Next: Characteristics
of a Cometary Up: No
Title Previous: A
key to testing
In fact the test of the theories described above is not so simple, since the ice might have suffered various alteration from the time of formation up to the present time. These processes may be classified into two types: one is the processing before or during accretion to form cometary bodies (i.e., at the stage of grains and their small aggregates), and the other is the processing of the ice after formation of a cometary nucleus.
For the first type of alteration, Donn (1991) discusses heating at collisions of grain aggregates during the accretion of grains to planetesimals (cometesimals) in the solar nebula (see Stage 4 in §2) or in the interstellar cloud (Stage 1 in §2). Another possible heating occurs during a high-temperature phase at formation of the solar nebula (Stage 2 in §2). Yamamoto (1985) briefly discusses the effect of this heating. Engel et al. (1990) discusses heating of grains in radial inflow in the solar nebula.
Irradiation by charged particles also leads to alteration of the ice (e.g., Moore et al., 1983; see Strazzulla, Pirronello in this volume). This processing is effective not only for the ice on grain surfaces before accretion to form cometary bodies, but also for the ice on the surface of the nucleus during residence in the Oort cloud.
The second type involves alteration of the ice in a cometary nucleus. One of the alteration processes is the thermal evolution of cometary nuclei during revolutions around the sun. This effect may lead to changes in both the crystalline structure and the chemical composition of the ice near the surface of the nucleus. The possibility that we can observe ``fresh" ice depends on the depth to which the ice suffers thermal alteration, which in turn depends on the thermal conductivity of a cometary nucleus.
Radiogenic heating in the interior of a cometary nucleus is another
possible alteration process of the ice. Whipple and Stefanik (1966) discussed
firstly this effect in their theory of splitting of a cometary nucleus
resulting from redistribution of volatiles in the nucleus. Various kinds
of possible alteration of the ice by radiogenic heating are discussed,
such as the presence of a liquid phase in the central region of a cometary
nucleus just after the accretion due to heating by the short-lived nuclides
Al (e.g. Wallis, 1980), transition of amorphous ice to crystalline
ice (e.g. Prialnik et al., 1987), and formation of the layered
structure in the chemical composition of the nucleus (Yabushita and Wada,
1988).