Rosetta mission scientists have discovered an unusual geological formation in the Aker region on the large lobe of 67P/Churyumov-Gerasimenko.
Image of the boulders taken by the Rosetta spacecraft on September 19, 2014 from a distance of about 18 miles (29 km). Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
“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,” said Dr Sebastien Besse from the European Space Agency, who first noticed the three unusual boulders on the comet’s surface.
Of the three boulders, the largest (labeled with a ‘3’) has a diameter of about 100 feet (30 meters). In images from the Rosetta orbiter, it stands out as having only a very small contact area with the surface. It also seems to be perched on the rim of a small depression.
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 traveled 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 67P/Churyumov-Gerasimenko was formed is not clear at this point,” said Dr Holger Sierks of the Max Planck Institute for Solar System Research in Germany.
The three curious boulders are found in the Aker region of 67P/Churyumov-Gerasimenko, on the comet’s large lobe. Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
“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 148 feet (45 meters) 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. Depending on the viewing angle, illumination, and spatial resolution, very different and sometimes even misleading impressions are created,” Dr Sierks said.
Further imaging of these formations should help provide more insight into their true nature and maybe even their origin.
