You are entirely forgiven for not knowing this, since they are not especially rare — cerium is about as common as copper. The problem is that they’re difficult to isolate and purify; it wouldn’t be ...

NASA’s Curiosity rover is attempting one of the rarest kinds of experiments ever tried on Mars, a painstaking chemical test that uses a limited supply of liquid reagents to hunt for traces of complex ...

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Wringing out clues that could change everything.