Electrons from Earth’s magnetic field may help to form water on the lunar surface during a full moon – although whether astronauts could harvest enough to drink it is unclear.
Water is made from hydrogen and oxygen, so can form when these two elements react. Because a hydrogen atom only has a single proton, sources of protons can serve as charged hydrogen atoms to bond with oxygen. Astronomers have previously observed that high-energy protons from the sun can make water on the moon by reacting with oxygen-containing compounds in rocks and glass spheres on the moon’s surface, but it was unclear if there were other cosmic sources.
Now, Shuai Li at the University of Hawai’i at Mānoa and his colleagues have used data from India’s Chandrayaan-1 mission to investigate water formation on the moon’s surface around the time of the full moon.
For a few days each month, the moon, when it is full, sweeps through a stretched-out region of Earth’s magnetic field called the magnetotail. As this happens, the moon is also shielded from protons from the solar wind, so Li and his colleagues thought that no new water should be formed then.
But the Chandrayaan-1 measurements for the moon’s rate of water formation over this period showed that it stayed constant.
“At first, it was a surprise,” says Li. “Second, I thought: ‘What is happening?’ It doesn’t make sense, because there’s no protons, so how can you form water? That means our previous understanding is partially wrong.”
He and his colleagues then looked at data on what was hitting the moon during those periods, finding that it wasn’t protons, but extremely high-energy electrons, which struck as the moon swept through Earth’s magnetotail. These have been shown in lab studies to be capable of triggering water-producing chemical reactions – in this case, electrons hit oxygen, which then allows hydrogen trapped in lunar material on the surface to bond – so the researchers think electrons are also a likely source of lunar water.
We don’t know what the relative contributions of Earth’s and the sun’s particles are to this process, though, or what form the lunar surface water is taking, says Li.
Answers to these questions could come from taking samples from the moon’s surface over a set period of time, says Romain Tartèse at the University of Manchester, UK. “You could envisage a mission where you go and pick up some soil sample in one area at one point of time in the lunar phase, and then pick up from the exact same place when the moon is at the opposite phase.”