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Ejection of Glycine Molecules Adsorbed on a Water Ice Surface by Swift-heavy Ion Irradiation

Scope

Classical molecular dynamics of a 2 MeV sulfur ion impacting glycine adsorbed on water ice, modeling magnetospheric irradiation of Europa-like surfaces and quantifying intact versus fragmented ejecta velocities.

Summary

Solar-system ice bodies host organic molecules that may be sputtered into exospheres by energetic ions. Focusing on glycine on crystalline water ice as a prototype amino-acid–ice system, the authors simulate a single swift-heavy-ion impact representative of magnetospheric sulfur ions impinging on Europa. The goal is to quantify radial zones of molecular survival versus fragmentation and to estimate ejecta speeds relative to Europa’s escape velocity for astrobiological transport models. The Astrophys. J. framing connects classical collision cascades to observable exosphere composition constraints without resolving electronic excitation explicitly.

Methods

MD engine / interactions. Molecular dynamics with ReaxFF (Monti et al. 2013 parametrization optimized for glycine-containing systems) so bond making/breaking can occur during the cascade. The two-body ReaxFF terms are splined to the ZBL potential at high pair energies to capture projectile–target nuclear collisions; electronic stopping for the 2 MeV S ion is represented with a thermal-track energy-deposition model consistent with their prior ion-track work (see ApJ Methods and citations). 3D PBC in the lateral x/y ice film (or as stated) with border thermostat zones; see paper for full boundary conditions on z.

Target construction. An amorphous water-ice slab is built with PACKMOL (~204 × 104 Å surface footprint, ~212 Å depth); 1000 glycine molecules are placed on the top surface (~2 Å average spacing). After relaxation and surface creation, the film is equilibrated 18 ps at 101 K (Europa-like surface T). Berendsen thermostats (10 Å border zones on sides/bottom) hold 101 K while the top remains free.

Impact protocol. A 2 MeV sulfur ion impacts at 45° to the surface normal (magnetospheric S on Europa-like ice). Electronic stopping power is taken as ~89 eV Å⁻¹ in the reported parameterization.

Analysis. Radial maps distinguish intact glycine desorption vs fragmentation; catalogs include CN⁻, CO, OCN⁻, CO₂, and water-radiolysis products (H⁺, H₃O⁺, HO⁻, H₂, O₂, H₂O₂, per abstract). Velocity distributions are extracted for organics and light fragments for comparison to Europa escape (~2 km s⁻¹).

MD protocol (additional coverage). N/A — no NPT barostat (non-equilibrium impact cascade; N/A for replica umbrella). N/A — static E-field. N/A — cumulative equilibration+cascade ps in one line: see ApJ; ~18 ps pre-impact NVT thermalization of the ice film is reported. Adaptive timestep during the ZBL+ReaxFF collision stage: N/A for a single number here. N/A — shear; 2 MeV S is ~45° to normal (magnetospheric) shock-analog in abstract framing.

FF training (block 2). N/A — uses a published Reaxff (Monti et al. 2013) with ZBL splice.

Static QM (block 3). N/A — DFT is not the engine of the MD cascade study.

Findings

1 — Outcomes and mechanisms

  • Spatial pattern. The authors map intact glycine ejection out to about 25 Å from the ion path, while a ~10 Å core experiences strong shattering; most ejecta in that core are fragments rather than whole molecules.
  • Glycine and water chemistry. The impact drives extensive bond breaking in the thermal spike; ~189 glycine units are destroyed in their accounting, with many Gly±H (protonation/deprotonation) products, while water is partially replenished by recombination after radiolysis (H⁺, OH⁻, H₃O⁺, then H₂ and O₂ within about 1 ps), consistent with laboratory water-ice radiolysis references they cite.
  • C–N–O fragments. CN⁻, CO, OCN⁻, and CO₂ appear among C/N/O products, with several species flagged as matching Portugal et al. experimental assignments where noted in their Table 2 / “Exp” column.
  • No di-glycine. In this ~32.1 ps track, they do not observe peptide-bond formation, and they argue the thermodynamic window for dimerization (vs prior shock studies at higher sustained pressure/temperature) is too brief for C–N coupling in the track.

2 — Comparisons

  • Direct nuclear damage vs thermal spike. Only about 0.4 keV of the 2 MeV S energy goes to nuclear stopping; the authors estimate O(10) direct-collision dissociations, versus hundreds from the electronic-stopping thermal track—so cascade chemistry is spike-dominated, not projectile-knock-on-dominated.
  • Experiments. Fragment identities and intact ejection of a minority of Gly±H are discussed alongside Hedin, Ens, and related swift-ion desorption work; H₂/O₂ from ice match Bar-Nun/Baragiola-class laboratory radiolysis expectations.

3 — Sensitivity and design levers

  • Ejecta velocity vs mass. Light fragments and volatiles readily exceed Europa’s ~2 km s⁻¹ escape speed, whereas most ejected intact glycine remains slower—a strong mass/energy partitioning for exosphere modeling.
  • Electronic stopping model. The thermal-track radius (R = 5 Å) and stopping power choice set the deposited energy density; the authors note not all electronic loss becomes lattice heat, which would lower yields but leave qualitative behavior from their prior ice studies.

4 — Limitations and outlook (as authored)

  • Single track, one impact geometry: they position the run as a detailed template for height/flux models of ejecta in Europa-like exospheres, not a full fluence average; O₂ ejection is suppressed at low fluence in their run compared to steady sputtering pictures with trapped O₂ accumulation.
  • Classical chemistry: electronic excitation is mimicked by heating in the track—appropriate for their rarefied chemistry goals but not a full first-principles nonadiabatic model.

5 — Corpus / KB honesty

  • Numbers and species lists follow pdf_path; this page does not duplicate Table 2 in full.

Limitations

Classical potentials cannot capture all electronic excitation channels; single-impact statistics require ensemble averaging for exospheric flux models.

Wiki prose here is a navigation aid. Definitive numbers, protocol details, and figure-level claims should be taken from the peer-reviewed article at pdf_path (and any Supporting Information cited there), not from this page alone.

Relevance to group

Astro-chemistry / impact cascade reference in the broader corpus; ReaxFF+ZBL classical RMD (not a DFT or continuum study).

Citations and evidence anchors

  • https://doi.org/10.3847/1538-4357/ab6efe