Mars is a cold, dry, desert-like planet. But billions of years ago, scientific evidence suggests that it had a thick atmosphere, which kept it warm enough to support flowing water on its surface. So, what happened to the Red Planet, and could it happen to Earth? "From everything we know about the history of Mars through robotic exploration, it had very similar chemistry [to Earth]. It had very similar periods of time and development. It had that thicker atmosphere, had standing water, fresh water. All the things that Earth had," said Casey Dreier, chief of space policy at the Planetary Society. "So, what went wrong? Why don't we see Mars as a rich planet now? That's exactly the kind of thing that helps us put Earth in context and hopefully makes us appreciate Earth a little bit better as a

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