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A rare chance to scrutinize a comet’s jet

On July 3, 2016 – as Comet 67P erupted with a plume of dust – the orbiting Rosetta spacecraft happened to pass right through the dust cloud.

On July 3, 2016, when Comet 67P sent a jet of dust into space, all 5 instruments aboard the orbiting Rosetta spacecraft were able to record the event. This image shows the dust plume, which originated from the Imhotep region on the comet. Image via ESA/ Rosetta/ UPD/ LAM/ IAA/ SSO/ INTA/ UPM/ DASP/ IDA/ MPS.

The Max Planck Institute for Solar System Research (MPS) in Germany reported on October 26, 2017 on scientists’ analysis of a very conveniently placed jet of dust that had erupted from Comet 67P/Chruyumov-Gerasimenko a year earlier. ESA’s Rosetta spacecraft, which was orbiting the comet at that time, passed serendipitously right through the jet and was able to use all five of its instruments to record it. Subsequent analysis of this goldmine of data from Rosetta is now complete. The scientists said it revealed a more intricate process driving the jets of comets than had been previously supposed.

It was known that the jets of comets are driven by the sublimation of frozen water, the process by which a solid turns into a gas without going through a liquid stage. But, in addition, these scientists said:

… further processes augment the outbreaks. Possible scenarios include the release of pressurized gas stored below the surface or the conversion of one kind of frozen water into an energetically more favorable one.

The analysis of the July 3, 2016 jet from 67P has now been published in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

Before the Rosetta spacecraft, who knew comets could look like this? This is Comet 67P/Churyumov-Gerasimenko – aka Chury – via Rosetta.

Thanks to Rosetta, researchers had previously discovered a day-night cycle of activity on Comet 67P. The comet’s “day,” that is, its day-night cycle (a single rotation on its axis) takes about 12.4 hours. Data from Rosetta had shown that, as the comet spins, and as the sun rises and shines on each new part of the comet, that area becomes most likely to produce jets. A statement from MPS explained:

When the sun rose over the Imhotep region of Rosetta’s comet on July 3, 2016, everything was just right: As the surface warmed and began to emit dust into space, Rosetta’s trajectory led the probe right through the cloud. At the same time, the view of the scientific camera system OSIRIS coincidentally focused precisely on the surface region of the comet from which the fountain originated. A total of five instruments on board the probe were able to document the outburst in the following hours.

Jessica Agarwal of MPS led the study. She said:

This was an amazing stroke of luck. It is impossible to plan something like this.

Prior to this event, the spacecraft had been able to point perhaps one of its instruments – from afar – toward an erupting jet on 67P. Agarwal said:

From the extensive measurement data of July 3, 2016, we were able to reconstruct the progress and the characteristics of the outburst as detailed as never before.

The July 3, 2016 dust plume was spotted within the ice-filled depression close to the large boulder near the bottom of this image. The image is a false-color composite, where the pale blue patches highlight the presence and location of water-ice. Image via ESA/ Rosetta/ UPD/ LAM/ IAA/ SSO/ INTA/ UPM/ DASP/ MPS.

The researchers were able to see the starting point of the jet as a circular area on the comet, about 30 feet (10 meters) in diameter, and located within a depression on the comet’s surface. As the data show, this area contains frozen water at the surface. In general, scientists assume that frozen gases on a comet’s surface, such as water, are responsible for dust production.

The new study shows, though, that sublimation of water ice by itself can’t explain the event of July 3, 2016. The dust production from this region was measured at approximately 40 pounds per second (18 kilograms per second), and hence the jet is much dustier than conventional models had predicted. Agarwal explained:

An additional energetic process must be at play – energy must have been released from beneath the surface to support the plume.

The scientists statement elaborated further:

It is conceivable, for example, that under the surface of the comet there are cavities filled with compressed gas. Upon sunrise, the radiation begins to warm the overlying surface, cracks develop and the gas escapes. According to another theory, deposits of amorphous ice beneath the surface play a decisive role. In this type of frozen water, the individual molecules are not aligned in a lattice-like structure, as is customary in the case of crystalline ice, but arranged in a far more disorderly fashion. Since the crystalline state is energetically more favorable, energy is released during the transition from amorphous to crystalline ice. Energy input through sunlight can start this transformation.

[However],exactly which process took place on July 3, 2016 is still unclear.

Matt Taylor, Rosetta Project Scientist at ESA, said:

There’s a particular focus now within the Rosetta science community on looking to combine data from 67P with modelling, simulations, and laboratory work here on Earth, to address the question of what drives such activity on comets.

Here’s a Rosetta spacecraft selfie with Comet 67P/Churyumov–Gerasimenko in background taken by the CIVA camera onboard the Philae Lander on September 7, 2014. The spacecraft and comet were separated by about 31 miles (50 km) at the time. Two frames were taken and merged due to the high contrast. Image via ESA/ Rosetta/ Philae/ CIVA. Read more about this image.

Bottom line: Outbursts of dust from comets appear without warning. But on July 3, 2016 – as Comet 67P erupted with a plume of dust – the orbiting Rosetta spacecraft happened to pass right through the dust cloud.

Via Max Planck Institute for Solar System Studies

Deborah Byrd

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