Statement

The 100 kGy sterilization dose is equivalent to approximately 2.0 million years of natural Mars surface radiation exposure. Jezero crater surface materials have an age measured in Gyrs (billions of years). Sterilization adds a small fractional increase (typically <1%) to the total radiation dose the samples have already accumulated on Mars. The youngest "exposure age" material found on Mars to date was by the Curiosity rover in Gale crater, with a surface exposure age of 78 million years (+/- 30 million years; Farley, 2014). The radiation-sensitive analytes most affected by sterilization (amino acids, carotenoid pigments, aromatic fluorophores, and other molecular biosignatures) have already been substantially degraded by natural Martian surface irradiation over geological time. For these measurements, sterilization damage is largely redundant with damage that nature has already inflicted.

This claim does not alter the workbook color assignments or the report's technique-level assessments, which intentionally evaluate sterilization effects in isolation (as if applied to a pristine sample). Here we claim that the potential effect on sample science by sterilization is strongly mitigated by surface exposure age.

Evidence

1. Mars surface radiation dose rates.
   Hassler et al., 2014, Science 343(6169):1244797. DOI: 10.1126/science.1244797
   Zhang et al., 2022, JGR Planets 127:e2021JE007157. DOI: 10.1029/2021JE007157
   Guo et al., 2021, Astronomy and Astrophysics Review 29(1):1-81. DOI: 10.1007/s00159-021-00136-5
   MSL/RAD measured ~58 ± 5 mGy/year absorbed dose in the silicon detector at Gale crater (Hassler et al., 2014). Zhang et al. (2022) modeled the full atmospheric and subsurface radiation environment using GEANT4/AtRIS, finding ~50-60 mGy/year in a water-sphere phantom at Gale-like surface pressures (~753 Pa), with the dose remaining roughly constant through the top ~10 cm of regolith before declining. Pavlov et al. (2022, see Evidence point 4) use 76 mGy/year from RAD measurements; Horne et al. (2022) cite 76-96 mGy/year. The range 50-76 mGy/year brackets conservative to nominal estimates for accessible sampling depths.

2. Exposure age equivalence.
   Using the conservative lower bound of 50 mGy/year:

   • 25 kGy (FDA standard) = 0.5 Myr equivalent
   • 100 kGy (G21 reference dose) = 2.0 Myr equivalent
   • 1,000 kGy (SRF worst-case) = 20 Myr equivalent

3. Jezero crater sample exposure ages are hundreds of millions to billions of years.
   Shahrzad, S. et al., 2019, GRL 46(5):2408-2416. DOI: 10.1029/2018GL081402
   Quantin-Nataf, C. et al., 2023, JGR Planets 128(6):e2022JE007628. DOI: 10.1029/2022JE007628
   Warner, N.H. et al., 2020, GRL 47:e2020GL089607. DOI: 10.1029/2020GL089607
   Farley, K.A. et al., 2014, Science 343(6169):1247166. DOI: 10.1126/science.1247166
   Crater-counting model ages for the Jezero floor unit give 2.6 ± 0.5 Ga (Shahrzad et al., 2019), but this is a spatial average. Quantin-Nataf et al. (2023) showed the floor unit has extreme heterogeneity: the NE sector has been continuously exposed for ~3 Ga, while the SW sector near the delta was exhumed most recently after progressive aeolian stripping of ~40 m of sedimentary cover. Even in this youngest sector, crater-counting resolution constrains exposure to hundreds of Ma, consistent with the Warner et al. (2020) vertical erosion rates of 10⁻³-10⁻⁴ m/Myr for the floor unit. The youngest exposure age encountered on Mars so far is 78 ± 30 Myr for the Cumberland drill site, part of the Sheepbed mudstone at Gale crater (Farley et al., 2014). Note that only Jezero crater samples are part of the Mars Sample Return effort; Gale crater exposure ages are discussed here for context only.

4. Amino acids already below detection thresholds for ancient Mars samples.
   Pavlov et al., 2022, Astrobiology 22(9):1099-1115. DOI: 10.1089/ast.2021.0166
   In silicate mineral matrices, amino acid radiolysis rates are enhanced 21× over pure compounds (k = 2.08 MGy⁻¹ with silica, 3.48 MGy⁻¹ with silica + perchlorate). At these rates, amino acids become undetectable within ~70 Myr at Mars surface dose rates. For samples with >100 Myr exposure, amino acids are already below detection limits before sterilization. Adding 100 kGy (~2 Myr equivalent) to a sample that has already accumulated ~10 MGy is a 1% increase in total dose/exposure "age."

