Cosmic-Ray Neutron Sensing in Ultra-Dry Environments: Linking Terrestrial Mars Analogs and Planetary Neutron Spectroscopy

Cosmic-ray neutron sensing (CRNS) has emerged as a powerful tool for monitoring near-surface water across a wide range of spatial scales, from soil moisture and snowpack on Earth to hydrogen mapping on planetary surfaces. While most terrestrial CRNS applications focus on environments with appreciable liquid water, far less is known about neutron behavior in extremely dry systems where hydrogen is sparse and primarily bound in minerals. These conditions are directly relevant to planetary neutron spectroscopy and provide an opportunity to connect environmental CRNS research with space science.

Here we present results from portable CRNS deployments at ultra-dry terrestrial analog sites, including Alvord Desert, Oregon, and the Namib Desert, Namibia. These campaigns targeted sites spanning very dry to dry conditions, dune and interdune settings, and minimal vegetation, allowing us to examine local-scale variability in moderated and bare neutron measurements under low-moisture endmember conditions. We apply state-of-the-art corrections for atmospheric pressure, water vapor, and incoming cosmic-ray intensity, and propagate counting statistics to assess uncertainty at rover-scale and field-scale integration times.

A central motivation for this work is the interpretation of passive neutron data acquired by the Dynamic Albedo of Neutrons (DAN) instrument on the Curiosity rover following the loss of its active pulsed neutron generator. Unlike terrestrial CRNS studies, Mars lacks direct ground-truth soil moisture measurements, and near-surface liquid water or ice is unstable at equatorial latitudes. As a result, the neutron signal is dominated by mineral-bound hydrogen and bulk composition effects. The terrestrial analog sites presented here provide a controlled framework for understanding neutron sensitivity, spatial variability, and correction strategies in similarly dry environments, while leveraging active neutron measurements and in situ sensors on Earth as calibration anchors.

Our results demonstrate that even under extremely dry conditions, corrected neutron counts exhibit measurable spatial and temporal structure, and that uncertainties associated with environmental corrections can be comparable to or exceed those from counting statistics. These findings highlight the value of cross-disciplinary collaboration between planetary science and environmental CRNS communities, and suggest that dry terrestrial analogs can play a key role in improving neutron-based water detection and modeling across Earth and planetary applications.