Using data from numerous missions,
including the Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater and Observation Sensing Satellite (LCROSS), researchers have gathered
evidence that the spin axis of Earth's moon actually shifted quite
substantially in the body's ancient past. The researchers were
looking at how ice is deposited at the two poles, when the mirrored
distribution revealed the secret.
The Moon might have been a constant fixture in the night sky since the beginning of human history, but we're almost constantly learning new things about it. Whether it's analysis of rocks brought home by Apollo 16 to shed light on the body's formation, or insights into where the Man in the Moon came from, we're still working to answer plenty of mysteries about Earth's lonely satellite.
The new study reveals a pretty huge revelation about the Moon – that the exact same face hasn't always been the one we see now, as the rotational axis actually shifted by some five degrees around 3 billion years ago.
In areas of the moon that are perpetually bathed in shadow, it's possible you'll find ice. But if sunlight hits a region, any ice deposits present will melt, meaning that the entire face of the moon as visible from Earth is ice-free. By looking at the distribution of ice at the Moon's poles, scientists noticed something unexpected – that the positioning of ice deposits appeared to have changed at some point in the body's history.
Using data collected by several NASA missions – namely the LRO, LCROSS, Lunar Prospector, and the Gravity Recovery and Interior Laboratory (GRAIL) – the researchers carefully studied the lunar poles. It quickly became apparent that the two regions had more in common with one another than was previously thought, with concentrations of ice displaced from each pole by the same distance, but in opposite directions, indicating that the body had shifted on its axis.
Such a huge shift would require a significant redistribution of mass. Modelling how changes in the lunar interior could have caused such a change, the team came to the conclusion that the formation and evolution of the Procellarum region on the lunar near-side, associated with high heat flow and ancient volcanic activity, was responsible. The volume of radioactive material in the region is thought to be sufficient to have heated the mantle, resulting in a density change that in turn would have reoriented the entire moon.
"The new findings are a compelling view of the moon's dynamic past," said Dr Yvonne Pendleton of the Solar System Exploration Research Virtual Institute (SSERVI), which supported the research. "It is wonderful to see the results of several missions pointing to these insights."
The results of the project are published in full in the journal Nature.