For years, scientists have believed frigid craters at the Moon’s poles hold water ice, which would be both a scientific boon and a potential resource for human missions. Now, researchers have discovered a reason why the ice has persisted on an otherwise bone-dry world: Some polar craters may be protected by ancient magnetic fields.
“It’s really exciting,” says Jim Green, NASA’s former chief scientist, who was not involved with the work. “It makes these areas even more fascinating.”
Hundreds of polar craters are in permanent shadow because of the Moon’s small tilt to the Sun, 1.5° compared with Earth’s 23.4°. The Sun never rises above their rims, keeping temperatures as low as –250°C. In some of the pits, radar instruments on orbiting spacecraft have detected the reflective signature of water ice, perhaps delivered by comet impacts. And in 2018, scientists using an instrument aboard India’s Chandrayaan-1 spacecraft reported measurements showing how molecules of polar ice absorbed infrared light—some of the most definitive evidence yet.
Explaining the ice’s survival has been a challenge, however. Although sunlight doesn’t reach the craters, the solar wind does, and these charged particles can destroy the ice, molecule by molecule, in a process called sputtering. “It’s highly erosive,” says Paul Lucey, a planetary scientist at the University of Hawaii, Manoa, who was also not involved with the work. “The ice would be gone in a few million years.”
Planetary scientist Lon Hood and his colleagues at the University of Arizona now think they know why the ice sticks around. In research presented last week at the Lunar and Planetary Science Conference in Houston, they showed that magnetic anomalies, remnants from the Moon’s ancient past, may be protecting some of these craters. “These anomalies can deflect the solar wind,” Hood says. “We think they could be quite significant in shielding the permanently shadowed regions.”
Researchers have known about the anomalies ever since the Apollo 15 and 16 missions in 1971 and 1972, when astronauts measured regions of unusual magnetic strength on the surface. Some anomalies are now known to be up to hundreds of kilometers across. Although their origin is debated, one possibility is they were created more than 4 billion years ago when the Moon had a magnetic field and iron-rich asteroids crashed into its surface. The resultant molten material may have been permanently magnetized.
Thousands of the anomalies are thought to exist across the lunar surface, but Hood mapped ones at the south pole in detail using data from Japan’s Kaguya spacecraft, which orbited the Moon from 2007 to 2009. He found at least two permanently shadowed craters that were overlapped by these anomalies, the Sverdrup and Shoemaker craters, and there are likely more. Although the remnant fields are thousands of times weaker than Earth’s, they could be sufficient to deflect the solar wind.
Craters with known anomalies could become prime targets for science and exploration. NASA is already planning to visit the south polar region with a rover due for launch next year, called VIPER, and the agency intends to send humans there later this decade as part of its Artemis program. Studying the ice could reveal how it was delivered, which may in turn shed light on how Earth got its water.
More data are needed to confirm the fields’ protective effect. Hood would like to put a solar wind instrument on the surface, which could measure the charged particles that pass the rim of the crater. “You would also need to collect samples and identify what is magnetized,” he says.
Currently no such mission is planned. But given the renewed focus on the Moon for many space agencies, Lucey thinks the mystery of the icy craters needs to be studied. “Why are some places icy and some not icy?” Lucey asks. “These magnetic fields need to be investigated as a possible explanation.”