The porosity of the moon's crust reveals the history of bombardment
About 4.4 billion years ago, the early solar system resembled a game of space rock dodgeball, such as large asteroids and comets, and, later, smaller rocks and galactic debris hit the moon and other terrestrial baby bodies. This period ended about 3.8 billion years ago. On the moon, this turbulent time leaves a heavily cratered surface, and a cracked and porous crust.
Now MIT scientists have discovered that the porosity of the moon’s crust, which reaches deep below the surface, can reveal a lot about the history of lunar bombardment.
In a study appearing today in Nature Geoscience, the team has shown through simulations that, at the start of the bombardment period, the moon was highly porous — nearly a third as porous as pumice. This high porosity is likely the result of an early and massive impact that destroyed much of the crust.
Scientists assume that a continuous onslaught of impacts will slowly build up porosity. Surprisingly, however, the team found that nearly all of the moon’s porosity formed rapidly with this massive impact, and the continuous strikes by smaller impactors actually solidified its surface. These smaller collisions then act to compress and compact some of the moon’s existing fissures and fractures.
From their simulations, the researchers also estimated that the moon experienced twice the number of collisions seen on the surface. This estimate is lower than what others have assumed.
“Previous estimates put that number much higher, as much as 10 times the impact we see on the surface, and we expect there to be much less impact,” said study co-author Jason Soderblom, a research scientist in MIT’s Earth Department. , Atmospheric and Planetary Science (EAPS). “That’s important because it limits the total material carried by impactors such as asteroids and comets to the moon and terrestrial bodies, and places a limit 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 at Purdue University and Auburn University.
A porous note
In the team’s new study, the researchers looked to track the moon’s changing porosity and use those changes beneath the surface to estimate the amount of impact it had on its surface.
“We know the moon is 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, the impact erases the previous impact,” Soderblom said. “What we found was that the way the impact created porosity in the Earth’s crust didn’t crumble, and that could give us a better constraint on the total number of impacts the moon was subjected to.”
To track the evolution of lunar porosity, the team looked at measurements taken by NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, the MIT-designed mission that launches twin spacecraft around the moon to precisely map surface gravity.
Researchers have turned the mission’s gravity map into a detailed map of the density of the moon’s base crust. From this density map, scientists can also map the current porosity across the moon’s crust. These maps show that the area around the youngest craters is highly porous, while the less porous areas surround the older craters.
Crater chronology
In their new study, Huang, Soderblom and their colleagues looked to simulate how the moon’s porosity changed when it was bombarded with first large impacts and then smaller ones. They included in their simulation the age, size, and location of the 77 largest craters on the lunar surface, along with GRAIL-derived current porosity estimates for each crater. The simulation covers all known basins, from the oldest to youngest impact basins on the moon, and between 4.3 billion and 3.8 billion years old.
For their simulation, the team used the youngest crater with the current highest porosity as a starting point to represent the early lunar porosity in the early stages of lunar heavy bombardment. They reasoned that the older craters that formed in the early stages would have started out very porous but would have been further impacted over time compacting and reducing their initial porosity. In contrast, younger craters, although formed later, will experience less if there is a subsequent impact. The underlying porosity will then be more representative of early month conditions.
“We used the youngest basins we have on the moon, which haven’t been affected too much, and used that as a way to start as initial conditions,” Huang explained. “We then used the equations to adjust the amount of impact needed to get from that initial porosity to the denser, current porosity of the oldest basin.”
The team studied 77 craters in chronological order, based on a predetermined age. For each crater, the team modeled the amount of change in the underlying porosity 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 would result in each crater’s current porosity.
These simulations show a clear trend: At the start of the heavy bombardment of the moon, 4.3 billion years ago, the crust was very porous – about 20 percent (by comparison, the porosity of pumice was about 60 to 80 percent). Approaching 3.8 billion years ago, the crust became less porous, and remains at its current porosity of about 10 percent.
This shift in porosity is likely the result of smaller impactors working to compact the cracked crust. Judging by this shift in porosity, the researchers estimate that the moon experienced about twice the number of small collisions seen on its surface today.
“This puts an upper bound on the rate of impact in the solar system,” said Soderblom. “We also now have a new appreciation of how impact regulates terrestrial body porosity.”
This research was supported, in part, by NASA.
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