A sample of asteroids highlights how the Earth gets its water

Water is essential to life on Earth and is one of our most precious natural resources. But given the formation of our planet, it is quite surprising how much water we have left. The Earth was aggregating from a cloud of gas and dust – a protoplanetary disk – and was very hot for the first million years. Its surface has been merged by the impacts of comets and asteroids. The interior of the Earth was also (and still is) kept liquid by a combination of gravitational heating and radioactive isotope disintegration.

This means that if there was an initial water (and organic compounds) on the Earth, it should have boiled quickly. So, how come there is a lot of water on our planet today? A startling new study, published in Science Advances, suggests that a type of asteroid that we did not expect to contain too much water could be responsible for it – at the same time demonstrating that the solar system is probably much wetter than thought it before.

Scientists have long debated the exact source of water from the Earth. One theory suggests that it could have been captured by asteroids and comets that collided with it. Another states that water was always present in the rocks of the Earth's mantle and was gradually released to the surface by volcanoes.

Thanks to the Japanese Hayabusa mission, we now have new evidence. The spacecraft brought back a valuable cargo of recovered grains to the surface of the 25,143 Itokawa asteroid in 2010. The researchers behind the new study were able to analyze the water content of two grains. They used a sophisticated kit called an ion microprobe, which bombards a sample with a beam of ions (charged atoms) to probe the composition of its surface.

The experiment was not easy – the grains are tiny, less than 40 microns (one millionth of a meter) in diameter and each grain was composed of several different minerals. The ion microprobe had to be centered on a specific mineral in each grain so that the authors could gather the required data. The mineral species analyzed was a silicate containing iron and magnesium, called pyroxene, which is almost completely free of calcium.

This type of substance is not usually associated with water – it is actually considered an anhydrous mineral (NAM). The network of a pyroxene crystal does not contain vacant sites for water molecules in the same way as, for example, a clay mineral, so its structure is not necessarily conducive to water absorption. However, the sensitivity of the technique used by the authors was such that they could detect and measure minute quantities of water.

The results were surprising: the grains contained up to 1000 parts per million of water. Knowing the composition of Itokawa, the researchers were then able to estimate the water content of the entire asteroid, accounting for between 160 and 510 parts per million of water. This is more than expected – remote measurements of two similar bodies (also asteroids of type S) revealed that one contained 30 and the other 300 parts per million of water .

Unlikely source

The water is made of hydrogen and oxygen. But these elements come in the form of different isotopes, which means that they can have a different number of neutrons in their atomic nucleus (neutrons are particles that make up the nucleus with protons). The researchers examined the isotopic composition of water in the water and found that it was very close to that of the Earth, suggesting that the water of the Earth had the same source as that of Hayabusa grains.

The results raise several interesting questions, the first being how much of this amount of water has arrived in the nominal anhydrous minerals? The authors suggest that, during their formation, the grains absorbed hydrogen from the protoplanetary disk which, at the high temperatures and pressures of the solar nebula, was combined with the oxygen of the minerals to produce of water.

Until here, so reasonable. But how is it possible that water has remained in the minerals? After all, they came from an asteroid S-type – an asteroid that forms in the inner and warmer part of the solar system. Itokawa had a complex history of thermal metamorphism and collision, reaching temperatures at least as high as 900 ° C. But researchers used computer models to predict the amount of water lost during these processes – it's has been found to be less than 10% of the total.

The water of the earth

But how does all this relate to the water of the Earth? The researchers assume that, following the absorption of water by the grains of the protoplanetary disk, the minerals aggregate and stick together to form pebbles and possibly larger bodies such as asteroids.

If this mechanism works for asteroids, it could also be the same for Earth. Maybe its original water came from these minerals that came together to help form the Earth. While water was lost at the beginning of Earth's history, it was added in collisions by many S-type asteroids – as implied by the similarity of the isotopic composition of hydrogen between Earth and Itokawa.

This fresh look at an old problem – the origin of the Earth's water – has resulted in a surprising conclusion, which suggests that a large population of asteroids in the internal solar system could hold much more water than what had been imagined.

So, while there is water everywhere in the solar system, the fact that it is hidden in minerals means that there is not always a drop to drink.

Monica Grady, Professor of Planetary and Spatial Sciences, The Open University

This article is republished from The Conversation under a Creative Commons license. Read the original article.