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Wednesday, August 5, 2020

Do we now understand why the near and far sides of the Moon are so radically different?

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The Moon is easily the most studied object in the solar system (other than Earth, of course). But it still contains puzzles for scientists. Why, for example, is one side of the Moon so different from the other?

The Moon is locked to Earth, so before space flight, we only knew one side. There was no reason why the Earth side should be different from the far side. But now we know different.

As the space race warmed, NASA sent a series of lunar orbits to verify the Moon before the Apollo missions. Lunar orbits 1 to 5 were sent to the Moon in 1966 and 1967. The orbiter focused mainly on the landing sites, but Lunar Orbiter 4 photographed 95% of the the other side of the moon.

One of the many images of the far side of the Moon captured in 1967 by Lunar Orbiter 4. Look at all these craters! Image credit:

Now, of course, we have mapped the entire surface of the Moon, and we have also collected all kinds of scientific data from our natural satellite. We sent astronauts there and brought hundreds of kilograms of the lunar regolith to study here on Earth. And NASA plans to return to the Moon in the near future.

But the puzzling question remains: why is one side so crater and the other not? Why is the near side so much smoother and dominated by vast basaltic volcanic plains? Why was this ancient volcanic activity mainly limited to one side, the side facing the Earth?

The near side of the Moon (left) and the far side of the Moon (right) Image credit: NASA
The near side of the Moon (left) and the far side of the Moon (right) The near side contains 97% of the dark volcanic plains, while the far side is much more cratered? Image credit: NASA

The main theory for the formation of the Moon is the giant impact hypothesis. This theory suggests that a large planetary body the size of Mars, called Theia, struck Earth. The collision sent a huge amount of molten material into orbit around the Earth, which eventually merged into the Moon.

A new study has examined the disparity between the two sides of the Moon. Its title is “Improvement of the construction of the crust at the beginning of the Moon by the depression of the melting point of the mantle. “It is published in the journal Nature Geoscience, and the main author is Stephen Elardo, assistant professor of geology at the University of Florida.

When Theia’s impact occurred and the Moon merged with the debris, the Moon was not large enough to continue volcanic activity as long as Earth. The Moon was much smaller and was cooling faster. However, for some reason, the volcanic activity lasted much longer on the near side than on the far side. This goes against grain, because a body should cool at the same rate overall.

The study is centered on a strange geochemical anomaly on the Moon. The near side contains a region called Storms of Kreep Terrane;. This region contains a large amount of specific elements. KREEP stands for K (the atomic symbol for potassium), REE (rare earth elements) and P (the atomic symbol for phosphorus). The KREEP terrane also contains the elements thorium and uranium, which decay radioactively and produce heat.

Distribution of thorium on the lunar surface of the Lunar Prospector mission.  Thorium is strongly correlated with other radioactive elements (producing heat), most of which are present on the side facing the Earth (near side).  The relationship between this region and many observed features of lunar history is a key issue in lunar science.  Credits: Laneuville, M. et al (2013) Journal of Geophysical Research: Planets.
Distribution of thorium on the lunar surface of the Lunar prospector mission. Thorium is strongly correlated with other radioactive elements (producing heat), most of which are present on the side facing the Earth (near side). The relationship between this region and many observed features of lunar history is a key question in lunar science. Credits: Laneuville, M. et al (2013) Journal of Geophysical Research: Planets.

The authors of this study wanted to know if the presence of KREEP could create the conditions for more sustained volcanic activity. KREEP could lower the melting point of the mantle, and the presence of radioactive elements could have worsened the effect, generating enough heat to fuel volcanic activity in the region long after the rest of the moon had cooled.

The research team created an analog rock that had the same properties, called the Mg-suite rock. They created six versions of this analog, one with zero KREEP content and one with 5%, 10%, 15%, 25% and 50% KREEP and the associated radioactive elements. They then subjected the samples to elevated temperatures.

The study showed that compositional asymmetries between the near side and the far side of the Moon had an effect early in the life of the Moon. The authors wrote: “Our results show that asymmetries in hemispherical composition on the Moon began to have a dramatic effect on magma production immediately after lunar differentiation.”

This study figure shows the effects of KREEP on the analog.  Results from high temperature experiments show that adding KREEP to an analog source rock of Mg-suite significantly lowers its melting temperature.  Each line is an isotherm showing the quantity of molten material present at a given temperature as a function of the fraction of KREEP by weight in the starting mixture.  In simple terms, the more KREEP, the lower the melting temperature.  Image credit: Elardo et al, 2020.
This study figure shows the effects of KREEP on the analog. Results from high temperature experiments show that adding KREEP to an analog source rock of Mg-suite significantly lowers its melting temperature. Each line is an isotherm showing the quantity of molten material present at a given temperature as a function of the fraction of KREEP by weight in the starting mixture. In simple terms, the more KREEP, the lower the melting temperature. Image credit: Elardo et al, 2020.

It is not a minor anomaly, and as they wrote, its effect was dramatic. “The large concentration of heat producing elements near the Moon not only had the potential to act as a heat source for fusion, but also to lower melting temperatures at the crust-mantle interface of a way that could have produced about 4 to 13 times more pudding of crusts than what would have happened on the hidden side. “

Previous research claimed to show that “the crust-forming magmatism that immediately followed the LMO was a Moon-scale event, “as the authors write. But these results call into question this conclusion. Most LMO models see the planets at this stage of evolution as homogeneous globes of well-mixed materials. These models do not take KREEP into account in the same way as this work.

The authors of the article write: “Our results demonstrate that the abnormal enrichment in incompatible elements of this nearby reservoir considerably lowers the melting temperature of the source rock for these magmas and may have resulted in a production of magma 4 to 13 times more significant under the near crust, even without any contribution from radioactivity. “

This figure from the study shows how the radioactive decay of potassium, thorium and uranium prevented the Procellarum KREEP terrane from cooling as quickly as the rest of the Moon, prolonging volcanic activity in the region.  The?  T increases with increasing KREEP content due to the radiogenic heat produced by the decay of K, Th and U. Image credit: Elardo et al, 2020.
This figure from the study shows how the radioactive decay of potassium, thorium and uranium prevented the Procellarum KREEP terrane from cooling as quickly as the rest of the Moon, prolonging volcanic activity in the region. The ?T increases with increasing KREEP content due to the radiogenic heat produced by the decay of K, Th and U. Image credit: Elardo et al, 2020.

When they added the heating effect of nuclear decay, the business got stronger. “From digital thermal modeling, we show that radiogenic heating exacerbates this effect and may have resulted in an asymmetric concentration of post-magma-oceanic crust on the lunar side,” added the official.

“Our results suggest that the near geochemical anomaly has influenced the thermal and magmatic evolution of the Moon throughout its post-differentiation history,” they wrote.

Since the Earth and the Moon are inextricably linked, these results could also tell us something about Earth.

It is believed that 4.4 billion years ago, a celestial body (Theia) struck the Earth and produced the Moon. Image credit: NASA / JPL-Caltech

In one Press release, Matthieu Laneuville, co-author of the study, commented: “Due to the relative lack of erosion processes, the surface of the Moon records the geological events of the early solar system. In particular, regions near the Moon have concentrations of radioactive elements like U and Th unlike anywhere else on the Moon. Understanding the origin of these local U and Th enrichments can help explain the early stages of the formation of the Moon and, therefore, the conditions on primitive Earth. “

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