A collision with a dwarf planet probably rocked our moon

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Artistic representation of a massive collision involving two large objects.
Artistic representation of a massive collision involving two large objects.
Image: NASA / JPL-Caltech

Our moon has a near side and a far side with radically different geological features. This anomaly has been troubling scientists for years, but new computer simulations suggest that the asymmetrical disposition of the Moon can be attributed to an old collision with another object, perhaps a dwarf planet.

For thousands and thousands of years, humanity had no idea what the hidden face of the moon looked like. Our natural satellite is tightly locked on the Earth, forcing us to watch one of its two hemispheres perpetually. However, from the Apollo missions, we finally acquired the ability to investigate the invisible face of the Moon. To the surprise of astronomers, it was found that the two lobes of the Moon showed striking differences in topography, crust thickness, and chemical composition. Scientists have assumed that this pronounced asymmetry is the result of old but unknown physical processes.

Maps showing the differences of lunar topography (A), thickness of the Earth's crust (B) and distribution of thorium (C). The star at the top right indicates the point of possible impact on the near side.
Maps showing the differences of lunar topography (A), thickness of the Earth's crust (B) and distribution of thorium (C). The star at the top right indicates the point of possible impact on the near side.
Image: JGR: Planets / Zhu et al. 2019 / AGU.

New research published in the Journal of Geophysical research: planets suggests that this unexpected asymmetry was caused by an old collision with a rather bulky object, probably a dwarf planet.

"This is a very provocative article," Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University, said in a statement, who was not involved in the study. "To understand the origin of the differences between the near and the far side of the moon is a fundamental problem in lunar science",
Hauck, who is the editor of JGR: Planets, added.

The main author of the new study, Zhu Meng-Hua from the Institute of Space Sciences of the University of Science and Technology of Macao, had the intuition that the Left and left side asymmetries were the result of a celestial collision after examining the data collected in 2012 by Gravity Recovery and Internal Laboratory (GRAIL). These data showed that the crust on the opposite side was about 10 kilometers (6 miles) thicker than the crust on the near side. In addition, the hidden side also had an additional crust layer made of magnesium and iron rich materials.

Using the GRAIL data, Zhu conducted a series of computer simulations to test the hypothesis that a giant collision would have tilted the moon. In total, 360 different computer models were made to determine if an impact could produce the same type of physical characteristics as those observed on the Moon today.

The modeled impact of an object 780 kilometers wide containing an iron core 200 km wide hitting the moon at 22,500 km / h. The right halves represent the temperature changes during the impact process and the black arrows of (C) and (D) indicate the movement of debris around the Moon.
The modeled impact of an object 780 kilometers wide containing an iron core 200 km wide hitting the moon at 22,500 km / h. The right halves represent the temperature changes during the impact process and the black arrows of (C) and (D) indicate the movement of debris around the Moon.
Image: JGR: Planets / Zhu et al. 2019 / AGU.

Of the simulations performed, two were in agreement with the GRAIL data. It has been shown that the asymmetry between the opposite side and the opposite side is caused by a large object measuring 780 kilometers (480 miles) reaching the near-moon at 22,550 km / h or by a slightly smaller object measuring 720 kilometers 450 miles) at a top speed of 24,500 km / h (15,000 mph). By comparison, the dwarf planet Ceres is 945 kilometers (590 miles).

Both of these scenarios have raised an abundant amount of debris that has fallen on the lunar surface, particularly on the opposite side of the collision. Falling materials buried the primitive crust on the opposite side with a layer 5 to 10 kilometers thick, which is consistent with GRAIL observations. The authors of the new study stated that the offending object was probably a dwarf planet orbiting the Sun and not a second Earth moon.

It is important to note that the new study could solve a persistent mystery regarding the differences observed in the isotopes of potassium, phosphorus and various elements of rare earths between the Earth and the Moon. This new theory happily explains this gap by suggesting that the elements arrived later on the Moon via the impact.

This new result could also explain, with excitement, the similar asymmetrical features observed on other planets in the solar system, including Mars.

"Indeed, several planets have hemispherical dichotomies, but for the Moon, we have plenty of data to test models and hypotheses, so the implications of this work may be broader than the Moon," Hauck said.

This is an intriguing conclusion, but one that relies heavily on computer models. Other researchers should conduct their own simulations, preferably with data collected from GRAIL and other sources. It would be interesting to see, for example, if the spillover models observed in the new study can be compared by others. Future research should also focus on the presence of "alien" material on the Moon (that is, the remains of the so-called dwarf planet) that may further corroborate the conclusions of the new document. Until then, the new theory will have to remain like a theory.

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