Earth's pull 'shaped Moon's surface'

By Neil Bowdler
Science reporter, BBC News

image captionThe Moon, showing part of its far side, photographed from the Apollo 16 spacecraft in April 1972

The Earth may have played a major role in shaping the lunar surface, according to a new research study by US researchers.

The team members say our planet's gravitational pull distorted the shape of the Moon in ancient times.

This led to "bulging" at the equator and could explain why the far side is more elevated than the near side of the Moon even today.

Details of the study are published in the journal Science.

The far side of the Moon remains a mystery in many ways. Densely cratered, it has few of the volcanic plains that characterise the near side with which we are all familiar, and it is much higher - several kilometres higher in places.

Now the authors of a new study think they know why it might have become so high and are blaming the Earth.

Over four billion years ago, soon after the Moon's formation and before it had solidified to its core, its crust floated on a sea of magma.

It was during this time that the Earth was able to tug on that floating crust, distorting it, much like the Moon tugs on the Earth's oceans today creating the tides.

This "tidal bulge" in the Moon's shape led to the crust melting and thinning at the poles where the strain was greatest, while the crust around the equator remained thicker and fatter, say the scientists.

Tidal bulge

Professor Ian Garrick-Bethell of the University of California in Santa Cruz, who led the study, believes tidal processes can explain between 25% and 40% of the Moon's topography.

But while the evidence for this remains in the elevated surfaces of the far side, the evidence on the near side has been obliterated by subsequent volcanic activity.

"Since the Apollo missions... people have known that the topography on the far side of the Moon is much more elevated than anywhere else on the moon," said Professor Garrick-Bethell.

"Our study is the first to quantify the structure and shape on the far side and in doing that, we were able to demonstrate that this topography bears the signature of a tidal process."

The process by which the stretching and pulling of the Moon by the Earth leads to variations in crust thickness is called "tidal heating".

"You can imagine that the part of the Moon that's pointing towards the Earth will start to flex and begin to be drawn towards the Earth and away from the Earth on the far side, and during this, the poles will be stretched and heated."

"The amount of strain in the rock is going to be greater at the poles so there will be more heating at the poles and there'll be thinner crust at the poles."

Tidal bulging probably occurs across the solar system today. The oceans on Earth bulge as a result of the Moon's gravitational pull and the same process is thought to occur on Jupiter's moon Europa.

Europa consists of an ice crust on an ocean of water, and Jupiter's massive gravitational pull is thought to distort the moon's shape much like the process described in this new paper.

A similar process could be occurring on the moon Titan, Saturn's largest moon.

More on this story