‘Porosity’ of Moon’s crust reveals history of celestial bombardment

The Moon was routinely pummelled by massive asteroids and comets during the creation of the solar system around 4.4 billion years ago and it still get hits by celestial objects today, albeit less frequently.

This period of intense bombardment ended around 3.8 billion years ago, leaving behind a heavily cratered face and a cracked and porous crust.

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. The team found that nearly all the Moon’s porosity formed rapidly with these massive impacts 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 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,” said study co-author Jason Soderblom, 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.”

The researchers looked to trace the Moon’s changing porosity and use those changes below the surface to estimate the number of impacts that occurred on its surface.

“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,” Soderblom said.

“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.”

To trace the evolution of the Moon's porosity, the team looked to measurements taken by Nasa’s Gravity Recovery and Interior Laboratory (GRAIL), an MIT-designed mission that launched twin spacecraft around the Moon to precisely map the surface gravity.

Researchers have converted the mission’s gravity maps into detailed maps of the density of the Moon’s underlying crust. From these density maps, scientists have also been able to map the current-day porosity throughout the lunar crust. These maps show that regions surrounding the youngest craters are highly porous, while less porous regions surround older craters.

The team studied 77 craters in chronological order, based on their previously determined ages. For each crater, the team modelled the amount by which the underlying porosity changed compared to the initial porosity represented by the youngest crater. They assumed a bigger change in porosity was associated with a larger number of impacts and used this correlation to estimate the number of impacts that would have generated each crater’s current-day porosity.

These simulations showed a clear trend. At the start of the lunar heavy bombardment, 4.3 billion years ago, the crust was highly porous; about 20 per cent. Closer to 3.8 billion years ago, the crust became less porous and remains at its current-day porosity of about 10 per cent.

This shift in porosity is likely the result of smaller impactors acting to compact a fractured crust. Judging from this porosity shift, the researchers estimate that the Moon experienced about double the number of small impacts as can be seen on its surface today.