Formation Of Earth's Moon Likely Caused By Huge Planetary Collision

Published: July 27, 1997

A rogue planet three times as massive as Mars probably sideswiped Earth 4.5 million years ago, vaporizing enough material from Earth’s upper layers to form the moon, according to a University of Colorado at Boulder study.

While many theories on the moon’s formation have been proposed, detailed analyses of lunar rocks obtained during NASA’s Apollo missions were key in creating the “giant impact theory” in the 1970s that is now widely accepted, said Robin Canup of CU’s Laboratory for Atmospheric and Space Physics. But the Mars-sized “impactor” proposed by Harvard University researchers in the late 1980s is not large enough to account for the formation of our unusually large moon, according to calculations by Canup and several other CU planetary scientists.

The modeling work by the CU-Boulder group, which is collaborating with the Harvard group, indicates the impactor must have been at least 2.5 to 3 times the mass of Mars to create the volume of debris required to eventually coalesce into the moon, Canup said. The “protoplanet” probably was orbiting the sun somewhere between Earth and Mars when the collision occurred.

“This was a surprising result,” said Canup. “Our calculations indicate a lot more impact energy than previously believed would have been required to produce enough material to form the moon.”

Canup presented a talk on the subject at the American Astronomical Society’s annual Division of Planetary Sciences Meeting held July 28 to Aug. 1 in Cambridge, Mass.

The CU research indicates an “oblique impact” between Earth and the ancient planet vaporized the upper portions Earth’s crust and mantle, spraying the material into Earth’s orbit. The material appears to have spread into a gaseous disk around Earth, then formed a handful of small, extremely hot moonlets that eventually coalesced into the single, large moon we see today, said Canup.

“Large-scale impacts like this one probably played a crucial role in shaping the solar system,” said Canup. The puzzling size and composition of Mercury, the extreme tilt in Uranus’ axis and the peculiar, “double-planet” system of Pluto and its large moon, Charon, indicate such impacts may have been relatively common.

“We believe this theory is a linchpin to understanding how planets formed in our solar system and in solar systems that may exist around other stars,” she said.

Questions regarding the formation of Earth’s 2,160-mile-in-diameter moon still remain, Canup said. Although the size of the impactor proposed by the CU team provides the correct amount of material required to form our unusually large moon, the model also yields “an Earth that is spinning too quickly.”

The angular momentum, or intensity of rotational momentum, in the Earth-moon system depends on the spin of Earth and the moon and their distance apart, said Canup. Although the total amount of rotational spin in planet-moon systems must remain constant over time according to Newton’s laws, the CU-Boulder model produces a system with roughly twice as much rotational spin as the Earth-moon system exhibits today.

“The Harvard model produced the right amount of initial spin for the Earth, but not enough material to have formed the moon,” she said. “Our closest celestial neighbor remains a mystery in many ways.”

The ongoing moon-origin study may pave the way for searches of newly forming solar systems, planets and moons elsewhere in the universe, she said. Because the disk material formed by planetary collisions would be extremely hot, such impacts would likely be “very bright” and might be detectable using new generations of sophisticated Earth- and space-based telescopes.

The moon, which orbits Earth at about 239,000 miles distant, appears to have formed at roughly 15,000 miles from Earth, according to the CU researchers.