Image Credit: Paulette Richardson


"What would happen if the Moon was split apart by a collision, as Mimas almost was (giving rise to its Death Star look)? Assuming none of the debris hit us and wiped out life as we know it, would Earth be significantly different if, instead of one moon, we had a number of little moons?"

                                                          Asked By: Tom McAteer


Since we don’t have two moons up in the sky (I just checked—only one where I live), this article is going to be completely speculative. Keep that in mind as you read through, this is what I believe could or might happen per the laws of physics.

Well, we kind of can't get away from catastrophe here on Earth if we had two moons. Let’s say the Moon was hit by a meteoroid, asteroid or comet and split in two, and let's ignore the impacts from the debris here on Earth from the impact. Let’s call one the Moon, and the other Luna. Let’s also say that the Moon stayed relatively close to where it is now, but when they split, Luna was pulled into a closer orbit around Earth. The gravity from Luna would tug on the Earth; causing large tsunamis with the rise and fall of the tides more often than they currently occur, and an increase in earthquakes (look out California!), and an increase in volcanic activity, which would pump ash and carbon dioxide into the atmosphere. The ash would block out the sun, killing a lot of plant and animal life. The carbon dioxide would also increase the greenhouse gas effect, speeding up lots of bad things such as melting of the polar caps, changing habitats, etc.

Not many people realize that as the Earth spins, and the Moon orbits the Earth, the oceans are pulled up to the Moon and the continents crash into the swell of the oceans. This tidal force is speculated to be one of the reasons that Earth's rotation has slowed from 6 hours to 24. Another reason is the law of angular momentum. The law of angular momentum can be seen in practice at any skating rink. When a spinning skater spreads out his or her arms, his or her spin slows. When the skater brings his or her arms closer to the body, the skater spins faster. The same is the case with the Moon (the skater's arms) and the Earth (the skater's body): As the Moon gets further and further away, the Earth slows in its spin. But with the creation of Luna, half the Moon's mass is suddenly pushed in closer to the Earth. The planet will start spinning more rapidly, screwing up all animal life’s day/night cycle.

Image Credit: NASA


A second moon in the night sky would certainly make our nights brighter. The reflection of the Sun’s light in Luna would bathe our nights in light, and nocturnal life would suffer. Unable to successfully hunt in the darkness of night any longer, many nocturnal animals would die out.

Luna would be a beautiful sight in the night sky, however. Directly after the impact, it would be a glowing ball in the sky, but after it cooled, there would be a lot of volcanic activity. Luna would be a lot like Io around Jupiter: We’d probably be able to see volcanoes erupting with our naked eye on clear nights, which would be an amazing sight. Unfortunately, the material from the eruptions would be ejected into space and make a ring around the Earth. This ring would be a beautiful, but deadly addition, and a constant source of meteors raining down on the Earth.


Image Credit: NASA/JPL-Caltech

Luna, however beautiful in the sky she would be, would be doomed. There are two ways Luna could be ultimately destroyed, and it all depends on how far away from Earth she ended up. Every planet has a boundary called the Roche limit. This is the area outside of which smaller bodies can remain whole while orbiting around a larger one—in this case, a moon around a planet. When a body crosses inside the Roche limit, tidal forces rip the body into pieces to form rings. This is how Saturn, Jupiter, Uranus and Neptune got their rings. If Luna were pushed inside this limit, it would be ripped apart. When a body is inside the Roche limit, it is doomed to continue to fall into the planet. Some things take only minutes, others can take millions of years. Objects outside the Roche limit are doomed to someday fly out into space. The Moon is destined for this fate as it is slowly but surely drifting away from the Earth.

If Luna stayed outside the Roche limit, it would be doomed to collide with the Moon. As the Moon and Luna passed by one another, Luna would be pulled farther from the Earth by the Moon, and the Moon pulled closer to the Earth by Luna. They would continue their tug-of-war until their inevitable collision. Upon the collision, we would regain our one single moon, but the debris from this collision would rain down on the Earth once again.

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