Madrid. Scientists at the Massachusetts Institute of Technology (MIT) discovered that the porosity of the lunar crust, which reaches far below the surface, can reveal much about the history of the bombardment of the Moon.
In a study published in Nature Geoscience, they showed through simulations that, at the beginning of the bombardment period, the Moon was very porous, almost a third of what pumice is. This elevated drilling was probably the result of the first massive impacts that shattered much of the crust.
About 4.4 billion years ago massive asteroids and comets and, later, smaller rocks and galactic debris hit the Moon and other infant terrestrial bodies. This period ended about 3.8 billion years ago. On Earth’s natural satellite, that tumultuous time left behind a heavily cratered face and cracked, porous crust.
Scientists assumed that a continuous avalanche of impacts would slowly increase the porosity. But surprisingly, the team found that almost all of it formed very quickly with these massive impacts, and that continued onslaught from smaller objects actually compacted its surface. These later, smaller, violent shocks acted instead to squeeze and compact some of the existing cracks and faults in the satellite.
From their simulations, the researchers also estimated that the satellite experienced twice as many impacts as can be seen on the surface. This estimate is lower than what others have assumed.
“Previous calculations put that number much higher, up to 10 times the impacts that we see on the surface, and we are predicting that there were fewer impacts. This is important because it limits the total material that impactors such as asteroids and comets brought to the Moon and to terrestrial bodies, and places constraints on the formation and evolution of planets throughout the solar system,” the co-author explained in a statement. of the study Jason Soderblom, a research scientist in the Department of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT.
In the team’s new study, the researchers tried to track the changing porosity of the Moon and use those modifications below the surface to estimate the number of impacts that occurred on its surface.
“We know that it has been so heavily bombarded that what we see on the surface is no longer a record of all the impacts it has had, because at some point, the previous ones were erased. What we’re finding is that the way the shocks created the porosity in the crust is not destroyed, and that may give us a better constraint on the total number of shocks the Moon was subjected to.”
To track the evolution of the satellite’s porosity, the team turned to measurements made by NASA’s Gravity and Interior Recovery Laboratory, or Grail, an MIT-designed mission that launched two spacecraft around the Moon. to draw an accurate map of surface gravity.
The researchers have turned those maps into detailed maps of the density of the underlying lunar crust. From them, scientists have also been able to map the current porosity. These show that the regions surrounding the younger craters are highly porous, while less porous areas surround the older craters.
In their new study, Soderblom, Ya Huei Huang, an EAPS postdoc and the study’s lead author, and collaborators at Purdue and Auburn Universities tried to simulate how porosity changed as it received impacts, first large and small. then smaller. They included in their work 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. It includes all known basins, from the oldest to the youngest, and spans ages between 4.3 billion and 3.8 billion years.
For their simulations, the team used the youngest craters with the highest current porosity as a starting point to represent the initial one in the early stages of heavy bombardment. They thought that the older craters that formed in the early stages would have started out very perforated, but would have been exposed to more impacts over time that compacted and reduced their initial porosity. In contrast, younger craters, although formed later, would have experienced fewer, if any, later impacts. Its underlying porosity would then be more representative of the initial conditions of the satellite.
“We used the youngest basin that we have on the Moon, which hasn’t been subjected to too many impacts, and we used that as a starting point as initial conditions. We then used an equation to fine-tune the number of impacts needed to go from the initial to the current, more compacted porosity of the older 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 one represented by the youngest crater. They assumed that a larger change in borehole was associated with a larger number of impacts, and used this correlation to estimate the number of shocks that would have generated the current porosity of each.
These simulations showed 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 (compared to pumice, it’s 60 to 80 percent). ). About 3.8 billion years ago, the crust became less porous, and remains at its current perforation of about 10 percent.
This change is likely the result of colliding smaller objects acting to compact a fractured crust. Judging from this change in porosity, the researchers estimate that the Moon experienced about twice as many small impacts as can be seen on its surface today.
“This puts an upper limit on impact rates throughout the solar system. We also now have a new appreciation for how impacts govern the porosity of terrestrial bodies.”
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