[meteorite-list] Rosetta's OSIRIS Discovers Balancing Rock on Comet 67P

[Ron Baalke]

http://www.mps.mpg.de/3990751/Aktuelles_2015_05_18_OSIRIS_entdeckt_Wackelstein

OSIRIS discovers balancing rock on 67P
Max Planck Institute
May 18, 2015

Scientists from Rosetta's OSIRIS team have discovered an extraordinary 
formation on the larger lobe of comet 67P/Churyumov-Gerasimenko in the 
Aker region. From a group of three boulders the largest one with a diameter 
of approximately 30 meters stands out: images obtained on 16 September 
2014 from a distance of 29 kilometers with the help of Rosetta's scientific 
imaging system OSIRIS show it to perch on the rim of a small depression. 
There seems to be only a very small contact area with the nucleus.

Similar geological formations are found also on Earth. So-called balancing 
rocks touch the underground 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." Impressive examples of balancing rocks occur in Australia or 
the southwest of the USA. Often these boulders travelled to their current 
location onboard of glaciers. In other cases, wind and water eroded softer 
material surrounding the rock.

"How the potential balancing rock on the comet 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. It 
is possible that also on 67P transport processes did their part. The comet's 
activity may cause such boulders to move and thus reach a new location.

"We had noticed this formation already in earlier images", says OSIRIS 
scientist Sebastien Besse from ESA, who discovered the possible balancing 
rock. "However, at first the boulders did not seem to differ substantially 
from other we had seen." Scattered boulders can be found in many places 
on the comet's surface. One of the largest ones measures approximately 
45 meters. In reference to the Egyptian pyramids, the scientists dubbed 
it "Cheops". Other regions on 67P resemble a rubble pile and are practically 
covered by boulders. 

"Interpreting images of the comet's surface can be tricky", says 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 (top image) from a larger 
distance of 105 kilometers the second boulder in the balancing rock formation 
appears to be protruding like a pillar. Images of the same region taken 
on 19 September 2014 (bottom image), however, cannot confirm his impression.

The OSIRIS scientists intend to continue to monitor the potential balancing 
rock carefully. New images might give insights into its true nature and 
maybe even its origin.

Rosetta is an ESA mission with contributions from its member states and 
NASA. Rosetta's Philae lander is provided by a consortium led by DLR, 
MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous 
with a comet, escort it as it orbits the Sun, and deploy a lander to its 
surface.

The scientific imaging system OSIRIS was built by a consortium led by 
the Max Planck Institute for Solar System Research (Germany) in collaboration 
with CISAS, University of Padova (Italy), the Laboratoire d'Astrophysique 
de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC 
(Spain), the Scientific Support Office of the European Space Agency (The 
Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), 
the Universidad Politéchnica de Madrid (Spain), the Department of Physics 
and Astronomy of Uppsala University (Sweden), and the Institute of Computer 
and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially 
supported by the national funding agencies of Germany (DLR), France (CNES), 
Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.