New NASA-funded research provides evidence that the spin axis of Earth’s moon shifted by about five degrees roughly three billion years ago.
The evidence of this motion is recorded in the distribution of ancient lunar ice, evidence of delivery of water to the early solar system.
"The same face of the moon has not always pointed towards Earth," said Matthew Siegler of the Planetary Science Institute in Tucson, Arizona.
"As the axis moved, so did the face of the ‘man in the moon’. He sort of turned his nose up at the Earth," Siegler noted in a paper appeared in the journal Nature.
Water ice can exist on Earth’s moon in areas of permanent shadow.
If ice on the moon is exposed to direct sunlight it evaporates into space.
The team show evidence that a shift of the lunar spin axis billions of years ago enabled sunlight to creep into areas that were once shadowed and likely previously contained ice.
The researchers found that the ice that survived this shift effectively "paints" a path along which the axis moved.
They matched the path with models predicting where the ice could remain stable and inferred the moon’s axis had moved by approximately five degrees.
This is the first physical evidence that the moon underwent such a dramatic change in orientation and implies that much of the polar ice on the moon is billions of years old.
"The new findings are a compelling view of the moon’s dynamic past," added Yvonne Pendleton, director of NASA’s Solar System Exploration Research Virtual Institute (SSERVI) in California.
"It is wonderful to see the results of several missions pointing to these insights," he said.
These findings may open the door to further discoveries on the interior evolution of the moon, as well as the origin of water on the moon and early Earth.