Scientists think they've pieced together how many impacts the celestial body has sustained over its lifetime.
Some of the moon's many craters, as photographed from the Apollo 8 spacecraft. The large crater in the foreground is Goclenius, and it has a diameter of 40 miles.
You can see them with the naked eye: massive pockmarks that scar the surface of the moon. Magnify your vision with binoculars or a telescope and those round blemishes multiply rapidly. The moon is covered by impact craters large and small, evidence of a violent history of bombardment by celestial bodies like comets and asteroids.
To determine the lunar crust's porosity, researchers analyzed the 77 largest craters on the moon, comparing their age, size, location and porosity, creating simulations to see how these impacts might have altered the crust's porosity over time. "We use the youngest basin that we have on the moon, that hasn't been subject to too many impacts, and use that as a way to start as initial conditions," Ya Huei Huan, the study's lead author and an MIT postdoc, said in the statement."We then use an equation to tune the number of impacts needed to get from that initial porosity to the more compacted, present-day porosity of the oldest basins.
The simulations indicated that in order to get to its present-day state, a much younger moon had a crust that was 20% porous. Then it was subjected to a period of intense bombardment — a period called lunar heavy bombardment, which occurred between 4.3 billion and 3.8 billion years ago. As older craters were pummeled by new impacts, the lunar surface became more compact. Today, the lunar surface is only about 10% porous.
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