What Made the Moon?

New simulations explain how a single impact on Earth could have yielded a moon

Several frames from the new simulation show the formation of a protolunar disk of material after a Mars-sized object obliquely impacts the Earth. Most of the impacting object eventually ends up on Earth, but a fraction of it remains dispersed in a cloud of debris orbiting the planet, from which the moon coalesces. The greens and blues indicate a cooler disk of material orbiting in the equatorial plane, while the oranges and reds indicate a hot cloud of material.

The "giant impact" theory, first proposed in the mid 1970s to explain how the moon formed, has received a major boost as new results demonstrate that a single impact could have yielded the current Earth-moon system.

Simulations performed by researchers at Southwest Research Institute (SwRI) and the University of California at Santa Cruz show that a single impact by a Mars-sized object in the late stages of Earth's formation could account for an iron-depleted moon and the masses and angular momentum of the Earth-moon system. This model is the first that can simultaneously explain these characteristics without requiring that the Earth-moon system be modified substantially after the lunar forming impact.

The Earth-moon system is unusual in several respects. The moon has an abnormally low density compared to the terrestrial planets, indicating that it lacks high-density iron. If the moon has an iron core, it constitutes only a few percent of its total mass compared to Earth's core, which is about 30 percent of its mass. The angular momentum of the Earth-moon system, contained in both the Earth's spin and the moon's orbit, is quite large and implies that the terrestrial day was only about five hours long when the moon first formed close to the Earth. This characteristic provides a strong constraint for giant impact models.

Previous models had shown two classes of impacts capable of producing an iron-poor moon, but both were more problematic than the original idea of a single Mars-sized impactor in the last stages of Earth's formation. One model involved an impact with twice the angular momentum of the Earth-moon system; this would require that a later event (such as a second large impact) alter the Earth's spin after the moon's formation.

The second model proposed that the moon-forming impact occurred when Earth had only accreted about half its present mass. This model required that the Earth accumulated the second half of its mass after the moon formed. However, if the moon also accumulated its proportionate share of material during this period, it would have gained too much iron-rich material -- more than can be reconciled with the moon today.

The new models use the modeling technique known as smooth particle hydrodynamics (SPH), which also has been used in previous formation studies. In SPH simulations, the colliding planetary objects are modeled by a vast multitude of discrete spherical volumes, whose thermodynamic and gravitational interactions are tracked as a function of time.

The new high-resolution simulations show that an oblique impact by an object with 10 percent of the mass of the Earth can eject sufficient iron-free material into Earth orbit to yield the moon, while also leaving the Earth with its final mass and correct initial rotation rate. This simulation also implies that the moon was created near the very end of Earth's formation.

"The model we propose is the least restrictive impact scenario, since it involves only a single impact and requires little or no modification of the Earth-moon system after the moon-forming event," said Dr. Robin M. Canup, assistant director of the SwRI Space Studies Department in Boulder.

UCSC Professor Erik Asphaug added, "Our model requires a smaller impactor than previous models, making it more statistically probable that the Earth should have a moon as large as ours."

"It is now known that giant collisions are a common aspect of planet formation, and the different types of outcomes from these last big impacts might go a long way toward explaining the puzzling diversity observed among planets," Asphaug said.

The moon also is believed to play an important role in Earth's habitability because of its stabilizing effect on the tilt of Earth's rotational pole.

"Understanding the likelihood of moon-forming impacts is an important component in how common or rare Earth-like planets may be in extrasolar systems," Canup said.

Contact Canup at (303) 546-6856 or robin@boulder.swri.edu.

Published in the Fall 2001 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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