'Slushy' magma ocean led to formation of the Moon's crust

Scientists have shown how the freezing of a 'slushy' ocean of magma may be responsible for the composition of the Moon's crust.

Producing the large volumes of anorthosite found on the Moon would have required a huge global magma ocean.

One of the outcomes of this collision was that the Moon was very hot - so hot that its entire mantle was molten magma, or a magma ocean.

"Since the Apollo era, it has been thought that the lunar crust was formed by light anorthite crystals floating at the surface of the liquid magma ocean, with heavier crystals solidifying at the ocean floor," said co-author Chloe Michaut from Ecole normale superieure de Lyon.

The range of anorthosite ages - over 200 million years - is difficult to reconcile with an ocean of essentially liquid magma whose characteristic solidification time is close to 100 million years.

In the low lunar gravity, the settling of crystal is difficult, particularly when strongly stirred by the convecting magma ocean.

This increase of crystal content occurs most dramatically near the surface, where the slushy magma ocean is cooled, resulting in a hot, well-mixed slushy interior and a slow-moving, crystal-rich lunar 'lid'.

"We suggest that cooling of the early magma ocean drove such vigorous convection that crystals remained suspended as a slurry, much like the crystals in a slushy machine."

The results suggest that the timescale of lunar crust formation is several hundreds of million years, which corresponds to the observed ages of the lunar anorthosites.

Serial magmatism was initially proposed as a possible mechanism for the formation of lunar anorthosites, but the slushy model ultimately reconciles this idea with that of a global lunar magma ocean.