Variety of fracture networks on comet 67P/C-G. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Extreme thermal stresses experienced by a comet as it orbits around the Sun could explain the extensive fracturing thought to drive its long-term surface erosion, say Rosetta scientists analysing high-resolution images of Comet 67P/Churyumov-Gerasimenko’s surface.
M. Ramy El-Maarry and his team identified 3 distinct settings in which the fractures occur: networks of long narrow fractures, fractures on cliffs and fractured boulders. In addition, several unique features were identified: the parallel fractures running across Hathor’s 900 m-high cliffs, an isolated 500m-long crevice in Anuket region of the comet’s neck, and a 200 m-long complex crack system in Aker on the large lobe.
The most prevalent setting appears to be networks of narrow fractures that extend for a few metres to 250 m in length, typically on relatively flat surfaces. Interestingly, in some locations, the fractures appear to cross cut each other in polygonal patterns at angles of 90° – on Earth and Mars this is often an indicator of ice that has contracted below the surface.
Fractures found on cliffs on comet 67P/C-G. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Another family of cross-cutting fractures is observed on cliff faces, eg Seth region on the comet’s large lobe. Fractured cliff faces were also observed at Abydos, the final landing site of Philae on the small lobe. The fact that the fractures cut across each other in different directions suggests that the stress direction changes over time. The fracturing points to an erosional sequence leading to the boulders’ eventual fragmentation.
Scientists think that the majority of these fracture patterns are most likely linked to the thermal history of the comet and result from stresses that stretch the comet’s surface apart. On Earth and Mars at least, these ‘tensile’ fractures can develop through several common processes: loss of volatile materials, thermal contraction or contraction and expansion cycles, and tectonic processes.
Fractured boulders on comet 67P/C-G. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Besides throwing off volatile materials as they near the Sun, comets are known to undergo high fluctuations in surface and subsurface temperature on daily and seasonal timeframes. This continuous thermal ‘shock’ leads to weakening or “fatigue” of the surface both on the short term due to daily heating cycles, and on the long term as the seasons change along the comet’s 6.5 year orbit around the Sun.
“But the presence of fractures in different settings, in addition to the isolated fractures in Anuket and Aker, suggests that other mechanisms may also be at work,” comments Ramy. eg mechanical forces related to the comet’s rotation or orbit around the Sun are responsible for the crack in Anuket, while the fractured cliffs of Hathor could have resulted from the comet’s formation, perhaps when 2 smaller cometisimals collided.”
A 200-m complex fracture system in the Aker region, on the comet large lobe. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Regardless of the origin of individual fracture systems, it is clear that fracturing plays an important role in the evolution of the comet’s surface.
Fractures observed in cliffs, with debris observed at their bases, imply that this phenomenon represents the first stage in the overall ‘mass-wasting’ of the comet: the cliff top is weakened and a landslide-type event ensues.
http://phys.org/news/2015-08-comet-fractures-surface-evolution.htmljCp
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