![LDPI–ReTOF mass spectrum obtained after the deposition of 40 ML pure NH3 ice at 15 K (red), without VUV irradiation. A reference mass spectrum obtained for a bare, clean substrate is presented for comparison (black). The spectra are offset for clarity -- [astro-ph.IM]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/Quantifying.png?fit=1200%2C676&ssl=1)
[astro-ph.IM] Gas–grain astrochemical models predict that a substantial fraction of elemental nitrogen in dense cores and protoplanetary disks is locked in molecular nitrogen (N2), in the gas or ice phase, although interstellar N2 ice has not been directly identified.
Rosetta measurements at comet 67P/Churyumov–Gerasimenko showed strong N2 depletion relative to CO, while NH3 was the dominant detected nitrogen-bearing ice. We experimentally quantify the conversion of NH3 ice into N2 and N2H4 under astronomically relevant conditions by studying its temperature- and fluence-dependent VUV photochemistry.
Experiments were performed under ultra-high vacuum using 40-monolayer NH3 ice at 15, 25, and 70 K irradiated at 115–170 nm to a total fluence of 3.8×1018 photons cm−2. Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was used to monitor N2 and N2H4 simultaneously.
VUV photolysis efficiently forms both products, with formation kinetics and product ratios depending on temperature and photon fluence. Initial-growth fits gave the highest apparent ice-retained N2 formation yield at 15 K, approximately an order of magnitude above those at 25 and 70 K.
The N2/NH3 column-density ratio was determined as a function of fluence and compared with astronomical constraints. These results suggest that NH3 ice photochemistry may provide an additional pathway to N2 ice in cold outer planetary environments.
P. Samarth, G. Fedoseev, S. Ioppolo, L. Hornekær, E. F. van Dishoeck, K.-J. Chuang
Comments: 22 pages, 7 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.17878 [astro-ph.IM] (or arXiv:2609.17878v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2609.17878
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Journal reference: A&A, 712, A235 (2026)
Related DOI:
https://doi.org/10.1051/0004-6361/202658992
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Submission history
From: Pranjal Samarth
[v1] Tue, 15 Sep 2026 22:06:22 UTC (1,747 KB)
https://arxiv.org/abs/2609.17878
Astrobiology, Astrochemistry,
