Lunar Origins Explained

07 November 2016

The Moon is the result of a cosmic car crash and a furiously spinning Earth according to new work published this week in Nature. Moon

Our previous model for lunar genesis was the "giant-impact" theory.  4.5 billion years ago a collision between the embryonic Earth and a Mars-sized object occurred.  Although the astronomical equivalent of a glancing blow, it was powerful enough to strip the Earth of a large amount of its mass.  This material eventually coalesced to form the Moon.

This theory is unable to explain everything, however.  It does not illuminate why the Moon orbits at an angle of 5 degrees above the equator, why the Earth and the Moon seem to be made from remarkably similar stuff or why the Earth is tilted.  The Earth's poles are not perpendicular to the Sun but are instead shifted 23 degrees so that one pole is always facing the Sun.  It is this tilt that allows the Earth to have the seasons that we enjoy (and loathe).

New work, led by Matija Cuk from the Seti Institute, has provided twists on this model and offers solutions to these conundrums.

In this new theory, a much higher energy collision occurred between the proto-Earth and an object of unknown size.  This collision had ten times the energy of the one suggested previously.  It vapourised and thoroughly mixed pre-Earth and the impactor together and then both the Moon and Earth formed from this molten mixture, explaining their similar compositions. 

After this violent encounter the Earth was left spinning incredibly fast, such that a single rotation only took 2.5 hours; just over a tenth of the current 24 hours required. The Earth was spinning so fast that it bulged at the middle and the diameter at the equator was twice that of the pole to pole length.  The Earth was also tilted at 70 degrees, much higher than the 23 degrees of today.

The collision left the  Moon orbiting around the Earth's tilted equator but over time its position shifted to instead be relative to the plane of the Sun.

This transition transferred half of the Earth's momentum to the Sun resulting in the Earth slowing down to its current 24 hour revolution, adopting its spherical shape and flipping up right to the modern day 23 degrees.

The Moon's orbit was now inclined 30 degrees to the plane of the Sun.  The Moon then began to move away from the Earth and at about 223,000 km away, it switched from one stable state to another causing its inclination to flip from 30 degrees to the 5 degrees we see today.

One colossal cataclysmic collision set the Earth, Moon and seasons in motion.

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