In a study published in the journal Nature Astronomy, researchers make the first definitive link between an outburst of dust and gas from the nucleus of 67P/Churyumov-Gerasimenko and the collapse of a prominent cliff, which also exposed the comet’s pristine, icy interior.
Left: images of the 230-foot (70 m) long, 3.3-foot (1 m) wide fracture at the top of the 440-foot (134 m) high Aswan cliff in the Seth region of 67P/Churyumov-Gerasimenko (marked with arrow). Center: a broad plume of dust is imaged by Rosetta’s navigation camera on July 10, 2015, which can be traced back to an area on the comet that encompasses the Seth region; the Aswan cliff is included within the marked rectangle. Right: two example images taken after the cliff collapse, showing the exposed material in the cliff face (top) and the new outline of the cliff top (bottom). The same boulder is circled in all images to guide the eye when viewing the scene from different orientations. Image credit: ESA / Rosetta / NavCam / MPS / MPS for OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / CC BY-SA IGO 3.0.
Short-lived outbursts occur commonly on comets with different frequencies and scales. Despite multiple observations suggesting various triggering processes, the driving mechanism of such outbursts is still poorly understood.
ESA’s Rosetta mission observed several outbursts from its target comet, which were attributed to dust generated by the crumbling of materials from collapsing cliffs.
Amongst the many features observed by the orbiter on the nucleus of the comet, one peculiar fracture, 230 feet (70 m) long and 3.3 feet (1 m) wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan, in the Seth region of the comet, on its large lobe.
Over the course of the following year as 67P/Churyumov-Gerasimenko drew ever closer to the Sun along its orbit, the rate at which its buried ices turned to vapor and dragged dust out into space increased along the way.
Sporadic and brief, high-speed releases of dust and gas punctuated this background activity with outbursts.
One such outburst was captured by Rosetta’s navigation camera on 10 July 2015, which could be traced back to a portion of the comet’s surface that encompassed the Seth region.
The next time the Aswan cliff was observed, five days later, a bright and sharp edge was spotted where the previously identified fracture had been, along with many new foot-sized boulders at the foot of the 440-foot (134 m) high cliff.
“The last time we saw the fracture intact was on 4 July, and in the absence of any other outburst events recorded in the following ten-day period, this is the most compelling evidence that we have that the observed outburst was directly linked to the collapse of the cliff,” said study lead author Dr. Maurizio Pajola, from the University of Padova in Italy and NASA’s Ames Research Center.
The event also provided a unique opportunity to study how the pristine water-ice otherwise buried tens of feet inside the comet evolved as the exposed material turned to vapor over the following months.
Indeed, after the event, the exposed cliff face was calculated to be at least six times brighter than the overall average surface of the comet nucleus.
By December 26, 2015, the brightness had faded by half, suggesting much of the water-ice had already vaporized by that time. And by August 6, 2016, most of the new cliff face had faded back to the average, with only one large, brighter block remaining.
In addition, Dr. Pajola and co-authors had a clear ‘before and after’ look at how the crumbling material settled at the foot of the cliff.
By counting the number of new boulders seen after its collapse, the researchers estimated that 99% of the fallen debris was distributed at the bottom of the cliff, while 1% was lost to space. This corresponds to around 10,000 tons of removed cliff material, with at least 100 tons that did not make it to the ground, consistent with estimates made for the volume of dust in the observed plume.
Furthermore, the size range of the new debris, between 10 and 33 feet (3-10 m), is consistent with the distributions observed at the foot of several other cliffs identified on the comet.
“We see a similar trend at the foot of other cliffs that we have not been so fortunate to have before and after images, so this is an important validation of cliff collapse as a producer of these debris fields,” Dr. Pajola said.
“Rosetta’s images already suggested that cliff collapses are important in shaping cometary surfaces, but this particular event has provided the missing ‘before-after’ link between such a collapse, the debris seen at the foot of the cliff, and the associated dust plume, supporting a general mechanism where comet outbursts can indeed be generated by collapsing material,” said Rosetta project scientist Dr. Matt Taylor, who was not involved in the study.
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M. Pajola et al. 2017. The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse. Nature Astronomy 1, article number: 0092; doi: 10.1038/s41550-017-0092
This article is based on text provided by the European Space Agency.
