Nucleus with Amorphous

The level of amorphous ice present in the nucleus is an important parameter for comet sublimation. At cold temperatures and low pressure, e.g., during the formation of cometary nuclei in the outer parts of the pre-planetary disc, water ice is expected to condense in its amorphous form. However, amorphous ices crystallizes in an irreversible and exothermic process at temperatures above 136 K. Thus, to be still present in comets today, the temperature of the ice grains built into the nucleus should have never exceeded this crystallization temperature.

The energy released during the exothermic crystallization process provides an additional energy source that can lead to enhanced sublimation of the ice. Thus, when the critical crystallization temperature is reached, the comet will show gas activity. This can result, for example, in a sudden increase of activity (outbursts), which is indeed often observed in comets. The transition of amorphous to crystalline ice also changes the physical parameters of the nucleus, like heat conduction (the heat conduction of amorphous ice is about four times lower than for crystalline ice [174]), porosity, and density. This will result in a different evolution of gas activity over the orbit as compared to a nucleus made purely of crystalline ice. In addition, amorphous water ice can efficiently trap gases of more volatile ices. These are released at the moment of water ice crystallization and provide an additional source of volatiles [15, 16,169,170].

To illustrate the effect of amorphous ice, let us assume that at least on its first orbit into the inner solar system, a comet is made purely of amorphous water ice. If we further assume the extreme case that all volatiles are trapped in the amorphous water ice, then even highly volatile ices can be released only at the moment of water ice crystallization. When our hypothetical new comet further approaches the Sun, the sublimation front moves deeper into the nucleus and the surface layers will be crystallized. We will therefore find sublimation in the crystalline ice layer as outlined above, and this layer may eventually deplete from the highly volatile species (Fig. 7).

At present, it is unclear whether water ice is contained in the nucleus in crystalline or in amorphous form and what would be the ratio of the two. However, also comets containing amorphous ice will be porous, and they also may contain grains of frozen crystalline volatiles. Such non-trapped highly volatile ices will therefore be able to sublime through the pores in the water ice and show activity also at large rh, similar to the case of a pure crystalline nucleus. Obviously, the outgassing of a cometary nucleus is a complex interplay between porosity, the presence of amorphous ice, and the abundances of volatiles. In addition, a critical factor for the internal layering of the nucleus is the rate of surface erosion by water sublimation in comparison to the time scale for penetration of the orbital heat wave. If surface erosion is fast, no equilibrium for the internal structure is reached even after many orbits.

Fig. 7. Illustration of differentiation of a nucleus consisting originally of amorphous water ice and its evolution after several orbits. The meaning of colours is as in Fig. 5. Blue represents an amorphous water ice phase. Other volatiles are trapped in the amorphous ice and are also present as a separate crystalline phase (left). After several orbits, the surface layer crystallizes and is depleted from volatile ices. In a porous nucleus, the separate non-trapped crystalline ice phases may also sublime from the deeper interior

Fig. 7. Illustration of differentiation of a nucleus consisting originally of amorphous water ice and its evolution after several orbits. The meaning of colours is as in Fig. 5. Blue represents an amorphous water ice phase. Other volatiles are trapped in the amorphous ice and are also present as a separate crystalline phase (left). After several orbits, the surface layer crystallizes and is depleted from volatile ices. In a porous nucleus, the separate non-trapped crystalline ice phases may also sublime from the deeper interior

So far, we neglected the presence of dust particles in the ice in our discussion. We already mentioned that large dust particles may accumulate to form a crust at the surface. The sublimation of underlying ices then depends on the thermal conductivity and the porosity of the dust layer. The presence of dust has a strong effect on the surface temperature. Pure ice surfaces use most of their energy for sublimation. Dust-covered surfaces can heat up; for example noon temperatures with dust are expected around 360 K in comparison to about 200 K for a pure ice surface [174]. Because a dust crust may form an effective obstacle for the sublimating ices, it has been proposed that pressure built up by sublimated gases unable to penetrate through a dust crust can lead to cracks in the surface and small outbursts of gas/dust activity.

Several groups attempting to model the nucleus activity exist in addition to the examples already given. They all provide predictions on the outgassing behavior of comets (e.g., [38,39,114]). In general, the models predict that the evolution of gas activity seen in the coma of a comet can be quite different for a comet where the upper layers contain crystalline ice or a comet where amorphous ice is still present close to the surface. In addition, the dust content affects the activity evolution. Unfortunately, many of the parameters entering the simulations of the outgassing processes are not well known. They have to be derived by comparing model predictions to in situ and ground-based observations of the long-term activity evolution or in situ data from landers.

Was this article helpful?

0 0

Post a comment