China’s Chang’e 6 mission, currently on its way to retrieve a sample of material from the far side of the Moon, will test theories C of why the lunar near and far side are so different. Having launched on 3 May, Chang’e 6 is expected to land in early June within the double-ringed Apollo basin, which lies inside the even larger South Pole-Aitken basin (SPA). The immense SPA is the largest impact feature of its kind in the Solar System, spanning 2,400 by 2,050 kilometres (1,490 by 1,270 miles) in area. It was formed some 4.3 billion years ago, which is very early in the history of the Solar System. Though Apollo is younger, it is also the largest impact site to be superimposed on the SPA. Apollo sports a double-ringed structure, with its inner ring of mountain peaks having a diameter of 247 kilometres (153 miles) and an outer ring about 492 kilometres (305 miles) across.
As the first sample-return mission to the far side, Chang’e 6 aims to bring back to Earth about two kilograms of lunar material. The far side is a relatively unknown place; its mystery is also enhanced by the fact that we cannot see it from Earth. It was photographed for the first time by the Soviet Luna 3 in 1959. And with this photograph in hand, scientists around the world were amazed to learn how different the far side of the Moon is in comparison to the side we’re familiar with. While both the near and far side sport a multitude of craters, the near side also displays vast, volcanic plains called lunar maria, which cover something like 31 per cent of the whole near-side area. Meanwhile, the far side is quite the opposite. It’s only about one per cent covered by volcanic plains.
But how did the near and far side come to be so different? The thickness of the crust seems to be a factor. Indeed, in 2011, NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission found that on average the far side’s crust is 20 kilometres (12 miles) thicker than the near side’s crust. The reason for this is thought to date all the way back to when our Moon was formed from debris ejected when a Mars-sized protoplanet struck Earth about 4.5 billion years ago. As the Moon coalesced from the debris around a wounded Earth, it became tidally locked, meaning it always shows the same face to our planet. Earth’s surface was rendered completely molten by the giant impact, and it radiated heat onto the nearside of the Moon, keeping itself molten for longer. Rock would vaporise on the near side and condense on the cooler far side, scientists suggest, making the crust on the far side thicker. “The fundamental finding indicates that the crustal thickness discrepancy between near side and far side may be the primary cause of lunar asymmetrical volcanism,” said Yuqi Qian of the University of Hong Kong.