The oldest known zircon mineral from any major planetary body, including Earth, has been found on the surface of the Moon. At 4,417 million years old, the microscopic zircon crystal provides an important landmark in the history of the Moon’s surface, according to a paper online this week in Nature Geoscience.

Alexander Nemchin and colleagues measured the lead and uranium isotopes contained in the tiny grain to determine the age of crystal formation. Zircons are minerals composed of zirconium and silica, and are formed through the cooling of molten and heated rocks. The Moon is generally believed to have formed through a collision between Earth and a Mars-sized asteroid, which formed a Moon covered by a pool of magma. As zircons should have formed when about 90 per cent of this lunar magma ocean solidified, the age shows that this degree of solidification was reached about 100 million years after the Moon’s formation.