Crystals collected by Apollo astronauts have proved the Moon is much older than thought – and may have even helped answer the biggest lunar mystery.
How did the Moon get here?
The Moon, Earth’s natural satellite, determines the length of an Earth day and gives us tides. But no one is quite sure where it came from.
Giant-impact theory, the most widely-accepted idea, suggests that in its early years Earth was hit by another small planet about the size of Mars. Some call this planet Theia. The massive amount of debris created by the collision is believed to have melted together and reformed into the Moon.
Capture theory suggests the Moon was a ‘wandering body’, such as an asteroid, that got a little too close to Earth and was trapped by its gravity, while the ‘accretion hypothesis’, by far the least exciting, argues it was simply created at the same time as the Earth.
Fission theory, the wildest idea, suggests at one point Earth was spinning so fast that some bits flew off, forming the Moon.
However, analysis of crystals brought back from the lunar surface by the last Apollo mission to the Moon in 1972 not only supports the giant-impact theory, but have helped pinpoint when the massive collision occurred.
If Theia hit Earth, the energy of the impact would have melted the rock and debris that went on to form the Moon’s surface.
‘When the surface was molten like that, zircon crystals couldn’t form and survive,’ said senior author Professor Philipp Heck, senior director at Chicago’s Field Museum of Natural History.
‘So any crystals on the Moon’s surface must have formed after this lunar magma ocean cooled. Otherwise, they would have been melted and their chemical signatures would be erased.’
This means that if the team could reveal the age of the crystals, they could reveal the point at which the Moon cooled and became fully formed.
To do this, the team used radiometric dating, which involves counting the number of uranium and lead atoms found in the crystals. Over a very specific time, uranium decays into lead, so by counting the ratio of uranium to lead in the crystals, they can be aged precisely.
‘Radiometric dating works a little bit like an hourglass,’ said Professor Heck. ‘In an hourglass, sand flows from one glass bulb to another, with the passage of time indicated by the accumulation of sand in the lower bulb.
‘Radiometric dating works similarly by counting the number of parent atoms [uranium] and the number of daughter atoms [lead] they have transformed to.
‘The passage of time can then be calculated because the transformation rate is known.’
Using this ‘cosmic clock’, the team found the crystals were 4.46 billion years old – and therefore so is the Moon, making it 40 million years older than previous estimates.
‘It’s amazing being able to have proof that the rock you’re holding is the oldest bit of the Moon we’ve found so far,’ said lead author Jennika Greer. ‘It’s an anchor point for so many questions about the Earth. When you know how old something is, you can better understand what has happened to it in its history.’
Professor Heck added: ‘The Moon is an important partner in our planetary system – it stabilises the Earth’s rotational axis, it’s the reason there are 24 hours in a day, it’s the reason we have tides. Without the Moon, life on Earth would look different.
‘It’s a part of our natural system that we want to better understand, and our study provides a tiny puzzle piece in that whole picture.’
The study is published in the journal Geochemical Perspectives Letters.
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