A shift in Jupiter’s orbit early in the life of the Solar System dislodged thousands of rocks from the Asteroid Belt, causing them to hit the inner planets, including Earth.
Evidence for this cataclysmic bombardment comes from a reanalysis of lunar rocks brought back by the Apollo astronauts and a careful study of lunar craters, said David Kring, a planetary geologist at the Lunar and Planetary Institute in Houston, Texas.
Kring presented his findings this week at the Geological Society of America’s annual meeting in Portland, USA.
Popularly known as the Late Heavy Bombardment, the cataclysm occurred during a relatively short period of time – just 100 million years or so – representing a spike in the number of large objects hitting the Earth, Moon, Mercury, Mars, and Venus.
“Samples taken from the Apollo mission showed that a large number of impact melts (lavas created by impacts) were generated at the same time,” said Kring.
Moon rocks held in storage for more than four decades were restudied with 21st-century technologies to better date impact events during the cataclysm. “We now understand impact rocks better,” said Kring. “Forty years ago, when we went to the Moon, we didn’t know whether all the circular features were volcanic or impact craters. Until we got those samples [from Apollo], we didn’t have any experience with impact craters.”
These techniques allow radiometric age dating, such as argon dating, with smaller samples than before, effectively increasing the number of possible tests. These tests (and others on asteroid fragments that have fallen to Earth as meteors) revealed that the bombardment came from asteroids, not comets as originally thought.
This is significant because an asteroid bombardment suggests an inner Solar System cataclysm and not something that came from beyond the asteroid belt. “Geochemical fingerprints point to asteroids,” said Kring.
The scientists also calculated the size of the asteroids that formed lunar craters known to be from the Late Heavy Bombardment, which occurred approximately 3.9 billion years ago.
They then compared these to the size of the asteroids in the asteroid belt. The conclusion: “The size distribution of craters on the Moon suggests the impacting objects came from the Asteroid Belt,” said Kring.
Models suggest a change in Jupiter’s orbit would generate the resonances needed to knock the asteroids off-course. “We now have increasing evidence in support of the hypothesis that the giant planets’ orbits have shifted over their history,” said Renu Malhotra, a planetary scientist at the University of Arizona in Tucson.
Adds Kring: “The data tells us that asteroids were the dominant source. And for the asteroids to move, Jupiter would have had to have moved.”