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Comets would have amorphous ice rather than crystalline one at the epoch oftheir accretion. Cometary ice contains some impurities that govern the latentheat of ice crystallization, $L_{\rm cry}$. However, it is still controversialwhether the crystallization process is exothermic or endothermic. In thisstudy, we perform one-dimensional simulations of the thermal evolution ofkm-sized comets and investigate the effect of the latent heat. We find that thedepth where amorphous ice can survive significantly depends on the latent heatof ice crystallization. Assuming the cometary radius of 2 km, the depth of theamorphous ice mantle is approximately 100 m when the latent heat is positive(i.e., the exothermic case with $L_{\rm cry} = + 9 \times 10^{4}$ J/kg). Incontrast, when we consider the impure ice representing the endothermic casewith $L_{\rm cry} = - 9 \times 10^{4}$ J/kg, the depth of the amorphous icemantle could exceed 1 km. Although our numerical results indicate that thesedepths depend on the size and the accretion age of comets, the depth in a cometwith the negative latent heat is a few to several times larger than thepositive case for a given comet size. This work suggests that the spatialdistribution of the ice crystallinity in a comet nucleus depends on the latentheat, which can be different from the previous estimates assuming pure waterice.

Survivability of Amorphous Ice in Comets Depends on the Latent Heat of Crystallization of Impure Water Ice