The Lunar Cataclysm
During Earth’s earliest days, our planet and others in the inner solar system, including the Moon, experienced repeated impacts from debris that formed the building blocks of the planets. Over time, as material was swept up and incorporated into the inner planets, the rate of impacts decreased. Then, roughly 4 billion years ago, a second wave of impacts appears to have taken place, with lunar projectiles hitting at much higher speeds. This increase could reflect the origin of the debris, where main belt asteroids were dislodged and sent into the inner solar system by shifts in the orbits of the giant planets. Understanding the history of impacts, and the role they played in the formation of the inner planets, can help astrobiologists understand the processes that lead to the creation of small, habitable worlds like Earth.
The team is composed of Simone Marchi, an NLSI postdoctoral fellow, William Bottke, the NLSI Team Lead at Southwest Research Institute, Boulder, Colo., David Kring, the NLSI Team Lead at the Universities Space Research Association (USRA) Lunar and Planetary Institute in Houston, and Alessandro Morbidelli from the Observatoire de la Cote d’Azur, France. Their research paper, “On the Onset of the Lunar Cataclysm as Recorded in its Ancient Crater Populations,” was recently published in the journal Earth and Planetary Science Letters.
The increase in velocities may indicate a change in the solar system when the craters were created. The analysis supports the “lunar cataclysm” hypothesis that the brief pulse of impacting objects 4 billion years ago was due to gravitational disturbances caused by the reorganization of the giant planets as their orbits changed. Nectaris, a crater close to the Apollo 16 landing site, appears to have recorded the spike in asteroid impacts during the “lunar cataclysm.”
Determining the magnitude and duration of any impact cataclysm and testing that hypothesis is a top science priority for future exploration of the Moon, according to a previously published report by the National Research Council.
“This is an exciting time for lunar research with LRO and other spacecraft providing so much new data,” said lead author Simone Marchi. “Collaborating with scientists of different disciplines allowed us to link these observational data to dynamical models to put new constraints on solar system history.”
The inferred increase in velocity seems to have occurred after the Moon’s largest impact basin was produced, the 1,550 mile (2,500 kilometer) diameter South Pole-Aitken Basin, but before the formation of the largest lava-filled impact basins on the lunar nearside, visible from backyards around the world.
“It is fascinating that the surface of our own Moon records evidence of orbital changes in Jupiter and Saturn that took place so long ago,” said NLSI Director Yvonne Pendleton.