  Comets would have amorphous ice rather than crystalline one at the epoch of
their accretion. Cometary ice contains some impurities that govern the latent
heat of ice crystallization, $L_{\rm cry}$. However, it is still controversial
whether the crystallization process is exothermic or endothermic. In this
study, we perform one-dimensional simulations of the thermal evolution of
km-sized comets and investigate the effect of the latent heat. We find that the
depth where amorphous ice can survive significantly depends on the latent heat
of ice crystallization. Assuming the cometary radius of 2 km, the depth of the
amorphous 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). In
contrast, when we consider the impure ice representing the endothermic case
with $L_{\rm cry} = - 9 \times 10^{4}$ J/kg, the depth of the amorphous ice
mantle could exceed 1 km. Although our numerical results indicate that these
depths depend on the size and the accretion age of comets, the depth in a comet
with the negative latent heat is a few to several times larger than the
positive case for a given comet size. This work suggests that the spatial
distribution of the ice crystallinity in a comet nucleus depends on the latent
heat, which can be different from the previous estimates assuming pure water
ice.
