Crystallization of Gas-Laden Amorphous Water Ice, Activated by Heat Transport to its Subsurface Reservoirs, as Trigger of Huge Explosions of Comet 17P/Holmes

摘要

Thick terrain layers, of the type recognized on the Deep Impact mission's close-up images of the nucleusof comet 9P/Tempel, and each 10~(13)to10~(14) grams in mass, are suggested to be attractive candidate carriers of solidmaterial 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 mechanismbeing a transformation of gas-laden water ice from low-density amorphous phase to cubic phase (crystallization) in areservoir 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, themass 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~100 °K at aphelion and nearly 120 °K (with~10percentof the ice in cubic phase) some 10 days before the megaburst andexplosively afterwards, due to the release of the trapped volatiles and completion of the phase transition. The proposedmodel is in agreement with a wide range of relevant observations of the 2007 megaburst of comet 17P, including theevent's post-perihelion timing, the water production rate, the CO-to-H_2Oproductionrateratio, the dust halo's expansionrate, and the energy involved. The observed recurrence rate of super-massive explosions of comet 17P is explained byheat transport through the terrain layers whose effective thermal conductivity is about 0.2 W m~(-1)°K-~(1).