Not too long after the planets began forming, a Mars-sized object slammed into Earth, creating the debris that would later coalesce into the moon. Some of the debris from this giant impact escaped all the way out to the asteroid belt. Collisions there left shock-heating signatures—a permanent record of the impact event—that can still be detected billions of years later in meteorites that have fallen to Earth.
Planetary scientists, including Professor Ed Scott at the University of Hawaiʻi at Mānoa’s Hawaiʻi Institute for Geophysics and Planetology, have found that a significant number of these altered meteorites have ages clustering at 100 million years after the solar system’s birth—the true age of the moon-forming impact, they say. The result is an independent check on other estimates for the moon’s age, and it suggests that the asteroid belt can provide important clues to the timing and nature of major events in the inner solar system when planets were forming. The team’s work—“Dating the Moon-forming impact event with asteroidal meteorites”—appears in the current issue of the journal Science.
“Meteorites provide an extraordinary record of major events in the formation of the planets and the solar system,” said Scott. “We study meteorites and asteroids as they provide the key to understanding how and when planets formed.”
From Earth to the asteroid belt and back
The team of NASA-funded researchers modeled the evolution of giant impact debris and analyzed ancient impact heating signatures in stony meteorites to conclude that pieces from the giant impact did indeed strike the asteroid belt.
Subsequent, less violent collisions between asteroids have since ejected some shocked remnants back to Earth in the form of fist-sized meteorites. By determining the age of the shock signatures in those meteorites, scientists were able to infer that their origin likely corresponds to the time of the giant impact, and therefore to the age of the Moon.
A window into the “land before time”
Director of NASA’s Solar System Exploration Research Virtual Institute (SSERVI) Yvonne Pendleton notes, “This is an excellent example of the power of multidisciplinary science. By linking studies of the Moon, of main belt asteroids, and of meteorites that fall to Earth, we gain a better understanding of the earliest history of our Solar System.”
“It is even possible that tiny remnants of the Moon-forming impactor or proto-Earth might still be found within meteorites that show signs of shock heating by giant impact debris. This would allow scientists to explore for the first time the unknown primordial nature of our home world,” said lead author of the paper and principal investigator Bill Bottke of the Institute for the Science of Exploring Targets team at the Southwest Research Institute, a U.S. team member of SSERVI.
To watch a model animation of the moon-forming impact, visit www.swri.org/press/2015/moon-forming-impact.htm.
—By Marcie Grabowski