7. ----------------------------
Title - Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope
By - William Reach et al, 2010
Extracts - The driving mechanism for the 1892 and 2007 eruptions from comet Holmes is not understood but it is thought that the heating of subsurface pockets of amorphous ice, causes subsurface cavities to build pressure until they rupture the comet’s surface. The existence of an energy source within Holmes suggests other comets, and possibly volatile-bearing asteroids, may also harbor regions of explosive potential. The ability of the nucleus to survive explosions with such great energy also indicates it has significant tensile strength. The hemispherical ejection models can match the morphology of the outer shell, but they do not generate the pronounced peak in ejecta located just anti-sunward of the nucleus. This peak has been referred to as the ‘false nucleus’ or ‘pseudonucleus’ or more prosaically, the ‘blob’. This feature was evident even to naked-eye or binocular observers in 2007, and it is evident in the 1892 images and visual observation notes (Barnard 1913). That comet Holmes survived the two largest outbursts (in 2007 and 1892) ever witnessed on a comet outside a planetary atmosphere indicates that this comet has significant internal strength as well as internal structure.
Activity of comets, at distances so far from the Sun that sublimation of common volatiles cannot occur, is evidence for the amorphous-crystalline transition driving activity (Meech et al, 2009).
Repetitive outbursts discovered in the detailed observing campaign for comet 9P/Tempel 1 have been explained with fluidized flows of material driven by subsurface amorphous ice.
The frequent splitting of cometary nuclei remains unexplained. If pockets of amorphous ice are heated and liberate 1021 erg of energy, such as we observed from the Holmes ejecta, there is more than enough energy to drive loosely bound subunits of cometary nuclei apart at their observed separation speeds, with total fragment kinetic energies of order or less than 1019 erg. The freshly exposed surfaces of split comets, may be more likely to have explosive outbursts of the type exhibited by comet Holmes in 1892 and 2007. Indeed, fragments from the interiors of split comets may have very short lifetimes for disruption due to explosive phase transitions of exposed or near-surface amorphous ice.
But older comets may lose a significant amount of their surface due to sublimation on multiple perihelion passages, insolation may eventually reach their amorphous ice layer resulting in erratic and possibly eruptive activity similar to that of comet Holmes.
Activation of pockets of subsurface ice could possibly explain impulsive mass loss from the comets in the asteroid belt, though this application is less likely to apply in the same manner as for cometary outbursts because asteroidal ice was not formed under outer solar system conditions that led to the unstable, exothermic phase transition of amorphous ice that led to the explosion of comet Holmes.
2. ----------------------------
Title - Outburst of Comet 17P/Holmes Observed With The Solar Mass Ejection Imager
By - Jing Li et al, 2010 (David Jewitt)
Extracts - One feature that is largely independent of the many unknowns is the step-wise progression of the crystallization front into the nucleus. Thermal runaways triggered by crystallization near the surface propagate downward into colder ice. Eventually, the heat released by crystallization is insufficient to drive additional ice to crystallize, and the runaway stops. The vertical distance is related to the thermal skin depth impressed in the nucleus by sunlight added at the surface and typically measured in meters. Crystallization is therefore at least qualitatively consistent with a scenario in which a disintegrating dusty surface shell is launched from the 17P/Holmes nucleus.
Kossacki & Szutowicz (2010) computed thermal models of 17P/Holmes and reached the opposite conclusion, namely that runaway crystallization is unlikely to have been responsible for the outburst. However, their conclusion relied, in part, on the very high ejected mass estimates of 10^12 to 10^14 kg by Montalto et al. (2008). As noted earlier, the latter mass estimates are based on a reported detection of extinction in the coma, which sits uncomfortably with the large-aperture SMEI data presented here. In fact, the upper end of the Montalto et al. (2008) mass estimate considerably exceeds our best guess as to the mass of the entire nucleus of 17P/Holmes, and therefore cannot be correct. For this reason, and because our own mass estimates are considerably smaller, we consider that to reject crystallization as the energy source for the 17P/Holmes outburst would be premature.
Crystallization of a subsurface layer of amorphous ice with the associated release of trapped super volatile gases could supply the mass, energy and momentum of the ejecta responsible for the remarkable outburst of comet 17P/Holmes. However, why 17P/Holmes should be uniquely afflicted by 3 such extraordinary outbursts, whereas most other comets show none, remains a complete mystery.
The mass of the dust coma was (2 to 90)×10^10 kg, corresponding to 0.2% to 10% of the nucleus mass. The ejected mass is equivalent to that contained within a surface shell on the 3.4 km diameter nucleus having a thickness 1.4 m to 60 m. Comparison with the 25 m thermal skin depth for heat conducted inwards since the previous outbursts in 1892/93, is consistent with conducted heat being the trigger responsible for the outbursts.
There is a 1.2 days lag between the start of the outburst and the time of peak activity, that may measure the timescale for the disintegration of fragments in the coma. The kinetic energy of the outburst was in the range (3 to 140)×10^15 J, far too large for sunlight to play any more than a triggering role in the expansion of the ejecta. The energy per unit mass of the ejecta (10^5 J/kg) is of the same order as the energy per unit mass released upon the crystallization of amorphous water ice.
No comments:
Post a Comment