As Comet 67P/Churyumov–Gerasimenko sneaks closer to the sun, the Rosetta orbiter is capturing dramatic outbursts from the ever-more active comet. This jet was so powerful, it momentarily out-puffed the solar wind, creating a rarely-observed diamagnetic cavity.
The quiescent comet on August 6, 2014 and the far more active comet year later on August 6, 2015. Image credit: ESA/Rosetta/NavCam
Comet 67P/Churyumov–Gerasimenko will reach perihelion on Thursday, August 13, the closest that it will get to the sun during its 6.5 year orbit. As the comet creeps closer to the sun, it is growing more active. Ice is sublimating directly into gas that explodes in dramatic dust-loaded jets, while chunks of ice ranging from a meter to a whopping 40 meters in diameter are shucking off the comet and falling into space. Fractures are splintering across the surface, and up to 40% of the previously-smooth plains have remoulded since Rosetta arrived in orbit last year.
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The Anuket region on the neck of Comet 67P/Churyumov–Gerasimenko produced a short-lived, powerful jet on July 29, 2015. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
This beautiful jet appeared, spurted gas and jet into space, and vanished again all in under an hour on July 29, 2015. The jet originated from the rugged Anuket region on the comet’s neck. The Rosetta spacecraft was 186 kilometers above the comet’s center of mass at the time of the outburst.
This isn’t the first jet the team has observed from the comet, but it is the brightest. Normally the jets are significantly dimmer than the rest of the comet, requiring an extreme contrast stretch to make them visible in photographs. This time, the jet was brighter than the comet’s nucleus.
Comet 67P/Churyumov–Gerasimenko seen by Rosetta’s OSIRIS camera at 13:06 GMT, 13:24 GMT, and 13:42 GMT on July 29, 2015. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The outburst produced a diamagnetic cavity, a temporary drop in the comet’s magnetic field. The comet is not magnetic, so its magnetic field is entirely the result of the solar wind. With gas escaping from the comet at a velocity of at least 10 m/s, researchers suspect the jet’s outburst was powerful enough to deflect the solar wind. The outburst of gas temporarily shoved the perpetually-smothering solar wind farther from the comet’s nucleus than usual, changing the pressure balance. It was powerful enough to push this cavity all the way out to Rosetta, creating a magnetic field-free region that stretched at least 186 kilometers away from the comet.
This is the first time a diamagnetic cavity has been observed since the Giotto satellite zipped past Comet Halley at 4,000 kilometers distance in 1986. Magnetometer team member Charlotte Götz grins:
“Finding a magnetic field-free region anyway in the Solar System is really hard, but here we’ve had it served to us on a silver platter – this is a really exciting result for us.”
The science team was hoping to find diamagnetic cavities on Comet 67P/Churyumov–Gerasimenko, although smaller than the one observed on Comet Halley. The new observations of this brief pocket in the solar wind will provide important data on comet/solar wind interactions.
The July 29 jet coincided with a temporary drop in the comet’s magnetic field strength, producing a diamagnetic cavity. Image credit: ESA/Rosetta/RPC/IGEP/IC
In the aftermath of the jet, other instruments on the Rosetta spacecraft picked up changes in the structure and composition of the gas surrounding the comet. Principle investigator for the spacecraft’s pressure sensor ROSINA, Kathrin Altwegg, explained:
“This first ‘quick look’ at our measurements after the outburst is fascinating. We also see hints of heavy organic material after the outburst that might be related to the ejected dust. But while it is tempting to think that we are detecting material that may have been freed from beneath the comet’s surface, it is too early to say for certain that this is the case.”
The gas envelope surrounding the comet, or its coma, had twice the carbon dioxide (CO2), four times the methane (CH4), and seven times the hydrogen sulphide (H2S) after the outburst compared to two days earlier. The nasty-smelling combination has draped the comet in the stench of rotting eggs and farts, offering faint mercies that the highly-anthropomorphized Rosetta spacecraft isn’t actually alive and possessing a keen sense of smell to go with the mass spectrometer. Of all the gases monitored by the instrument, only the water (H2O) content stayed roughly constant.
Changes in the gas content of Comet 67P/Churyumov–Gerasimenko’s coma compared to two days before the outburst. Image credit: ESA/Rosetta/ROSINA/UBern/ BIRA/LATMOS/LMM/IRAP/MPS/SwRI/TUB/UMich
The dust also stepped up its game, increasing by a factor of ten after the outburst. The dust counter typically picked up 1 to 3 hits per day in early July, which increased to 30 hits per day 14 hours after the outburst, and briefly peaking at 70 hits within a 4-hour window the following day. The GAIDA dust counter’s principle investigator Alessandra Rotundi points out it wasn’t just the sheer amount of dust that increased, but also its velocity:
“It was not only the abundance of the particles, but also their speeds measured by GIADA that told us something ‘different’ was happening: the average particle speed increased from 8 m/s to about 20 m/s, with peaks at 30 m/s – it was quite a dust party!”
Comet 67P/Churyumov–Gerasimenko will reach perihelion, its closest approach to the sun, on Thursday August 13, 2015. Image credit: ESA
The comet is most active at perihelion because sunlight is flooding into areas that have been shadowed for years, suddenly bumping surface temperatures. The comet’s activity is expected to lag, peaking in the weeks following perihelion on Thursday. While in this highly active phase, the Rosetta spacecraft is pulling up to 300 kilometers away from the surface to hopefully avoid the worst of the shedding boulders, jets, and any other unpredictable activity.
Top image: An outburst from Comet 67P/Churyumov–Gerasimenko on July 29, 2015 seen from 186 kilometers altitude above the comet’s center of mass. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/Mika McKinnon