The rover tucked a core sample about the size of a piece of chalk into a treasure chest in its chassis. There the rock will remain until a future robot parachutes down onto the Martian surface, grabs the chest, and launches it back to Earth. If scientists are able to inspect it in person, and they find that Mars was indeed once alive with microbes, we would know that life on our planet is no cosmic one-off. We would have reason to believe that it has emerged on many of the hundreds of billions of planets that exist in our galaxy alone. The cosmos that we look up into at night would no longer seem a cold void. It would shimmer with a new vitality.