¹Sang Woo Kim, ^1,2Kyeong Ja Kim
Journal of Geophysical Research: Planets, 131, e2026JE009892 Link to Article [DOI: 10.1029/2026JE009892]
¹Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Republic of Korea
²University of Science and Technology (UST), Daejeon, Republic of Korea

Published by arrangement with John Wiley & Sons

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 (1⁢08$$ source neutrons per configuration) to quantify what a pulsed neutron generator with time-resolved prompt gamma-ray neutron activation analysis (PGNAA) adds to neutron-only measurements, for eight representative Martian regolith compositions under idealized detection. The model reproduces the established first-order sensitivity of the epithermal die-away constant to WEH (𝜏epi =57$$–77 𝜇$$s for WEH =15$$–1 wt%; power-law 𝑅2 =0.87$$)—an internal benchmark. The central experiment holds WEH fixed at 3 wt% across six compositionally distinct host endmembers: 𝜏epi$$ 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 ≥7.6⁢𝜎$$ separation under the present idealized uncertainties. In a native-WEH benchmark, 27 of 28 pairs are separated at >5𝜎$$ by 𝜏epi$$ 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.