Why did Comet 17P/Holmes burst out? Nucleus splitting or delayed sublimation?

Based on millimeter-wavelength continuum observations we suggest that the recent spectacle of comet 17P/Holmes can be explained by a thick, air-tight dust cover and the effects of H(2)O sublimation, which started when the comet arrived at the heliocentric distance <= 2.5 AU. The porous structure inside the nucleus provided enough surface for additional sublimation, which eventually led to the break up of the dust cover and to the observed outburst. The magnitude of the particle burst can be explained by the energy provided by insolation, stored in the dust cover and the nucleus within the months before the outburst: the subliming surface within the nucleus is more than one order of magnitude larger than the geometric surface of the nucleus - possibly an indication of the latter's porous structure. Another surprise is that the abundance ratios of several molecular species with respect to H(2)O are variable. During this apparition, comet Holmes lost about 3% of its mass, corresponding to a dirty ice layer of 20 m.