Combining Rosetta's ROSINA volatile and COSIMA refractory measurements, Marschall, Morbidelli and Marrocchi (2025) constrained comet 67P/Churyumov–Gerasimenko's refractory-to-ice mass ratio to 0.5<χ<1.70.5<\chi<1.7 and found a distinctive noble-gas pattern: xenon near solar abundance, krypton slightly depleted, and argon heavily depleted. They proposed that this pattern records formation from pebbles trapped for millions of years in disk pressure bumps at 25–35 au (30–40 K), where slow internal diffusion lets noble gases escape at pebble surfaces and sublimate — argon fully, krypton partially, xenon not at all. This scenario, however, is only sketched qualitatively. The open problem is to convert it into a quantitative, laboratory-anchored account of where and under which disk conditions 67P formed: measured diffusion/desorption kinetics for Ar, Kr, and Xe in realistic icy pebbles; a model reproducing the observed depletion pattern as a function of pebble size, ice composition, temperature, and residence time; a sharpened formation-distance constraint relative to the krypton sublimation front; and a quantified argon budget.

Background

Comet 67P/Churyumov–Gerasimenko (67P) is a Jupiter-family comet (JFC), i.e. a short-period comet whose orbit is controlled by Jupiter, widely held to originate in the scattered disk — a population of icy planetesimals on Neptune-crossing orbits that was emplaced during the early dynamical rearrangement of the giant planets. ESA's Rosetta spacecraft orbited 67P for two years (2014–2016). Its ROSINA mass spectrometer measured the volatile (ice) composition of the escaping coma gas, including the noble gases argon, krypton, and xenon, and its COSIMA instrument measured the elemental composition of refractory (non-volatile) dust particles. The refractory-to-ice mass ratio χ\chi was not measured directly; Marschall, Morbidelli and Marrocchi (2025) combined the two datasets by Monte Carlo sampling, leaving χ\chi free and constraining it a posteriori to 0.5<χ<1.70.5<\chi<1.7 from the requirement that elemental ratios such as C/O be physically reasonable and not super-solar.

Noble gases are classic formation tracers because they are chemically inert and condense at characteristically low temperatures: in disk thermal profiles, each species has a sublimation (snow) line — the heliocentric distance beyond which it can condense into solids. Using condensation temperatures from Öberg and Wordsworth (2019) (roughly 20 K for Ar, 30 K for Kr, 45 K for Xe), the authors place the Xe line near 13 au and the Ar line near 70 au in evolved-disk temperature profiles. Laboratory work underpins the relevant ice physics: Ligterink et al. (2024) studied temperature-programmed desorption (heating an ice sample on a controlled temperature ramp while recording which gases leave it) of noble gases trapped in H2_2O:CO2_2 ices and found that all noble gases are retained in bulk ice until transformations at about 50–60 K, so simple inward drift cannot fractionate them; Yokochi (2022) showed experimentally that adsorption (binding of gas molecules onto ice surfaces, as distinct from condensation into the bulk) at 36–45 K can reproduce 67P's measured Ar/H2_2O ratio; Balsiger et al. (2015) measured solar Ar isotopic composition in 67P's coma, consistent with adsorbed (gas-phase) argon.

The puzzle Marschall et al. address is that Xe is near solar abundance in 67P while Ar is heavily depleted and Kr slightly depleted — a pattern hard to produce if grains simply drifted inward, since all noble gases would be lost together. Their proposed resolution rests on the pebble picture of planet formation: dust grows into pebbles (mm–dm aggregates) that would rapidly spiral inward by aerodynamic drag, but observations of dust rings in protoplanetary disks imply long-lived dust traps at gas pressure bumps (local maxima in the disk's radial gas-pressure profile, where inward drift of solids reverses) where pebbles can be retained for millions of years. If 67P's building blocks sat in such a trap at 25–35 au (disk temperatures 30–40 K) until late cometesimal formation — several Myr after calcium-aluminum-rich inclusions (CAIs), the oldest solar-system solids — then Ar, whose surface sublimation temperature is exceeded there, diffuses out and is lost; Xe, too cold to desorb, is retained at solar abundance; and Kr, near its sublimation front, is partially lost. This mechanism is presented only as a sketch (their Fig. 4): no diffusion coefficients, rate laws, or timescale calculations are given, and the consistency with disk-evolution and scattered-disk dynamical models (cf. Nesvorný 2018) is asserted qualitatively. The residual uncertainties in the underlying measurements — COSIMA refractory abundances (e.g., hydrogen-to-carbon assumptions) and ROSINA coma sampling variability — were tested only by substituting the composition of CI chondrites (the most chemically primitive meteorite class, taken to approximate solar elemental abundances) for the refractories, which left the conclusions qualitatively robust.

