How the Moon was Created

There is probably not a single person on the planet who has not looked up at the Moon and wondered how did it get there? Whose is that face that looks down at us? Is it made of green cheese?

For most of us it is just idle curiosity, but for William Hartmann it became his life’s work.
His crowning achievement arrived when he was still a young man. In 1984 hundreds of scientists from all over the world met in Kona, Hawaii to discuss the creation of the Moon. This meeting has been described as:

‘..perhaps the most successful in the history of planetary science’.
(‘Dr. Michael Drake, director of the University of Arizona’s Planetary Science Department

The scientists came to the meeting with support for all the existing theories but with very little argument these scientists were convinced by the evidence presented by Hartmann and others so that we now have the ‘Kona Consensus’ on how the Moon was created.

When Hartmann was a child, there were several theories.

The first, of course is the biblical theory of creation which must be believed by those who believe it but cannot be part of a scientific discussion.

Another had first been put forward by George Darwin, son of Charles. This theory was that the Earth and the Moon had begun to form together as one body. He thought that while the proto-Earth was still in a nebulous molten state, a large amount of material became separated by the Earth’s spin and became the Moon. In Darwin’s time (1845 1912) there was not really any scientific evidence for or against this theory.

Another theory which has had a lot of popularity in the past is the ‘capture theory’. This is the idea that the Moon was on a different orbit around the Sun which at some point brought it near to the Earth so that it got caught up and began to orbit around the Earth instead. This was in accord with Newton’s theories of gravity so it was a strong possibility. The problem was that this body from elsewhere would have had to have been traveling at a precise speed, a precise distance from Earth. Otherwise it would have collided, or floated on by.

There was one other theory, which wasn’t at the time given any credence. It was first put forward in the 50s by Reginald Daly. His studies of the Moon through a telescope led him to believe that the craters were, rather than the solidified burst bubbles of lava as was widely thought, were actually impact sites. On this basis he put forward the ‘giant impact hypothesis’. He thought that perhaps another body had struck the Earth, breaking a piece off which then became the Moon. At the time, this seemed ludicrous and was largely ignored. Until William Hartmann came to the same conclusion around 20 years later.

It was not until the 60s, when the Moon became a hot political topic that more evidence was brought to bear. When the Apollo missions brought back samples from the Moon it was found that their chemical composition was not similar enough to be simply an offshoot of the Earth. And yet, strangely, there were too many similarities to support the capture theory.

Around this time, Hartmann was studying at the Lunar and Planetary Laboratory in Tucson, Arizona. He considered himself to be a ‘crater-counter’, concerned mainly with the mapping of the Moon.

In addition he and his colleagues were studying yet another theory – co-accretion.
Accretion is the method by which planets came to be formed out of the debris of the ‘Big Bang’. 4.5 billion years ago our solar system would have been full of debris which accreted together into chunks under the influence of each one’s gravity.

The new evidence added feasibility to the theory that both the Earth and the Moon had formed by accretion at the same time. Yet there were still problems. The main one was that all of the other bodies that formed at that time in our solar system have an iron core which the Moon lacks.

Studying work on accretion being done by Safronov in Russia, Hartmann et al. realized that there must have been many large bodies in Earth’s vicinity during this period. This, along with their studies of the Moon’s craters, suggested that the ‘giant impact’ theory wasn’t as silly as it had first seemed.

In 1974 they published their hypothesis in Icarus, expecting to be laughed at. On the contrary, they discovered that other scientists, most notably Alastair Cameron, had been approaching the same conclusion from a different direction.

Cameron was a planetary physicist who had been studying the relationship of the movement between the two bodies. He realized that the maths didn’t fit any of the previous theories. He was also working on the idea that a third body must have hit the Earth. This theory explained all of the difficulties with previous ones and has become almost universally accepted.

This third body is usually referred to as ‘the impactor’, Theia (in Greek mythology the mother of Selene, the moon goddess) or sometimes Orpheus (an ancient Greek astrologer). It must have been around the size of Mars, one tenth of the size of the Earth.

‘Earth blew up into an amoeba shape, then settled back into a sphere.’ (Dana Mackenzie).

About a third of the Earth’s volume was pulverized and sent up in an enormous plume, forming a spiral as the planet rotated. The iron core of the impactor melted into the iron core of the Earth. The volatile elements in the exploded material boiled off. Over time the Earth settled down and the plume of debris accreted to form the Moon. This theory makes sense of all of the anomalies discovered in the constitution of Moon rocks, and of the absence of an iron core in the Moon.

Since 1984, and the Kona Consensus, all the further evidence gathered has supported the giant impact theory.

William Hartmann is still studying the formation of bodies in space as well as being even more well known as a space artist. You can see his work on his web-pages at the Planetary Science Institute site.


Dana Mackenzie 2002 ‘The Big Splat or How our Moon came to be.’

Hartmann, W. K., R.J. Phillips, and G.J. Taylor, eds. 1986. Origin of the Moon. (Houston: Lunar and Planetary Institute.)

Robin Canup Ph.D. 1999, ‘Big Bang, New Moon’ published in ‘Technology Today’