Rosetta's Comet Offered Brief and Powerful Summer Fireworks

Compilation of the brightest outbursts seen at Comet 67P/Churyumov–Gerasimenko by Rosetta’s OSIRIS narrow-angle camera and Navigation Camera between July and September 2015. Credit: OSIRIS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; NavCam: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0. Download the high resolution image.

Brief but powerful outbursts seen from Comet 67P/Churyumov-Gerasimenko during its most active period last year have been traced back to their origins on the surface.

In the three months centred around the comet's closest approach to the Sun, on 13 August 2015, Rosetta's cameras captured 34 outbursts.

These violent events were over and above regular jets and flows of material seen streaming from the comet's nucleus. The latter switch on and off with clockwork repeatability from one comet rotation to the next, synchronised with the rise and fall of the Sun's illumination.

By contrast, outbursts are much brighter than the usual jets - sudden, brief, high-speed releases of dust. They are typically seen only in a single image, indicating that they have a lifetime shorter than interval between images - typically 5-30 minutes.

A typical outburst is thought to release 60-260 tonnes of material in those few minutes.

On average, the outbursts around the closest approach to the Sun occurred once every 30 hours - about 2.4 comet rotations. Based on the appearance of the dust flow, they can be divided into three categories.

One type is associated with a long, narrow jet extending far from the nucleus, while the second involves a broad, wide base that expands more laterally. The third category is a complex hybrid of the other two.

"As any given outburst is short-lived and only captured in one image, we can't tell whether it was imaged shortly after the outburst started, or later in the process," notes Jean-Baptiste Vincent, lead author of the paper published today in Monthly Notices of the Astronomical Society .

"As a result, we can't tell if these three types of plume 'shapes' correspond to different mechanisms, or just different stages of a single process.

"But if just one process is involved, then the logical evolutionary sequence is that an initially long narrow jet with dust is ejected at high speed, most likely from a confined space.

"Then, as the local surface around the exit point is modified, a larger fraction of fresh material is exposed, broadening the plume 'base'.

"Finally, when the source region has been altered so much as not to be able to support the narrow jet anymore, only a broad plume survives."

The other key question is how these outbursts are triggered.

The team found that just over half of the events occurred in regions corresponding to early morning, as the Sun began warming up the surface after many hours in darkness.

The rapid change in local temperature is thought to trigger thermal stresses in the surface that might lead to a sudden fracturing and exposure of volatile material. This material rapidly heats up and vaporises explosively.

Image above: Locations of the summer outbursts observed by Rosetta's OSIRIS (blue dots) and NavCam (red dots) between July and September 2015, superimposed on a regional map of Comet 67P/Churyumov-Gerasimenko. Insets highlight the topography and terrain for a selection of outbursts (note that some locations can be seen in multiple insets from different angles).

Many outbursts occur at distinct boundaries, for example along terraced landforms between Anuket and Sobek (top centre); in pits and niches in Maftet and Wosret (top right); and along the boundary between Anhur and Aker (bottom right and left). Anhur in particular is weakly consolidated and shows a variety of niches, alcoves, boulders and debris indicating this region is susceptible to mass-wasting and cliff collapse, leading to exposure of volatile-rich materials that trigger outbursts.

Clockwise, the images were taken by the OSIRIS narrow-angle camera on 1 May 2016 from a distance of 18 km; 27 January 2016 from 76 km; 2 May 2016 from 18 km; and 23 January 2016 from 76 km, respectively.

- Download the high resolution image.

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