The Moon's violent, volcanic youth may have knocked the satellite right off its axis.
That’s according to a group of US scientists, who claim hydrogen-rich deposits on the lunar surface mark the Moon’s ancient poles. The findings could help understand how the Moon evolved below its surface.
As the Moon's axis is perpendicular to the direction of the Sun, its poles are extremely chilly. Temperatures at the Moon’s north pole have been recorded at -247 °C, which is among the coldest in the Solar System – even colder than Pluto.
This means the poles are capable of preserving ice for billions of years. In the 1990s, scientists pinpointed ice deposits in craters at the lunar poles by measuring hydrogen atoms. These ice deposits uncovered how water moved across the Moon’s surface.
In this study, the team reanalysed the hydrogen-deposit data around the Moon’s poles.
They noticed that each of the Moon’s poles has a deposit – or a patch of ice – displaced by the same distance from the “true” pole today. The team believe these patches mark lunar poles from billions of years ago, shifted by 6 ° from the Moon’s current axis.
This shift likely happened through “true polar wander”. If mass in a planet or moon reshuffles, the axis of rotation shifts to keep most of the mass along the spin axis – like an ice skater drawing in his arms before a spin.
Temperatures at the Moon’s north pole have been recorded at -247 °C
Scientists believe shifting mass once toppled Jupiter’s moon Europa off its axis by 80 °.
“The idea that true polar wander caused the shifts […] makes a lot of sense,” writes University of California, Santa Cruz, planetary scientist Ian Garrick-Bethell in a News & Views article.
So how did the Moon’s mass shift?
The authors believe the best candidate is the Procellarum KREEP Terrane (PKT) – the Moon’s most radioactive area, and believed to be once the most volcanic.
There are still a couple of holes to the theory. For one, there is no explanation for why these ancient “palaeopoles” would have formed in concentrated spots, rather than a drifting line towards the Moon’s current poles.
And given that these old ice deposits are now exposed to sunlight, the team wondered why they lasted billions of years since the proposed shift. (One possible explanation is that the ice is hidden underground, like the permafrost in Alaska.)
But if the PKT is indeed responsible for the Moon’s spin axis shift, Garrick-Bethell believes, the finding could expose more of the Moon’s past.
“It would help to constrain the magnitude of the heating events that occurred in that region […] tell us how deep into the Moon the PKT reaches, and limits the timescales over which it became hot,” he writes.
“This is important for understanding how the PKT became so radioactive in the first place,” he adds, “an enduring mystery in lunar science.”
The work was published in the journal Nature.
Viviane Richter is a freelance science writer based in Melbourne.
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