When we look up at the full moon, we only ever see one face: the “man in the moon” is always gazing back at us. Scientists believe that the far side, eternally hidden from view, may hold the key to fundamental mysteries about the moon’s formation and its earliest history. China’s Chang’e four mission could reveal new clues to the cataclysmic collision that created the moon and uncover the origins of the water that is unexpectedly abundant in lunar soil. The moon’s far side is sometimes known as the dark side, although it is not darker than the near side in any literal sense. It undergoes the same phases of illumination by the Sun as the side facing Earth. But because the moon spins on its axis at the same rate as it orbits Earth, one side remains permanently out of view.
It was only in 1959 when the first images of the far side were beamed back by the Soviet Union’s Luna 3, that intriguing differences were revealed. The far side is pockmarked by more craters and appears almost devoid of the seas of solidified lava, known as maria, that form the shadowy shape of a face that we see from Earth. Scientists believe that as asteroids pummelled the lunar surface during the solar system’s early history, giant lava flows on the moon’s near side filled impact craters, obscuring them from view. On the far side, fewer lava flows occurred, leaving intact a pristine record of ancient impacts. The oldest, largest and deepest of these is the Aitken Basin, where Chang’e four is now poised for exploration. Testing the composition of the soil could help narrow down theories about how the moon formed and the beginnings of the solar system.
The mission will also conduct the first astronomy observations from the moon’s far side, which is seen as a uniquely attractive site for monitoring radio waves coming from deep space. Astronomers operating Earth-bound radio telescopes have to constantly grapple with electromagnetic interference from human activity: shortwave broadcasting, maritime communication, telephone and television signals. The far side of the moon is shielded from such signals, making it far easier to pick up faint fingerprints left by the Big Bang. These traces left across the cosmos could help tell us how the universe inflated at unimaginable speed in the first trillionth of a second after the Big Bang.
Chang’e four could also help pave the way for China’s human spaceflight ambitions. One theory for the abundance of water in lunar soil is that it is produced by reactions between the solar wind (flow of charged particles including hydrogen) and minerals in the soil (containing oxygen). “People are trying to reproduce this process in the lab, but it’s really difficult to do correctly,” said Martin Wieser, a researcher at the Swedish Institute of Space Physics and the principal investigator on an instrument onboard Chang’e designed to measure how the solar wind interacts with the lunar surface. “The only way is to go to the lunar surface and look at it there. Chang’e 4 is in the ideal place to study this process.” Understanding how water is produced and distributed in the lunar soil could be crucial for establishing a more permanent human lunar outpost. The amount in the soil is not large – an area of 10 sq metres might contain only enough water to fill a glass – but harvesting it could eventually be cheaper than bringing water from Earth. The craft is also carrying a mini-greenhouse, which will test how well plants, specifically potatoes and small flowering Arabidopsis plants, related to cabbage, grow on the moon.
Credit: Hannah Devlin for The Guardian, 3 January 2019.