Early in our Solar System’s history, the Earth was slammed by a Mars-sized body. The collision effectively disassembled both bodies and created a swirling mass of debris from which the present Earth and its Moon condensed. The process of forming these two bodies was violent, as debris of various size rained down on their surfaces. As a result, the Moon’s surface started out as a global ocean of molten rock.

Eventually, as this ocean cooled, it formed the Moon’s crust. But the process was complex. Different minerals solidified at different temperatures and depths. We’ve had some models of how this might have happened, based on a limited number of experiments, as well as our early understanding of the Moon’s composition. But scientists from VU Amsterdam have revisited this issue in light of what we now know of the Moon. The scientists have tested how various mineral mixes behave under extreme temperatures and pressures. Their results indicate that the Moon must have started out with significant amounts of water mixed into its global magma ocean.

How do you model an entire ocean of molten rock? You start with the known composition of the Moon and use that to create a mix of the appropriate minerals. Then you expose those minerals to extreme pressures and temperatures well beyond the melting point of rock. For these experiments, the temperatures ranged up to 1,550°C. Since the magma ocean was potentially hundreds of kilometers deep (current estimates range from 400 to 1,000 kilometers), pressures ranged up to 3 GigaPascals, which is nearly 30,000 atmospheres.