Statement

Gamma irradiation of rock and regolith samples produces exogenous volatile species (H₂, CO₂, CH₄, organic fragments) that migrate into the headspace gas, contaminating indigenous volatile inventories. Water radiolysis in hydrous mineral phases produces H₂ with a well-established yield (G = 0.45 molecules/100 eV), and organic radiolysis produces CO₂ and CH₄. This mechanism drives approximately 15 "Partially Affected" cells across SCI 1.5, SCI 3.4, SCI 3.6, and SCI 4.3. The dedicated atmospheric tube (Roubion, M2020-164-2) is exempt from this mechanism because it contains no rock, no mineral-bound water, and no organics; the only relevant pathway for the atmospheric tube is wall-surface radiolysis of adsorbed species, which is second-order and characterizable via analog experiments.

Evidence

1. Water radiolysis G-values for volatile production: Spinks & Woods, 1990, An Introduction to Radiation Chemistry (3rd ed.), Wiley-Interscience. ISBN 978-0-471-61403-6 G(H₂) = 0.45 molecules/100 eV. At 100 kGy in water-bearing mineral phases, this yields measurable H₂ production that contaminates the headspace gas inventory. OH• and H₂O₂ co-products drive secondary reactions with organic matter, producing CO₂ and CH₄.
2. Organic radiolysis volatile products: Fox et al. 2023, JGR Planets 128:e2022JE007624. DOI: 10.1029/2022JE007624 Radiation-induced fragmentation of organic matter produces volatile species including CO₂ and low-molecular-weight hydrocarbons. The magnitude depends on organic content and mineral matrix (see Silicate Matrix Enhancement of Radiolysis claim).
3. Atmospheric tube gas density precludes direct gas-phase radiolysis: Carrier et al. 2025, Mars Sample Return: Sample Receiving Project Measurement Definition Team final report (supplement). NASA/JPL. Zorzano et al., 2024 (atmospheric tube gas content estimate). The atmospheric tube contains ~4.9 µmol of gas (~0.22 mg) at ~6.1 mbar. At this density, the gamma interaction cross-section with the gas is negligible: essentially all gamma energy is deposited in the titanium tube walls, not in the gas.

Argument

A1: The mechanism is sample-dependent. Volatile production scales with water content, organic content, mineral matrix, and surface area. Samples with hydrous phases or organic-bearing minerals produce more radiolytic headspace contamination than anhydrous igneous samples. This sample-dependence explains why the workbook assigns "Partially Affected" rather than "Affected": some tubes will be minimally affected.

A2: Two distinct sub-mechanisms require separate analysis. Sub-mechanism A (rock/regolith tubes): water and organic radiolysis in the solid sample produces exogenous gases that migrate into the headspace. Sub-mechanism B (atmospheric tube): gas-phase radiolysis is negligible due to the low gas density; the relevant pathway is wall-surface radiolysis releasing decomposition products of adsorbed species into the headspace. Sub-mechanism B is second-order relative to A and can be bounded by analog irradiation experiments.

A3: Headspace measurements require an uncontaminated baseline. All headspace gas measurements require comparison against the atmospheric tube to distinguish sample-derived gas contributions from the ambient Mars atmosphere captured at sealing. If the atmospheric tube's molecular mixing ratios are shifted by wall-surface radiolysis (sub-mechanism B), the baseline comparison is degraded. Noble gas isotope ratios in the baseline are unaffected (see Isotope Ratio Preservation claim).

Implication

Supports "Partially Affected" for techniques analyzing volatile inventories in rock and regolith tubes:

• GAS SOURCE MS × SCI 1.5 (Atmospheric History)
• GC-MS × SCI 2.1 (Habitability/Preservation)
• EGA × SCI 3.4 (Prebiotic Chemistry)
• TGA × SCI 3.4
• IR SPECTROSCOPY × SCI 1.5, SCI 3.6
• GC-MS × SCI 2.2, SCI 2.3 (headspace monitoring) Supports "Not Affected" for noble gas isotope ratios in both rock tube headspace and the atmospheric tube.

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