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HCN sublimation front e i All';: some orbits

CO sublimation front

Fig. 5. Illustration of the differentiation of an originally homogeneous crystalline nucleus (left) over subsequent orbits. Circles indicate separate ice phases of volatiles (H2O: blue, HCN: red, CO: green) and a dust component (brown). After several orbits, the surface regions are depleted of volatile ices. Their sublimation fronts have moved into the interior of the nucleus (right), and these ices sublimate through the pores of the water ice matrix to the surface

We assume a thermal conductivity of K = 0.6 J m-1 s-1 K-1, a density of p = 700 kgm-3, a specific heat c = 8 x 102 Jkg-1 K-1, and an orbital semimajor axis, a, between 5 AU for a Jupiter family comet and several hundred to thousand AU for long-period comets. Then the orbital skin depth is between 5 m up to several hundred meters. Thus, it may well be that, for example, the interior of a large Jupiter family comet is never reached by solar energy and remains at its original state (neglecting radioactive heating). The outer meters of a comet, however, are definitely modified during its subsequent orbits around the Sun.

For short-period comets, the temperature at aphelion is still sufficient to sublime very volatile ices, such as CO. Some models (e.g., [18]) predict that after many orbital revolutions, the sublimation front of this minor volatile has moved deep (several meters) into the interior. At such depths, the available energy depends only little on the orbital position of the comet. The resulting gas production rate is then expected to be almost constant along a cometary orbit in this model, whereas less volatile ices show a clear orbital variation of gas production (Fig. 6). Such an extreme scenario is most likely to occur for highly volatile species, such as CO and CO2, which might be present only in the lowest layers of differentiated nuclei.

Cometary rotation leads to diurnal variations of the solar energy input on the surface. Again, we have a look at the skin depth:

Fig. 6. The effect of differentiation of a crystalline porous nucleus on its gas production rates over several orbits for a Jupiter family comet [21]. The evolution of CO activity is almost constant, whereas water and other less volatile ices show clear variations over the orbit of the comet

Fig. 6. The effect of differentiation of a crystalline porous nucleus on its gas production rates over several orbits for a Jupiter family comet [21]. The evolution of CO activity is almost constant, whereas water and other less volatile ices show clear variations over the orbit of the comet

With a rotation period of P = 10 h, the diurnal skin depth is only 0.1m [174]. Thus, rotational modulation of solar energy influences only the top layers of a nucleus.

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