Martian Proton Albedo as Signature of Near-Surface Water

Understanding the Martian soil water budget is crucial not only for in situ resource utilization in future human missions to Mars, but also for reconstructing the geological and climatic history of the planet, as well as to assess the potential of ancient or even present microbial life. Here, we present a methodology to study near-surface water using albedo protons, based on measurements from the Radiation Assessment Detector (RAD) onboard the Mars Science Laboratory (MSL). With this approach, water can be investigated beneath dust layers at approximately 3–7 cm, representing a new observational depth range compared to existing methods. In combination with data from the Dynamic Albedo of Neutrons (DAN) experiment, also part of MSL, we show that MSL/RAD has so far been unable to resolve small variations in regolith water of 2–7 %. However, supporting simulations suggest that larger water reservoirs, such as those at higher latitudes or locally near the equator, may be detected by MSL/RAD with measurement times of approximately one to two months. We demonstrate that a future Mars detector, specifically redesigned to measure albedo protons, could detect changes in near-surface water content of about 20 % within roughly 5–17 days, and variations exceeding 40 % within only a few days, with statistical significance. We therefore propose including albedo proton measurements in future missions to Mars or other extraterrestrial bodies, as they represent a promising complement to existing methods for probing near-surface water.