Today, the planet Mars presents itself as a cold, arid desert, but its soil retains the imprint of a far more welcoming past. Meandering valleys and water-altered minerals attest to a period when the ...

Astronauts train to scale a Mars volcano at an analogue site in France. Lava tubes that crisscross the Martian surface might provide sanctuaries for the first human explorers on the Red Planet. (Photo ...