The largest of these are called impact basins. A new paper argues that these pools were created not by asteroids, but by planetesimals.
Previous studies have shown that main belt asteroids are responsible for impact basins on the Moon.
“It was assumed that the impactors that formed the basin are asteroids released from the inner expansion of the main belt (1.8-2.0 AU),” the article notes.
But the authors of the study believe that most of the impactors were rocky planetesimals left at a distance of ~0.5–1.5 AU. after the accretion of a terrestrial planet.
Scientists have created models to determine the role of planetesimals in the formation of lunar basins.
The models are based on previous accretion studies of terrestrial planets, which show how planetesimals have changed over time due to collisions with other objects.
The researchers also relied on dynamic simulations of asteroids and comets to see what role they played in impacts with the moon.
The authors’ work shows that asteroids have had the most impacts on the Moon in the past 3.5 billion years. But before that, planetesimals had done most of the damage.
“The integrated history of lunar collisions shows that the remaining planetesimals dominated the stream of early collisions,” the researchers said.
The results also show that about 500 planetesimals with a diameter of 20 km collided with the Moon when there was a lunar magma ocean on it, and these impacts did not leave a lasting trace.
The conclusions of scientists are also confirmed by the effects on the Earth. Researchers rely on the study of spherical layers. When the impactors collide with our planet, they create a plume of vaporized rock.
The rock condenses into tiny spherical rocks called spherols that fall back to Earth. They form spherical layers embedded in the rock.
“Our model predicts an impact of ~20 d > 10 km on Earth for T = 2.5–3.5 Ga,” the study says. During this period of time, both the main belt asteroids and the remaining planetesimals hit the Earth.
The scientists note that while asteroid impacts were evenly distributed throughout the Late Archean, nearly all planetesimal impacts must have occurred before 3 billion years ago.
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