Study of 4.3 billion-year-old lunar rock overturns theory on formation of the Moon's crust

15 May 2020

Apollo 17, Station 6, the sampling site for the sample studied, troctolite 76535, which the NASA Lunar Sample Compendium calls ‘the most interesting sample returned from the Moon’ (https://curator.jsc.nasa.gov/lunar/lsc/76535.pdf).
(Credit: NASA image as17-140-21496. https://www.hq.nasa.gov/alsj/a17/images17.html)

Scientists studying lunar rock collected during the NASA Apollo 17 Mission in 1972 have found new evidence that large portions of the moon’s crust were formed by massive impact events.

This evidence overturns previous theories that the magmas rising from the moon’s interior were responsible for helping form the lunar crust. It was thought that impacts from colliding asteroids and comets were only destructive, but the research has shown that they also helped to build the outer layer of the moon.

The discovery made by scientists funded by the Science and Technology Facilities Council (STFC) also provides a unique record of how the terrestrial planets in our solar system were formed and shaped by geological processes over time.

Radiometric age dating of the sample of moon rock at the Swedish Museum of Natural History revealed that it formed over 4.3 billion years ago. Scientists found that the sample contains unique evidence of mineral formation at incredibly high temperatures (in excess of 2300 °C). This can only be achieved by the melting the outer layer of a planet in a very large impact event.

$Zirconia in Apollo 17 sample 76535 has a complex structure of interlocking crystal orientations, which the researchers used to identify that the grain had once been ultra-high temperature cubic-zirconia. Data from electron backscatter diffraction mapping at the University of Portsmouth. Colours represent different crystallographic orientations$

Zirconia in Apollo 17 sample 76535 has a complex structure of interlocking crystal orientations, which the researchers used to identify that the grain had once been ultra-high temperature cubic-zirconia. Data from electron backscatter diffraction mapping at the University of Portsmouth. Colours represent different crystallographic orientations.
(Credit: authors’ own image (dataset, but not this image included in Nature Astronomy paper))

The STFC-funded researchers from the University of Portsmouth, The University of Manchester and The Open University, used a technique called electron backscatter diffraction (EBSD) to discover the former presence of cubic zirconia, a mineral phase that would only occur in rocks heated above 2300 °C. EBSD is a technique that can determine the structure, crystal orientation and phase of materials.

Dr James Darling from the University of Portsmouth said:

“The discovery reveals that unimaginably violent impact events helped to build the lunar crust, not only destroy it. Going forward, it is exciting that we now have laboratory tools to help us fully understand their effects on the terrestrial planets.”

The research paper is now available in Nature Astronomy.

An interactive image of the complex crystal analysed in the study can be viewed online using the Virtual Microscope.

Resources on lunar geology for schools, museums and outreach organisers are available from our STFC Lunar Rocks and Meteorites Loan Scheme.

Last updated: 26 May 2020