Computer Science
Scientific paper
Oct 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009icq....31...99s&link_type=abstract
International Comet Quarterly, vol. 31, p. 99-124
Computer Science
Scientific paper
Thick terrain layers, of the type recognized on the Deep Impact mission's close-up images of the nucleus of comet 9P/Tempel, and each 10^(13) to 10^(14) grams in mass, are suggested to be attractive candidate carriers of solid material released into the atmosphere during super-massive explosions (megabursts) and/or major fragmentation events. The properties of the 2007 megaburst of comet 17P/Holmes are shown to be consistent with the triggering mechanism being a transformation of gas-laden water ice from low-density amorphous phase to cubic phase (crystallization) in a reservoir located beneath a layer tens of meters thick. Molecules of highly volatile gases, carbon monoxide in particular, trapped in amorphous water ice and released during the phase transition (at 130 K to 150 K), are superheated, generating -- almost instantly in a runaway process -- a momentum needed to lift off, from the comet's nucleus, the mass of the layer and, after its collapse, to accelerate the pile of mostly microscopic dust debris to subkilometer-per-second velocities. Strongly temperature dependent, the crystallization rate increases progressively between about 100 K at aphelion and nearly 120 K (with about 10 percent of the ice in cubic phase) some 10 days before the megaburst and explosively afterwards, due to the release of the trapped volatiles and completion of the phase transition. The proposed model is in agreement with a wide range of relevant observations of the 2007 megaburst of comet 17P, including the event's post-perihelion timing, the water production rate, the CO-to-H_2O production rate ratio, the dust halo's expansion rate, and the energy involved. The observed recurrence rate of super-massive explosions of comet 17P is explained by heat transport through the terrain layers whose effective thermal conductivity is about 0.2 W m^(-1) K^(-1).
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