The mysterious tilt of the moon's orbit might come from an angled, giant impact that vaporized most of the early Earth, creating the moon in the process, a new study finds.
Earth and the other major planets of the solar system follow orbits around the sun that mostly lie within a thin, flat zone defined by the sun's equator. This is likely because these worlds arose from a protoplanetary disk of gas and dust encircling the sun's midriff.
Oddly, the moon's orbit is slightly inclined compared to Earth's orbit around the sun, by about 5 degrees. Until now, scientists could not reconcile the moon's tilt with the leading theory of how the moon formed. [Here's How The Moon Was Made (Video)]
Previous research suggested that during the early days of planet formation, the newborn Earth grazed a Mars-size rock called Theia (named after the mother of the moon in ancient Greek mythology). Debris from the impact later helped form the moon.
This giant-impact hypothesis seemed to explain many details about the moon and Earth, such as the large size of the moon compared with Earth and the rates of rotation of the two bodies. However, in the past 15 years, new evidence has challenged scientists to rework the details of this scenario.
In 2001, scientists began discovering that terrestrial and lunar rocks had more in common than expected: Earth and the moon possessed extremely similar levels of many isotopes. (Isotopes are versions of the same element with different numbers of neutrons.)
Prior work suggested that planetary bodies that formed in different parts of the solar system generally have different isotopic compositions. The isotopic similarities of Earth and the moon threw the giant-impact hypothesis into crisis because previous computer simulations of the collision predicted that 60 to 80 percent of the material that coalesced to form the moon came from Theia rather than Earth. The likelihood that Theia happened to have virtually the same isotopic composition as Earth seemed unlikely.
The latest version of the giant-impact hypothesis seeks to resolve this crisis by suggesting that an extraordinarily high-energy impact created the moon — one so violent that it vaporized not just Theia but also most of Earth, down to the young planet's mantle region (the layer just above the core). This dense vapor then formed a cloud more than 500 times bigger than today's Earth. Much of this material would have fallen back onto Earth as it cooled, but some of the debris would have gone on to form the moon. Previous research suggested that the material from Earth and Theia would have mixed together in the cloud, helping to explain why Earth and the moon have similar isotopic compositions.
One feature of this new model is that Earth was spinning very quickly after it got hit, taking maybe only 2 to 3 hours to complete a day. Prior work suggested that over billions of years, gravitational interactions between Earth and the moon slowed down both their rates of rotation, helping to explain why Earth now takes about 24 hours to complete a day.
However, until now, this new model could not explain the strange tilt of the moon's orbit.
"The inclination of the moon's orbit has been a major unsolved problem with the Earth-moon system," said Sarah Stewart, a planetary scientist at the University of California, Davis and a senior author of the new study. "With a giant impact, the moon forms from a disk around the moon's equator, and even though the dynamical evolution of the system is complicated, if the moon started near the Earth's equator, we expect that it should stay near the Earth's equator as it moves away from the Earth over time — but we instead see this 5-degree inclination," she told Space.com.
Now, Stewart and her colleagues suggest that the answer may be that the giant impact that created the moon hit Earth at a highly slanted or oblique angle. [How the Moon Evolved: A Photo Timeline]
"What's beautiful about this work is that we can end up with the current state of the moon — its orbit, its chemistry — with just one step, without invoking any other event," Stewart said. "We don't invoke a sequence of events that needs to be just right to explain the moon's current state."
The scenario the researchers modeled involves a complicated dance among Earth, the moon and the sun. It begins with the giant impact that formed the moon. That collision left Earth spinning very quickly, so much so that its shape became squashed, with its diameter at its equator twice that as its diameter from pole to pole. The impact also tilted Earth so its axis of spin was highly slanted compared with the sun's axis of spin by about 70 degrees.
As the moon slowly pulled away from Earth over time and both their rates of rotation slowed down, the moon reached a point called the "Laplace plane transition," where the influence of Earth on the moon became less important than gravitational forces from the sun. This led the sun to help slow the Earth's rate of spin, the researchers explained.
The process of the moon crossing the Laplace plane transition slanted Earth so that its axis of spin was more upright, to about its current tilt of 23.5 degrees compared with the sun's axis of spin. This in turn led the moon to orbit Earth at a high angle of about 30 degrees, Stewart said.
As the moon continued to slowly move away from Earth, it reached another milestone, the "Cassini state transition," wherein the gravitational pull of Earth influenced the angle of the moon, the researchers said.
"Because the Earth is tilted, gravitational forces between the Earth and moon are not equal at the poles and equator," Stewart said. "The net effect of that is to lower the inclination of the moon's orbit to its current 5 degrees."
The likelihood that the early Earth was hit with the right properties to explain the current tilt of the moon's orbit "is something like 30 percent," Stewart said. "It's reasonably likely."
Future research will pinpoint whether such an impact can help to explain the current chemistry of the moon and Earth, Stewart said.
The scientists detailed their findings online Oct. 31 in the journal Nature.