General Overview

The sublimation of nucleus ices is governed by the balance between the effective incident solar radiation energy on the surface, the reflected radiation, the thermally re-radiated energy, and the energy used for volatile sublimation and internal heat conduction. The sublimated gas produces a drag force on the dust particles at the surface, which then expand into the coma with the gas (Fig. 3, see also Sect. 3). In the coma, the parent gas molecules are subject to various chemical destruction processes (see Sect. 5).

Surface

Fig. 3. Sketch to illustrate the surface energy balance and the processes in the coma

Nucleus i

Surface

Fig. 3. Sketch to illustrate the surface energy balance and the processes in the coma

The energy balance at the nucleus surface is given by:

ri dz

Here Fq denotes the incident solar flux, 0 the solar zenith angle, rh the heliocentric distance, AB the comet Bond albedo (see Sect. 7 for a definition of albedo), and e the infrared emissivity, respectively. Z(T) is the surface sublimation rate of the ices at temperature T, L(T) the latent heat used for sublimation, ks is the coefficient for heat conduction into the interior along z, and the internal temperature gradient. Unfortunately, many of the critical parameters that govern the sublimation processes are only poorly known. In particular the heat conduction into the interior is uncertain because it depends on porosity, composition, conductivity of the ices, etc., of which we have only limited knowledge. Reference [174] provide an overview on heat conduction in a porous medium and the approximations used in various comet models.

A first estimate on the activity evolution of cometary nuclei with heliocentric distance can be obtained when solving the energy balance for a pure ice surface facing the Sun and neglecting internal heat conduction. On a log-log scale (Fig. 4), the sublimation of ices shows a sudden rise at the heliocentric distance, where sufficient solar energy for their sublimation becomes available. This sharp rise in Z is often referred to as the "onset of activity" for a given species. When further approaching the Sun, the sublimation increases proportional to the increasing incoming solar energy as rbecause almost all energy is converted into sublimation. The heliocentric distance, at which the onset of activity is seen, depends on the volatility of the ices. Highly volatile ices,

i i i i

r , , r , i

,1,1! A„ = 0.04

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