Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.2005b&link_type=abstract
American Astronomical Society, DPS meeting #41, #20.05
Astronomy and Astrophysics
Astronomy
Scientific paper
A survey of activity on four comets visited by spacecraft indicates that repetitive outbursts and collimated jets tend to occur in geographically special regions: near one, or both, rotation poles and/or near the ends of the long axis. In addition, evidence of similar types of surface flow on three of the four cometary nuclei indicates the processes that form them maybe shared by many comet nuclei. We use this evidence to argue that each of these phenomena is the surface manifestation of on-going activity that occurs in the interior of many cometary nuclei and that produces substantial volumes of volatile gas. This conclusion parallels that of Prialnik and her colleagues (Prialnik et al., 2004) who have shown how the phase change of gas-laden amorphous ice in the interior of comets could be responsible for massive outbursts and the splitting of cometary nuclei.
We propose that directed jets, like those seen on 19P/Borrelly, are caused by a spatially localized effusive release of cold volatiles (CO and/or CO2) from the interior into the comets H2O atmosphere. In the day time, the latter provides a constraining channel through which a mixture of H2O and cold volatile gas is accelerated as in a jet. In the vicinity of the nucleus the jet is made visible largely by ice crystals condensed out of the H2O in the jet. At night, the H2O atmosphere disappears and the relatively small flux of volatile gas from the interior continues to be released, no longer in the form of a highly collimated jet, but, probably, in the form of a broad directed outflow similar to that studied by Kitamura (1987). We present a crude hydrodynamic model for such a jet based on Bernoulli's equation for an inviscid, compressible gas in the adiabatic approximation.
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