Did you know the Moon has two faces and that they are completely different? Well, this had been an unknown mystery for quite a long time. However, recent research has revealed that the solution to this riddle lies in an ancient asteroid collision that rocked the Moon 4.3 billion years ago.
The collision was so huge and powerful that it altered the shape of Earth’s natural satellite, resulting in an irregular balance between the side visible from the globe to us and the other side that remains hidden. The nearside, the face of the Moon that is visible to us from the Earth is dominated by the lunar mare — the vast, dark-coloured remnants of ancient flowing lava. Meanwhile, the far side is packed with craters and virtually devoid of large-scale lava flow.
What has been found
In the new study published in the journal Science Advances, scientists have explained that this strange geographical contrast between the two sides of the Moon is due to a giant impact billions of years ago near the lunar south pole.
The South Pole-Aitken Basin (SPA), which is the second-largest impact crater in the solar system, found on the Moon was also formed from the same gigantic impact.
This basin is the largest and oldest recognised impact basin on the Moon. Its diameter is roughly 2,500 km or 1,550 miles. The moon’s circumference is just under 11,000 km, meaning the basin stretches across nearly a quarter of the Moon. The new research also confirms how important the massive impact that formed this basin was to the history of the Moon’s formation.
“We know that big impacts like the one that formed SPA [South Pole-Aitken] would create a lot of heat,” said Matt Jones, a PhD candidate at Brown University and lead author of the new study published in Science Advances. “The question is how that heat affects the Moon’s interior dynamics. What we show is that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside. We expect that this contributed to the mantle melting that produced the lava flows we see on the surface.”
The study further points out that the asteroid collision leading to this massive impact would have created a gigantic plume of heat that propagated through the lunar interior. The researchers have also argued that this plume would have carried certain materials — a suite of rare-Earth and heat-producing elements — to the Moon’s nearside, visible to the Earth. Then that concentration of elements would have contributed to the volcanism that helped create the nearside volcanic plains.
Moon’s cratered history
The difference between how the two faces of the Moon look, had been first revealed during the US-led Apollo missions and the Soviet Luna missions. The analysis had further revealed differences in the geochemical composition and that the nearside is home to a compositional anomaly known as the Procellarum KREEP Terrane (PKT) — a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with heat-producing elements like thorium. The KREEP terrane contains elements like thorium and uranium, which decay radioactively and produce heat. This heat would have allowed that part of the Moon to remain volcanic long after the rest of the Moon had cooled.
The new study found that when the object collided with the Moon it led to major lava flows on the nearside filling the older impact craters. “What we show is that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside. We expect that this contributed to the mantle melting that produced the lava flows we see on the surface,” Matt Jones said in an official release, adding, “And the South PoleAitken impact is one of the most significant events in lunar history. This work brings those two things together, and I think our results are really exciting.”
Finally, the study concluded that any uniform distribution of the lunar crust would have been disrupted by the heat plume from the SPA impact. Thus, the two sides of the Moon are different, where one side has many more craters than the other.
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