Thermal evolution of cometary nuclei by radioactive heating and possible formation of organic chemicals

Thermal evolution of large cometary nuclei (radii ≥ 40 km) due to heating by decay of ²³⁸U, ²³⁵U, ⁴⁰K and ²³²Th is investigated by adopting chemical models based on observations of P/Halley and mineral compositions of C1 chondrites. If the thermal diffusivity of crystallized ice is sufficiently small $$\kappa_{c}\leq 0.0015 \,\text{cm s}^{-1}$$ the central temperature may reach the melting point of H₂O in large nuclei (radii ≃ 200 km), while if the diffusivity is 0.003 or greater, the central temperature will not reach the melting point. The high temperature attained within cometary nuclei allows molecules such as HCN and NH₃ to form various organic compounds such as amino acids through oligomerization, and such compounds might be delivered to the Earth through disintegrations of the large nuclei, which subsequently impact the Earth. The extraterrestrial amino acids found in the K/T boundarylayer and in the dust of P/Halley might have been processed in cometary interiors.