Constraining the formation region of comets.

There is a general consensus that cometesimals formed in an original reservoir, the primordial Kuiper-belt (PKB), between 20-40 au from the Sun (e.g., review by Nesvorny 2018). From there, they were scattered into the current trans-Neptunian region. In particular, the so-called scattered disk acts as the source reservoir for Jupiter family comets, JFCs (e.g., Duncan & Levison 1997). Yet, the current formation region is primarily constrained by the dynamics that lead to the current day structure of the Kuiper-belt.

Here, we discuss independent constraints coming from noble gas abundances. We combine the data from the two instruments ROSINA (Balsiger et al. 2007; Rubin et al. 2018) and COSIMA (Kissel et al. 2007; Bardyn et al. 2017) onboard ESA's Rosetta mission to derive the bulk elemental composition of comet 67P. These measurements show that Krypton and Xenon are only slightly depleted compared to the solar composition, while Argon is strongly depleted. For comparison, e.g., CI chondrites are heavily depleted in these three noble gases. This suggests that comets formed further out in the protoplanetary disk at a distinctly different distance than the parent bodies of CI meteorites. It also suggests that comets formed between the Xenon and Argon condensation lines. The Krypton line, which lies between the Xenon and Argon lines, is at about 25 au for a passive disk. We will show our latest protoplanetary disk models and their implications on where the respective temperatures occur in the disk in more detail.