The Chinese lunar program has brought interesting information that can help answer the question that has long tormented scientists about why the reverse side of the moon is so different from the visible. Science News explains how the flip side of the moon looks like a layer cake and why scientists blame meteorites for cruelty.
The discovery allows us to explain why the visible side looks different.
The reverse side of the moon is like a layer cake. According to new data from the Chang’e-4 interplanetary station and the Yutu-2 lunar rover, it consists of alternating layers of stone and soil to a depth of 40 meters, which suggests a strong impact in the past, scientists said in a February 26 science journal Advances.
“We know a lot about the visible side of the moon thanks to the Soviet Lunokhod and the American Apollo program, but little is known about the reverse side,” says Yan Su, a researcher at the Beijing branch of the Chinese Academy of Sciences in Beijing. “For the first time, Chang’e-4 gives us the opportunity to obtain reliable stratigraphic data about it.”
Chang’e-4 and Yutu-2 were the first spacecraft to land on the far side of the moon. In January 2019, they descended into the 186-kilometer crater Von Karman. Exploring a crater located in the 2500-km South Pole – Aitken basin, the lunar rover sent down radar pulses, figuring out the composition of the surface under its wheels.
A scientist from the Chinese Academy of Sciences Chunlai Li and his colleagues analyzed the 106-meter path traveled by a lunar rover in the first two lunar days (approximately two Earth months) of data collection. They found that on top lies a layer of loose soil (regolith) about 12 meters thick.
“It looks like very clean sand, like on a beach,” said study coordinator Elena Pettinelli from University of Rome III.
Below, the apparatus found another 12-meter layer of coarser material, stuffed with cobblestones like a cherry pie. Further to a depth of 40 meters – the radius of the radar – lie alternating layers of coarse and granular matter.
These layers, according to scientists, arose due to impact emissions. Basically, the surface of the Von Karman crater is an even layer of cooled lava created by long-standing volcanic activity. However, meteorites crashing into the lunar surface repeatedly hit this lava, leaving craters and covering the surface with emissions from impacts.
“This is a very brutal process,” said moon geologist Daniel Moriarty, who works at the Goddard Space Flight Center (NASA’s Greenbelt, Maryland lab) and was not involved in the study. Part of the emission material comes from the deep bowels of the moon, possibly even from the mantle.
The upper layer of smooth sand probably arose as a result of crushing of the surface by micrometeorites and the influence of strong temperature shifts.
The picture of the lunar surface structure, obtained thanks to Chang’e-4, differs from the data obtained from its predecessors. Chang’e-3 and its Yutu lunar rover landed in the Sea of Rains on the visible side of the Moon in 2013. Then the lunar rover radar was blocked by dense volcanic rock at a depth of only about 10 meters. Probably, on the visible side of the zone of volcanic eruption are closer to the surface than on the back.
“The structure of the Chang’e-4 landing site is more complex … and implies a completely different geological context,” says Su. According to the researchers, the lava base of the Von Karman crater can lie so deep that Yutu-2 is simply not able to “probe” it.
In the future, scientists expect to understand why on the visible side of the moon there are many lowlands with a smooth surface of volcanic rock (seas), and the reverse side is uneven and covered with craters.
“The question of why the visible side of the moon is so different from the back is considered one of the key issues in the science of the moon,” Moriarty explains. “If the new information allows us to partially clarify the history of volcanic activity on the reverse side, this will be a very important step.”