Earth is the only planet known to have liquid water on its surface, a key feature when it comes to explaining the emergence of life. However, was this water always present in the rocks that made up our planet? Alternatively, was it later delivered by asteroids and comets that bombed the Earth? Or did Earth̵
In the diary Science, scientists from the Center de Recherches Pétrographiques et Géochimiques in Nancy (CNRS / Université de Lorraine) contribute to this debate by showing that most of the water present on Earth today was probably there from the very beginning. Yet the Earth formed in a region of the Solar System where temperatures were too high for water to condense and clump together with other solids such as ice, long supporting the hypothesis of a late addition of water.
However, the amount of water present in the rocks that made up the Earth had never been accurately estimated. Scientists focused on meteorites with a similar composition to Earth, called enstatite chondrites, and more specifically on a small number of these that have undergone little warming in their lifetime and therefore still show a primitive composition. Using two complementary techniques, they measured their hydrogen content and determined exactly where some of this was.
Their results show that Earth’s primitive rocks likely contained enough water to provide at least three times the amount of water in Earth’s oceans, and possibly much more.
Furthermore, the hydrogen in these meteorites has the same isotopic composition like that of the water stored in the Earth’s mantle, while the isotopic composition of the oceans is consistent with a mixture containing 95% water from enstatite chondrites and just 5% water supplied by water-rich comets or asteroids. The Earth therefore appears to have obtained the vast majority of its water from its constituent materials.
Notes These are very rare and make up less than 2% of meteorites. Although thirteen had been brought together for this study, some of them had been altered and only eleven of them were considered with their original water content.  A water molecule is made up of two hydrogen atoms and one oxygen atom. Like many chemical elements, hydrogen can exist in several forms called isotopes, which differ in their mass.
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Reference: “Earth’s water may have been inherited from material similar to enstatite chondrite meteorites” by Laurette Piani, Yves Marrocchi, Thomas Rigaudier, Lionel G. Vacher, Dorian Thomassin and Bernard Marty, 28 August 2020, Science.
DOI: 10.1126 / science.aba1948