5. Carotenoid and aromatic biosignatures degrade on geological timescales.
   Dartnell et al., 2012, Analytical and Bioanalytical Chemistry 403(1):131-144. DOI: 10.1007/s00216-012-5829-6
   Blanco et al., 2018, Astrobiology 18(12):1497-1516. DOI: 10.1089/ast.2016.1645
   Fox et al., 2023, JGR Planets 128:e2022JE007624. DOI: 10.1029/2022JE007624
   Carotenoid Raman biosignatures are completely destroyed by ~60 kGy equivalent exposure (Dartnell et al., 2012), corresponding to <1 Myr at Mars surface rates. Aromatic amino acid fluorescence (tyrosine, phenylalanine) is reduced to 9-25% at 50 kGy (Blanco et al., 2018). Irradiated organics converge toward indistinguishable macromolecular signatures regardless of starting material (Fox et al., 2023). For samples with exposure ages exceeding a few million years, these molecular biosignatures have already been degraded by natural radiation.

6. Radiation type comparison: GCR vs. gamma.
   Zhang et al., 2022, JGR Planets 127:e2021JE007157. DOI: 10.1029/2021JE007157
   The high-energy ions comprising Galactic Cosmic Rays (GCRs) cause the same electron ionization and chemical bond scission as gamma rays for the majority of their energy deposition. However, GCR ions also cause nuclear spallation events that produce secondary neutrons. Zhang et al. (2022) show that secondary neutrons peak at ~30 cm depth and contribute ~50% of the biologically-weighted effective dose at the surface. Spallation and subsequent neutron capture cause nuclear transmutations (atoms gain or lose neutrons) that 662 keV gamma radiation cannot produce. This distinction is minor for the science measurements most affected by sterilization (organic and biosignature analyses), but it means that sterilization pauses the isotope aging clock because it does not cause nuclear effects.

Argument

A1: The arithmetic is straightforward and conservative. At the lowest published Mars surface dose rate (50 mGy/year), 100 kGy equals 2 Myr. At nominal RAD-measured rates (76 mGy/year), it equals 1.3 Myr. Against expected sample exposure ages, this is a very small increase in total accumulated dose.

A2: The measurements most affected by sterilization are the same measurements most affected by natural Mars radiation. Amino acid detection, carotenoid identification, aromatic fluorescence discrimination, and viable organism recovery are all compromised by radiolytic bond scission, whether the source is GCR or gamma. Pavlov et al. (2022) show amino acids become undetectable within ~70 Myr at Mars surface radiolysis rates. Dartnell et al. (2012) show carotenoids destroyed within <1 Myr equivalent. The youngest Mars materials encountered yet have ~78 Myr of exposure age (Farley, 2014).

A3: This argument strengthens with sample age and weakens for young or shielded samples. The "damage already done" argument is strongest for surface samples with long exposure histories (>100 Myr), which includes all Jezero samples. It is weakest for recently excavated or deeply shielded material with low prior radiation exposure. For such hypothetical samples, sterilization would represent the dominant radiation event, and the workbook color assignments (which assess sterilization in isolation) would be the appropriate measure of science impact. The workbook deliberately does not incorporate this surface-age context in order to remain conservative and agnostic to sample history (mechanism-focused).

A4: GCR radiation has already done things gamma cannot. Natural GCR exposure includes spallation-induced nuclear transmutations and neutron activation that 662 keV gamma radiation cannot replicate. This means the natural radiation environment has already altered the samples in ways that go beyond what sterilization adds. Sterilization produces strictly a subset of the damage mechanisms that GCR exposure produces (no nuclear effects).

A5: This reframes the sterilization trade-off. The appropriate comparison is not "pristine sample vs. sterilized sample" but "Mars-irradiated sample vs. Mars-irradiated-plus-sterilized sample." For the radiation-sensitive science objectives, the practical difference between these two states is small for ancient samples.

Links

Concept Home
Workbook
Report