There was plenty of reason to celebrate when the Perseverance rover successfully touched down in Mars's Jezero Crater this afternoon. But in some ways, the rover showed up too late—3.5 billion years too late, in fact.

Long ago, in an earlier epoch, as studies of Mars have shown, Jezero Crater was Jezero Lake, a 45 km (28 mi.) depression in the northern Martian hemisphere, fed with water via a channel that cut through the crater rim and spread into the lake in a graceful delta.