H–D Exchange in Ultraviolet-irradiated Water: Methane Ice Mixtures
- 1Center for Astrophysics ∣Harvard & Smithsonian
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.
Related
- Quantifying the temperature-dependent yields of N2 and N2H4 formation in vacuum-ultraviolet-irradiated NH3 ice
- Irradiated Atmospheres VII. Effect of Mixing Energy Flux on Atmosphere-Interior Coupling
- Atmospheric dynamics and variability of periodically irradiated tidally locked planets
- Optical Follow-Up of Two Spider Pulsar Candidates: An Irradiated Redback and a Possible Double Giant Impostor
- Spatial variability of CH4-to-CO conversion pathways in irradiated gas giant atmospheres: implications for chemical relaxation and 1D quench schemes
BibTeX
@article{simon2026exchange,
title = {{H–D Exchange in Ultraviolet-irradiated Water: Methane Ice Mixtures}},
author = {Alexia Simon and Elettra L. Piacentino and Mahesh Rajappan and Karin I. Öberg},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea57c},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea57c},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - H–D Exchange in Ultraviolet-irradiated Water: Methane Ice Mixtures
AU - Alexia Simon
AU - Elettra L. Piacentino
AU - Mahesh Rajappan
AU - Karin I. Öberg
PY - 2026
DA - 2026/10/07
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea57c
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea57c
AB - 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.
SN - 0004-637X
SN - 1538-4357
ER -