A rogue planet three times as massive as Mars probably sideswiped Earth 4.5 million years ago, vaporizing enough material from Earth聮s upper layers to form the moon, according to a University of Colorado at Boulder study.
While many theories on the moon聮s formation have been proposed, detailed analyses of lunar rocks obtained during NASA聮s Apollo missions were key in creating the 聯giant impact theory聰 in the 1970s that is now widely accepted, said Robin Canup of CU聮s Laboratory for Atmospheric and Space Physics. But the Mars-sized 聯impactor聰 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 聯protoplanet聰 probably was orbiting the sun somewhere between Earth and Mars when the collision occurred.
聯This was a surprising result,聰 said Canup. 聯Our calculations indicate a lot more impact energy than previously believed would have been required to produce enough material to form the moon.聰
Canup presented a talk on the subject at the American Astronomical Society聮s annual Division of Planetary Sciences Meeting held July 28 to Aug. 1 in Cambridge, Mass.
The CU research indicates an 聯oblique impact聰 between Earth and the ancient planet vaporized the upper portions Earth聮s crust and mantle, spraying the material into Earth聮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.
聯Large-scale impacts like this one probably played a crucial role in shaping the solar system,聰 said Canup. The puzzling size and composition of Mercury, the extreme tilt in Uranus聮 axis and the peculiar, 聯double-planet聰 system of Pluto and its large moon, Charon, indicate such impacts may have been relatively common.
聯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,聰 she said.
Questions regarding the formation of Earth聮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 聯an Earth that is spinning too quickly.聰
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聮s laws, the CU-Boulder model produces a system with roughly twice as much rotational spin as the Earth-moon system exhibits today.
聯The Harvard model produced the right amount of initial spin for the Earth, but not enough material to have formed the moon,聰 she said. 聯Our closest celestial neighbor remains a mystery in many ways.聰
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 聯very bright聰 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.