It was in the late 1950s and 1960s that Soviet and American missions revealed two very different faces of the Moon.
The visible face is covered by so-called lunar seas, which appear as large dark patches that reveal ancient lava flows. On the hidden face, however, these seas are barely visible.
The reason for this difference is a long-standing mystery.
But a new scientific study from the United States proposes an explanation to solve the riddle: a gigantic impact billions of years ago would have generated this difference.
Two very different faces
“The biggest differences between the visible face and the hidden face of the Moon have to do with the appearance and with the chemical composition of these lunar regions”, told BBC News Mundo, the BBC’s Spanish service, José María Madiedo, an astrophysicist at the Instituto de Astrofísica de Andalucía (IAA-CSIC), a specialist in asteroid impacts on the Moon. Madiedo did not participate in the American study.
“On the visible face there are many large areas covered with solidified lava that are called mares. On the far face, however, these mares are very scarce.”
Madiedo also highlighted that, in terms of composition, the space missions carried out so far have found great contrasts in the presence of certain elements.
“For example, on the visible face there are higher concentrations of potassium, titanium, thorium, phosphorus and other elements of the so-called rare earth group (REE).
“All this reveals that throughout its evolution the Moon must have suffered some kind of phenomenon that could give rise to these differences.”
The new study, published in the journal Science Advances, points out that the differences between the faces of the Moon occur because of the large impact, which created the so-called South Pole-Aitken basin (SPA).
The South Pole-Aitken Basin “is a large impact crater located in the vicinity of the Moon’s south pole,” Madiedo explained.
“With a diameter of about 2,500 km and a depth of 12 km, this is one of the largest impact structures found on Solar System objects.”
The location of this basin lies in the area corresponding to the far side of the Moon, which is not visible from Earth, added the expert.
“From our planet, you can only see the edge of the basin, which is formed by a mountain range about 9 kilometers high.”
The impact that created the South Pole-Aitken basin would have caused a huge cloud of heat that spread through the Moon’s interior.
As it spread, this cloud carried several rare earths to the visible side of the Moon. And the abundance of these heat-generating elements would have contributed to the volcanic activity that created the lava flows.
“We know that large impacts like the one that formed the SPA would generate a lot of heat,” says Matt Jones, lead author of the study and a postdoctoral student at Brown University in Rhode Island.
“The question is how this heat affects the Moon’s interior dynamics. What we show is that under any plausible conditions at the time the SPA formed, these heat-producing elements end up being concentrated on the near side.”
“This contributed to the melting of the mantle that produced the lava flows we see on the surface.”
‘Like a surfer’
The Moon’s near side hosts a compositional anomaly known as Procellarum KREEP Terrain (PKT) — a concentration of potassium, rare earth elements and phosphorus, along with heat-producing elements like thorium, explained in a statement to Brown University.
This KREEP appears to be concentrated in and around Oceanus Procellarum, the largest of the nearby volcanic plains.
Scientists ran computer simulations of how the heat generated by a giant impact would alter convection patterns inside the Moon and how this could redistribute KREEP material in the lunar mantle.
Models of the lunar interior suggest that the material should have been more or less evenly distributed below the surface. But this new model shows that the uniform distribution would be interrupted by the heat cloud generated by the SPA impact.
“According to the model, the KREEP material would have ‘surfed’ the heat wave that emanated from the SPA’s impact zone as if it were a surfer,” reads the Brown University statement.
As the heat cloud extended below the Moon’s crust, this material was drawn towards the nearer side.
The researchers say the work provides a credible explanation for one of the great mysteries about the Moon.
“How PKT formed is possibly the biggest open question in lunar science,” Jones said. “And the South Pole-Aitken impact is one of the biggest events in lunar history. This work brings those two things together and I think our results are really exciting.”
In addition to Brown University, researchers from Purdue University, Stanford University, the Lunar and Planetary Science Laboratory in Arizona, and NASA’s Jet Propulsion Laboratory participated in the study.
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