Massive Impact With Dwarf Planet Traveling at 14,000 MPH Caused Differences Between Moon's Two Faces: Study

moon, impact, collision
Artist’s depiction of a collision between two planetary bodies. New research suggests the stark differences between the moon’s heavily cratered far side and the lower-lying open basins of the near side were caused by a wayward dwarf planet colliding with the moon in the solar system's early history. NASA/JPL-Caltech

For decades, scientists have struggled to explain the asymmetrical nature of the moon's two hemispheres, which display striking differences in elevation, composition and crust thickness.

Now, a study published in the Journal of Geophysical Research: Planets suggests that the differences were caused by a massive collision with a wayward dwarf planet early in the solar system's history.

All the way back to the NASA Apollo missions in the 1960s and 1970s, observations of the moon have cast light on the asymmetry between its two faces. We now know that the far side—which always faces away from Earth—is covered in craters, while the near side, the one we can see, is characterized by low-lying open basins.

In 2012, NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission obtained gravity data on the moon, which revealed that the crust is actually thicker and contains an extra layer of material on the far side.

Several ideas have been proposed to try to explain the moon's asymmetry. One hypothesis suggests that there were once two moons orbiting the Earth, and these merged to make one object, very early in its formation. Another suggests that a large body—such as a dwarf planet or perhaps a large asteroid—collided with the moon, although this would likely have occurred later in its history than the merging-moons scenario, and after it had formed a solid crust.

If the second scenario is correct, there should be signs of such an impact in the structure of the lunar crust today, according to Meng Hua Zhu, lead author of the paper. He is with the Space Science Institute at the Macau University of Science and Technology.

For the study, Zhu and his team used computer simulations to model different moon impact scenarios to see what kind of event could reproduce the crust as we understand it today, with the help of the gravity data obtained by GRAIL.

These simulations showed that the most likely candidate for producing these conditions was a large world measuring around 480 miles in diameter crashing into the near side of the moon at 14,000 miles per hour.

The simulations also demonstrated that a slightly smaller body, measuring around 450 miles in diameter but traveling slightly faster at 15,000 mph, would also have produced similar effects.

The researchers say that both of these impacts would have thrown up vast amounts of material that fell back down to the moon's surface, burying the crust on the far side in up to 6 miles of debris. This would explain the extra layer of crust material detected by GRAIL.

According to the study, it is likely that the colliding object was some kind of dwarf planet or asteroid that was on its own orbit around our sun—not an early second moon of the Earth.

"This is a paper that will be very provocative," Steve Hauck, editor-in-chief of the Journal of Geophysical Research, who was not involved in the study, said in a statement.

"Understanding the origin of the differences between the near side and the far side of the moon is a fundamental issue in lunar science," he said. "Indeed, several planets have hemispherical dichotomies, yet for the moon we have a lot of data to be able to test models and hypotheses with, so the implications of the work could likely be broader than just the moon."