The moon’s poles have slightly shifted over the last several billion years, a new study suggests. And extinct lunar volcanoes might be to blame.
Ancient deposits of water ice mark where the poles used to be, researchers report online March 23 in Nature. These deposits are probably left behind by water that collected in shadowed locales that used to be at the poles, Matt Siegler of the Planetary Science Institute in Tucson and colleagues suggest. The deposits are on opposing sides of the moon, offset from the contemporary poles by about 6 degrees and can be connected by a straight line drawn through the moon’s center.
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Siegler and colleagues found the ancient poles by reanalyzing data from NASA’s Lunar Prospector mission, which launched in 1998 and spent 18 months orbiting the moon, alongside a bevy of insights from recent moon missions. One of Prospector’s instruments recorded the speeds of neutrons streaming from the lunar surface. The spacecraft found regions that lack high-speed neutrons, which presumably have been slowed by hydrogen atoms from water molecules. Those hydrogen deposits are useful proxies for mapping water ice.
Poles can wander as mass shifts inside a planet or moon. In this case, a region within Oceanus Procellarum — a vast dark landscape that faces Earth — is the most likely culprit. Procellarum is a sea of hardened magma left behind by volcanic eruptions. After lunar lava relocated to the surface more than 3.5 billion years ago, the poles meandered as the moon rebalanced itself, the researchers suggest.