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 ...

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 ...