The secret is hydrogen, not the oxygen
A neutron has roughly the same mass as a hydrogen nucleus, a single proton. When a fast neutron strikes a hydrogen atom, it can transfer a large share of its energy in one collision, like one billiard ball hitting another of equal size. Repeated collisions slow the neutron down until it reaches thermal energy, the low speed where it wanders among atoms and is easily captured. Because water is two-thirds hydrogen by atom count and holds it densely, it is an efficient place for that slowing to happen [1][3].
The oxygen in water matters less for this slowing. The evidence points to hydrogen as the key actor: when researchers model how neutrons die away in Martian soil, they specifically need data on how hydrogen bound in liquid water and water ice scatters neutrons, and those details shift the inferred water content [1]. That is a sign that the hydrogen in water, not the molecule as a whole, drives the neutron behavior.
Slowing down is half the job; capture finishes it
Being a good absorber is really two steps: slow the neutron, then capture it. Water excels at the first step. For the second, the capture itself often happens on hydrogen, but other elements can be far more voracious. A detector study notes that chlorine-35 has a thermal-neutron absorption cross section up to two orders of magnitude larger than hydrogen, which is why doping a water detector with salt boosts its capture signal [3]. In plain water, hydrogen does the capturing, but slowly.
This two-step picture explains why water works as a shield and as a detector medium. In a water-filled phantom used for neutron dosimetry, the neutron flux dropped by roughly a factor of forty after passing through the water, from about 4.0 × 10^8 to 1.0 × 10^7 neutrons per square centimeter per second [4]. That is a concrete measure of how effectively a modest thickness of water removes neutrons from a beam.
Why this makes water findable from orbit and on Mars
The same physics runs in reverse when scientists hunt for water. On Mars, the Curiosity rover's DAN instrument fires neutrons into the ground and watches how they die away; water in the soil slows and absorbs them in a recognizable pattern, letting researchers estimate water-equivalent hydrogen [1][2]. Along a 27-kilometer traverse of Gale crater, these measurements showed water-equivalent hydrogen averaging between about 2 and 3 percent by weight in many rock units, and above 3 percent in several later units [2].
The method is sensitive enough that the modeling details matter. Using low-temperature and thermal-scattering data for hydrogen in water and ice generally raised the inferred water content and shifted estimates of other neutron absorbers [1]. That does not mean earlier estimates were wrong; it means the way water's hydrogen scatters neutrons is a real, measurable part of the signal, and getting it right changes the numbers [1].
Water is a great moderator, but not the strongest capturer
It helps to separate two reputations water has. It is an excellent moderator and a practical shield because of its hydrogen density, but it is not the most aggressive neutron capturer per atom. Elements such as chlorine capture thermal neutrons far more strongly [3]. This is why Martian soil studies track water and chlorine as separate neutron-relevant ingredients rather than treating water as the whole story [2][5].
In the Greenheugh area of Gale crater, the DAN instrument measured both water-equivalent hydrogen and absorption-equivalent chlorine in the shallow subsurface, about 50 centimeters down [5]. The fact that both are reported separately reflects the underlying physics: water's hydrogen shapes how neutrons slow, while other elements like chlorine shape how they are finally captured. Water earns its reputation mainly by being dense in hydrogen and very good at putting the brakes on neutrons.
Sources used in this answer
The Impact of Thermal Neutron Scattering and Temperature Variations on Martian Water Content Estimates with Dynamic Albedo of Neutrons
Modeling Martian soil neutron die-away with low-temperature and thermal-scattering data for hydrogen in water and ice generally raises inferred water-equivalent hydrogen and shifts estimates of neutron absorbers.
Water and Chlorine in the Martian Subsurface Along the 27 km Traverse of NASA's Curiosity Rover According to DAN Measurements: 2. Results for Distinct Geological Regions
DAN measurements along a 27-km Gale crater traverse show water-equivalent hydrogen averaging about 2–3 wt.% in many units and above 3 wt.% in several later units, with absorption-equivalent chlorine near 1% throughout.
Enhanced water Cherenkov detector for soil moisture detection
A NaCl-doped water Cherenkov detector can measure thermal neutrons for soil-moisture sensing, and chlorine-35 has a thermal-neutron absorption cross section up to two orders of magnitude larger than hydrogen.
Determination of the absorbed dose of epithermal neutrons in a phantom object filled with water
In a water-filled phantom, epithermal neutron flux fell from about 4.0 × 10^8 to 1.0 × 10^7 n·cm⁻²·s⁻¹ after transmission, showing water's strong attenuation of neutrons.
STUDY OF WATER AND CHLORINE CONCENTRATIONS USING DAN INSTRUMENT IN THE GREENHEUGH AREA OF GALE CRATER
DAN measurements in the Greenheugh area of Gale crater assess water-equivalent hydrogen and absorption-equivalent chlorine in the shallow subsurface at about 50 cm depth.