How was the Moon formed?Various possibilities have been put forward thru the years
- The moon was formed independently, and captured by the Earth
- The moon and Earth formed independently, side-by-side
- The Earth spun so fast that the Moon flew off
- Formed as a result of the collision of two large bodies
THe following information was known before Apollo
- The Earth-Moon system is unusual in the relative size of the two bodies, some references refer to it as a 'double planet'.
- The Moon is made of relatively lightweight material - it has an abnormally low density compared to other rocky bodies.
- The Earth-Moon system has an unusually high amount of angular momentum.
We now know these pieces of information about the Moon.
- It is as old as the Earth
- It is made of the same type of rock as the Earth. Although the Moon is richer in silicates and poorer in metals.
- The Moon is poor in volatiles
- The isotopic composition is the same as the Earth. (Although an element has different isotopes, the percentage composition of these isotopes would be the same in any sample of the element, taken from any place on the Earth. This isotopic composition is different for samples of the element from other parts of the Solar System e.g. from meteorites.)
For example, the three stable isotopes of oxygen 16O, 17O and 18O occur on the Earth and Moon in relative proportions that are closely similar.
- It is very dry - there appears to be not a single molecule on water on the Moon.
- The moon had once been completely molten.
- The Core has a radius of about 700 kms. If the core is very small, then it occupies a significantly smaller fraction by volume than the core of any other terrestial planet.
In 1975, Will Hartmann and Donald Davis proposed the theory that the Moon formed as the result of a major collision.
- A planetary embryo strikes the Earth.
- Its core accretes onto the Earth's core.
- Its mantle is fragmented and vaporized (along with a lot of the outer surface of the Earth).
- Some returns to Earth, but the rest clumps together gravitationally.
The nagle of impact is very important here - by no means every impact will produce the required outcome.
This is likely to have been the last giant impact suffered by the Moon, because any further impacts would have destroyed the Moon.
A central tenent of lunar formation appears to be the magma ocean suurounding the new-born Moon. Observations from Clementine in 1994 confirmed this part of the theory. Initially the entire Moon may possibly have been molten.