Discussion

We focus our discussion on the elemental abundances of the noble gases (Fig. 3). Krypton is slightly depleted relative to xenon, and argon is heavily depleted relative to both krypton and xenon. The important new constraint we introduce here is that xenon is present in the comet in solar abundance and about four orders of magnitude more abundant than in CI chondrites. Noble gases can either be adsorbed in gaseous form in ice, or they can condense on icy grains if the disk temperature is low enough. Adsorption can only ever be partial. It cannot explain the solar abundance of xenon in 67P nor the fact that the isotopic properties of xenon in the comet are radically different from those of the Sun or the atmosphere of Jupiter (Marty et al., 2017). Thus, we think that the data from 67P rule adsorption out for xenon. Condensation of noble gases on grains can, in principle, explain solar abundance. Moreover, if at least part of the noble gases had been injected into the disk already in condensed form, they would have avoided isotopic equilibration with the gas and preserved a distinct isotopic composition.

The condensation temperatures of noble gases on grains are given in Öberg & Wordsworth (2019): roughly 20 K for argon, 30 K for krypton, and 45 K for xenon.

Explaining the depletion pattern in 67P, however, is not trivial.

The reason is that once the original grains aggregate into pebbles, noble gases are not expected to sublimate at the corresponding temperatures.

They remain trapped in the icy matrix and are desorbed only at specific temperatures, primarily during the transformation of CO2 and H2O ices (Ligterink et al., 2024).

These transformations start at about 50 −60 K and equally for all noble gases.

Thus, in a scenario where grains drift towards the Sun from the outer disk, they would contain xenon, krypton and argon in solar abundance until they reach this temperature.

According to this, any object formed in a region of the disk colder than 50 −60 K should have all noble gases in solar abundance, while objects formed in warmer portions of the disk can start fractionating their elemental ratios.

Nevertheless, at any temperature, the strong depletion of argon relative to krypton and xenon observed in 67P is difficult to understand (Ligterink et al., 2024), even more so considering our result that xenon is in solar abundance.

We propose to solve this conundrum taking into account that comets very likely formed late in the protoplanetary disk phase.

In fact, comets and other related large icy small bodies such as Trojans and Kuiper-belt objects have very low densities (between 300 kg m−3 and 1500 kg m−3; Groussin et al., 2019; Berthier et al., 2020; Spencer et al., 2020) and contain highly-volatile ices, in particular CO and CO2 (Eberhardt et al., 1987; Gasc et al., 2017; Kelley et al., 2022), which suggests that comets have never been significantly heated or even differentiated.

Thus, they must have formed late (Neumann et al., 2018) at a time when most of the short-lived radiogenic material has already decayed, i.e., several million years after the formation of the oldest solids (CAIs; Amelin et al., 2010; Connelly et al., 2012).

At that time, dust should have been trapped in disk structures, rather than still drifting towards the star.

Evidence for this comes from the ubiquitous observation of dust rings in protoplanetary disks (Andrews, 2020) and the ratio between the disk sizes in dust and gas does not seem to shrink over time (Najita & Bergin, 2018).

The observed evolution of disk sizes over time is totally inconsistent with models where dust continuously drifts towards the star (Appelgren et al., 2020; Birnstiel, 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).