Laboratory Evidence that Methanol-Rich Ice Mantles Lower Methyl Formate Binding Energies

[astro-ph.IM] Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H₂O ice matrix.

However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice (T ~ 100K), challenging this view.

To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H₂O and CH₃OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions.

We find that MF binds more weakly to CH₃OH ice than to H₂O-rich substrates, with representative binding energies of ~6247K for amorphous H₂O, ~6213K for crystalline H₂O, and ~5469K for amorphous CH₃OH. MF deposited on crystalline CH3OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333K.

Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ~85K on CH3OH and ~95-100K on H₂O.

Together, these effects support a two-step thermal desorption pathway in which MF associated with CH₃OH-rich ice can be released prior to bulk H₂O sublimation.

Rachel E. Gross, Jeroen Terwisscha van Scheltinga, L. Ilsedore Cleeves, Catherine A. Dukes, Robin T. Garrod, Alexia Simon, Shane Sawyer, Adam Woodson

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:2609.27031 [astro-ph.IM] (or arXiv:2609.27031v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2609.27031
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Submission history
From: Rachel Gross
[v1] Tue, 22 Sep 2026 20:22:26 UTC (8,330 KB)
https://arxiv.org/abs/2609.27031

Astrobiology, Astrochemistry,