Other smaller planetary entities were growing at the same time that Earth was forming, 4.5 billion years ago. Late in Earth’s growth, one of them struck, blowing forth rocky debris. A portion of that debris gathered on the moon after entering Earth’s orbit.
Why this is a sound theory The moon lacks a substantial iron core, but the Earth has. This is due to the fact that by the time of the massive impact, Earth’s iron had already drained into the core. Consequently, the impactor’s and Earth’s blown-out debris originated in their rocky, low-iron mantles. Computer simulations suggest that the iron core of the impactor melted upon impact and fused with the iron core of the planet.
The average density of the Earth is 5.5 grams per cubic centimeter, while that of the moon is only 3.3 g/cc. The lack of iron on the moon is the same cause.
The oxygen isotope composition of the moon is identical to that of the Earth, in contrast to that of Mars rocks and meteorites from other areas of the solar system. This demonstrates that the moon originated from material created nearby to Earth.
∙ If an evolutionary process is proposed as the origin of the moon, it becomes difficult to explain why other planets do not have moons that are similar to the moon. Only Pluto has a moon that is significantly smaller than it. The advantage of our huge impact theory was that it suggested a stochastic catastrophic event that might only occur on one or two planets out of nine.
