Of the three boulders, the largest (labelled with a ‘3’) has a diameter of approximately 30 metres. In an image taken on 16 September 2014 (below), it stands out as having only a very small contact area with the comet’s surface. It also seems to be perched on the rim of a small depression.

“We had noticed this formation already in earlier images, however, at first the boulders did not seem to differ substantially from others we had seen,” says OSIRIS scientist Sebastien Besse from ESA, who first noticed the formation.

Similar geological formations are also found on Earth. Balancing rocks touch the underlying ground with only a tiny fraction of their surface and often look as if they may tilt or topple over any moment. Some can actually be rocked back and forth and are then referred to as “rocking stones.” Many of these boulders are so-called “erratics” that travelled to their current location within glaciers. In other cases, wind and water eroded softer material in the local bedrock, leaving only the more resistant material behind.

“How this apparent balancing rock on Comet 67P/C-G was formed is not clear at this point,” says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany.

One possibility is that transport processes related to cometary activity played a role, causing such boulders to move from their original site and reach a new location.

Scattered boulders are seen in many places on the comet’s surface, sometimes in otherwise relatively smooth regions. One of the largest – Cheops – measures approximately 45 metres in size and sits in the middle of the smooth part of Imhotep on the underside of the comet’s large lobe. In other regions, it is more common to see rubble piles comprising hundreds of boulders.

“Interpreting images of the comet’s surface can be tricky,” adds Sierks. Depending on the viewing angle, illumination, and spatial resolution, very different and sometimes even misleading impressions are created.

For example, in an image taken on 16 August 2014 from a distance of 105 kilometres, one of the smaller boulders [labelled ‘2’] in the balancing rock formation appears to be protruding like a pillar. However, this impression is not confirmed in an image of the same region taken on 19 September 2014 from the much closer distance of 29 km. Similarly, thanks to the change in viewing geometry between images, the appearance of the large boulder also changes quite dramatically, even between the 16 and 19 September images.

Further imaging of these formations should help provide more insight into their true nature and maybe even their origin.

Unlabelled versions of the images are also provided below (all credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)