Thin crust hints at moon's watery youth

Experiments show the lunar crust needed water to crystallise to the thickness it is today. Jana Howden reports.

NASA's Galileo spacecraft snapped images – stitched together here – of the moon's north pole in December 1992. A new study suggests the moon's early magma ocean contained water when it crystallised and produced the crust.

Looking up at the lump of rock circling our planet, it’s hard to imagine our moon ever harboured water.

But a team in the Netherlands calculated that if the moon’s crust formed without water, it would be around twice the thickness it is today.

The work, published in Nature Geoscience, provides an insight into the Earth’s composition while the moon was still a baby. Did it acquire water from comets and asteroids or was it there all along?

Astrobiologist Jonti Horner, from the University of Southern Queensland, Australia, who wasn’t involved in the research, commented that the study presented “important data for people discussing early versus late hydration [of Earth]”.

It’s widely thought that our largest satellite was born from a collision between Earth and another planetary body, which left a disc of superhot material – part of which eventually coalesced into a magma-rich moon.

Over time, dense minerals on the moon sank to the bottom while those less dense floated.

Eventually, the top layer – a mineral called plagioclase – cooled and crystallised to form the rocky crust.

Yinhao Lin and colleagues at Vrije University in Amsterdam wanted to see if they could reproduce these early conditions in the lab and use those results to model the evolution of the moon’s crust.

They mixed combinations of minerals you’d find on the moon and heated them in a high-pressure chamber. Plagioclase emerged from the mix, as expected.

But when they measured the rate plagioclase was produced and popped that in their modelling of a magma-rich moon, calculations indicated that the present-day moon should sport a crust almost 68 kilometres thick – much thicker than measurements taken by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission of between 34 and 43 kilometres.

What was missing? The answer, it seemed, was water.

The team didn’t add water to their initial experiments, so they decided to see how plagioclase production – and therefore crust thickness – would change when water was added to the mix.

Indeed, adding the equivalent of up to 3,150 parts per million of water to a 700-kilometre-deep magma ocean produced plagioclase at a slower rate.

This decreased their model of the crust’s thickness to 42 kilometres – within the range of NASA’s observations.

While the researchers acknowledge that there could be many ways for the moon’s crustal thickness to reach between 34 and 43 kilometres, they maintain that, in every scenario, water must have been present as the magma ocean cooled and crystallised.

And because the moon and Earth coagulated from the same hot disc, the moon’s wet start suggests that Earth also contained water at the time.

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