Why the moon has TWO faces

An ancient collision between the moon and a dwarf planet left half the far side of the lunar surface heavily-cratered, scientists have suggested.  

The new research suggests that a collision between the moon and an object ‘slightly smaller’ than a dwarf planet in the solar system’s early stages is the best explanation.

The difference between the Earth-facing near-side of the moon and the scarred far side of the moon has been debated among scientists since the Apollo era. 

Previously, scientists had suggested that Earth had two moons billions of years ago which merged – creating the uneven surface of the far side that exists today.  

Measurements made by the Gravity Recovery and Interior Laboratory (GRAIL) mission in 2012 gave scientists more information about the structure of the Moon.

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New research suggests the Moon (left) collided with the moon and an object slightly smaller than a dwarf planet (right) in the solar system’s early stages. Here, an artist’s depiction of a collision between two planetary bodies

These included how the moon’s far side has a crust which is thicker than the near side and includes an extra layer of material while the near side boasts a thinner and smoother crust.  

‘The detailed gravity data obtained by GRAIL has given new insight into the structure of the lunar crust underneath the surface,’ said Dr Meng Hua Zhu, a co-author of the study. 

Dr Hua Zhu and his team ran 360 computer simulations of different scenarios where impacts with the moon could result in the crust of today’s moon.

The topographic (A), crustal thickness (B), and thorium distribution of the Moon show a dramatic difference between the nearside and farside. The star on the nearside represents the center of the proposed impact basin. The black dashed lines represent the boundary of Imbrium (Im), Orientale (Or), and Apollo (Ap) basin, respectively.

They found the best solution is a large body, about 480 miles (780 kilometers) in diameter, smacking into the near side of the moon at 14,000 miles per hour (22,500 kilometers per hour). 

The model shows the impact would have thrown up vast amounts of material that would fall back onto the moon’s surface, burying the crust on the far side in 3 to 6 miles (5 to 10 kilometers) of debris. 

The model also provides a good explanation for the unexplained differences in isotopes of potassium, phosphorus and rare-earth elements like tungsten-182 between the surfaces of the Earth and Moon. 

China’s Chang’E-4 mission’s Yutu-2 rover (pictured) has found presence of material from the moon’s deep layers at its landing site on the far side of the moon. Scientists are hoping to add to their models with data from China’s Chang’e4 mission

‘Our model can thus explain this isotope anomaly in the context of the giant impact scenario of the Moon’s origin.’ the researchers write.  

‘This is a paper that will be very provocative,’ said Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University. 

‘Understanding the origin of the differences between the nearside and the farside of the Moon is a fundamental issue in lunar science. 

‘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.’

Scientists are hoping to add to their models with data from moon missions, including China’s Chang’e-4 mission, which landed on the far side of the moon in January.

Analysis of data from the Chang’e-4 mission revealed substances from the moon’s interior on the surface of the crater where the probe landed. 

This provides further evidence that an enormous collision occurred at least 3.9 billion years ago. 

The research was published in AGU’s Journal of Geophysical Research: Planets.