About three billion years ago, the Earth's moon may have turned on a significantly different axis - then it shifted so that the "Man on the Moon" turned up his nose at us.
That is, a new study from planetary scientists at Texas' Southern Methodist University made this finding, which cites volcanic action as a possible cause, while looking at NASA data that held information on lunar polar hydrogen. That hydrogen was picked up by orbital instruments and is thought to be ice that is sheltered from the sun's heat by craters around the moon's south and north poles.
Led by scientist Matt Siegler, the team noted in their report that ice boils off into space when exposed to direct sunlight. Therefore, any ice that is intact in NASA data is possibly billions of years old and is a significant indicator of the direction in which the moon formerly faced.
Siegler picked up on a strange offset - or irregularity - in the pattern of ice, in the path from what is now the moon's north and south poles. After that, he and the team looked more closely at NASA's Lunar Prospector and Lunar Reconnaissance Orbiter mission data. After doing statistical modeling and analysis, they found that the ice occurs in an offset pattern by the same distance at each pole - in precisely opposite directions.
Seeing such exact opposition showed them that the imaginary pole running from north to south through the moon (its axis) had moved by a minimum of six degrees. It is likely that this happened more than one billion years ago, Siegler said.
"This was such a surprising discovery. We tend to think that objects in the sky have always been the way we view them, but in this case, the face that is so familiar to us - the Man on the Moon - changed," noted Siegler, who also works as a scientist at the Planetary Science Institute in Arizona.
"Billions of years ago, heating within the moon's interior caused the face we see to shift upward as the pole physically changed positions," he pointed out. "It would be as if Earth's axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth."
The team believes that the shift occurred partly because the moon's mass changed. This likely happened because volcanic activity around 3.5 billion years ago caused melting in part of the moon's mantle. As a result, the mantle bubbled toward the surface, likely moving the surface.
The finding was published in the journal Nature.
