years after the ending of the Rosetta mission in 2016 and is published in the journal Nature Astronomy.

Chury’s Ultraviolet Aurora Identified for the First Time

The finding represents the first time that an ultraviolet aurora has been correctly identified around a comet, and it wouldn’t have been possible without the data form the ESA’s Rosetta mission. In fact, the UV glow around Chury — a Jupiter-family comet that was originally located in the Kuiper belt — had been spotted before but had been wrongly attributed as photons from the Sun, much like the ‘airglow’ of the Earth — a faint emission of light by a planetary atmosphere.

This image shows the key stages of the mechanism by which this aurora is produced: as electrons stream out into space from the Sun and approach the comet, they are accelerated and break down molecules in the comet’s environment. Some of the atoms of hydrogen and oxygen are produced in an excited state and de-excite by producing ultraviolet emissions, the observed aurora. The auroral nature of the emissions has been revealed from the analysis of observations from a set of in situ and remote-sensing instruments aboard Rosetta (RPC, ROSINA, VIRTIS, MIRO and Alice). © ESA ( ESA/ATG medialab)

Thus the key question for the Bern team, was, could they prove that the ultraviolet glow was actually caused by solar wind electrons accelerated towards the comet and striking the gas in the coma and not ‘airglow’ — or ‘nightglow’ as it is often called?

Martin Rubin from the University of Bern Physics Institute and co-author of the paper, explains: “Since this process is a very high energy one, the resulting glow is also highly energized and therefore in the ultraviolet range, which is invisible to the human eye.”

Galand continues, empathising the importance of Rosetta’s data: “By analyzing the Rosetta data though, it was revealed that solar wind electrons are the reason for the glow and not in fact photons, as previously assumed.”

Animation of ultraviolet aurora being produced at comet Chury. ©ESA (spacecraft: ESA/ATG medialab)

One interesting difference between the glow observed around Chury and Earth’s aurora is that the former doesn’t require a magnetic field around the comet. Therefore, this data could give astronomers and astrophysicists a new way of studying the Sun’s solar winds.

“The observation of cometary aurora phenomena definitely has an aesthetic value,” says Rubin. “Beyond that, the UV observations from Earth could one day also provide information about the solar wind at these comets — even without a space probe like Rosetta being on site.”

The analysis conducted by the team required data from various with the Rosetta Orbiter Spectrometer for Ion and Neutral Gas Analysis (ROSINA) mass spectrometer, in particular, providing vital data on the Chury’s composition and the density of the coma surrounding the comet.

The team combined this with data from other instruments on Rosetta such as the ALICE UV spectrograph, Rosetta Plasma Consortium (RPC) Ion and Electron Spectrometer (IES) and the Langmuir Probe (LAP), the Microwave Instrument for the Rosetta Orbiter (MIRO) and the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS).

The Rosetta Mission: Still Delivering the Goods

The controlled impact of the Rosetta lander on comet 67P/Churyumov-Gerasimenko — Chury — that occurred on 30th September 2016 brought an end to the ESA’s historic mission. It followed two years of remote observations on the target comet, leaving the craft to rest in the comet’s Ma’at region.

After entering into an orbit around Chury on 6th August 2014 — following a ten-year journey that saw it also collect data on the asteroids 867 Steins (in 2008) and 21 Lutetia (in 2010) — it launched the Philea lander. The arrival of the lander at Chury’s surface in November that year marked the first ‘soft landing’ of a human-made spacecraft on a comet and the first scientific activities conducted on such a body.

Artist’s im­pres­sion of the Roset­ta or­biter de­ploy­ing the Phi­lae lan­der to comet 67P/Churyu­mov–Gerasi­menko (not to scale) ( © ESA–C. Carreau/ATG medialab.)

Early in the Rosetta mission, data from the craft had allowed astrochemists to detect the presence of important chemical elements, necessary for the early development of life such as the amino acid glycine — a key component of DNA and cell membranes. The finding added credence to the theory that some of the ingredients for life arrived at the surface of Earth by comet collision.

And Rosetta has continued to make history.

In February this year, a study based on a synthesis of Rosetta data showed that during the two years it observed Chury, the comet’s nucleus changed colour, becoming progressively less red. Researchers found that Rossetta saw this shift as Chury approached the Sun and that it was reversed — with the comet once again reddening — as it moved away from our star.

Comet, comet, comet Chameleon: Colour changing comet. (ESA)

Such observations simply wouldn’t have been possible without Rosetta and the ESA’s long term in situ dedication to tracking a comet as it orbits the Sun. No ‘snapshot’ of Chury could have delivered either of the findings described here and so-many more not mentioned. Further to that; this new research shows that secrets still lurk in the data Rosetta collected and that the mission is still capable of delivering surprises. It’s just up to researchers to tease it out.

As Kathrin Altwegg, head of ROSINA, the mass spectrometer at the University of Bern, points out: “The analysis was complicated and required data from various instruments.