Impact of Radiogenic Heating on the Formation Conditions of Comet 67P/Churyumov–Gerasimenko

Because of the high fraction of refractory material present in comets, the heat produced by the radiogenic decay of elements such as aluminum and iron can be high enough to induce the loss of ultravolatile species such as nitrogen, argon, or carbon monoxide during their accretion phase in the protosolar nebula (PSN). Here, we investigate how heat generated by the radioactive decay of Al-26 and Fe-60 influences the formation of comet 67P/Churyumov-Gerasimenko, as a function of its accretion time and the size of its parent body. We use an existing thermal evolution model that includes various phase transitions, heat transfer in the ice-dust matrix, and gas diffusion throughout the porous material, based on thermodynamic parameters derived from Rosetta observations. Two possibilities are considered: either, to account for its bilobate shape, 67P/Churyumov-Gerasimenko was assembled from two primordial similar to 2 km sized planetesimals, or it resulted from the disruption of a larger parent body with a size corresponding to that of comet Hale-Bopp (similar to 70 km). To fully preserve its volatile content, we find that either 67P/Churyumov-Gerasimenko's formation was delayed between similar to 2.2 and 7.7 Myr after that of Ca-Al-rich Inclusions in the PSN or the comet's accretion phase took place over the entire time interval, depending on the primordial size of its parent body and the composition of the icy material considered. Our calculations suggest that the formation of 67P/Churyumov-Gerasimenko is consistent with both its accretion from primordial building blocks formed in the nebula or from debris issued from the disruption of a Hale-Bopp-like body.