Science Interviews


Sun, 14th Nov 2004

Where Our Moon Came From

Dana Mackenzie

Part of the show Origin of Earth, The Moon and Life

Chris - Where did the moon come from?

Dana - The moon came from a collision between the earth and another planet about 4.5 billion years ago, about 50 million years after earth first formed. When the earth was in the process of forming, you still had a great amount of debris still in the same orbit as the earth. The planet that ran into the earth was the largest piece of this debris, about the size of Mars.

Chris - Why was such massive planet in the same orbit as Earth in our developing solar system?

Dana - For a period of 50 to100 million years, all the debris between Venus and Mars were collecting into planetessimals -small planets -and the y got bigger and bigger. Eventually you have the biggest chunk running into Earth.

This would have been trillion times larger than the impact that killed the dinosaurs. This collision was big enough to melt the surface of the earth. The Earth was polarised, and had rings like Saturn, but they were much denser than Saturn's rings so they condensed and formed the moon.

Chris - How do you know that's true?

Dana - There is evidence that the moon had a magma ocean that was completely covered by molten rock early in its history. If you have all this debris coalescing to form the moon, a lot of heat is released and that explains the magma ocean. This giant impact theory also explains other things. The moon is similar to the Earth's mantle -its surface- but the moon doesn't have an iron core like the Earth does. If you have the moon and Earth growing up in the same part of the solar system, why did we get all the iron? The giant impact would have stripped a lot of the Earth's exterior, which would have gone into orbit and formed the moon, but its interior was not affected.

Most of the moons of Saturn and Jupiter were likely formed by accretion, but the moon of Pluto -Charon -was probably formed the same way as our moon. It's very close in size to our moon, which is anomalously large -it's almost a double planet. The fact that there are two 'double planets' in our solar system suggests that their origin was very similar.

Chris - What does the moon do for us today?

Dana - It stabilises the Earth's axis which means climate on earth has been much steadier than the climate on Mars. We have had ice ages of course, but we would have had much more severe ice ages than if we had not had the moon. It's possible it would have impacted the development of life.


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