Moon rock trait offers new way to understand impact history

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. 

But these craters only record some of the moon's history. 

A team of researchers from the Massachusetts Institute of Technology (MIT) has pieced together possible impact scenarios, tracing changes in how compact the moon's surface has been over the ages to estimate the barrage it took. The findings suggest that the moon likely sustained at least double the number of impacts as there are craters currently on its surface.

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"We know the moon was so bombarded that what we see on the surface is no longer a record of every impact the moon has ever had, because at some point, impacts were erasing previous impacts," Jason Soderblom, a planetary scientist at MIT and co-author of the study, said in a statement.

In order to "see" the previous craters, so to speak, the researchers turned to the porosity of the moon's crust — how loosely packed the material is — for clues. "What we're finding is that the way impacts created porosity in the crust is not destroyed, and that can give us a better constraint on the total number of impacts that the moon was subject to," said Soderblom.

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.

Through these simulations, the team has estimated the total number of impacts to be roughly double the number of craters that exist on its surface today, which is a rather conservative estimate.

"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," Soderblom said. "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."

The study was published Thursday (July 7) in the journal Nature Geoscience.

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