Giant Impact On Lunar Surface Billions Of Years Ago Resulted In Two-Faced Moon: Study

The near side is characterised by the presence of the lunar mare, which represents the vast, dark-coloured remnants of ancient lava flows. The far side of the Moon is crater-pocked.

Two-faced Moon: Near side of the Moon (left); Far side of the Moon (right) | Photos: NASA

New Delhi: The side of the Moon which is visible to people on Earth appears far different from the side which the Moon hides on its far side. 

The side of the Moon which faces our planet is known as the near side of the Moon, while the other side is called the far side. 

Difference Between Near And Far Sides Of The Moon

The near side is characterised by the presence of the lunar mare, which represents the vast, dark-coloured remnants of ancient lava flows. 

The far side of the Moon is crater-pocked, and is virtually devoid of large-scale features like those present on the near side.  

The two sides of the Moon being different, or, in other words, the Moon being 'two-faced', is one of the most enduring mysteries of Earth's natural satellite.

Researchers have now provided a new explanation for the 'two-faced Moon'. It states that a giant impact billions of years ago near the Moon's south pole is responsible for the two sides of the Moon being different.

The study, led by researchers at Brown University, United States, was recently in the journal Science Advances. 

What Created The Near Side Volcanic Plains?

The research shows that the impact that formed the Moon's giant South Pole–Aitken (SPA) Basin would have created a massive plume of heat that propagated through the lunar interior. 

The plume would have carried certain materials to the Moon's near side, including a suite of rare-Earth and heat-producing elements. Due to the concentration of the elements, volcanism would have occurred, which, in turn, created the near side volcanic plains.

In a statement issued by Brown University, Matt Jones, the study's lead author, said it is known that big impacts like the one that formed SPA would create a lot of heat. 

He said that researchers are trying to figure out how that heat affects the Moon's interior dynamics. 

The scientists have shown that under any plausible conditions at the time that SPA was formed, it ends up concentrating the heat-producing elements in the near side. This could have contributed to the mantle melting that produced the lava flows one can see on the lunar surface, according to Jones.

In the 1960s, the Soviet Luna missions and the United States Apollo program revealed the differences between the near and far sides of the Moon. 

The differences in volcanic deposits have been determined. However, future missions will reveal differences in the geochemical composition also.

Compositional Anomaly On Near Side Of Moon

There is a compositional anomaly on the near side, known as the Procellarum KREEP terrane (PKT). This is a concentration of potassium (K), rare-Earth elements (REE), phosphorus (P), and heat-producing elements like thorium. 

The largest volcanic plain on the nearside, known as Oceanus Procellarum, is rich in KREEP, according to the study.

There could be a connection between the PKT and the near side lava flows, some scientists suspected. However, the reason behind the suite of elements being concentrated on the near side remained unknown. 

The new study provides an explanation to the suite of elements being concentrated on the near side. The explanation is connected to the South Pole–Atiken basin, the second largest known impact crater in the solar system.

The scientists conducted computer simulations of how heat generated by a giant impact would alter patterns of convention in the Moon's interior, and how this occurrence might redistribute KREEP material in the lunar mantle. 

The last part of the mantle which solidified after the Moon's formation is thought to be represented by KREEP. 

It likely formed the outermost layer of the mantle, just beneath the lunar crust, according to the study.

The KREEP material would have ridden the wave of heat emanating from the SPA impact zone like a surfer, according to the model. 

The material was distributed en masse to the near side, as the heat plume spread beneath the Moon's crust.

According to the scientists, the research provides a credible explanation for one of the Moon's most enduring mysteries.

Jones said that how the PKT forms arguably the most significant open question in lunar science. He added that the South Pole–Atiken impact is one of the most significant events in lunar history, and that this work brings those two things together.