Overberg Geoscientists Group
4500 Mya
Formation of the Moon
The Moon is thought to have formed after a smaller planetoid, approximately the size of Mars, impacted the early Earth. The planetoid has been called ‘Theia’ after the goddess who was the mother of ‘Selene’ the moon goddess in Greek mythology.
There were a number of outcomes from this impact, apart from the disc of debris from both the Earth and from ‘Theia’. The tilt of the earth’s rotation axis is one of the more obvious outcomes. This, obviously, is very noticeable as its main effect is to give us the seasons that we experience during the year.
Lunar exploration during the NASA Apollo program collected many rock samples, some of which are stored at the University of Cape Town and are still used for research into the origins of the Moon and Earth.
Altogether more than 200kg in fact. These have been analysed in great detail, and have proved to be very similar to Earth rocks and minerals. This leads to a possible conclusion that most of the debris that formed the moon was derived from the Earth and not Theia. It is thought that due to the very high energy of the impact, that most of the central heavy, dense core of the planetoid sank and combined with the Earth’s core. The moon is much less dense than the Earth, and the Earth logically enough has a more massive core. One of the things that this may have led to is the Earth’s magnetic field, which is generated by the movement of the liquid Outer Core.
This not only allows us to navigate using a compass, but perhaps more importantly it shields us from the Solar Wind. This is a stream of charged particles leaving the Sun's corona and accelerating to a speed of 200 to 1000 kilometres per second.
The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it. However, it is kept away by the pressure of the Earth's magnetic field.
The moon has been captured by the Earth's gravity and we only see one side of the moon that is always facing us. The most obvious features we can see are the ‘Mare’ (plural ‘Maria’) the flat, darker areas and surrounding them the many meteorite impact craters.
The Maria are major floods of lava that have probably been caused by huge meteor impacts. The age of these is younger than the surrounding mountainous regions, which date back to shortly after the impact event.
The smaller numerous meteorite craters signify the continuous bombardment subsequently. These meteorite ‘storms’ would also have impacted the Earth, but with the effects of erosion and plate tectonics they have mostly been removed.
Further reading:
Wiechert, U.; Halliday, A. N.; Lee, D.-C.; Snyder, G. A.; Taylor, L. A.; Rumble, D. (October 2001). "Oxygen Isotopes and the Moon-Forming Giant Impact". Science. 294 (12): 345–348
De Meijer, R.J.; Anisichkin, V.F.; Van Westrenen, W. (2013). "Forming the Moon from terrestrial silicate-rich material". Chemical Geology. 345:
Melanie Barboni; Patrick Boehnke; Brenhin Keller; Issaku E. Kohl; Blair Schoene; Edward D. Young; Kevin D. McKeegan (January 11, 2017). "Early formation of the Moon 4.51 billion years ago". Science Advances. 3 (1):
Hartmann, W. K., R.J. Phillips, and G.J. Taylor, eds. 1986. Origin of the Moon. (Houston: Lunar and Planetary Institute.)
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D J Mourant July 2023