Analysis of the porosity of the lunar crust has enabled a more accurate assessment of the history of meteorite bombardment of our satellite.
The study was conducted by scientists from the Massachusetts Institute of Technology (USA). They used data from the GRAIL space mission to create a density map of the lunar crust. And with the help of computer simulation they recreated how the density changed throughout the history of the Moon. According to the new model, five times fewer meteorites have fallen on the satellite than some previous estimates.
By the abundance of craters on the Moon, one would think that the entire history of meteorite impacts is 'recorded' on the surface. In fact, in the turbulent first hundred million years after the formation of the bodies of the solar system, there were so many collisions that craters overlapped each other. The heavy bombardment lasted from 4.4 to 3.8 billion years ago.
"We know that so many meteorites fell on the Moon that what we see on the surface no longer reflects the entire history of the falls, because at some point new collisions began to erase old ones," explains Jason Soderblom, one of the authors. - We found that the crustal porosity created by these collisions is preserved and this allows us to estimate the total number of meteorite impacts much more accurately.
Scientists believed that all impacts necessarily increased the porosity of the crust. Computer simulations have shown that this is not the case. Almost all of the "porosity" of the Moon was formed during the first massive impacts. And further small meteorites compressed the fractures and cracks that were once formed.
In the model, the team took into account the age, size and location of the 77 largest craters on the lunar surface as well as their present-day porosity, compiled from surface gravity data from the GRAIL mission. The authors selected the youngest crater, which had not been affected by further impact, as the benchmark for the moon's initial density. They then calculated how many collisions were required to reach the density of the remaining, older craters.
The results are interesting. It turns out that at the beginning of the late heavy bombardment period the moon had a porosity of about 20% (compared with 60-80% for pumice). By the end of the period it had declined to the present 10% due to small meteorite impacts.
As the model showed, this reduction in porosity required about twice as many small meteorites as craters on the surface of the modern moon. "According to previous estimates, there could have been many more, even ten times more than the craters on the surface, and we predict fewer collisions," Soderblom added.
Such an estimate is important not only for the Moon, but for all bodies in our system. The amount of additional matter and various compounds that arrive on the Earth's satellite and other bodies after they have formed depends on the number of meteorites that have fallen. This, in turn, affects the formation and evolutionary history of the planets in the solar system.