2023-10-27 13:45:56
The age of the Moon is an important parameter for understanding its joint history with the Earth. A modern study of samples from Apollo 17 suggests that it is older than previously thought.
A new publication in Geochemical Perspectives Letters shows that the legacy of the Apollo missions is very much alive, even more than 50 years after the memorable arrival of representatives of the noosphere on the surface of the MoonMoon. The cosmochemists and planetologists of this noosphere have therefore had hundreds of rocks and samples of lunar soil brought back by the astronautsastronauts of Apollo 11Apollo 11 to Apollo 17. They have always been sources of inspiration for these researchers, who benefit modern tools for analyzing the isotopic composition of the minerals minerals constituting these rocks.
Already, at the beginning of the 1970s, the lunar samples studied by the famous “Four Horsemen” of the Apollo program, geochemists by training – namely Gerald Wasserburg alongside Bob Walker, Jim Arnold and Paul Werner Gast – had already started to appear talkative. Gerald Wasserburg and his colleagues had carried out work that would lead to advancing the model generally accepted today for the origin of the Moon, namely the collision between a planet the size of Mars called Theia and the young Earth.
This collision must have taken place no more than a few hundred million years after the start of the formation of the Solar SystemSolar system which the study of meteoritesmeteorites indicates must date from 4.56 billion years ago, to a few million almost years.
The study of lunar rocks tells us that shortly after the beginning of its birth, the Moon must have been covered by an immense global ocean of magmamagma which, as it cooled, crystallized according to various processes that we know how to reproduce on Earth by studying the magmatic processes leading to terrestrial igneous rocks.
Have you ever wondered how the dark and bright regions of the Moon are formed? In this short video you will learn how they emerged from an ancient Moon, surrounded by an ocean of magma. The video was produced by Professor Tim Swindle, of the Lunar and Planetary Laboratory at the University of Arizona, part of the LPI-JSC Center for Lunar Science and Exploration. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © Lunar and Planetary Laboratory, University of Arizona
The memory of zircons
However, among the samples brought back by Apollo 17 in 1972, a group of researchers found crystals produced during the cooling of the magma ocean. These are zircons and on Earth we know them well. These are minerals forming a group of silicatessilicates, more precisely nesosilicatesnesosilicates, whose chemical formula is ZrSiO4. It appears as one of the early products of the primary crystallization of igneous rocks and is particularly stable, which means that not only does it survive long periods of time without being altered, but above all, it keeps the quantity of isotopesisotopes intact. that it contained during its formation, with of course the products of radioactive decay. We can therefore faithfully and precisely date zircons. On our Blue Planet, this also provides information on ancient plate tectonics and other processes that may date from the Archean, or even the Hadean.
In this case, the zircons found and studied with an analytical method, using what is called a tomographic atomic probe, push back the age of the Moon by 40 million years, to at least 4.46 billion years. years. They were necessarily formed during the crystallization of the lunar magma ocean.
Uranium-lead radiometric dating
« These crystals are the oldest known solids that formed after the giant impact. And because we know the age of these crystals, they serve as an anchor for the lunar timeline », explains in a press release Philipp Heck, curator of the Field Museum Robert A. Pritzker for meteoritics, a professor at the University of Chicago and lead author of the study published with Jennika Greer, while she was a doctoral student at Field Museum and the University of Chicago.
The dating that the researchers carried out is typical of that used with isotopes of uraniumuranium and lead, the former disintegrating into the latter by radioactivityradioactivity. During their crystallization, the zircon crystals will trap a mixture of different isotopes in a stable manner and without external contamination over several billion years. Those of uranium will then behave like the sand in an hourglass in front of lead cores. The more time passes, the more lead nuclei we will have. This is a classic example, among others, of radiometric dating.
This is the proportion of lead isotopes found by the researchers which indicates that the sample studied was approximately 4.46 billion years old and that therefore the Moon must be at least that old.
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