The same face of the Moon has not always pointed towards the Earth.
The spin axis of the Moon has moved by at least six degrees, and that motion is recorded in ancient lunar ice deposits, reports Matthew Siegler of the Planetary Science Institute and his colleagues in a new paper in the journal Nature.

This motion is believed to have resulted from a warm, low-density region of the lunar mantle below the dark patch of lunar mare called Oceanus Procellarum. The same heat source that caused the volcanic mare to form also warmed the mantle. This is the first physical evidence that the Moon underwent such a dramatic change in orientation and implies that the ice on the Moon is billions of years old.

Siegler is lead author on the Nature paper “Lunar True Polar Wander Inferred from Polar Hydrogen.” The new findings help explain the earliest dynamical and thermal history of the Moon and shed light on the origin of lunar water.

“We found that the polar shift required to explain the distribution of ice matches perfectly with the existence of a fossilized mantle plume below the lunar mare,” said Siegler, a PSI Associate Research Scientist. “So, the same thing that caused the dark lavas that make up the face of the Man on the Moon also caused the axis of the Moon to move — and it is recorded in the polar ice.”

“This ice distribution tells us the near side of the Moon shifted towards the north pole — so the Man on the Moon is sort of turning his nose up at the Earth. This gives us a way to model exactly where the ice should be, which tells us about its origin and where astronauts might find a drink on future missions to the Moon.”

A physical change of the lunar spin axis, known as true polar wander, can only result from a very large change in the mass distribution of the Moon. According to models by co-author and University of Arizona graduate student James Keane, this change was provided by a large, warm region of the near-side lunar mantle, which still exists, controls the current orientation of the Moon, and the face we see from Earth.

This also provides an explanation for a longstanding mystery of the odd distribution of lunar hydrogen that has been painstakingly mapped by co-author Richard Miller of the University of Alabama, Huntsville. Compared to similar temperature environments on the planet Mercury, the Moon has far less ice. As this polar migration occurred, ice formerly hidden from the Sun in shadowed craters near the lunar poles would have moved into sunlight and boiled away.

The paper shows the Moon may have once had much more ice near its poles and the ice we see today is the tiny portion that has survived this polar migration. Large amounts of ice could have been brought to the Moon by comets and icy asteroids early in the Moon’s history or potentially outgassed from the lunar maria themselves. Figuring out the origin of this ancient lunar water might also help scientists understand how water was delivered to the early Earth.

Reference: “Lunar True Polar Wander Inferred from Polar Hydrogen,” M. A. Siegler et al., 2016 March 24, Nature [].

Siegler’s work was funded by a grant to PSI from the NASA SSERVI Vortices project and NASA’s Lunar Reconnaissance Orbiter mission.

The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to solar system exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972. PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork on all continents around the world. They also are actively involved in science education and public outreach through school programs, children’s books, popular science books and art. PSI scientists are based in 22 states and the District of Columbia, and work from various locations around the world.