Boulder close-up

The scientific imaging system OSIRIS on board ESA’s spacecraft Rosetta has caught a spectacular glimpse of one of the many boulders that cover the surface of comet 67P/Churyumov-Gerasimenko.

Boulder Cheops, taken by Rosetta’s OSIRIS narrow-angle camera on 19 September 2014, from a distance of 28.5 km. The boulder has a maximum dimension of about 45 m. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

With a maximum dimension of approximately 45 metres, it is one of the larger boulders seen on the comet. It stands out among a group of boulders in the smooth region located on the lower side of 67P/C-G’s larger lobe.

This cluster of boulders reminded scientists of the famous pyramids at Giza near Cairo in Egypt, and thus it has been named Cheops for the largest of those pyramids, the Great Pyramid, which was built as a tomb for the pharaoh Cheops (also known as Kheops or Khufu) around 2550 BC.

This choice also introduces a more general Egyptian naming scheme agreed by Rosetta scientists that will be used for many of the features on 67P/C-G, in keeping with the spirit of the overall mission and spacecraft names.

Cheops was seen for the first time in images obtained in early August upon Rosetta’s arrival at the comet (see below – can you spot it?). In the past few weeks, as Rosetta has navigated closer and closer to the comet, OSIRIS imaged the unique structure again – but this time with a much higher resolution of 50 centimetres per pixel.

The lower side of 67P/C-G’s larger lobe. The image was presented on the occasion of arrival on 6 Aug; it was taken from a distance of 130 km and the image resolution is 2.4 metres per pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The boulder-like structures that Rosetta has revealed in many places on the surface of 67P/C-G are one of the comet’s most striking and mysterious features (see yesterday’s CometWatch for a focus on the boulders on the comet’s ‘neck’ region).

Just like the many other boulders seen by both OSIRIS and the spacecraft’s NAVCAM, Cheops stands out not just physically, but also as a slightly brighter feature compared to the darker surface around it.

OSIRIS Principal Investigator Holger Sierks, from the Max Planck Institute for Solar System Research (MPS) in Germany, describes the surface of Cheops as “very craggy and irregular.”

Interspersed between the lighter lumps on the boulder’s surface are intriguing small patches of darker material, similar in brightness and texture to the ground upon which the boulder lies.

“It almost looks as if loose dust covering the surface of the comet has settled in the boulder’s cracks. But, of course, it is much too early to be sure,” says Sierks.

Apart from their size distribution, which is being measured through careful analysis of the images, almost all other properties of 67P/C-G’s boulders are still a mystery to researchers. What are they made of? What are their physical properties, including density and stability? How were they created? As Rosetta continues to survey and monitor the comet’s surface in the next months, the scientists will be looking for clues.

“For example, if the boulders are exposed by cometary activity or are displaced following the comet’s gravity field, we should be able to track this down in our images,” adds Sierks.

Tomorrow, Rosetta will begin its Close Observation Phase reaching a distance of only 10 kilometres from the comet’s surface, giving the cameras an even closer view of these features.