Title
Rewriting the Origin of the Moon: How Synestias Explain What the Canonical Model Cannot
Abstract
A central challenge in planetary science is understanding planetary accretion and the remarkable chemical and isotopic diversity observed among planets. Giant impacts are now recognized as a universal feature of planet formation, yet the traditional “canonical” Moon forming impact model struggles to reproduce several defining properties of the Earth–Moon system. In particular, canonical simulations predict a lunar disk dominated by impactor material, a result incompatible with the near identical Earth–Moon isotopic compositions and with the Moon’s characteristic depletion in moderately volatile elements.
To resolve these inconsistencies, we developed a new high energy Moon formation scenario in which the Moon forms inside an impact generated structure known as a synestia. A synestia is a rapidly evolving, vapor rich planetary body. In this environment, the post impact Earth and the surrounding disk become thoroughly mixed and largely vaporized. As the synestia cools, condensates form within a chemically homogeneous silicate vapor, enabling the Moon to accrete while immersed in this gas. This process naturally imprints the Moon with the observed pattern of moderate volatile depletion and explains why the Earth and Moon share indistinguishable isotopic signatures across multiple systems.
Our new high precision Ca isotope measurements further reinforce this picture. Lunar rocks and minerals exhibit mass dependent Ca isotope compositions identical to those of the bulk silicate Earth. Modeling of the lunar magma ocean confirms that such isotopic equivalence requires extensive Earth–disk equilibration—readily achieved in a synestia but not in canonical impact scenarios. Likewise, our modeling of refractory major and trace elements shows a striking match between the bulk silicate Earth and Moon, a similarity that canonical models cannot reproduce due to their limited mixing efficiency. Whether or not pre impact bodies experienced mantle overturn, the canonical model consistently yields a fractionated proto lunar disk, contradicting observations.
In contrast, the synestia model inherently produces a fully mixed protolunar disk, complete vaporization of the outer layers of the colliding bodies, and an initially molten Moon—conditions that satisfy the full suite of isotopic, chemical, and volatile constraints. Emerging evidence for Earth–Moon similarity in refractory lithophile elements therefore strongly favors high energy, mixing intensive impact scenarios such as the synestia. These results underscore the need for a new framework for testing Moon formation models, one that incorporates the thermodynamic, chemical, and dynamical consequences of fully vaporized, post impact planetary structures.