Survival and Detectability of Mars-Derived Organic Carbon in Phobos Regolith: A Quantitative Hardness Taxonomy for MMX Sample Analysis

Aryan Hussain Sahir 

The Martian Moons eXploration (MMX) mission will return ≥ 10 g of Phobos regolith containing a small fraction of Mars-derived impact ejecta (Ramsley and Head, 2013; Chappaz et al., 2013). We quantify what fraction of this material could plausibly carry detectable ancient Martian organic matter, correcting an uncited back-of-envelope estimate that assumed percent-level survival of degraded organics. Two independent estimates of ejecta antiquity — impact-flux chronology (96.7–99.75% of launching events pre-Noachian) and present-day geologic-mapping area fractions (45–75% of Mars's surface is Noachian/Hesperian; Tanaka et al., 2014; Carr and Head, 2010) — answer different physical questions and are both reported rather than collapsed into one figure. A two-axis mechanical/radiolytic hardness taxonomy across six biosignature classes shows ejection shock (< 5 GPa; Hyodo et al., 2019) is non-lethal for all classes, while radiolytic dose over Phobos's 3.8 Gyr exposure is lethal for all classes except refractory macromolecular carbon: Monte Carlo propagation (N = 2×106) gives a median labile-biomolecule survival fraction of 10−310.2, and cellular viability windows of 6.0×105–2.8×108 yr, both far short of 3.8 Gyr, independently corroborated (with caveats on shared underlying data) by planetary-protection risk models (Fujita et al., 2019; Kurosawa et al., 2019). Refractory carbon yield is 39.0 ng per 10 g sample (median; 5th–95th percentile 2.6 ng–0.6 μg), validated by bootstrap resampling and reproduced across independent random seeds, with its dominant uncertainty (χC, 90.4% of variance) independently anchored near 4–12 ppm via a Gale Crater back-calculation. Detection probability against real instrumental thresholds ranges from 48.7% (best validated nano-isotope instrumentation) to effectively zero (conventional bulk EA-IRMS), and is itself an upper bound given that returned sample is subdivided across multiple analyses in practice. MMX's realistic organic target is structurally and isotopically diagnostic refractory carbon, not intact biosignatures.