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The Moon over Earth – extremes of planetary evolution

July 10, 2020 

The Moon has been orbiting Earth for almost four-and-a-half billion years. The two bodies represent two extremes of planetary evolution. The Moon was formed 4.425 billion years ago from material ejected from Earth during a massive collision with a protoplanet. It subsequently developed a magma ocean more than 1000 kilometres deep on which a crust formed. Beneath the crust, the mantle rock solidified over the course of 200 million years. Earth also had a magma ocean, but did not form a floating crust. Once the Moon had solidified, magma formed in its interior and rose to the surface. However, volcanic activity came to a halt three billion years ago and the face of the Moon has hardly changed since then.

One of the oldest Moon rocks

July 10, 2020 

For the Apollo 16 mission, a location in the central highlands of the lunar near side was selected. One goal was to bring samples from the rocks found there, the oldest on the Moon, to Earth. The objective was to determine when the Moon originated and when its first crust was formed. Sample 60025 is an iron-rich anorthosite and one of the three oldest rocks brought to Earth by the six Apollo missions. It is a feldspar rock with a high content of aluminium and calcium that solidified 4.36 billion years ago, reflecting the crystallisation of an early crust covering the magma ocean. Astronauts John Young and Charles Duke picked up the rock just 14 metres from the Lunar Module. It is almost 20 centimetres across and weighs 1836 grams. It is coarse-grained and a small section is coated by black ‘glass’, which solidified very quickly as a melt after an impact event and did not form any crystals.

Rock and dust samples from the Moon

July 10, 2020 

One of the most important scientific tasks of the six moon landings that took place between 1969 and 1972 was the collection of lunar rock and regolith samples. The goal was to find out how old the Moon is and how it evolved. The analysis of lunar samples has shown that the bright components of the lunar surface represent the primary layer of lighter rocks such as anorthosite. These rocks are over four billion years old and have been redistributed multiple times due to the impact of asteroids. The dark areas visible to the naked eye, however, originated from iron- and magnesium-rich volcanic activity that did not begin until several hundred million years later. The image shows Alan Bean, the Lunar Module Pilot of Apollo 12, the second mission to land on the Moon, with a dust-filled sample capsule. Commander Charles Conrad is reflected on the visor.

Anatomy of the early Moon

July 10, 2020 

This model shows the interior of the Moon at an early stage, when its components began to solidify. The floating crust (grey) formed a thermally insulating layer over the remaining magma ocean (yellow) and the still partially molten lunar mantle below. The cooling of the lunar mantle occurred both by convection, that is by the circulation of material in huge cells (seen here as blue-white downward currents), and by the rising of mantle melt, which were formed locally by convection. This hot magma mixed into the remaining magma ocean and slowed down its cooling. Due to this process and the additional insulation provided by the crust, it took about 200 million years for the magma ocean to completely solidify.

Schematic structure of the early Moon’s interior

July 10, 2020 

After the Moon was formed 4.425 billion years ago, heavy material from the solidifying magma ocean settled. The heaviest components formed a metallic core, and the lighter ones a silicate mantle. Such rocks formed from the magma ocean are also called cumulates and are heavier than the remaining melt. The exception – in a late phase of evolution of the magma ocean the mineral plagioclase, an aluminium-calcium-silicate, which belongs to the group of feldspars, was formed. Plagioclase was lighter than the melt and rose to the magma ocean’s surface, where it formed the early lunar crust. The Moon cooled down through various heat transport processes, including convection in the magma ocean and in the solid lunar mantle, but also through the transport of melts from the mantle cumulates through magmatic heat channels.