ABSTRACT

The origin of the Moon is one of the oldest questions in planetary science. Nearly all recent work on lunar origin has focused on the "giant impact" theory, which proposes that the collision of a planet-sized body with the forming Earth produced a disk of debris orbiting the Earth that later accumulated into the Moon. The impact theory is strongly favored because it provides a natural explanation for many properties of the Earth-Moon system, including the Moon's lack of iron and the early Earth's rapid rotation rate. But there remains a major unresolved problem. Most impacts produce a disk that originates primarily from the impacting planet, and would as such most naturally produce a Moon whose composition was measurably different from that of the Earth. Instead, the Earth and Moon are now known to have essentially identical isotopic compositions across a variety of elements. New impact scenarios have been proposed to explain how the merger of two separately formed planets might have ultimately produced an Earth and Moon with equal isotopic compositions. But all involve what appear to be rather unlikely events, suggesting that the Moon-forming event was a rare occurrence or that we are misinterpreting key constraints.

About the Speaker:

Dr. Robin Canup is Associate Vice President of the Planetary Science Directorate at Southwest Research Institute in Boulder, Colorado. She received her BS in physics from Duke University and her PhD in astrophysics from the University of Colorado in Boulder. Her work focuses on the dynamics of planet and satellite formation, including the development of leading models for the origin of satellites of solid and gaseous planets. She is the recipient of the Urey prize of the Division of Planetary Sciences and the Macelwane medal of the American Geophysical Union, and is a member of the National Academy of Sciences and the American Academy of Arts and Sciences.