TIL: Why Mars's Ocean Disappeared | Today I Learned

This is what Mars looks like today, and this is what it may have looked like 3 to 3.5 billion years ago. Notice the difference? Well, the planet was warmer and wetter, and it even had an ocean that covered the entire Northern Hemisphere. So where did that ocean go?

I'm Brendan Mullen, an astrobiologist and emerging explorer with National Geographic, and I'm going to tell you what happened 4 and 1/2 billion years ago when the solar system first formed. Earth and Mars formed from basically the same sort of stuff, like carbon, silicates, oxygen, nitrogen, you know, stuff like that. They're basically the same, except for one key difference: that's size. If we shrunk Earth and Mars down to scale and let's say, let's make Mars the size of a softball, Earth would be the size of a bowling ball. It's a big difference!

And in the universe, size matters. When Earth and Mars and all the other planets first formed, they were very hot, and they've been cooling off since. But the size difference means everything. Earth still has a churning liquid metal core in it, while Mars is essentially frozen solid. Without a turning molten core like on Earth, Mars doesn't have a magnetosphere, a protective magnetic field around the planet. Without the magnetosphere, the solar wind or charged particles from the Sun hit the atmosphere and strip off molecules and atoms over time. So billions of years later, we have far less atmosphere on Mars than we used to.

So what does that have to do with Mars's disappearing ocean? Well, without that pressure of the atmosphere on top of it, that water evaporates out into space or freezes beneath the surface. But we can still see the role that it played in shaping the Martian terrain. Is there a chance we'll find life on the surface of Mars? The answer is actually yes. If we found that life, what I would really be ashamed to say is that we did something bad to it.