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A collision with a dwarf planet has probably caused our moon to go awry



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

Our moon has a near and a far side with dramatically different geological features. This anomaly has puzzled scientists for years, but recent computer simulations suggest that the asymmetric disposition of the moon is due to an age-old collision with another object – possibly a dwarf planet.

For thousands and thousands of years, humanity had no idea what the other side of the moon looked like. Our natural satellite is tethered to the earth, forcing us to constantly look at one of its two hemispheres. However, starting with the Apollo missions, we have finally acquired the ability to study the invisible face of the moon. To the surprise of the astronomers, it was found that the two lunar lobes show conspicuous differences in topography, crustal strength and chemical composition. The scientists suggested that this pronounced asymmetry is the result of old but unknown physical processes.

Maps with differences in lunar topography (A), crustal strength (B) and thorium distribution (C). The star on the top right shows the possible impact point in the vicinity.
Image: JGR: Planets / Zhu et al. 2019 / AGU.

Recent research published in the Journal of Geophysical Research: Planets suggests that this unexpected asymmetry was caused by an age-old collision with a rather large object, probably a dwarf planet.

"This is an article that will be very provocative," said Steve Hauck, a professor of planetary geodynamics at Case Western Reserve University, who was not involved in the study, in a press release. "Understanding the origin of the differences between the near and the far side of the moon is a fundamental issue in lunar science," added Hauck, Editor-in-Chief of JGR: Planets. The lead author of the new study, Zhu Meng-Hua from the Space Science Institute Macau University of Science and Technology surmised that the near-to-far-side asymmetries were the result of a sky collision after investigating the data collected by gravity in 2012 Mission of the Restoration and Interior Laboratory (GRAIL). These data showed that the crust on the other side was about 10 kilometers (6 miles) thicker than the crust on the near side. In addition, the other side showed an additional crust layer consisting of magnesium and iron rich materials.

Using the GRAIL data, Zhu performed a series of computer simulations to test the hypothesis that a huge collision caused the moon to go wrong. A total of 360 different computer models were run to determine if an impact can produce the same physical features observed on the Moon today.

The modeled impact of a 780-kilometer-wide object with a 200-kilometer-wide iron core on the moon at 22,500 km / h. The right halves represent the temperature variations during the impact, and the black arrows in (C) and (D) show the movement of the debris around the moon.
Image: JGR: Planets / Zhu et al. 2019 / AGU.

Of the simulations performed, two agreed with the GRAIL data. Specifically, it has been shown that the asymmetry from the near side to the far side is caused by a large object with a diameter of 780 kilometers (480 miles) that hits the moon near 22,550 km / h (14,000 miles per hour) or through a slightly smaller object with a diameter of 720 kilometers (450 miles) at a higher speed of 24,500 km / h. For comparison: The dwarf planet Ceres has a diameter of 945 kilometers.

In both scenarios, large amounts of debris were thrown onto the lunar surface, especially on the opposite side of the collision. The falling material buried the primary crust on the other side with a 5 to 10 kilometer thick layer, which corresponds to the observations of GRAIL. The authors of the new study stated that the questionable object was probably a dwarf planet in orbit around the Sun and not a second Earth Moon.

Importantly, the new study has the potential to solve a mystery about the differences in the isotopes of potassium, phosphorus and various rare earth elements between the Earth and the Moon. This new theory happily explains this discrepancy by suggesting that the elements will later reach the moon as a result of the impact.

Interestingly, the new result may also explain similar asymmetric features found on other planets in the solar system, including Mars.

"Indeed, some planets have hemispheric dichotomies, but for the moon we need to have a lot of data. They are able to test models and hypotheses, so the implications of the work could probably be wider than just the moon," said Hauck.

It's a fascinating conclusion, but one that depends heavily on computer modeling. Other researchers should do their own simulations, preferably with data from GRAIL and other sources. For example, it would be interesting to see if the fallout patterns observed in the new study can be compared by others. Future research should also focus on the presence of "extraneous" material on the moon (ie, the remains of the alleged dwarf planet), which could further validate the findings of the new publication. Until then, the new theory must remain just that – a theory.


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