October 19, 2021

I, Science

The science magazine of Imperial College

Four and half billion years ago, a meteorite the size of Mars slammed into Earth, melting the entire surface of our planet and throwing billions of tonnes of rock into space.  In time, the ejected rubble – baked dry by the explosion’s searing heat – condensed into a parched, lifeless satellite, the Moon. 

This is scientists’ best-received theory on how our nearest neighbour came to be.  Or it was… until this week’s discovery of water in rocks from a lunar volcano.

Most rocks on Earth contain a small amount of water, even those deep below the crust in a layer known as the mantle.  Water dissolved in molten rock is partly responsible for explosive volcanic eruptions.  As magma rushes to the surface, the pressure drops, the water vaporises and then violently expands – a process known as degassing.

In contrast, scientists thought Moon rock was dry and took this as key evidence for the giant impact theory.  But work published in Science by US geologists shows that volcanic rock samples collected from the Moon during the Apollo 17 mission are up to 0.14% water – similar to levels found in rocks from the Earth’s upper mantle.

Solidified lava from volcanoes provides a rare window into what makes up the inside of Earth and the Moon, but degassing removes water and other key elements from the rocks, making it difficult to understand the exact composition of a planet’s interior.

Tiny melt inclusions tell big stories

To get round this problem the US team, led by Dr Erik Hauri of Washington’s Carnegie Institution, looked for melt inclusions – tiny geological time capsules of the original molten rock that become trapped inside crystals as they start to grow before an eruption.   The crystal shields the trapped melt and stops it losing water.

By testing melt inclusions in lunar olivine crystals, Hauri’s team found water, fluorine, sulphur and chlorine at levels close to that of Earth’s mantle rocks.

Their discovery doesn’t just cause problems for the big impact theory, it also challenges the idea that light elements were brought to the Earth and Moon by a procession of smaller meteorite strikes after the giant impact.  The meteorite history of Moon and Earth are radically different, so if the composition of their mantle rocks is similar, as these findings suggest, another explanation is needed.  Dr Hauri’s team proposes two.

First, the big impact could have created a hot, turbulent atmosphere that enveloped the Earth and all the shattered debris while the Moon formed.  The light elements may have been evenly distributed throughout this atmosphere and, once everything had calmed down, the Moon and Earth ended up with a similar quota.

Second, a large portion of the rocks that make up the Moon’s interior could have simply escaped the melting and drying out proposed in the original theory.

Whatever the true story, Dr Hauri’s team has certainly opened up the field to new speculation.  And while their discovery might confuse the issue of the Moon’s birth, it could neatly explain why, in 2001, Feldman et al found water ice in its craters.