For decades, scientists have been perplexed by the sharp contrast between the Moon’s highly cratered far side and the lower-lying wide basins of the Earth-facing nearside.
According to fresh data concerning the Moon’s crust, the discrepancies were generated by a stray dwarf planet impacting with the Moon early in the solar system’s history. The Journal of Geophysical Findings: Planets of the American Geophysical Union has issued a report on the new research.
The enigma of the Moon’s two faces arose during the Apollo period when the first observations of the Moon’s far side revealed startling discrepancies. The Gravity Recovery and Interior Laboratory (GRAIL) mission, which launched in 2012, uncovered new data about the Moon’s structure, such as how its crust is thicker and has an extra layer of material on its far side.
There have been a variety of theories proposed to explain the Moon’s asymmetry. One theory is that there were once two moons orbiting Earth, which joined during the Moon’s formation.
Another theory is that a massive object, possibly a young dwarf planet, found itself in an orbit around the Sun that put it in direct collision with the Moon.
According to Meng Hua Zhu of the Space Science Institute at Macau University of Science and Technology, the new study’s principal author, this latter massive impact scenario would have occurred after the Moon had acquired a solid crust, which would have occurred later than a merging-moons scenario. Today, signs of such an impact should be present in the lunar crust’s structure.
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.
Steve Hauck
“The detailed gravity data obtained by GRAIL has given new insight into the structure of the lunar crust underneath the surface,” Zhu said.
GRAIL’s new results provided Zhu’s team of researchers a more specific goal to shoot for with the computer simulations they used to assess various early-Moon impact scenarios. The authors of the study ran 360 computer simulations of huge impacts with the Moon to see if a similar event millions of years ago might replicate the crust of today’s Moon as detected by GRAIL.
They discovered that a huge body, about 480 miles (780 kilometers) in diameter, crashing into the Moon’s nearside at 14,000 miles per hour is the best fit for today’s asymmetrical Moon (22,500 kilometers per hour).
That would be the equivalent of an object somewhat bigger than Ceres moving at a quarter the speed of meteor pebbles and sand grains that burn up as “shooting stars” in Earth’s atmosphere.
A slightly smaller, 450-mile (720-kilometer) diameter object crashing at a slightly quicker 15,000 miles per hour is another strong fit for the impact combinations the team predicted (24,500 kilometers per hour).
The impact would have thrown up large volumes of material that would have fallen back on the Moon’s surface, burying the primordial crust on the far side in 3 to 6 miles (5 to 10 kilometers) of debris in both of these scenarios, according to the model. According to Zhu, this is the additional layer of crust identified on the far side by GRAIL.
According to the latest research, the impactor was most certainly not Earth’s early second moon. The impactor, whether it was an asteroid or a dwarf planet, was most likely on its own orbit around the Sun when it collided with the Moon, according to Zhu.
According to the researchers, the huge impact model also explains the inexplicable discrepancies in potassium, phosphorus, and rare-earth elements like tungsten-182 between the Earth’s and Moon’s surfaces.
According to the study’s authors, these components could have come from the big impact, which would have added that material to the Moon after its formation.
“Our model can thus explain this isotope anomaly in the context of the giant impact scenario of the Moon’s origin.” the researchers write.
According to the researchers, the new study not only provides a solution to long-standing puzzles regarding the Moon, but it may also shed light on the construction of other asymmetrical worlds in our solar system, such as Mars.
“This is a paper that will be very provocative,” said Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University and Editor-in-Chief of the JGR: Planets.
“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.”
