Astrochemistry

Icy Volatile Enhancements in Evolving Protoplanetary Disks

By Keith Cowing
Status Report
astro-ph.EP
April 17, 2026
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Icy Volatile Enhancements in Evolving Protoplanetary Disks
Outer disk regions become increasingly H2O ice poor as compared to dominating N2, CO, and CO2 ices over a disk’s lifetime. The leftmost pie displays the assumed initial abundances across all species that arrive in the disk frozen-out on solid particles. Individual species making up the total volatile ice composition are labeled and color-coded. Radial ice abundances from 1 to 300 au are displayed at 3 Myr in the six small pies to the right of the initial abundances. Species labels are only included in the radial 3 Myr pies if their ice abundance makes up at least 10% of the total ice composition. Displayed to the right of the 3 Myr radial ice composition pies are those of representative solar system carbon-depleted and carbon-enriched comets adapted from D. Jewitt & D. Z. Seligman (2023), which use the average of measured H2O, CO2, and CO ice production rates from a number of solar system and interstellar comets for the carbon-depleted comet, detailed in D. Z. Seligman et al. (2022), and comet C/2006 W3 (Christensen) from T. Ootsubo et al. (2012) for the carbon-enriched case. Refractory species are not considered in these pies, only volatile species. — astro-ph.EP

Protoplanetary disk ice lines shape a multitude of planet formation processes, setting the environmental composition through evolution.

Ice line locations depend on molecular sublimation and deposition properties, but in dynamic disks where temperature and density structures change, so do the expected compositions of planets and planetesimals.

In turbulent viscous disks with particle drift, thermal evolution, and desorption/adsorption, Price et al. 2021 demonstrated that the CO/H2O ice ratio beyond the CO ice line can become enhanced by ∼10×.

We expand on their work by incorporating additional carbon, nitrogen, and oxygen species, more particle sizes, and a broader disk parameter exploration. We find that before ∼0.5Myr, volatile ices are enhanced relative to H2O as the outer disk is desiccated by drift, while at later disk times outward advection and volatile deposition further increase relative volatile icy enhancements beyond the evolving critical disk radius.

The outcome of these combined relative icy enhancement to H2O mechanisms is solid C/O ∼ N/O ∼1 beyond the hypervolatile ice lines, much higher than expected in static disks. Hypervolatiles (N2, CO, and CH4) robustly increase to ∼100× across the explored parameter space, while mid-volatiles (CO2 and NH3) are sensitive to model choices, with enhancements ranging from ∼2−50×.

Together these results demonstrate that coupling disk dynamics with simple sublimation and deposition chemistry is fundamental to predicting grain, planetesimal, and planetary compositions, particularly the role of advection in redistributing volatiles across disk radii.

Elizabeth Yunerman, Ellen Price, Karin Öberg

Comments: 30 pages, 16 figures, 1 table. Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2604.14124 [astro-ph.EP] (or arXiv:2604.14124v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.14124
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Submission history
From: Elizabeth Yunerman
[v1] Wed, 15 Apr 2026 17:49:26 UTC (2,609 KB)
https://arxiv.org/abs/2604.14124
Astrobiology, exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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