The Earth and the Moon

A theory explaining the origin of the Moon needs to take into account three things: (1) the physical characteristics of the Earth–Moon system, i.e., why the Earth and Moon orbit the way they do; (2) the similarities in the Earth’s and the Moon’s compositions; and (3) the differences in composition. Each consideration alone is easy enough to account for, but combining all three can be challenging. If the Earth and the Moon were utterly different, it would be easy to suppose they formed separately and the Moon was captured by the Earth. Alternatively, if they were exactly similar, it would be easy to suppose they had somehow formed simultaneously from the same source material.

As it turns out, the Moon matches the composition of the Earth’s crust and mantle fairly well, while overall the Moon lacks the Earth’s iron content—and on Earth, the most iron-rich portion of the planet is its interior.

The most commonly accepted theory is that a hypothetical, approximately Mars-sized body called Thea struck the Earth obliquely early in the solar system’s formation, knocking off a significant portion of the mantle, while leaving the iron-rich core relatively intact. The resulting debris from the Earth’s mantle and the remnants of Thea formed the Moon and settled back on the Earth’s remaining surface, accounting for the similarity between the two.

Sounds good as far as it goes, but to stand up, the theory needs to account for the Earth’s rotation and the Moon’s orbit, as well as allow for sufficient mixing between Thea and the material blasted off the early Earth so as to eliminate any noticeable difference in composition.

This article from Nature details some of the latest considerations in evaluating this theory.

Meanwhile, the Juno spacecraft has returned a new video, shown above, of the Earth-Moon system, showing both objects in motion. Juno is heading for Jupiter, but used an Earth flyby on October 9th to gain speed on its way. Read more about the video here.


2 comments on “The Earth and the Moon

  1. Nicholas says:

    Cool video.

    Yeah I think there is a more fundamental problem with the theories attempting to explain the Moon’s formation. It is the nebular hypothesis. Nebular hypothesis is chalk full of contradictions especially in light of all the new Kepler data. Its a hypothetical cul-de-sac. A dead end. A tautological trap. It simply cannot be used to explain all the phenomenon of our system.

    This may sound crazy, but I suppose the Earth and the Moon are old dark stars. Call them black dwarfs. They are cooled and compressed stars. They are cinders of what were once active fusing stars. The Moon is older than the Earth. Both the Earth and the Moon marched around the galaxy prior to being captured by the newer star we call the Sun. In a religious context I would say that when the Earth was an old dark star moving near the Sun, God transfigured its face and guided it into the orbit around the Sun. Then God made the Moon to orbit around the Earth. This could have happened billions of years ago but I think the scientists are mistaken in their solar nebular assumption.