Introduction

It is commonly argued that studying small Solar System bodies (asteroids, comets, etc.) directly informs planet formation theories because they are remnants of the original planetesimal population that triggered Solar System formation. Comets are considered among the most pristine objects in our Solar System because their physical and chemical properties are most easily explained by a lack of internal processing. For example, comets have very low densities (∼500 kg m−3; Groussin et al., 2019) and contain significant amounts of highly-volatile ices (incl. CO and CO2; Eberhardt et al., 1987; Gasc et al., 2017). Although most comets have typical diameters between 1-5 km (Snodgrass et al., 2011), the mentioned properties appear to hold even for large comets (e.g., Biver et al., 1997; Kelley et al.,

2022; Spencer et al., 2020; Brown, 2013), suggesting that these bodies experienced minimal heating or other planetary processes. This view that comets are, to a large extent, primordial is consistent with our current understanding of their dynamical past. They have presumably formed beyond Neptune in a massive primordial disk of precursory bodies, commonly referred to as planetesimals or cometesimals (e.g. Nesvorný, 2018, for a review of the dynamics in the early Solar System). When Neptune migrated through that disk, these bodies were scattered into the current day Kuiper Belt – in particular, the so-called Scattered Disk – where they have remained until recently (e.g. Nesvorný et al., 2016; Nesvorný, 2021). The Scattered Disk is widely considered as the main source reservoir of Jupiter family comets (JFC; Duncan et al., 2004; Duncan, 2008; Dones et al., 2015). From the Scattered Disk, comets are injected into the inner Solar System by the giant planets. Once close enough to the Sun, they exhibit activity driven by the sublimation of ices (in particular H2O). Because JFCs can have very low perihelia (∼1 au), they are also the most easily accessible comet population for interplanetary spacecraft. Over the past three decades, they have thus become the target of multiple international missions, including 19P/Borrelly (Deep Space 1, Rayman, 2002), 81P/Wild 2 (Stardust, Reichhardt, 1995), 9P/Tempel 1 (Stardust, and Deep Impact, A’Hearn et al., 2005), 103P/Hartley 2 (Deep Impact/EPOXI, A’Hearn et al., 2011), and most recently 67P/Churyumov-Gerasimeko (Rosetta, Taylor et al., 2017). Rosetta was the first mission to go into orbit around a comet. Two years at 67P/Churyumov-Gerasimenko (hereafter 67P) gave us an unprecedented up-close look at the physical and chemical properties as well as the evolution of 67P along its orbit from 3.7 au on the inbound orbit to perihelion at 1.25 au and back out to 3.8 au. Rosetta has revolutionised our understanding of comets (see Keller & Kührt, 2020, for a review). Yet many questions remain open. In particular, the hope that we can determine how comets formed has not been fulfilled. Proponents of conflicting formation scenarios have claimed Rosetta’s observations confirm their theories (see Weissman et al., 2020). One main objective, finding the formation location of comet 67P, has not yet been addressed, and it is one of our goals. In this work, we combine measurements from two analytical instruments from the Rosetta space mission: i) “Rosetta Orbiter Spectrometer for Ion and Neutral Analysis" (ROSINA) and ii) “COmetary Secondary Ion Mass Analyzer" (COSIMA) instruments to deduce the bulk chemical composition of 67P. We will show that the noble gas xenon is present in solar abundance, which strongly suggests that it has been accreted in solid form, while argon was accreted out of the gas phase. Krypton being slightly depleted is suggestive that the comet formed close, but slightly inward, of the krypton sublimation line. This implies the formation of comet 67P and JFCs, more generally, at roughly 25 −35 au in the protoplanetary disk. This constraint is solely driven by composition but is consistent with dynamical models on the origin and evolution of Jupiter family comets.