First evidence that asteroids delivered water to the Moon

31 May 2016

An artist's impression of the lunar magma ocean

An artist's impression of how the Moon looked while it was still partially molten. This is the period during which asteroids delivered the Moon's water. © NASA/GSFC

Water on the surface of the Moon was supplied by asteroids, a study has shown.

Until now, the origin of water on the moon has been a contested topic.

But a new investigation of lunar samples, published in Nature Communications, shows that asteroids were the main source.

Professor Sara Russell, Head of Mineral and Planetary Sciences at the Museum and a co-author on the study, says the findings give us an insight into the origin of water in our solar system.

She says, 'We have shown that asteroids were responsible for the majority of water and nitrogen delivered to the Moon, between 4.5 and 4.3 billion years ago.

'It is an exciting finding, because the Earth probably got its water in exactly the same way.

'We now know much more about the types of objects impacting on both the Moon and the Earth in the time just after they formed.'

A giant time capsule

Little was known about exactly where the Earth and the Moon got their water from.

Since the Earth's formation, its geological record has been worn away because of moving plate tectonics, volcanic activity and weathering.

But the Moon has no tectonic movements, active volcanoes or weather systems, meaning any evidence of past impacts is left on its scarred surface. 

Professor Sara Russell studies a meteorite

Prof Sara Russell studies meteorites that hold valuable information about the formation of our solar system

 

By studying the preserved geology, scientists can learn more about the kind of bodies that crashed into both the Moon and the Earth during their early history.

Chemical data

Previous studies have shown that water on the Moon has a similar chemical makeup to Earth's water.

This suggests that the Moon's water was either inherited from the Earth before the two bodies split, or was delivered to the Earth-Moon system shortly after they formed.

Prof Russell and her fellow researchers looked at chemical data from lunar samples. They also examined data from meteorites and comets.

Meteorites that originate from asteroids are mainly made up of metals and rocky material, whereas comets are made up of ice, dust, rocks and organic compounds.

Prof Russell says, 'It is perhaps surprising that asteroids were responsible for the water on the Moon, since they are mostly made up of rock and metal. Small amounts of water can be contained within the minerals, but they contain much less water than comets, which are mostly ice and dust.

'But the chemical compounds of the comets we studied did not match the chemicals in the lunar samples.'

The team found that comets were responsible for less than 20% of the Moon's water.

Future work

The Moon may have received its water when it was still partially molten. At that time, it was being pummelled by asteroids and comets from across space.

The asteroids would have crashed into the magma ocean, preventing the water they carried from dissipating into space.

Dr Jessica Barnes, lead author of the study, says, 'We suggest that this water was delivered to the Moon very early on, within the first 200 million years of its lifetime.

'We are now able to say that water in the Earth and Moon shares a common origin, but there is still much left to do.'

Astronauts have not explored the far side of the Moon or its polar regions. Dr Barnes says that future exploration may reveal even more information.

She adds, 'Our work is timely, especially in light of the plans to send robotic, as well as human, prospecting missions to previously unexplored regions of the Moon.

'There exists every possibility that rocks from the unexplored regions may provide further clues to the origin of lunar water, its characteristics and distribution.

'Sending future missions to prospect for lunar volatiles could also help us determine whether water-ice on the Moon’s surface could be a useful resource for other space exploration activities.'

  • By Katie Pavid

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