I would have to say that the spectacular event we are currently
witnessing in conjunction with P/Holmes is on a scale far beyond what
simply can be explained by any normal outgassing, yet still not so
totally cataclysmic as to destroy the comet. Remember, P/Holmes did
this in 1892 as well and has survived down to today. At the moment,
P/Holmes appears about 1,000,000 times brighter than it did 48 hours
ago, so whatever caused this mega-outburst would have to be on a truly
grand scale.
While there may be other possible explanations, I think one I saw
proposed quite a number of years ago (perhaps by Z.Sekanina?) to
explain cometary mega-outbursts, still seems the most plausible in my
eyes. It is perhaps even more so today since we have, via space probe,
actually seen (and sampled!) some cometary nucleii close up.
The explanation has been bolstered by the fact that we now know
cometary nucleii come in a considerable range of densities, running the
gamut from a fluffy snow and ice matrix, to an accumulation of "dust
bunnies"(!). Toward the less dense end of this broad spectrum, one can
conceive of a low density nucleus that, over time and through
outgassing, develops a large region with a very fragile fairly castle
or highly tenuous honeycomb-like structure.
Cometary nucleii being irregular in shape, rotation or perhaps even the
minute gravity of the nucleus itself, will create increasing shear
forces if the area is remote from the center of rotation. At some point
the highly fragile bonds connecting the honeycomb of material will
reach the failing point and a collapse, or more likely a sudden
crushing/consolidat ing event on a grand scale perhaps covering several
square kilometers wide and deep, will occur. This crushing collapse
would expel a truly gigantic volume of dust in the process. However,
such an event would not necessarily be nearly so catastrophic as to
cause the nucleus to disrupt or fragment. Actual loss of total mass
might be only a few percent.
Such a scenario also fits well with the observed occurence of a
secondary brightening some weeks after the first event. Those who are
familiar with terrestrial structural collapse situations are aware that
the primary event often leaves a large amount of instability in the
material involved. This remaining instability will only remain for a
short time before a further adjustment toward stability occurs,
resulting in a major second collapse, with an outward physical
appearance very similar to that of the initial event.
Additionally, such a mega-outburst would be almost totally dust-driven
and this fits very nicely with the B-V color index we see in P/Holmes'
coma and nucleus currently.
JBortle |
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置顶卡
变色卡
千斤顶
显身卡
