There are many factors that allowed life to develop on TerreTerre. But the presence of volatile elements is certainly one of the most essential. Volatile elements are chemical elements that have the ability to change from solid or liquid state to gaseous state at relatively low temperatures. The six most common elements in the composition of living organisms are all volatile elements, like water.
These elements have been present on Earth since its formation. However, it is commonly accepted that our Planet was mainly formed from the material composing the internal protosolar disc. From there, many researchers have assumed that the volatile elements entering the composition of the Earth come from impacts of asteroids formed in this inner zone of the Solar System.
The major contribution of carbonaceous chondrites
But perhaps the story is not so simple. In a previous article (see below), we already presented the debates animating the scientific community about the origin of volatile elements. A study, based on the chemical signature of the Earth’s mantle and chondriteschondritesalso suggested that the volatile elements captured at the time of the formation of the Earth came from at least two different sources: the nebulanebula primitive solar, but also the outer part of the Solar systemSolar systemlocated beyond the asteroid belt.
A new study goes further this time, suggesting that the proportion of external intake is much higher than previously thought.
Scientists have studied the chemical composition of 18 meteoritesmeteorites from various origins. Eleven would have been formed inside the Solar System (so-called non-carbonaceous meteorites), and seven would have come from the outer zones of the Solar System (so-called carbonaceous meteorite). The researchers were particularly interested in the abundance of different isotopesisotopes zinc, an element considered moderately volatile. The isotopic compositions of the different meteorites were then compared to samples of terrestrial rocks. And the results are quite surprising! They suggest that if only 10% of the Earth’s mass comes from carbonaceous meteorites, this material from the confines of the Solar System would have provided no less than half of the zinc that makes up the Earth today. However, meteorites rich in zinc would also be rich in other volatile elements and in particular in water. A result that would suggest that the Earth’s water comes, in large part, from the outer region of the Solar System.
Without this external input, the Earth would have been drier
Chondrites from outside the Solar System could therefore have significantly influenced the composition of the Earth and played a major role in the development of life by providing the necessary ingredients. The results, published in Science, suggest that without the contribution of these carbonaceous chondrites, the Earth would have been significantly poorer in volatile elements. The planet would certainly have been drier, even unable to support the development of life.
In this context, the researchers now wish to study the composition of Martian and lunar rocks, in order to compare them with their results. The objective is to see if Mars benefited from the same contribution as the Earth and to know if the collision which gave rise to the LuneLune may have played a role in the supply of volatile elements, as some hypotheses suggest.
Where does the carbonecarbonel’nitrogennitrogen, and the water that made the Earth habitable? Even today, this question remains intensely debated. However, a chemical elementchemical element in particular could help to solve this enigma, or at least to advance on the path of knowledge. It’s about kryptonkrypton. Its presence within the deep terrestrial mantle sheds light on the formation of our planet.
Article by Morgane Gillard published on December 17, 2021
When, and from what source, did volatile elements such as carbon, nitrogen and water arrive? Several models exist. Some studies suggest that the assimilation of these chemical elements took place before the giant impact that gave birth to the Moon, others that they would have arrived after. Three main sources are also evoked to explain the composition in volatile elements of the Earth: the SoleilSoleilchondrites (carbon-rich meteorites from outside the Solar System) and cometscomets.
Krypton, a good chemical tracer
In an attempt to constrain the question of the origin of volatile elements, a team of researchers from the University of California focused on the composition of the Earth’s lower mantle. The scientists more particularly tracked down the presence of the different isotopes of krypton, a chemical element of the family of gazgaz nobles. Being inert, noble gases are indeed good tracers of sources of volatile elements. Krypton, in particular, is very useful in differentiating solar sources from chondritic sources thanks to their fairly clear isotopic differences.
For their study, the researchers sampled volcanic rocksvolcanic rocks produced by volcanoesvolcanoes hotspot in Iceland and the Galápagos. Indeed, the magma feeding these volcanoes comes from a very deep zone in the Earth’s mantle, close to the limit with the outer core. The composition of the erupted magmatic rocks is therefore representative of a primitive mantle, whose chemical composition has changed little since the formation of the Earth, 4.4 billion years ago. The magmas rising from these very deep zones are in particular composed of certain noble gases. When they come to the surface through the processes eruptiveeruptivethe magmas will then imprison these gases in the form of small enclaves, which will form small bubbles during the crystallization of the is madeis made. The researchers thus went in search of a particular element within these bubbles: krypton. The measurements required the development of a new analysis technique, based on the mass spectrometrymass spectrometry. The different isotopes of krypton have thus been quantified, in particular the rarest, such as Kr78 and the Kr80.
The composition of the Earth’s mantle is very close to that of chondrites
The results, published in Nature, show that the chemical signature of the current lower mantle is very close to that of chondrites, primitive carbon-rich meteorites, which come from outside the Solar System. On the other hand, studies on the neonneon, another noble gas present in the lower mantle of the Earth, shows that it would come from the Solar System. These two results suggest that the volatile elements having been aggregated during the formation of the Earth come from at least two different sources: the rocky bodies coming from deep space but also the material composing the primitive solar nebula. These elements would have been integrated into the composition of the Earth very early on.
Indeed, krypton isotopes suggest that planetesimals from outside the Solar System bombarded Earth millions of years before the Moon was formed. But, at the same time, our planet would also have absorbed dust and gases from the solar nebula. This hypothesis is not only valid for the Earth, but would also make it possible to explain the formation of the other planets of the Solar System, even those orbiting around other starsstars.
These new findings also have implications for how the Earth’s atmosphere formed. Because the researchers discovered that the ratio between the different isotopes of krypton is not the same for the atmosphere as for the lower mantle. This suggests that some gases from the atmosphere were also brought to Earth after the giant impact that gave birth to the Moon. In the opposite case, the two environments, atmosphere and mantle, would have the same isotopic composition.
