About 4.4 billion years ago, the early solar system resembled a game of space rock bouncers, as massive asteroids and comets, and later smaller rocks and galactic debris, pummeled the Moon and other terrestrial bodies. This period ended approximately 3.8 billion years ago. On the Moon, this turbulent time left behind a heavily cratered face and a cracked and porous crust.
Now MIT scientists have discovered that the porosity of the lunar crust, which goes deep below the surface, can reveal a lot about the moon’s bombardment history.
In a study published in Natural science, the team showed through simulations that early in the bombardment period, the Moon was very porous—almost one-third as porous as pumice stone. This high porosity was probably the result of early massive impacts that destroyed much of the crust.
The scientists hypothesized that the continuous pressure of the shocks would slowly create porosity. But surprisingly, the team found that almost all of the moon’s porosity was formed quickly by these massive impacts, and that the continuous onslaught of smaller impactors actually compacted its surface. These later, smaller impacts acted instead to compress and seal some of the Moon’s existing cracks and faults.
From their simulations, the researchers also estimated that the Moon has experienced twice as many impacts as seen on the surface. This estimate is lower than what others have suggested.
“Preliminary estimates indicate that this number is much larger, 10 times the impact we see on the surface, and we predict that the impact was smaller,” says study co-author Jason Soderblom, a research associate in MIT’s Department of Earth, Atmospheric and Planetary Sciences. (EAPS). “This matters because it limits the total amount of material brought to the Moon and terrestrial bodies by impactors such as asteroids and comets, and places constraints on planet formation and evolution throughout the Solar System.”
The lead author of the study is EAPS postdoc Ya Huei Huang, along with collaborators from Purdue University and Auburn University.
In the team’s new study, the researchers sought to track changes in the moon’s porosity and use those changes below the surface to estimate the number of impacts that have occurred on its surface.
“We know that the Moon has been so bombarded that what we see on the surface is no longer a record of all the impacts the Moon has ever had, because at some point the impacts have erased the previous impacts,” Soderbloom says. “We found that the porosity in the crust was not destroyed by the impact, and this may give us a better constraint on the total number of impacts the Moon has experienced.”
To trace the evolution of the moon’s porosity, the team turned to measurements made by NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, a mission designed by MIT that launched twin spacecraft around the moon to accurately map the surface’s gravity.
The researchers converted the mission’s gravity maps into detailed density maps of the crust underlying the moon. Using these density maps, scientists were also able to map the present-day porosity of the entire lunar crust. These maps show that the regions surrounding the youngest craters are highly porous, while the less porous regions surround the older craters.
Chronology of craters
In their new study, Huang, Soderblom and their colleagues sought to model how the Moon’s porosity changed as it was subjected to first large and then smaller impacts. They incorporated the age, size, and location of the 77 largest craters on the moon’s surface into their simulations, as well as GRAIL-derived estimates of the present-day porosity of each crater. The simulations include all known basins, from the oldest to the youngest impact basins on the Moon, ranging in age from 4.3 billion to 3.8 billion years.
For their simulations, the team used the youngest craters with the highest porosities to date as a starting point to map the initial porosity of the Moon during the early stages of the Moon’s heavy bombardment. They argued that the old craters that formed in the early stages were initially very porous, but were subjected to further impacts over time that compacted and reduced their original porosity. In contrast, younger craters, although formed later, would have experienced fewer subsequent impacts. Their core porosity would then be more representative of initial lunar conditions.
“We’re using the youngest pool we have on the moon that hasn’t been impacted too much, and we’re using that as a way to start as the initial conditions,” Huang explains. “We then use the equation to adjust the number of impacts required to go from the original porosity to the more compacted modern porosity of the oldest basins.”
The team studied 77 craters in chronological order based on their previously determined ages. For each crater, the team modeled the amount of change in core porosity compared to the original porosity of the youngest crater. They hypothesized that greater porosity change was associated with more collisions, and used this correlation to estimate the number of collisions that would have resulted in the present-day porosity of each crater.
These simulations showed a clear trend: At the beginning of the heavy bombardment of the Moon, 4.3 billion years ago, the crust was very porous, about 20 percent (by comparison, pumice has a porosity of 60 to 80 percent). Closer to 3.8 billion years ago, the crust became less porous and remained at its current porosity of about 10 percent.
This shift in porosity is likely the result of smaller impactors acting to seal the crustal rift. Judging from this shift in porosity, the researchers estimate that the Moon has experienced about twice as many small impacts as can be seen on its surface today.
“This puts an upper limit on the level of impact across the solar system,” Soderblom says. “Now we also have a new understanding of how impacts regulate the porosity of terrestrial bodies.”
The research was supported in part by NASA.