In puffs of gas from rocks more than 3 billion years old dug up by one of NASA's robotic explorers on Mars, scientists have identified several complex organic molecules — possible building blocks for ancient life. It's not aliens. (It's never aliens.) But it is “consistent with the past presence of biology,” said Ken Williford, an astrobiologist at NASA's Jet Propulsion Laboratory. “And it makes us more confident that if biomarkers” — or direct evidence of biologic activity — “are there, we might find them.” In two studies published today in the journal Science, this new finding from NASA's Curiosity rover is paired with another discovery: The planet's methane — another organic molecule usually (but not always) produced by living beings — varies with the seasons. In the past, scientists have seen plumes and patches of this intriguing substance, but this is the first time they've been able to discern a pattern in its presence...