To the Editor —

We disagree with the conclusion by Cooper et al.1 that stored lunar soils from the Apollo missions have significantly degraded over time. Specifically, they directly compared particle size distributions of lunar soils obtained by two different techniques — a wet-sieving technique applied several decades ago versus a recent laser-diffraction technique applied to a selection of the same soils. They observed differences in the particle size distributions that led them to infer degradation of lunar soils over time. However, we argue that a direct comparison of the lunar soil analyses obtained from these two measurement techniques is not valid.

Significant differences between the two techniques have been documented when applied to terrestrial soils2. Although Cooper et al. compared the techniques on a synthetic soil designed to be lunar-like3, this synthetic soil does not have the complexity of a space-weathered lunar soil or even a terrestrial-weathered soil on Earth4. We argue that the observed differences in particle size distributions result from differences between the two techniques, including the use of different carrier fluids (alcohol and water), as well as sample-size and sample-integrity biases. Thus, we claim that the Apollo era lunar soil samples have not degraded as reported.