In this episode of Our Island Universe, we look at the origins of the Moon.
Shanil Virani, Director of the John C. Wells Planetarium in Harrisonburg, VA.
Follow on Twitter as shanilv
Transcript:
Our Moon is our constant companion. Sometimes you see it during the day, sometimes you see it during the night but its always there. Our ancestors figured out thousands of years ago that the pattern of lunar phases repeats every ~30 days. In fact, that’s where the word “month” comes. Think "MOONth" next time you hear the word month! So where did our Moon come from? Have we always had a Moon?
To answer this question, we need to collect some clues. An important clue was provided by the Apollo program. Between 1969 and 1972, we send 12 men to the Moon. They returned with samples of lunar rock. Age dating of the moon rocks tell us that the samples are about 4.5 billion years old. Hmmm…. that’s about the same age as the oldest Earth rocks. By measuring the length of time its take laser light to return from the Moon, it turns out the Moon is actually moving AWAY from us! The rate at which it is moving away from Earth is about a couple of inches a year, the same rate at which your finger nails grow! So those are 2 important clues. Ages are the same and the Moon is moving away from us. If were to play this movie in reverse, then that means the Moon would be getting closer and closer! Indeed our best model for how we got a Moon suggests that about 4.5 billion years ago, just as a young Earth was beginning to take shape, it was hit by something! Now this wasn’t any old object, in order to get the physics to work, the Earth was likely hit by a Mars-sized object!
This model is known as the “giant impact hypothesis”. One problem, however, is that if the moon indeed formed from the collision between Earth and another object, why is the moon a near chemical match to our planet? Shouldn't it contain chemically distinct remnants of that other object too? A group of researchers say no, not necessarily! By running computer simulations of solar system formation, they learned that since planets and the likely objects that hit them form at similar distances from the Sun, they would also have similar chemical compositions! the giant impact hypothesis is still our best model for how the Moon formed. So The next time you see the Moon, remember that it was likely formed in a cataclysmic collision with our young earth 4.5 billion years ago!