Now MIT scientists have found that the porosity of the moon’s crust, reaching well beneath the surface, can reveal a great deal about the moon’s history of bombardment.

In a study appearing in Nature Geoscience, the team has shown through simulations that, early on in the bombardment period, the moon was highly porous — almost one-third as porous as pumice. This high porosity was likely a result of early, massive impacts that shattered much of the crust.

Scientists have assumed that a continuous onslaught of impacts would slowly build up porosity. But surprisingly, the team found that nearly all the moon’s porosity formed rapidly with these massive imapcts, and that the continued onslaught by smaller impactors actually compacted its surface. These later, smaller impacts acted instead to squeeze and compact some of the moon’s existing cracks and faults.

From their simulations, the researchers also estimated that the moon experienced double the number of impacts as can be seen on the surface. This estimate is lower than what others have assumed.

“Previous estimates put that number much higher, as many as 10 times the impacts as we see on the surface, and we’re predicting there were fewer impacts,” says study co-author Jason Soderblom, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “That matters because that limits the total material that impactors like asteroids and comets brought to the moon and terrestrial bodies, and gives constraints on the formation and evolution of planets throughout the solar system.

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