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One side of the moon is filled with far more craters than the other, and scientists finally know why: A massive asteroid that hit the moon about 4.3 billion years ago created chaos in the moon’s mantle, according to a new study.
More than 9,000 visible craters mark it moonthanks to barrage of meteor showers, asteroids and comets over billions of years, according to International Astronomical Union (opens in new tab). However, these craters are not evenly distributed over the surface of the moon. The other side of the moon that people never see from the earth because the moon is tide-locked (meaning it takes the same amount of time for the moon to rotate and orbit the Earth), it has a significantly higher concentration of craters than the visible near side.
The near side of the moon has fewer pits because the surface is covered by the moon’s maria – large stretches of solid lava that we can see with the naked eye on Earth as dark spots on the moon. These lava fields probably covered the craters that would otherwise have marked the near side of the moon. The other side of the moon has almost no lunar maria, which is why its craters are still visible.
Scientists have long suspected that the lunar Maria was formed in the wake of a massive collision about 4.3 billion years ago. This collision created the South Pole-Aitken Basin (SPA), a huge crater with a maximum width of about 1,600 miles (2,574 kilometers) and a maximum depth of 5.1 miles (8.2 km), which is the largest pit on the moon. and second largest confirmed impact crater in solar system. But until now, scientists have not been able to explain why only the nearest side of the moon has lava fields.
Related: How many space rocks hit the moon each year?
The new study finds that the SPA influence created a unique phenomenon inside the mantle of the moon, the magma layer beneath the crust, which only affected the nearest side.
“We know that big influences like the one that formed the SPA would create a lot of heat,” lead author Matt Jones, a doctoral student in planetary science at Brown University, said in a statement (opens in new tab). “The question is how the heat affects the inner dynamics of the moon.”
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Scientists already knew that near-surface lava fields originated from the lunar mantle because lunar samples brought back by the Apollo missions contained radioactive, heat-generating elements such as potassium, phosphorus and thorium, all of which are suspected to be found in abundance within the lunar mantle, according to the statement.
In the new study, computer simulations revealed that the SPA impact would have created a heat tab in the mantle that pushed the radioactive elements against the crust. The researchers repeated the simulation for a number of possible scenarios of the SPA impact, including direct hits and lightning strikes, and found that no matter how the asteroid hit, the mantle impacts would have only affected the near side of the moon.
Put another way, when a space rock collided with the moon, it caused lava from the mantle to flow out on the near side and buried many of its older impact craters.
“What we are showing is that under all plausible conditions at the time the SPA was formed, it ends up concentrating these heat-producing elements on the nearest side,” Jones said. “We expect this to have contributed to the mantle melt that produced the lava flows we see on the surface.”
The researchers are pleased to have solved what they described as “one of the most significant issues in lunar science,” according to the statement.
“The SPA impact is one of the most significant events in the history of the moon,” Jones said. Being able to better understand how it shaped the two sides of the moon we see today is “really exciting,” he added.
The study was published online April 8 in the journal The progress of science (opens in new tab).
Originally published on Live Science.
Editor’s note: This story was updated at. 12:15 EDT to correct a description of tide locking.