Among the many mysteries of Mars, perhaps the most challenging is what happened to the Red Planet’s water. It once was wet and fertile. “It’s now bone dry,” said Cosmos host, Neil deGrasse Tyson, “Something bad happened on Mars. I want to know what happened on Mars so that we may prevent it from happening here on Earth.” Long-Dead Waterways Mars is laced with distinctive signs of big, long-dead rivers but scientists still don’t know what kind of weather fed them, because their understanding of the Martian climate billions of years ago remains incomplete. Early in the planet’s history, Mars only received a third of the sunlight of present-day Earth, which shouldn’t be enough heat to maintain liquid water. That is but one of the long list of its unsolved mysteries.