Daniel CleryOn Earth, deep time is an open book. By measuring trace radioactive compounds in rocks that decay with metronomic regularity, dating experts have learned when oceans opened, volcanoes erupted, and mass extinctions struck.
But the story is muddled elsewhere in the Solar System because records are sparse. Scientists estimate ages on the Moon and the rocky planets from the number of craters that pock their surfaces.
They have fixed dates from just nine places, all on the Moon: the six Apollo and three Soviet Luna sites from which samples were returned to laboratories on Earth.China’s Chang’e-5 mission, set to launch on 24 November, aims to make it 10, by returning the first Moon rocks since the last Luna mission in 1976.