Year: 2026
Type: article
Abstract: The deuterium-to-hydrogen (D/H) ratio in solar system volatiles is commonly used to diagnose volatile formation conditions. This ratio may be altered by a range of processes under solar nebula conditions, however, including energetic processing of icy grains. In this study, we investigate the effects of ultraviolet (UV) irradiation on the H/D exchange in mixed methane–water ices, a model system for water and organics ice mixtures. In an ultrahigh vacuum setup, we subject H 2 O:CD 4 and D 2 O:CH 4 ice mixtures of different thicknesses and mixing ratios to UV irradiation at different temperatures (10–70 K), and measure the formation rates of CH 3 D, CD 3 H, and HDO. Across the different ice mixtures, we find that H-to-D exchange is 2–3x faster and proceeds further in D 2 O:CH 4 ices compared to H 2 O:CD 4 ices. We attribute this difference to a higher propensity for OD radicals to extract H atoms from methane, compared to OH radicals to extract D atoms. The exchange cross sections for both ice systems increase somewhat with temperature, while the steady-state abundances are less affected. Based on these experiments, interstellar ices will experience substantial H–D exchange during the nebular lifetime, but not complete isotopic scrambling. We hence predict that nebular icy volatiles will retain a memory of their initial D/H composition even after substantial UV processing. Together, these results suggest that water and organic D/H values in solar system volatiles can be used to constrain their formation conditions, but only if ice exchange chemistry is taken into account. In the case of UV-driven H–D exchange in organic-rich water ices, this chemistry will likely favor organic deuteration and water de-deuteration.
Source: The Astrophysical Journal
Institution: Center for Astrophysics Harvard & Smithsonian
Language: English
Topic: Astro and Planetary Science
Subfield: Space and Planetary Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W7220704010