About 4.4 billion years ago, the early solar system resembled a game of space rock dodgeball, as massive asteroids and comets, and later smaller rocks and galactic debris, pounded the moon and other young Earth bodies. This period ended about 3.8 billion years ago. On the moon, this tumultuous time left behind a heavily cratered face and a cracked and porous crust.
Now, MIT scientists have found that the porosity of the lunar crust, which extends far below the surface, can reveal a lot about the history of the moon’s bombardment.
In a study published today in Nature Geoscience, the team used simulations to show that early in the bombing period, the moon was highly porous — nearly a third as porous as pumice. This high porosity was likely the result of early, massive impacts that shattered much of the crust.
Scientists assumed that a continuous onslaught of impacts would slowly build up porosity. But surprisingly, the team found that almost all of the moon’s porosity formed quickly with these massive impacts, and that the continued attack from smaller impactors actually compacted its surface. These later, smaller impacts instead worked to compress and densify some of the moon’s existing fissures and fractures.
Based on their simulations, the researchers also estimate that the moon has undergone double the number of impacts seen on its surface. This estimate is lower than what others have assumed.
“Previous estimates put that number much higher, as much as 10 times the impacts we see on the surface, and we predict there were fewer impacts,” said study co-author Jason Soderblom, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “That matters because it limits the total material that impactors like asteroids and comets have brought to the moon and terrestrial bodies, and puts limits on the formation and evolution of planets throughout the solar system.”
The study’s lead author is EAPS postdoc Ya Huei Huang, along with collaborators from Purdue University and Auburn University.
A porous plate
In the team’s new study, the researchers sought to trace the moon’s changing porosity and use those changes below its surface to estimate the number of effects on the surface.
“We know that the moon was bombed in such a way that what we see on the surface is no longer a record of every impact the moon has ever had, because at some point previous impacts obliterated previous impacts,” Soderblom said. “What we’re finding is that the way impacts caused the porosity in the crust isn’t destroyed, and that may give us a better limit on the total number of impacts the moon was subject to.”
To trace the evolution of the moon’s porosity, the team looked at measurements from NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, an MIT-designed mission that launched two spacecraft around the moon to accurately map the surface gravity. bring.
Researchers converted the mission’s gravity maps into detailed maps of the density of the moon’s underlying crust. Based on these density maps, scientists have also been able to map the current porosity in the entire lunar crust. These maps show that regions around the youngest craters are highly porous, while less porous regions surround older craters.
In their new study, Huang, Soderblom and their colleagues attempted to simulate how the moon’s porosity changed when it was bombarded with first large and then smaller impacts. They included in their simulation the age, size and location of the 77 largest craters on the lunar surface, along with GRAIL-derived estimates of each crater’s current porosity. The simulation includes all known basins, from the oldest to the youngest impact basins on the Moon, and spans ages between 4.3 billion and 3.8 billion years old.
For their simulations, the team used the youngest craters with the highest current porosity as a starting point to represent the moon’s initial porosity in the early stages of the lunar heavy bombardment. They reasoned that older craters that formed in the early stages would have started out highly porous, but over time would have been exposed to further impacts that condensed and reduced their initial porosity. In contrast, younger craters, although formed later, would have experienced fewer or no subsequent impacts. Their underlying porosity would then be more representative of the moon’s initial conditions.
“We’re using the youngest basin we have on the moon, which hasn’t undergone too many shocks, and use that as a way to start as initial conditions,” explains Huang. “We then use an equation to tune the number of impacts it takes to get from that initial porosity to the more compact, current porosity of the oldest basins.”
The team studied the 77 craters in chronological order, based on their previously determined ages. For each crater, the team modeled the amount by which the underlying porosity changed compared to the initial porosity represented by the youngest crater. They assumed that a greater change in porosity was associated with a greater number of impacts, and used this correlation to estimate the number of impacts that each crater’s current porosity would have generated.
These simulations showed a clear trend: At the beginning of the heavy moon bombardment, 4.3 billion years ago, the crust was very porous – about 20 percent (for comparison, the porosity of pumice is about 60 to 80 percent). Closer to 3.8 billion years ago, the crust became less porous, remaining at its current porosity of about 10 percent.
This shift in porosity is likely the result of smaller impactors compacting a fractured crust. Judging by this porosity shift, the researchers estimate that the moon has undergone about double the number of small impacts seen on its surface today.
“This puts an upper limit on impact velocities across the entire solar system,” Soderblom says. “We also now have a new appreciation for how impact determines the porosity of terrestrial bodies.”
This research was supported in part by NASA.
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