UNITED STATES: The early solar system resembled a game of space rock dodgeball around 4.4 billion years ago as the moon, and other developing terrestrial bodies were battered by giant asteroids and comets, followed by smaller pebbles and galactic junk. This time span came to an end 3.8 billion years ago. This turbulent period left the moon with a highly cratered face and a broken, permeable crust.
MIT researchers have discovered that the moon’s crust’s porosity, which extends far below the surface, can provide a wealth of information on the moon’s bombardment history.
In a research published in Nature Geoscience, the team used simulations to demonstrate that the moon was highly porous early in the bombardment period—almost one-third as pumice. The early, powerful impacts that fractured much of the crust are probably to blame for this high porosity.
Scientists predicted that a steady stream of strikes would gradually increase porosity. Surprisingly, the scientists discovered that almost all of the moon’s porosity developed quickly with these huge impacts and that the subsequent onslaught of smaller impactors compacted the moon’s surface. Instead, some of the moon’s existing fractures and cracks were squeezed and compacted by these later, smaller impacts.
The scientists’ calculations also suggested that the moon had twice as many impacts as are visible on its surface. This estimate is lower than previous estimates.
A spotty record
The team’s most recent research goal was to track the moon’s changing porosity and calculate the number of impacts on its surface using changes below the moon’s surface.
The scientists used observations from NASA’s Gravity Recovery and Interior Laboratory, or GRAIL. This MIT-designed mission flew twin spacecraft around the moon to precisely monitor the surface gravity, to chart the evolution of the moon’s porosity.
Researchers have created comprehensive maps of the moon’s underlying crust density using the mission’s gravity measurements. Scientists have also been able to map the current porosity of the lunar crust using these density maps. These maps reveal that areas next to the youngest craters are quite porous, whereas areas close to older craters are less porous.
Crater timeline
In their latest work, Huang, Soderblom, and his associates replicated how the moon’s porosity was altered when it was subjected to successive bombardments of larger and smaller impacts. In their simulation, GRAIL-derived estimations of each crater’s present-day porosity included the age, size, and locations of the 77 biggest craters on the moon’s surface. The simulation spans ages between 4.3 billion and 3.8 billion years old and contains all known basins on the moon, from the oldest to the youngest impact basins.
To simulate the original porosity of the moon during the early stages of the heavy lunar bombardment, the researchers started with the craters with the highest current-day porosity, which are the youngest. They reasoned that older craters that formed in the beginning would have created highly porous but would have been exposed to more impacts over time that compacted and lowered their initial porosity. While younger craters developed later, they would have been subject to fewer other impacts. The underlying porosity would then be closer to the primordial conditions of the moon.
Based on the 77 craters’ previously established ages, the researchers investigated them in chronological order. The scientists calculated the change in underlying porosity for each crater relative to the starting porosity indicated by the youngest crater. They used this correlation to estimate the number of hits that would have produced each crater’s current porosity. They assumed that a larger change in porosity was linked to a more significant number of impacts.
These models revealed a definite pattern: the crust was quite porous during the beginning of the intense lunar bombardment, 4.3 billion years ago—roughly 20%. (by comparison, the porosity of pumice is about 60 to 80 per cent). The crust lost some of its porosity around 3.8 billion years ago and has since maintained its current porosity of 10%.
Journal Information: http://www.nature.com/articles/s41561-022-00969-4
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