09:15 〜 09:30
[PPS03-02] Thermal evolution of pebble-pile comets and its dependence on the latent heat of crystallization of impure H2O ice
キーワード：彗星、熱進化、結晶化、非晶質氷
Comets are small icy objects and they are thought to be formed in the outer region of the solar nebula. As comets spent a long time under cold conditions after their formation, they are "Rosetta Stone" to unveil the environment of the early solar system.
Comets are made of ice and refractory materials. The composition of ice is not pure H2O but mixture of H2O, CO, CO2, and so on. Moreover, it is widely believed that cometary ice is amorphous rather than crystalline at the epoch of their accretion. Then, the potential heat sources for the early thermal evolution of comets are (1) the decay heat of short-lived radionuclide, 26Al, and (2) the latent heat of ice crystallization.
We note, however, that the latent heat for the crystallization of cometary ice is poorly constrained. Kouchi and Sirono (2001) reported that the latent heat is drastically reduced by the impurities: while the crystallization of pure H2O ice is exothermic process, the crystallization of impure ice is endothermic process when the concentration of impurities exceeds approximately 2%.
In this study, we performed one-dimensional simulations of thermal evolution of km-sized comets to investigate the impacts of low or negative latent heat of impure ice. We found that the depth where amorpous ice can survive strongly depends on the latent heat of crystallization, i.e., the impurities. Ourresults suggests that the spatial disributon of the crystallinity of ice in a comet nucleus might be diffferent from that shown in earlier studies, depending on the presence/absence of the runaway crystallization.
Comets are made of ice and refractory materials. The composition of ice is not pure H2O but mixture of H2O, CO, CO2, and so on. Moreover, it is widely believed that cometary ice is amorphous rather than crystalline at the epoch of their accretion. Then, the potential heat sources for the early thermal evolution of comets are (1) the decay heat of short-lived radionuclide, 26Al, and (2) the latent heat of ice crystallization.
We note, however, that the latent heat for the crystallization of cometary ice is poorly constrained. Kouchi and Sirono (2001) reported that the latent heat is drastically reduced by the impurities: while the crystallization of pure H2O ice is exothermic process, the crystallization of impure ice is endothermic process when the concentration of impurities exceeds approximately 2%.
In this study, we performed one-dimensional simulations of thermal evolution of km-sized comets to investigate the impacts of low or negative latent heat of impure ice. We found that the depth where amorpous ice can survive strongly depends on the latent heat of crystallization, i.e., the impurities. Ourresults suggests that the spatial disributon of the crystallinity of ice in a comet nucleus might be diffferent from that shown in earlier studies, depending on the presence/absence of the runaway crystallization.