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As Rosetta has moved in on Comet 67P/C-G, engineers have focused in on its most interesting surface features, such as the nucleus’s neck as well as a collection of very large boulders on a relatively smooth area on the nucleus’s larger lobe. The biggest boulder, seen as the middlemost rock in the photo above, they have named Cheops. It is estimated to be 150 feet across with a height of about 80 feet.
It should be emphasized that calling these features boulders might actually be premature at this time.
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.
Cheops simply might not be a distinct boulder at all, but instead a protrusion of harder material that was left behind when the softer stuff around it got blown away during the comet’s many close encounters with the Sun. For example, the close up of the neck region also showed a smooth area with what looked like boulders, but the link notes that
It will be interesting to get a closer view of these features, to see how they fit into the overall picture of the evolution of the comet. As the comprehensive survey of 67P/C-G continues, efforts will also be made to learn more about the origin of the boulders and, of course, their composition. Are they relics from the comet’s interior, exposed by an erosive process that has removed material from around them; are they products of erosion from nearby cliffs; or were they exhumed by jet activity?
I had one interesting impression at my first glance at the overhead close-up of Cheops. To me, it seemed to be sitting very lightly on the surface.