Moving a Boulder on a Comet

Take a look at these images of a boulder that moved on Comet 67P/Churyumov-Gerasimenko as seen by the Rosetta spacecraft.  Fascinating stuff.  The boulder clearly moved.  But how?

Here’s ESA’s caption (via NASA):
A 100 foot-wide (30 meter), 28-million-pound (12.8-million-kilogram) boulder, was found to have moved 460 feet (140 meters) on comet 67P/Churyumov-Gerasimenko in the lead up to perihelion in August 2015, when the comet’s activity was at its highest. In both images, an arrow points to the boulder; in the right-hand image, the dotted circle outlines the original location of the boulder for reference.

The movement could have been triggered in one of two ways: either the material on which it was sitting eroded away, allowing it to roll downslope, or a sufficiently forceful outburst could have directly lifted it to the new location. Indeed, several outburst events were detected close to the original position of the boulder during perihelion.

The images were taken by Rosetta’s OSIRIS camera on May 2, 2015 (left) and Feb. 7, 2016 (right), with resolutions of 7.5 feet (2.3 meters) per pixel and 2.6 feet (0.8 meters) per pixel, respectively.

Rosetta is a European Space Agency mission with contributions from its member states and NASA. Rosetta’s Philae lander is provided by a consortium led by the German Aerospace Center, Cologne; Max Planck Institute for Solar System Research, Gottingen; French National Space Agency, Paris; and the Italian Space Agency, Rome. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the U.S. participation in the Rosetta mission for NASA’s Science Mission Directorate in Washington.

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So first I’m siding with the erosion idea by way of off gassing or jets. It seems that a jet strong enough to directly lift a “12.8 million-kg” boulder would lift more material than just the boulder and if it did the boulder would likely show more of an impact mark as the soil looks sand-like. True the same forces could erase those marks but you’d think there would be some physical sign of such a powerful event. Still there are no “boulder-tracks” and again off-gassing might erase the tracks.

The other thing that isn’t explained is this: Is the 12.8 million-kg / 28 million-lb boulder a true 67P weight or is that what it would weigh here on Earth?  OR is this a measure of mass but not stated as such? This is a case where knowing the mass would be helpful. Like I said – fascinating stuff!

Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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