Heat and Gas Diffusion in Comet Nuclei (pdf file 5.5 MB) - ISSI

20 2. The Structure of Comet Nuclei
have been consumed. In that case a comet becomes less active, or the activity
stops because a stable dust mantle forms that inhibits any detectable
activity, making the comet unobservable. Giotto measurements (Keller et
al., 1986) indicate that active areas are limited to a small fraction of the
surface of short-period comets.
Many comets show an extremely low activity; this is usually interpreted
as a sign of volatile depletion in the outer layers of the nucleus, and is thus
expected that over time, activity would decline to the point of being negligible.
Comet nuclei become in this way dormant or extinct, and they may
assume an asteroidal appearance that makes them indistinguishable from
true asteroids. It is not necessary that the body has no emission at all: there
may be bodies with a level of cometary activity below the threshold of detectability
(Luu and Jewitt, 1992). This hypothesis is especially important
in the study of the origin of NEOs: it seems that from dynamical evidence
and statistical considerations, a fraction of NEOs may be of cometary origin
(Wetherill, 1988; Binzel et al., 1992; Harris and Bailey, 1998).
From an observational point of view, the distinction between asteroids
and comets is not as clear as it was in the past. While there are only two
objects (2060 Chiron and 4015 Wilson-Harrington) with IAU double designations,
there is a list of objects classified as asteroids that are suspected to
be extinct or dormant comets on the basis of dynamical and observational
considerations, among them 2201 Oljato (McFadden et al., 1993). Oljato
had been singled out as an unusual asteroid because of its orbital elements
and the possible relation with the Orionid meteor shower. The UV photometric
data, modeled as fluorescent emissions of neutral species, seem to
indicate a plausible gas production of OH and CN. The previous characteristics,
together with the anomalous excess of UV in the reflectance spectra,
have been regarded as an indication that Oljato can be an extinct comet
(McFadden et al., 1993).
On the other hand, there are bodies for which a cometary origin appears
very unlikely, yet they exhibit fluorescent emission, such as 1862 Apollo,
1566 Icarus, and 1 Ceres (Bockelée-Morvan and Crovisier, 1992). There
are many examples of small bodies first designated as comets, which later
have been classified as asteroids. Recent cases are 119P/Parker-Hartley
that was named 1986 TF, 137P/Shoemaker-Levy 2 known as 1990 UL3,
and 107P/Wilson-Harrington that had been lost and was then found again
as an asteroid named 4015 1979 VA, before being definitively named 4015
Wilson-Harrington.