A study examines angular momentum constraints on the formation and orbital evolution of the Moon. Proposed scenarios for the Moon's formation typically involve a giant impact that would have left the Earth rotating much faster than its present rate. Several mechanisms for subsequent slowing of Earth's rotation have been proposed, including one that involves the postimpact Earth having high obliquity--the angle between the equatorial and orbital planes. ZhenLiang Tian and Jack Wisdom analyzed the high-obliquity scenario in the context of a constraint that has not been previously fully explored. The component of the total angular momentum for the Earth-Moon system that is perpendicular to the Earth's orbital plane (Lz) is conserved throughout the system's history, except during intervals at which specific conditions are present. Therefore, the postimpact Lz should be within a few percent of its present-day value. The authors demonstrated that having a high postimpact obliquity always results in angular momentum much larger than the present-day value. This occurs regardless of the tidal parameters or tidal models used to model the system. The results suggest that the high obliquity scenario is incompatible with the present-day Earth-Moon system. However, a low obliquity scenario could be consistent with the Lz constraint, suggesting a plausible mechanism for how the fast-rotating postimpact Earth could have slowed to its present rotation rate, according to the authors.
Article #20-03496: "Vertical angular momentum constraint on lunar formation and orbital history," by ZhenLiang Tian and Jack Wisdom.
MEDIA CONTACT: Jack Wisdom, Massachusetts Institute of Technology, Cambridge, MA; tel: 617-253-7730; e-mail: wisdom@mit.edu; ZhenLiang Tian, University of California, Santa Cruz, CA; e-mail: zlt@ucsc.edu
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Journal
Proceedings of the National Academy of Sciences