What If The Earth Had Two Moons?

The idea of an Earth with two moons has been a science fiction staple for decades. The properties of the Moon’s far side has many scientists thinking that another moon used to orbit the Earth before smashing into the Moon and becoming part of its mass. But what if the Earth actually had a second permanent moon today? How different would life be?

Our Earth-Moon system is unique in the solar system. The Moon is 1/81 the mass of Earth while most moons are only about 3/10,000 the mass of their planet. The size of the Moon is a major contributing factor to complex life on Earth. It is responsible for the high tides that stirred up the primordial soup of the early Earth. It’s the reason our day is 24 hours long. It gives light for the variety of life forms that live and hunt during the night and it keeps our planet’s axis tilted at the same angle to give us a constant cycle of seasons.

A second moon would change that. And it wouldn't be pretty. Imagine the Moon's identical twin comes hurtling by and is trapped by Earth's gravity. As it settles into orbit, halfway between Earth and our original moon, it yanks violently at the oceans. In the real world, this is how our original Moon helps generate tides.

So, the second moon would amplify the effect. Causing peak tides that would be 6 times higher, eroding shorelines and flooding many of our world's greatest cities including New York, Singapore, London, Mumbai - gone.

But not all destruction would happen on Earth. The combined pull of the planet and the original moon would also yank on the second moon. The second moon would be caught in a tug of war between Earth and the original moon. The gravitational pull back and forth from both ends would warp the second moon's surface triggering tremendous volcanic activity. Flooding the second moon's surface with red-hot rivers of lava. Just like hundreds of the volcanoes you see today on Jupiter's hellish moon, Io.

But even that's not the end of the spectacle. Right now, our current moon is spiralling away from Earth at 3.8 cm a year. That's about how fast your fingernails grow. At the same time, it pulls on the Earth slowing down the planet's rotation. Which is actually lengthening our days by around 1 second every 40,000 years. It may not sound like much, but with two moons in place, it would accelerate this process even more.

Millions of years from now, the day will have grown by 16%! Lasting longer than 28 hours! Now, a little extra time in the day may sound pretty nice, but here's the problem: the extra moon would drift towards the current Moon. And that's where the real danger comes in.

After millions of years, the two moons would collide! The impact would be so massive it would rip the very core of the moons apart. Lava would erupt from their centre - like a runny egg in space. Casting a vivid red light in the sky on Earth. Meanwhile, debris would go hurtling in all directions, where some of it would inevitably strike Earth, forming massive craters miles wide.

It would be an apocalypse for all life on Earth. And what didn't hit the planet would instead be trapped by Earth's gravity. Forming a ring of debris around the equator. Similar to the rings around Saturn - but not for long. Within just a few years, those chunks would clump together, forming one large, single body. 

Perhaps any life that survived will call it the Moon or maybe something even better. Anyhow, There would be many other indirect and far-reaching effects on Earth due to the presence of two moons. So as it turns out, it’s good that we have only one moon and it’s even better that it’s going to stay that way in the near future.