  Because of the high fraction of refractory material present in comets, the
heat produced by the radiogenic decay of elements such as aluminium 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. Here, we investigate how heat generated by the radioactive
decay of 26Al and 60Fe influences the formation of comet
67P/Churyumov-Gerasimenko, as a function of its accretion time and size of
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
~2 kilometer-sized planetesimals, or it resulted from the disruption of a
larger parent body with a size corresponding to that of comet Hale-Bopp (~70
km). To fully preserve its volatile content, we find that either
67P/Churyumov-Gerasimenko's formation was delayed between ~2.2 and 7.7 Myr
after that of Ca-Al-rich Inclusions in the protosolar nebula 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.
