Beyond Hydrogen: Simultaneous Depth Profiling of Water and Rock-Forming Elements in the Martian Shallow Subsurface via Combined Active Neutron and Prompt Gamma-Ray Spectroscopy

Abstract

The Dynamic Albedo of Neutrons (DAN) instrument on the Curiosity rover has mapped subsurface hydrogen along an extensively analyzed 27-km segment of its traverse in Gale Crater, revealing water-equivalent hydrogen (WEH) of 1–6 wt%. Neutron die-away measurements alone, however, cannot determine whether this hydrogen resides in adsorbed water, hydrated minerals, or ice. Here we use Geant4 Monte Carlo simulations ($10^8$ source neutrons per configuration) to quantify what a pulsed neutron generator with time-resolved prompt gamma-ray neutron activation analysis (PGNAA) adds to neutron-only measurements, for eight representative Martian regolith compositions under idealized detection. The model reproduces the established first-order sensitivity of the epithermal die-away constant to WEH (${\tau }_{\text{epi}}=57$–$77\mu$s for WEH $=15$–1 wt%; power-law $R^2=0.87$)—an internal benchmark. The central experiment holds WEH fixed at 3 wt% across six compositionally distinct host endmembers: ${\tau }_{\text{epi}}$ compresses to 61.4–67.0 µs, whereas simultaneously measured PGNAA ratios (Fe/Cl, apparent Si/Ca, Fe/H) provide complementary elemental constraints: for every host pair, at least one of the three ratios gives $\ge 7.6\sigma$ separation under the present idealized uncertainties. In a native-WEH benchmark, 27 of 28 pairs are separated at >5$\sigma$ by ${\tau }_{\text{epi}}$ alone, driven largely by the broad assigned WEH range. Fe (Si) capture-to-inelastic ratios decrease monotonically by 36% (44%) for a 20 cm dry overburden, giving model-dependent depth information. These results define the information content of pulse-synchronized neutron–gamma-ray packages and modeling priorities for future Mars, Titan, and lunar volatile payloads.