Some of the moon soil collected by Apollo astronauts has deteriorated significantly during its four-plus decades on Earth, a new study reports.

Scientists found that the median particle size in a set of 20 different Apollo soil samples held in laboratories for research use has decreased by more than half since the samples were first measured 40 years ago.

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The 20 soil samples Cooper and her team looked at belonged to this latter group. Between 2007 and 2012, they used laser-diffraction techniques to measure particle sizes in the samples, which were provided by JSC and the University of Tennessee in Knoxville. The researchers compared their results with the original soil measurements, which were made in the 1970s using sieves.

The differences between the two datasets are stark. For example, the median particle diameter has decreased from 78 microns (0.0031 inches) to 33 microns (0.0013 inches). And in the original sieve data, 44 percent of soil particles were between 90 and 1,000 microns (0.0035 to 0.039 inches) wide; today, just 17 percent of the particles are that large.

The most likely explanation for the degradation is damage caused by water vapor, the scientists say.

"Leaching by water vapor causes the specific surface area of a lunar soil sample to multiply, and a system of pores develops," they wrote in the study, which was published online last week in the journal Nature Geoscience. "These structural changes may be attributed to the opening of existing, but previously unavailable, pore structure or the creation of new surfaces through fracturing of cement or dissolution of amorphous particles."