If you could travel back in time to the early stages of the Solar System, some 4.5 billion years ago, you wouldn’t find a single life-friendly world, but three. Venus, Earth, and Mars all looked very similar from a planetary perspective, as they all had substantial surface gravity and atmospheres similar to Earth’s in thickness. There were volcanoes, watery oceans, and complex interactions that enabled these worlds to retain the heat they absorbed from the Sun.
Moreover, their atmospheric compositions were similar, all rich in hydrogen, ammonia, methane, nitrogen and water vapor. For a time, conditions were favorable to life arising on all three worlds, but it didn’t last. Venus experienced a runaway greenhouse effect, boiling its oceans after perhaps 200 million years. But Mars lasted far longer before becoming inhospitable: over a billion years. These are their stories.