  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 H2O 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 H2O are variable.
During this apparition, comet Holmes lost about 3% of its mass, corresponding
to a 'dirty ice' layer of 20m.
