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High precision isotopic measurements of heavy elements in extra-terrestrial materials: origin and age of the solar system volatile element depletion

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New cutting-edge techniques unravel the origins of the solar system

By measuring volatile elements in extraterrestrial materials, scientists are learning more about the formation of the Earth, the Moon and even our water.


Scientists are constantly adding to one of the greatest stories ever told: the origins of our solar system. Understanding how our planetary bodies formed will not only satisfy curiosity but also help in the search for other habitable worlds across the universe. One key character is volatile elements – chemicals that easily turn to vapour, such as hydrogen, carbon dioxide and water. Volatile elements play a major part in a planet’s functional processes and are central to the development of life. On Earth, they determine the physical characteristics of rocks and the movement of the mantle, and help generate our magnetic field. An EU-funded project, PRISTINE, has developed a new method to understand volatile elements further. Rather than measuring the abundance of volatile elements, the PRISTINE team analyses their isotopes – atoms of chemical elements that have a slightly different mass. Isotopes develop over time through physical and chemical changes in the environment, so analysing them can point to processes that happened in the past. PRISTINE focused on isotopic ratios in extraterrestrial samples collected during previous space missions. The project has already added to our understanding of the Moon’s origin. One major theory proposes that the Moon was formed following a collision between the early Earth and another small planet. “Through our data we can propose for the first time that following this event the Moon must have lost its volatiles, and that these volatiles likely fell back onto Earth,” says Frederic Moynier, a cosmochemist at the University of Paris.

New techniques seek out volatile elements

The team used a series of state-of-the-art instruments known as multicollector inductively coupled plasma mass spectrometers, which analyse isotopic ratios. “In these types of mass spectrometers, the elements are ionised in a plasma of argon, electromagnetically separated, and their abundance is then measured,” Moynier explains. One difficulty here is that the scientists must inject only the required element into the device, so they must use chemical methods to extract the pure element from samples and discard the rest. The team developed methods to analyse isotopic ratios of volatile metals such as zinc and rubidium in samples taken from the Moon and Earth. “We demonstrated that the depletion of Moon volatiles must have occurred during a large-scale evaporation process. In other words, the material that formed the Moon had volatiles, and lost them even during its creation,” he says.

A new project to dig further

The results greatly advance our understanding of the Moon’s formation, but there are many unanswered questions. “We suggested that, for example, the Moon volatiles fell back on Earth, but this is not a trivial mechanism. We are now working with colleagues performing astrophysical modelling to solve this question,” he adds. PRISTINE has ended, yet Moynier will deepen the research under a new grant from the European Research Council. The next project, METAL, will attempt to build up an even more complete picture of the early solar system. “Once we know how solar system planets acquired their volatiles, we could apply this to extrasolar planets,” Moynier notes. This is all the more exciting with new images expected to come with NASA’s recently launched James Webb Space Telescope, and samples such as those collected from the asteroid Ryugu by Japanese scientists.


PRISTINE, moon, earth, solar, system, origin, volatile, formation, space, planets, water

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