Planetologists investigate origin of heavy bombardment of the moon 3.9 billion years ago

Apollo sample 60335 with visible globules of metal: These metal globules are from asteroids which have struck the moon. Credit: AG Kleine
The moon was subject to heavy asteroids bombardment 3.9 billion years ago. However, the origin of this bombardment was not known. These hypotheses have been tested by Munster University's planetologists using precise isotope measurements from lunar rocks. Their conclusion: The moon's bombardment is due to the continuous impacts of asteroids that have remained from the main phase in Earth's formation. These results are published in Science Advances' current issue.

Numerous craters are visible on the lunar surface that date back to asteroids' impacts. Age determinations of lunar rocks that were formed during these impacts reveal a surprising clustering around ages of approximately 3.9 billion years ago, which is about 500 million years after the formation date. These observations led to the hypothesis of a Late Heavy Bombardment of the Moon, or LHB.

What was the source of the late bombardment? And where did the asteroids which hit the moon come form? Scientists discuss two possibilities. Scientists discuss two possibilities. One, these bodies could be leftover material from Earth's main phase that struck the moon with decreasing frequency. Another theory is that around 3.9 billion years ago, instability in the orbits gas giant planets caused a sudden increase in impacts from comets and asteroids from the outer solar systems.

Munster scientists examined lunar rocks formed by the bombardment that occurred 3.9 billion years back. These rocks contain small metal globules made from material from impactor asteroids. Researchers were able to determine the origin of these bodies by studying their isotopic composition. Because these elements have a pattern of changes in their isotopic compositions depending on where they were created in the solar system, researchers focused on molybdenum and ruthenium.

Dr. Emily Worsham, a planetologist and the lead author of the study, says that "our research shows that the bombardment on the moon was by a similar bodies that formed the Earth and Moon." Therefore, the impact craters found on the moon are the result of a constant bombardment of asteroids leftover from the main phase in the earth's creation. Scientists can also rule out an increase in impact rates due to bombardment from bodies outside the solar system. Then, where did the clustering that lasted 3.9 billion years occur? Emily Worsham explains that it has been suggested previously that most of the lunar rocks examined so far were made of material from one impact basin, the Mare Imbrium on the north-central Earth facing side of the moon.

Theoretical calculations have shown that the orbits and positions of the gas giants altered at some time in the history of the solar systems. This caused a scattering of large numbers of bodies from the outer solarsystem inwards which in turn led to collisions with the earth, the moon, and others. Professor Dr. Thorsten KLEINE explains that this event must have occurred earlier than previously believed. We don't find any evidence of comets or asteroids impacting the outer reaches in the lunar rocks. According to recent dynamical models, the change in orbits of gas giant planets probably occurred during the main formation phase for Earth-like planets. This is roughly the first 100 million years of our solar system.

Thorsten Kleine adds, "Our study also shows that Earth-like planets incorporated water rich bodies from the outer Solar System relatively early in their formation, thus creating conditions for the emergence and sustaining of life."

Continue reading: Studying the oldest geologic imprints of the moon

Further information: Emily A. Worsham and colleagues, Late accretionary histories of Earth, Moon, and Moon, Science Advances (2021). Information from Science Advances Emily A. Worsham and colleagues, Late accretionary histories of Earth, Moon, and lunar impactites (2021). DOI: 10.1126/sciadv.abh2837

Westfalische Wilhelms University Munster.