While Mars is known as a frozen, red planet today, it has all the evidence we could ask for of a watery past, lasting for approximately the first 1.5 billion years of the Solar System. Could it have been Earth-like, even to the point of having had life on it, for the first third of our Solar System’s history? (Kevin Gill / flickr)

What Was It Like When Venus And Mars Became Uninhabitable Planets?

Earth wasn’t the only potentially habitable planet in the early Solar System. What happened to Mars?

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.

Rather than the two moons we see today, a collision followed by a circumplanetary disk may have given rise to three moons of Mars. As the innermost, largest moon fell back to Mars, only two survive at present. Just as Earth’s moon was formed by a great impact long ago, so, too, were Mars’s moons. (Labex UnivEarths / Université Paris Diderot)

It’s remarkable that worlds that are so different from one another might have…