If you looked up at the Moon from Earth a few billion years ago, it might not have looked the same as today. New research has suggested that the Moon’s axis has shifted by six degrees, with its poles being located in different positions.
The theory was presented in a study published in Nature, led by Matthew Siegler from the Planetary Science Institute in Arizona. Using data from NASA's Lunar Prospector spacecraft, which orbited the Moon in the late '90s, scientists analyzed evidence that frozen hydrogen, likely water ice, is located near the poles.
The Moon’s poles are among the coldest reaches of the Solar System, where water ice can remain stable for billions of years trapped in permanently dark craters. But looking at available data, scientists found that some deposits were not located exactly at the poles today, but rather nearby. And deposits at both poles were shifted by the same amount. This suggests that, about 2 billion years ago, the Moon was angled slightly differently to what we see today.
“This is exciting because we have found that the history of the lunar interior is recorded in the last place you would think to look – the ice at the poles,” Siegler told IFLScience. “The fact that we have this record also means much of the ice on the Moon is very old. That means that if we can get a sample of the ice, it might tell us about the delivery of water to the early Solar System.”
How the view of the Moon from Earth may have shifted over time. James Keane
According to the researchers, the cause of this shift may have been a spike in temperatures beneath the surface of the Moon, caused by radiogenic heating – the decay of radioactive elements, which is a source of heat not only on the Moon, but other worlds like Pluto. And it may provide information on the Moon’s volcanic evolution.
If the distribution of mass inside a body significantly changes, it can change the orientation of the body – something evidenced recently on Mars. The authors of this latest study suggest that a region on the Moon called the Procellarum KREEP Terrane (PKT), the Moon’s most radioactive area and also once the most volcanic (forming many of the dark plains we see today), was once hotter and therefore less dense than it is now. This shift in density caused the so-called “polar wander.”
“If a huge pile of lead weights suddenly appeared in New York, the city’s latitude would eventually shift to a position slightly southward because of planetary reorientation,” Ian Garrick-Bethell from the University of California, Santa Cruz, wrote in an accompanying News and Views article. “The opposite is also true – if New York suddenly became lower in density, it would shift northward. This process is known as true polar wander.”
The study raises some interesting points. It could help scientists figure out the extent of the PKT, essentially how deep it goes, which may be a step towards working out how it got so radioactive. But it also raises questions, notably in that the Moon’s volcanism ended about 3 billion years ago, so the PKT should have been getting colder since then, not hotter. This should have meant the polar wander was in the opposite direction to that observed.
Other research has also suggested even larger shifts for the lunar axis, as much as 60 degrees, so the authors note their study will have to be combined with others to really get to the bottom of the story. One thing seems clear, though: The face of the Moon as seen from Earth has not always remained the same.