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 ( 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 (–s for WEH –1 wt%; power‐law )—an internal benchmark. The central experiment holds WEH fixed at 3 wt% across six compositionally distinct host endmembers: 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 separation under the present idealized uncertainties. In a native‐WEH benchmark, 27 of 28 pairs are separated at >5 by 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.