Theories of the Formation of the Moon

The non-radioactive, stable rocks of Earth and its Moon have a very different composition of isotopes (different chemical forms) of oxygen from those of all meteorites ever analysed.

Earth and Moon formed at the same distance from the Sun; possibly they grew up together.

Calculations suggest Earth has never spun fast enough to throw off a moon-like body.

Earth's rotational speed (angular momentum) would have to be four times faster than it is now - which models show to be very unlikely.

Impact craters show the Moon has been bombarded with many colliding objects. Because there is no free water on the Moon, these craters do not erode away as they would on Earth.

Evidence of large impacts indicates that gigantic projectiles were around in the distant past. If one hit the Earth, enough material could have been lifted into orbit to form the Moon.

Earth has a tilt and is not oriented on exactly the same ecliptic plane as most other planets in the Solar system. (Think of this plane as an invisible 'plate' on which nearly all planets lie as they orbit the Sun). However, Earth does orbit in the same direction as the other planets and, like them, has a nearly circular orbit.

Impact with an object the size (mass) of Mars would significantly and permanently alter Earth's orbit.

The density of the Moon is the same as that of the rocks just below the crust - that is the rocks of the upper mantle of Earth.

Similar rock densities indicate similar origins.

The Moon's crust is thinner on the side nearest Earth.

The Moon was close to Earth when its mantle cooled. (Earth's gravitational field pulled slightly more mantle Earth-wards before it 'set', so the crust is thinner on that side.)

Moon rocks show signs of all having been melted at some time in the ancient past, but this is not typical of Earth's rocks.

The Moon was formed very hot, possibly entirely molten.

The average thickness of Moon's rocky crust is 70 kilometres, compared to Earth's average of 20-60 kilometre thickness of crust over the continents and 8-10 km over the Ocean basins.

The Moon's greater crustal area (for its size) could indicate the addition of crustal rocks thrown out from Earth as the result of an impact.

Comets are space bodies that are captured when they wander too close to a star like our Sun and get caught in its gravitational field. They have highly elliptical orbits.

The chance of the Moon being captured by Earth in a similar way would require a highly unlikely combination of circumstances - even if a Moon-like object came near Earth, it would be more likely to pass by, or collide with it, than get captured; a successful capture would have resulted in an elongated comet-like orbit.

Rocks recovered from the Moon during the Apollo space programme have less of the types of elements that boil off at high temperatures than the rocks of Earth. For example, Moon rocks contain no water but volcanic rocks on our planet do have water in them.

The lack of volatile elements in moon rocks indicates that it was heated to a high temperature by, for example, an impactOR differences in rock composition could indicate that the Moon originated elsewhere.

Pictures taken during comet Shoemaker-Levy's 1994 collision with Jupiter showed huge effects that spread far into space beyond the planet. Compared with Jupiter's huge size, the comet was relatively small and it broke up into a number of fragments that all collided separately.

The pictures show what can happen when a space object collides with a planet; scientists who had predicted that the effects would be catastrophic were correct.

Techniques used by scientists to tell the age of ancient rocks reveal that Earth and its Moon are roughly the same age, 4.5 billion years old.

This is evidence of similar origins OR of Moon formation when the Earth was very young.

About one third of the bulk of Earth is made up of the iron core at its centre but only 2-4% of the mass of the Moon comes from an iron core.

a) Earth's iron could have already drained into the core by the time the giant impact happened. b) Iron from the colliding object could have ended up in the core of the Earth.

The Moon does not have an overall magnetic field like Earth, but some of its surface rocks show signs that there may have been a magnetic field early in the Moon's history

a) The Moon had a common origin with Earth, which gained extra iron from the impacting object. b) Scientists do not yet understand the significance of the Moon's loss of a magnetic field.