23 Jan 2015

Rosetta Mission: new results reveal comet 67P is an ‘icy dirtball’ covered in ‘goose bumps’

Pit on Comet 67P

Pit on Comet 67P

Scientists at The Open University (OU) have played an important role in the first results from the historic European Space Agency mission, Rosetta, to study Comet 67P/Churyumov-Gerasimenko. New images from the Rosetta orbiter’s camera OSIRIS and analysis from the craft’s GIADA instrument have given new insights into what comets are made of and how planets may have been formed.

Spectacular new detailed images from the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) instrument – the eyes of the Rosetta orbiter, have revealed ‘goose bump-like’ formations, a few metres in size, on the walls of a pit on the comets surface. This hole into the interior of the comet appears to show that the comet is built up of small spheres, which are revealed as lumps in the pit sides. Scientists believe that these lumps are possibly the metre-scale boulders that formed first in the solar system, and from which all comets (and planets) were eventually made. These images have also revealed that the pit is a source of activity, as small ‘jets’ of dust can be seen silhouetted against the dark interior in over-exposed images. OU research fellow Dr Colin Snodgrass was a member of the OSIRIS team. He said: “The OSIRIS images are much more detailed than any previous views of comets, and give us a completely new window on how our solar system formed.”

While OSIRIS can observe the largest dust particles emitted by the comet, the smaller grains that ultimately form the large-scale head and dust tail of the comet have been directly detected by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment. The slow speed of Rosetta relative to the comet allows the ejection speeds of the particles to be measured for the first time. The first detections of dust particles by GIADA have confirmed that solid material dominates the outflow at large distances from the Sun, with four times more mass of dust than gas being emitted. Rather surprisingly, the scattering of sunlight that allows us to see the dust is dominated by dust grains of around one tenth of a millimetre in diameter. This is about a hundred times larger than the dominant grains emitted when comets are closer to the Sun and much more active.

Comets are the primary source of dust in the solar system - about 50 tonnes of cosmic dust rains down on the Earth every day. GIADA measurements allow, for the first time, the properties of this dust to be measured throughout a comet’s passage of the Sun, providing insight into the how it is lifted from the surface of the comet and distributed throughout the solar system. OU senior lecturer Dr Simon Green is co-investigator on the GIADA instrument and said: “These early results have already shown that a comet may be better described as an ‘icy dirtball’ rather than the previous ‘dirty snowball’. We are looking forward to many exciting new results as we take the first close-up view of a comet becoming active.”

These findings are published in the journal Science.

The OU’s next Open Minds lecture will be discussing the latest results from Rosetta at its February session on 17 February 2015. The talk ‘Landing on a Comet: What we found from the Rosetta mission’ is free to attend and open to all. To find out more please visit:


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