A New Spin on the Origin of the Earth and Moon

Categories: Moon to Mars

Scientists have long thought that the Moon was formed after a small planet collided with the early Earth. Credit: Copyright Fahad Sulehria, 2005, www.novacelestia.com

Scientists have long believed the Moon formed as a result of a collision between the early Earth and a smaller planet, but computer models of the giant impact have always predicted the wrong composition for the Moon. Now researchers at Harvard University and the SETI Institute are proposing a new spin on the giant impact model to match the observed composition of the Moon. Understanding how the Moon formed is important for astrobiologists who are studying how the Earth became habitable for life as we know it.

The previous giant impact models have held that the small planet, Theia, hit the Earth, sending a cloud of debris from Theia into orbit that formed the Moon. But the chemistry of the Moon matches the Earth. Now Sarah T. Stewart, a professor in Harvard’s Department of Earth and Planetary Sciences, and her SETI colleague Matija Cuk propose a new giant impact model that resulted in pieces of the Earth breaking off and forming the Moon.

The researchers present a dynamic model of their theory, motivated by the results of chemical analyzes of isotopes from the Earth and Moon, in a paper published online today in Science. The results were also presented at the 44th meeting of the AAS Division for Planetary Sciences in Reno, NV.

Additionally, Stewart and Cuk propose that prior to the collision and creation of the Moon, the Earth was spinning much faster than it does now, and had a day that was only two to three hours long.

Many scientists believe that Earth itself emerged from a series of giant impacts. These impacts made the early Earth spin near its stability limit of about 2 hours per revolution. The last giant impact, they believe, formed a Moon that is a twin of the Earth. Stewart and Cuk posit that when the giant impact occurred between Theia and the fast-spinning Earth, the high speed of the Earth’s spin caused the ejection of material from Earth into orbit. The ejected material formed a Moon with chemical composition similar to Earth. After the impact, the rapidly rotating Earth was slowed down by the gravitational interaction between the Sun and the Moon.

A still from a simulation of the Moon-forming impact. Credit: Harvard University

Previous giant impact models could match the size of the Moon and the present angular momentum of the Earth and Moon but did not explain the similar chemistry of the Earth and Moon. But the new theory, with the discovery of a mechanism to slow the spin of the Earth after the impact, explains how a giant impact with a fast-spinning Earth could result in a Moon with a similar chemical composition

As part of their dynamic model, Cuk and Stewart found that a resonance between Earth’s orbit around the Sun and the Moon’s orbit around Earth can pass angular momentum to the Sun. Furthermore, Cuk and Stewart showed that if the Earth was fast-spinning before the impact then a giant impact would eject enough Earth material into orbit to make the Moon.

Today, tides between the Earth and Moon both slow Earth’s rotation and push the Moon’s orbit further away. But the total angular momentum of the system is conserved. The finding is significant because without a fast-spinning Earth preceding impact, "a giant impact could not make the Moon originate from the Earth’s mantle with today’s angular momentum," says Sarah Stewart.

The origin of the Moon had been called into question by isotope analyzes of material from both Earth and the Moon. The isotope signatures of celestial bodies differ greatly and often are used to ‘fingerprint’ different planets and meteorite groups. The data show that the Earth and Moon are ‘isotopic twins,’ a contradiction to the Moon origin story from the original giant impact model. If the original model were correct, then the Moon should have had a different isotopic fingerprint than the Earth.

Nineteenth century scientists speculated about a fast-spinning early Earth. George H. Darwin, son of Charles Darwin, studied the relation between tides and the Moon. In 1879, he suggested that the Moon formed by fission from the Earth, but he did not know how early Earth might have being spinning so quickly. A similar dynamic model for a great impact resulting in the formation of the Moon from Earth material is described in a second paper in the same issue of Science. This alternative dynamic model is presented by Dr. Robin Canup of the Southwest Research Institute (SwRI).