Asteroids, not comets, delivered most water to the Moon

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An artist’s illustration of what the lunar magma ocean may have looked like shortly after the formation of the Moon.

The lion’s share of the Moon’s water was ferried on asteroids – not comets – during its early history, new calculations suggest.

European and US researchers, led by Jessica Barnes from Open University in the UK, compared atomic differences in molecules from lunar samples to those in comets and asteroids and concluded at least 80% of the Moon’s water came from rocky asteroids called carbonaceous chondrites.

They published their work in Nature Communications.

Today a grey calm satellite, the Moon’s youth was turbulent. It’s thought to have formed around 4.5 billion years ago when a Mars-sized object slammed into Earth.

Initially a ball of magma, after a few thousand years it developed a crust – called a thermal lid – which kept any volatiles such as water locked inside. Over the next 200 million years or so, the Moon slowly solidified completely.

During that time, it was ripe to absorb any comets or asteroids that punched through the crust, along with any water on board. Today, lunar samples show around 100 parts per million water.

Comets are often described as “dirty snowballs” so one might assume most of the Moon’s water might come from them.

But a particular type of asteroid – carbonaceous chondrites – also happens to be particularly water-rich.

To figure out where the Moon’s water came from – comets or asteroids – Barnes and her colleagues examined hydrogen isotopes in water.

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Potential origins of the Moon’s water, delivered while it was still partially molten.
LPI / David A. Kring

Sometimes a hydrogen atom, which comprises one proton, one neutron and one electron, will sometimes accommodate a second neutron.

This “heavy water” is found in comets, but not in asteroids.

Analyses of lunar rock samples show they don’t contain much in the way of heavy water either.

Barnes and colleagues calculated that 80% of the Moon’s water was brought on carbonaceous chondrite asteroids while less than 20% came on comets.

Based on nitrogen isotopes in the Moon, they calculated the specific type of asteroid that probably carried the most water was what’s called a CO-type carbonaceous chondrite. (The Ornans meteorite that fell in France in 1868 is one such CO-type.)

Could water have been present in the hot disc of material from which the Moon coagulated, 4.5 billion years ago?

Until its crust formed, water would have boiled and hissed out of the Moon’s magma oceans into space.

But even if the newborn Moon contained 25% of today’s water, asteroids still would have supplied most of the shortfall, the researchers calculated.

The work doesn’t just apply to the Moon. Mars’ water is remarkably similar to that on early Earth and the Moon, they write, so the same types of asteroid likely “delivered a vast majority of water to the rocky planets in the inner Solar System”.

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