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New studies offer contrasting scenarios for making the Moon. One argues for a one big splat early in solar-system history; a second envisions a score of lesser blows that built up the Moon over time; and a third suggests water was involved.

Given the trove of lunar samples in hand and the power of modern laboratory analyses, you’d think that by now geochemists should have completely nailed exactly how the Moon formed. But not so — in fact, there’s still lots of debate on how Earth formed.

Artwork of a Mars-sized object colliding into the Earth early in solar system history. Many planetary scientists believe that an impact such as this threw off the debris which eventually formed the Moon.
Lynette Cook / Getty Images

Here’s the basic problem: about 30 years ago, dynamicists showed that a body roughly the mass of Mars could have struck Earth a glancing blow and ejected enough debris into orbit to collect into a Moon-size object. In virtually all of those simulations, most of what ends up in the Moon came from the impactor rather than from Earth.

 

Throwing Water on the Problem

As if the How and When of the Moon’s formation weren’t complicated enough, a third new analysis argues that — despite its extreme dryness today — the Moon likely contained a lotof water when it formed. In the same issue of Nature Geoscience, Yanhao Lin (Vrije Universiteit Amsterdam) and three others describe their experimental attempts to mimic how the Moon’s magma ocean solidified. Lower density minerals would have floated to the top, forming a crust.

They find that the suite of minerals found in the lunar crust today — combined with its thickness — argue that water was part of the mix at a concentration of 270 to 1,650 ppm. This might not seem like much — but if proven true there’d be significant implications.

“A wet start of the Moon, coupled with the strong similarities between the composition of the Moon and the composition of the silicate Earth,” Lin’s team concludes, “suggests that equally high concentrations of water were present in the Earth at the time of the Moon-forming event.”