Rock known as 'rusty' retrieved by the Apollo 16 mission reveals the hot, dry origin of the Moon

The Moon’s interior is likely very dry, revealed an analysis of a ‘rusty’ lunar rock, while contradicting a study that showed evidence of water on the Moon. The results suggest that when the Moon formed it was “very, very, hot… Essentially an ocean of magma”, said James Day, a geochemist at the University of California, San Diego. According to Day, it would have been so hot that any water or other compounds and elements that are volatile under conditions on the Moon, such as zinc, would have evaporated very early in the Moon’s history. [caption id=“attachment_3962631” align=“alignleft” width=“380”]  Image: NASA.[/caption] The study published in the journal Proceedings of the National Academy of Sciences is based on the analysis of fragments of the “Rusty Rock” – a rock collected from the Moon’s surface during the Apollo 16 mission in 1972 – which had rust on its outer surfaces. As water is one of the essential ingredients of rust, the Rusty Rock had mystified scientists for a long time. Some speculated that the water could have been terrestrial but further tests showed the rock and the rust were lunar in origin. However, the new chemical analyses by Day and his team revealed the Rusty Rock’s composition is consistent with it coming from a very dry interior. “It’s a bit of a paradox. It’s a wet rock that comes from a very dry interior part of the Moon,” Day noted. Day found the rust on the Rock is full of lighter isotopes of zinc, meaning it’s probably the product of the zinc condensing on the Moon’s surface after evaporating during the sweltering period of its formation. “Zinc is a volatile element, so it behaves a bit like water under conditions of Moon formation,” Day explained. In the same way, the interior of the Moon must be enriched in the heavy isotopes and have been depleted in the light isotopes and volatile elements, revealing that it is dry, Day said.