During the higher temperatures and energy of perihelion, the expansion, contraction and transferred forces along the surface of the nucleus cause what can only be described as "duck quakes". Smaller rocks fall into the many deep cracks, while large monoliths stay stuck on the surface. On relatively (gravitationally) horizontal areas, the medium term effect is for all smaller and medium size boulders to be consumed into the surface as rubble, and for larger boulders (eg. Cheops, Tekhenu) to remain stranded on an otherwise pebbly looking "plain" (eg, Hapi, Imhotep, Anuket).
The "sieve" effect also explains larger, more long term cracks such as those on Anuket and Hapi. Mid sized, "wedge shaped" boulders can get stuck in a crack and manage to hold it open, and stopping it from the dynamic process that tends to fill them in with smaller rubble. This appears to be the case in the longer term cracks in both Anuket and Hapi - see the OSIRIS images below. Cracks are punctuated by boulders that seem to be wedged stuck in the cracks.
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