Dave Bigelow

NASA launches its most ambitious hunt for traces of life on Mars, landing a car-sized rover in a rocky, ancient river delta. The rover will stow samples for possible return to Earth and test technology that may pave the way for human travel to Mars.

Four and a half billion years ago, the young Earth was a hellish place-a seething chaos of meteorite impacts, volcanoes belching noxious gases, and lightning flashing through a thin, torrid...