Does the water on Earth come asteroids?

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The formation of impact craters has been simulated in the laboratory since the 1960s with the Ames Research Center. By firing projectiles of a composition similar to water-rich asteroids, researchers have shown that a significant part of the water could remain trapped in the impact debris, illuminating the origin of the water found on Earth.

Two American planetologists have just added a new element to the debate, already filled with twists, about the origin of ocean water on Earth and indirectly on the Moon. The two questions are linked because of a common origin of these two bodies, revealed by the Apollo missions and which led to the famous hypothesis of the collision between our blue planet and the long lost planet Theia.

This is a problem that we can and have studied through geochemistry and cosmochemistry. For example, a process of degassing the mantle of the Earth by volcanoes has been done. But even if such an assumption were correct, it leaves behind the origin of the water in the mantle itself. It was also assumed that the Earth had probably lost its original water during the collision with Theia, which had led to the formation of an ocean of magma, and thus to the loss of water into space due to the high temperatures reached. A second contribution, in the form of meteorites or comets, or both has then been advanced.

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We can try to see more clearly, focusing in particular on the abundances of deuterated forms (HDO and D2O) of water in the small bodies of the Solar System. Deuterium oxide (D2O) is of course chemically similar to water (H2O), but its hydrogen atoms are heavy isotopes – namely deuterium whose nucleus contains a neutron in addition to the proton present in each hydrogen atom. It tends, for example, to avoid taking the form of vapor or to escape the attraction of a body, which may lead to signatures concerning its origin.

Astrochimists are aware of this, and that is why they measured the D / H ratio for comets and asteroids. In the case of terrestrial oceans, they found it included in the range of D / H ratios of asteroids between Mars and Jupiter and some comets, but not that found in comet 67P/Churyumov-Gerasimenko.

The article published in Science Advances by Peter Schultz of Brown University and Terik Daly of Johns Hopkins University is also moving in the direction of water intake from asteroids. To arrive at this conclusion, the two researchers concentrated on the problem of the quantity of water that could be brought by a collision between the young Earth, in formation or not, and an asteroid of greater or lesser size. During the collision, the energy released must melt the material of the celestial body and even lead to its evaporation, so again, a priori, to a loss of water into the space if the steam is hot enough.

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To be clear, the two researchers used the Ames Vertical Gun Range that was built in the 1960s for the Apollo project. This gun is used to draw Pyrex beads in a vacuum chamber in a soil sample intended to reproduce, for example, what happens on the Moon during an impact and thus to study the formation of craters.

Daly and Schultz began by making beads with a composition similar to those of water-rich carbon chondrites from asteroids. These beads were then shot at speeds of about five kilometers per second on a dry pumice material. The researchers then analyzed the debris produced by the impact with an armada of analytical tools, looking for signs of water that could be trapped there. They found that at realistic velocities and angles of impact, up to 30% of the water in the logs was trapped in post-impact debris.

Planetary geologists think they understand the reason. The water would vaporize well during the impact, but it would be partly recaptured, paradoxically, by a part of the melt generated by the impact in the column of debris produced. Part of the water from the Earth and the Moon may well have been brought by asteroids when they were very young.

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Carl Frantz

Polyglot, humanitarian, Carl was born in Germany but raised in the USA. He writes mostly on tech, science and culture.