"We tend to think that objects in the sky have always been the way we view them," says planetary scientist Matt Siegler. But that most viewable of objects—our moon—hasn't always looked as it does today, according to Siegler and his colleagues. They believe that some 3 billion years ago, in a process that took some 1 billion years to complete, the moon's axis was nudged 125 miles, or 6 degrees. Siegler provides a great visual: "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 finding was born of research into lunar polar hydrogen: Where the sun doesn't shine, this hydrogen should take the form of ice; where it does, the hydrogen boils off, a press release explains.
The Christian Science Monitor reports that in 1998, scientists found hydrogen deposits, believed to be composed of ice, where they seemingly shouldn't be: not at the north and south poles, but 6 degrees off those poles. Now, a deeper dive into data gathered by NASA established that the offset occurred in identical amounts, and in precisely opposite directions, leading to the team's theory, published Wednesday in Nature. The axis shift gains the moon entry into a small group of so-called "true polar wanderers": Mars, Enceladus and Europa (moons of Saturn and Jupiter, respectively), and Earth. Ian Garrick-Bethell, who reviewed the paper for Nature, says the theory raises two big questions: Why wasn't a dragging trail of ice formed over the billion-year shift, and why didn't the hydrogen boil off in its new sun-touched position? The BBC reports the team suggests the ice might have been buried as asteroids hit the planet. (Read more moon stories.)