Is the moon a child of Earth?

The moon is unusual. There are bigger moons in the Solar System, some with under-ice oceans and at least one with a thick atmosphere.

However, all those other moons orbit giant planets.

Our moon is so large compared with the Earth that sometimes the pair get described as a double planet.

This suggests that something happened that differs from what went on in the young Solar System, when the planets formed.

The standard picture is the collapse of a cloud of gas and dust into a rotating disc. The core forms a star, the Sun, the disc forms smaller discs that collapse into planets, and the left over material forms moons, all far smaller than the planet. 

There is a limit to how small one of these collapsing discs can be because the body in the centre has to have enough gravity to hold it together, and with lots of bodies forming nearby, all their conflicting gravitational attractions cause small discs to dissipate.

The moon is too big to be a by-product of the Earth's disc and too small to have a disc of its own. So, where did it come from?

One suggestion was that the young Earth was spinning too quickly to hold itself together and spun off the moon.

The theory that seems to best fit the bill as we see things at present is that soon after the Earth formed it was hit by an object around the size of Mars. The resulting chaotic debris then collapsed to form the Earth and moon.

The "pre-Earth" was a fully formed planet when it was hit. Heavy elements such as iron and nickel had settled down into the centre, forming the core, and the material surrounding it, volcanic rocks such as basalt formed a mantle.

This stuff was semi-molten, as it is today, with a cooler, solidifying surface of basalt rocks, rather like the floor of our oceans today. The impact was unlikely to have been dead centre, because the debris cloud would have been thrown off so fast it would never fall back to form only one or two bodies.

An off-centre impact would have removed a large lump of the pre-Earth's mantle and a bit of core. This agrees with the geological information and the rock samples brought back from the moon by the Apollo astronauts.

The moon 's rocks are very much like the basaltic rocks making up the Earth's ocean floor and mantle. The main difference is that the rocks the astronauts managed to grab were from the moon 's surface, and have been exposed to billions of years of vacuum and hot and cold, and as a result are extremely dry.

After things settled down again, we had a young Earth that rotated faster than it does today, and the moon orbited much closer. The two worlds pulled up huge tides in each other, and the rapid rotation of the Earth pulled the tidal bulge a bit ahead of the moon.

The gravitational pull of the bulge pulled at the Moon, speeding it up, and perversely, the way orbits work is that trying to speed an object up makes its orbit move further away and makes it move slower.

Try swinging a ball around your head on the end of a couple of metres of elastic. Once you have it moving in a nice circle, try to speed it up. It will move away and slow down. This is definitely a "do outdoors and use eye protection experiment.” Believe me, I've done it.

While the pull of the tidal bulge accelerated the moon, it slowed the Earth's rotation, so that today our day is 24 hours long. This tidal process has not finished.

The moon is still receding from us, and our days are still lengthening. 

One consequence of this is that one day there will no longer be any total eclipses of the Sun.

That our moon is at the right distance from us to exactly cover the Sun is probably unique in our galaxy, and this lucky coincidence won't last forever.

  • Venus shines brightly in the west after sunset
  • Left to right, Mars, Saturn and Jupiter lie close together low in the southeast before dawn
  • The moon will reach Last Quarter on the 14th. 


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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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