2024), demonstrating the need for dust traps due to radial pressure bumps.

It is, therefore, reasonable to envision that dust remains trapped in pressure bumps for millions of years until the conditions (i.e., the dust/gas ratio, which increases as gas is removed) become favorable for late cometesimal formation.

Over these long timescales, the noble gases, even if initially trapped in water or CO2 ice in the pebble, have plenty of time to diffuse to the surface of the pebble (Smith et al., 1997; Livingston et al., 2002; Ligterink et al., 2024).

Once the volatiles have reached the surface, they can desorb/sublimate if the disk temperature is high enough, i.e. larger than the condensation/sublimation temperatures of (Öberg & Wordsworth, 2019).

If a noble gas evaporates at the surface (e.g., argon), the diffusion continues, and over time the pebble becomes strongly depleted in that gas.

If instead the temperature in the disk is too cold for evaporation of the noble gas at the surface of the pebble (e.g., xenon), diffusion stops and the pebble preserves its original abundance.

Close but inwards of a sublimation line, we can envision that the process of diffusion and sublimation is slow, and in the end, the pebble is only partially depleted.

This could be the case of krypton.

This scenario is sketched in Fig. 4.

Figure 5 shows the temperature profiles from our models of disks.

The parameters of the model cover the same values as in Marschall & Morbidelli (2023) (e.g., in the assumed evolution of the disk viscosity).

Here, in contrast to the results presented in Marschall & Morbidelli (2023), we explicitly consider the stellar radiation onto the disk when solving for the temperature.

While the temperatures differ from model to model in the inner part of the disk, where viscous heating dominates, all of them show the same temperature profile beyond about 8 au.

Beyond that distance, the temperature is dominated by stellar radiation and thus follows almost exactly what would be expected from the analytical solution for a passive disk (Chiang & Youdin, 2010).

The xenon line is at around 13 au, the krypton line is at roughly 45 au, and the one of argon is at 70 au.

We argued above that 67P must have formed beyond the xenon line, thus beyond 13 au, to accrete xenon in solid form. 67P should not form too close to the xenon line, though, because, in this case, it would form in a xenon-enriched environment that would lead to super-solar abundances of xenon.

Additionally, 67P also had to form close but still inside of the krypton line, i.e., inside of 35 au.

How far inside of the krypton line 67P could have formed is unclear.

But a reasonable guess would be that the formation region is somewhere in the 25-35 au region.

This conclusion, obtained from compositional considerations, is well in line with the location of the reservoir of planetesimals that generated the scattered disk, the Oort cloud and the hot Kuiper belt population, according to dynamical models (Nesvorný, 2018), and thus with the dynamical origin of 67P.

[Figure: Figure 4: A sketch of a protoplanetary disk scenario. Initially, the dust and gas disk expand together through the viscous expansion of the gas (e.g., Drążkowska & Dullemond, 2018; Morbidelli et al., 2022; Marschall & Morbidelli, 2023) (a & b). At some point, the dust grows large enough that radial inward drift due to aerodynamic drag in the tangential direction starts to dominate on short timescales (e.g., Takeuchi & Lin, 2002, 2005), and the dust disk begins to contract (c). Eventually, structures in the disk form (Andrews, 2020) and trap dust, thus preventing the complete loss of the dust disk (d). Dust pebbles trapped within these pressure bumps remain there until they are accreted into planetesimals. Depending on the location in the disk and the accretion time, pebbles lose volatiles/retain the respective volatile species. The xenon, krypton, and argon lines are illustrated as examples.]