Looking at the origins of the Moon

The Moon: A short history

Around 4.5 billion years ago, the formation of the Sun's family of planets was in full swing.

Dust and lumps of rock and ice were colliding and joining together into growing lumps. The third planet out from the Sun had grown to around the size of the Earth today. Due to the enormous amounts of energy released in the impacts, the planet was a big ball of molten rock.

Over time the heavier materials, such as iron and nickel were gradually sinking to the core of the new planet, with the lighter stuff forming the upper layers. Then it was hit a heavy glancing blow by a body around the size of Mars. If the collision had been head-on, neither body would have survived. They would both have been reduced to dust and rubble for other planets to collect.

Most of the debris collapsed together to form the ball of molten rock that would become the Earth. Some of the material was blasted off into space to be collected by other planets, or maybe ended up as dust and gravel still floating around the Solar System today. The rest formed a great belt surrounding the Earth.

Over 100 million years that material collected together into a ball, the Moon. This bizarre, ancient accident is why we have such an unusual combination of worlds. Nowhere else in the Solar System is there a moon so large compared with the planet it orbits. For this reason, the Earth-Moon system is often called a "double planet".

This ancient collision theory is one of the more widely accepted explanations as to how we got our moon. There is certainly evidence to support it. Firstly, when the Apollo astronauts brought back those vital lunar rock samples, the rocks were found to be identical with basaltic rocks here on Earth, except that over billions of years of heating and cooling in a vacuum, they have become extremely dry.

Secondly, the Moon is maybe two-thirds the density of the Earth because the oblique impact blasted off lots of lighter, outer layer material from that primordial planet, but left the dense core fairly intact. So the Earth retained almost all of the heavy, iron and nickel making up the Earth's core. The smaller size of the Moon, together with its comparative lack of iron and nickel sealed another aspect of the Moon's future.

Circulation of iron-rich materials inside the Earth generates electrical currents, which generate the Earth's magnetic field. This in turn keeps the solar wind away from the top of the Earth's atmosphere. Otherwise it would be gradually scrubbing it away.

The combination of atmosphere and magnetic field protects us from dangerous high-energy radiation from the Sun and elsewhere. The Moon inherited less of this magnetic-field generating material, and in addition, being smaller, it cooled faster and allowed any atmosphere it had to expand further out into space.

With no magnetic field to protect it and less gravity to hold it down, the Moon rapidly lost its atmosphere. This allowed the high-energy radiation to reach the surface.

Here on Earth, the atmosphere traps heat, making our world warmer, and also smoothes out the daily temperature variations. The Moon's lack of atmosphere leads to a very different situation. Even though it is at the same distance from the Sun its average temperature is below freezing, and its daily temperature variations are extreme, ranging from over 100 C during the day to -120 C or colder during the night.

Future residents of the Moon will have to be protected from the temperature extremes and the high-energy radiation hazards. Fortunately there is a solution. It is very likely that the lunar bases, colonies and cities will be built underground. On the plus side, there will be enormous amounts of solar energy, and it is never cloudy on the Moon.


• Venus shines brightly, low in the sky after sunset. Jupiter, almost as bright, lies higher and to its left, with Mars, redder and fainter, high in the south.

• The Moon will be new on Feb. 19.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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