Numerous craters have formed on the lunar surface as a result of asteroids’ impacts. Age determinations on lunar rocks generated after these collisions reveal a startling grouping around 3.9 billion years ago, or roughly 500 million years after the Moon was formed. These findings have led to the notion of a Late Heavy Moon Bombardment (or LHB).
But where did the asteroids that hit the Moon come from, and what was the cause of this late bombardment? Scientists are debating two options. One theory is that these bodies are leftover material from Earth’s main creation phase, which has been hitting the Moon with decreasing frequency. Another theory is that instability in the orbits of the gas giant planets caused a rapid rise in asteroids and comets from the outer solar system some 3.9 billion years ago.
These assumptions have been explored by planetary scientists at the University of Münster using extremely precise isotope analysis of lunar rocks generated during the bombardment 3.9 billion years ago. Their findings and conclusions: There was no abrupt spike in the impact rate, and the bombardment of the Moon may be traced back to continuing strikes of asteroids left over from the Earth’s main creation phase.
The findings were published in the current edition of the journal “Science Advances.”
Münster scientists looked at lunar rocks that were created during the 3.9 billion-year-old bombardment. Tiny metal globules made up of material from impactor asteroids can be found in these rocks. Researchers can establish where these metal globules originated in the solar system by analyzing their isotopic makeup. They concentrated on the elements ruthenium and molybdenum since their isotopic compositions differ dramatically depending on where they were produced in the solar system.
“Our research shows that the bombardment of the Moon was by the same bodies that formed the Earth and Moon,” says planetologist and lead author of the study Dr. Emily Worsham.
As a result, the impact craters on the Moon are the result of a constant bombardment by leftover asteroids from the Earth’s main formation phase. This also allows scientists to rule out a rapid increase in the impact rate due to outer solar system bodies bombarding the planet. But where did the grouping of 3.9 billion-year-old ages come from?
“It has previously been suggested that the lunar rocks studied so far are mostly made up of material from a single impact basin – the Mare Imbrium in the north-central Earth-facing side of the Moon,” adds Emily Worsham.
The orbits of the gas giants changed at some point in the solar system’s early history, dispersing a significant number of bodies from the outer solar system inside, colliding with the Earth and the Moon, among others, according to theoretical calculations.
“This event must have taken place earlier than previously thought, because we find no evidence of impacts by asteroids or comets from the outer reaches of the solar system in the lunar rocks,” explains Prof. Dr. Thorsten Kleine.
The shift in the orbits of the gas giant planets most likely occurred during the major creation phase of Earth-like planets – around the first 100 million years of the solar system – which is consistent with recent dynamical models.
“Our study, therefore, also shows that the Earth-like planets incorporated water-rich bodies from the outer solar system relatively early, during their formation, thus creating the conditions for the emergence of life”, adds Thorsten Kleine.
The research was carried out as part of the German Research Foundation-funded Collaborative Research Centre SFB/Transregio 170 “Late accretion onto terrestrial planets.”
Image Credit: Getty
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