Ice deposits that formed in craters on opposite sides of the Moon three billion years ago indicate it may have once spun on a different axis.

Volcanic activity in the Moon’s interior billions of years ago may have moved the lunar poles to their present position, according to a team of scientists from the Planetary Science Institute in Tucson, Arizona.

They said their calculations, published in the journal Nature, open the way to better understand how water reached the inner solar system.

Lead author Dr Matthew Siegler said the Moon’s axis had moved about six degrees to the rotation we see today.

“It takes a huge change in the mass of the Moon to do that — something like a giant crater or volcano forming,” Dr Siegler said.

He said the team was able to source the change in rotation to the centre of the Procellarum region — the black part of the face of the moon — which is also the centre of nearly all of the volcanism on the Moon.

“By looking at how much the pole moved, we are able to see how this region, which is about 30 per cent of the Moon, and the interior of the Moon evolved,” Dr Siegler said.

Because the Procellarum region was most geologically active early in the Moon’s history, Dr Siegler said it was likely the change in spin axis, known as polar wander, occurred billions of years ago.

Hydrogen deposits evidence of ‘palaeopoles’

The team used modelling to determine what changes in density needed to happen in the Moon’s interior to cause a six-degree movement in the satellite’s orientation.

While analysing lunar hydrogen data from the Moon from almost a decade ago, Dr Siegler’s team noticed that each lunar pole had a hydrogen deposit that was slightly displaced from the true north and south poles.

These hydrogen deposits were directly opposite each other, so that a line drawn from one to the other would pass through the centre of the Moon — and were located equal distances from their respective poles, but in opposite directions.

This suggested these deposits were evidence of “paleopoles” and that the Moon’s spin axis had shifted to its current alignment.

A window on the solar system’s water

Dr Siegler said because the hydrogen was most concentrated in the Moon’s extreme cold regions, it was believed to be water.

“It is really cold in the shadowed craters near the lunar poles, most areas never get above minus 170 degrees Cesius,” Dr Siegler said.

“At these temperatures water ice acts like any other rock — it doesn’t melt or evaporate — so it can stick around forever.

“The ice we are observing, or at least most of it, had to have arrived before the spin axis changed. That likely happened around three billion years ago.

“That means that the moon can provide a record on how water reached the inner solar system, which is generally accepted to have started dry.”

He said future missions sampling the ice might be able to determine whether the water came from comets and asteroids, or from volcanoes on the moon itself.

NASA already had plans for a lunar rover mission in the 2020s that would drill for ice to one-metre depth, however bringing ice samples back was still in the distance.

“The hope is always that new findings like this might influence NASA’s plans,” Dr Siegler said.