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Moon’s Poles Changed Location Billions Of Years Ago

The poles of Earth’s moon were not always in their current position, according to a paper, entitled “Lunar True Polar Wander Inferred From Polar Hydrogen”, published in Nature.

The ancient and current north and south poles of the moon. Photo credits: JAMES TUTTLE KEANE.

The ancient and current north and south poles of the moon. Photo credits: JAMES TUTTLE KEANE.

The moon’s poles might not have always been in their current position. Rather, they are likely to have shifted spot throughout the last billions of years, according to a new paper that argues extinct lunar volcanoes are the culprit. The research was led by Matt Siegler from the Planetary Science Institute in Tucson.

The poles are no more where they used to be: their ancient spots are marked by water ice, say the researchers. These deposits might have once accumulated near the poles, and when the latter shifted, the water would have been left behind. They are currently located on exactly opposite sides of our satellite. The current poles (each of them) are found about 6 degrees from them. Both pairs can be linked via a straight line passing through the center of the moon. This precise opposite location characterising each pair (the ancient wandering pole and the current one) is indicative of how the moon axis must have shifted by at least 6 degrees.

Hydrogen deposits (greenish-blue glow) mark where the moon’s poles are today (blue arrow) and where they used to be (green arrow). Interior heat flow (cross-section colors, red is hotter) drove ancient volcanoes that threw the moon out of balance billions of years ago and shifted the poles’ locations. Photo credits: JAMES TUTTLE KEANE.

The current poles marked by hydrogen deposits (greenish-blue glow; blue arrow), and their ancient positions (green arrow). Interior heat flow shown in the cross section (red is hotter). Photo credits: JAMES TUTTLE KEANE.

The researchers, therefore, think the moon departed from its original axis, maybe about 3 billion years ago, by 6 degrees, which are equivalent to a distance of 200 km. This movement might have happened over the course of 1 billion years. To help readers to better understand the event, he explained that the equivalent of this shift would be is the Earth’s axis moved from Antarctica to Australia.

The ancient poles were identified when Siegler and his team re-examined data from NASA’s Lunar Prospector mission. Of this bulk of information are the recorded speeds of neutrons that flow from the surface of the moon. The Prospector had found areas lacking high-speed neutrones which would have been slowed down by hydrogen atoms that were part of water molecules. These very hydrogen deposits were used to map the water ice. The hydrogen is thought to be in the form of ice that lies in craters around the north and south poles of the moon. When it is exposed to direct sunlight, the ice boils off into space, thereby indicating the ancient orientation of the satellite.
Siegler points out how we might tend to think that heavenly bodies have always been as we see them today, but that, in the case of the moon, it is not actually so.

Now, what caused this shift? According to Siegler, the moon’s interior heated billions of years ago (the change in mass was on account of an internal event), thereby causing the physical change in position of the poles. A heavy volcanic area would have caused the polar wander, the shift of a large, single mantle plume. A proportion of the mantle allegedly melted around 3.5 billion years ago because of ancient volcanic activity, which caused it to bubble to the surface.

There is one region of the moon’s crust called Procellarum that is said to be radioactive; this would have served as an oven broiler that heated the mantle underneath. This melting would have led to the formation of dark patches (ancient lava) we see on the moon. Since this piece of hot mantle would be lighter than the other regions of mantle that were colder, the change in mass made Procellarum to move, and with it, the whole moon moved. This movement is the event that occurred over a billion years, thereby leaving the trace of the path in the moon’s ice.

This axis-shifting phenomenon constitutes a rare occurrence. Few heavenly bodies have gone through this change as a consequence of mass shift which scientists refer to as the “true polar wander”. According to researchers, the only other planetary objects falling into this group are our very own Earth, Mars, Europa (Jupiter’s moon), and Enceladus (Saturn’s moon). The moon is different, though, because of its polar ice that clearly indicates the path taken by the shifting poles.

The lunar ice might help us resolve certain mysteries of the Universe, says Siegel. The origin of the water on Earth is unknown. It is thought to have come from the outer solar system. But, nothing is confirmed. Ice on other planets like Mercury or on the moon might help answer these questions. This ancient ice is described as a “time capsule” that could lead us back to the original supply of Earth – something that is not available here; we do not have anything that old that has remained on our planet.

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