Problem Statement

The source paper asks, in effect: can the qualitative scenario — 67P assembled from pebbles that resided for millions of years in a disk pressure trap at 25–35 au (30–40 K), losing argon and partially losing krypton by slow internal diffusion followed by surface desorption while retaining xenon — be made quantitative and validated, so that comet 67P's measured composition yields a determinate formation location and disk environment? A resolution requires each of the following four parts, and an established negative result on any one part resolves the task: (i) diffusion and desorption kinetics for Ar, Kr, and Xe in realistic icy pebbles (H2_2O:CO2_2-dominated, porous), measured experimentally and/or derived theoretically, sufficient to predict noble-gas fractionation as a function of pebble size, ice composition and structure, local disk temperature, and residence time; (ii) a test of whether these kinetics reproduce 67P's observed pattern — Xe in near-solar abundance, Kr slightly depleted, Ar heavily depleted, for a refractory-to-ice ratio within the source's 0.5<χ<1.70.5<\chi<1.7 — including the timescales needed for the observed partial Kr loss and near-complete Ar loss; (iii) a formation-distance constraint sharpened from 'somewhere in 25–35 au' to a determinate position relative to the krypton sublimation front (or a justified revision of the band); and (iv) a quantified argon budget distinguishing gas-adsorbed Ar from any condensed Ar and its retention under later heating (stellar irradiation, collisional processing), which the source raises but does not quantify.

References

  1. Marschall, R., Morbidelli, A., & Marrocchi, Y. (2025). The refractory-to-ice ratio in comet 67P: Implications on the composition of the comet-forming region of the protoplanetary disk. Planetary and Space Science 259, 106061, DOI 10.1016/j.pss.2025.106061; arXiv:2501.17864, https://arxiv.org/abs/2501.17864
  2. Ligterink, N. F. W., Kipfer, K. A., Rubin, M., Altwegg, K., Hänni, N., Müller, D. R., Wurz, P., Galli, A., & Wampfler, S. F. (2024). Sublimation of volatiles from H2_2O:CO2_2 bulk ices in the context of comet 67P/Churyumov–Gerasimenko. Astronomy & Astrophysics 687, A78, DOI 10.1051/0004-6361/202449554, https://doi.org/10.1051/0004-6361/202449554
  3. Yokochi, R. (2022). Adsorption-driven Gas Trapping in Cometary Ice Analogs. The Astrophysical Journal 940, 153, DOI 10.3847/1538-4357/ac9621, https://doi.org/10.3847/1538-4357/ac9621
  4. Öberg, K. I., & Wordsworth, R. (2019). Jupiter's Composition Suggests its Core Assembled Exterior to the N2_2 Snowline. The Astronomical Journal 158, 194, DOI 10.3847/1538-3881/ab46a8, https://doi.org/10.3847/1538-3881/ab46a8
  5. Balsiger, H., Altwegg, K., Bar-Nun, A., et al. (2015). Detection of argon in the coma of comet 67P/Churyumov-Gerasimenko. Science Advances 1, e1500377, DOI 10.1126/sciadv.1500377, https://doi.org/10.1126/sciadv.1500377
  6. Rubin, M., Altwegg, K., Balsiger, H., et al. (2019). Elemental and molecular abundances in comet 67P/Churyumov-Gerasimenko. Monthly Notices of the Royal Astronomical Society 489, 594, DOI 10.1093/mnras/stz2086, https://doi.org/10.1093/mnras/stz2086
  7. Bardyn, A., Baklouti, D., Cottin, H., et al. (2017). Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta. Monthly Notices of the Royal Astronomical Society 469, S712, DOI 10.1093/mnras/stx2640, https://doi.org/10.1093/mnras/stx2640
  8. Nesvorný, D. (2018). Dynamical Evolution of the Early Solar System. Annual Review of Astronomy and Astrophysics 56, 137, DOI 10.1146/annurev-astro-081817-052028, https://doi.org/10.1146/annurev-astro-081817-052028
  9. Bannister, M. T., Pfalzner, S., Pearce, T., Mustill, A. J., Klahr, H., Nomura, H., Ohashi, N., Kokotanekova, R., Marino, S., Bodewits, D., Marschall, R., Seligman, D. Z., Jones, G. H., & Veras, D. (2025). The Origins & Reservoirs of Exocomets. Space Science Reviews 221, 90, DOI 10.1007/s11214-025-01219-w; arXiv:2509.22541, https://arxiv.org/abs/2509.22541
  10. Miller, J. W., Umurhan, O. M., Huber, C., Guilbert-Lepoutre, A., & Birch, S. P. D. (2026). Hypervolatile retention within small Kuiper Belt Objects. Icarus 460, 117290, DOI 10.1016/j.icarus.2026.117290, https://doi.org/10.1016/j.icarus.2026.117290
  11. Cassata, W. S., Parai, R., Borg, L. E., & Shearer, C. K. (2025). Comets and carbonaceous chondrites delivered noble gases to the Moon. Earth and Planetary Science Letters 671, 119654, DOI 10.1016/j.epsl.2025.119654, https://doi.org/10.1016/j.epsl.2025.119654
  12. Boitard-Crépeau, L., Ceccarelli, C., Beck, P., Vacher, L., & Ugliengo, P. (2025). Was Earth's Water Acquired Locally during the Earliest Phases of the Solar System Formation? The Astrophysical Journal Letters 987, L25, DOI 10.3847/2041-8213/ade5aa, https://doi.org/10.3847/2041-8213/ade5aa