A few grams of dust from the asteroid Ryugu, brought back to Earth in 2020, help to better understand the origin of the water on our planet.
Earth is the only planet in the Solar System with large expanses of liquid water on its surface. Covering nearly 70% of its surface, the oceans have enabled the emergence and development of life. But where does all this water come from? And what were the processes involved in the formation of the oceans?
Around 4.57 billion years ago, the Solar System was born from a cloud of gas and dust rotating around the young Sun. Over time, under the influence of gravity and numerous collisions, this material gradually coalesced to form the planets, including Earth.
Asteroids and the Genesis of the Solar System
Several scenarios, which are not mutually exclusive but complementary, have been proposed to explain the origin of water on what would later become the « Blue Planet. » One such scenario suggests that water was already present in the material within the protoplanetary disk that accreted to form our planet. In this case, water could have been trapped as small fluid inclusions within minerals or even present within the structure of these minerals, relatively evenly distributed.
Another hypothesis suggests that the delivery of water to Earth occurred later, through impacts from comets or asteroids rich in water. Due to their relatively small size compared to the other planets, asteroids did not undergo high-temperature events that could have altered their chemistry. As a result, they have evolved very little since their formation 4.57 billion years ago.
While the terrestrial planets formed closer to the Sun (between 0.4 and 1.5 astronomical units, with one astronomical unit being the distance from Earth to the Sun, about 150 million kilometers), asteroids are mainly found in more distant regions from the Sun, particularly in the « main belt » (between 2 and 4.5 astronomical units).
These colder regions are conducive to the aggregation of water ice and silicate dust, which gives rise to asteroids. After their formation, some asteroids are driven to migrate within the solar system, reaching orbits closer to the Sun, similar to Earth’s orbit: these are known as Earth-crossing asteroids. Among them, the carbonaceous asteroid Ryugu (1 kilometer in diameter) was the subject of a study during the Hayabusa 2 space mission conducted by the Japanese space agency JAXA between 2014 and 2020.
The spacecraft first closely studied Ryugu, then collected samples in an impressive maneuver before heading back to Earth. On December 6, 2020, a few grams of dust (5.4 grams) from this asteroid landed in Australia aboard a capsule. With great excitement, we received a small portion of this treasure at the University of Lille: a few grains, barely visible to the naked eye, possibly holding clues to understanding the formation of Earth’s oceans.
Water from the Ryugu Asteroid
Our research work aims to decode this celestial material, down to the atomic scale, in search of every clue! The instrument we use to probe this scale is a transmission electron microscope. It emits a beam of electrons that passes through the sample. The signal collected from the interaction between this matter and the electron beam provides structural and chemical information about the mineralogical assemblages it comprises. This technique allows us to observe details down to the nanometer scale, which is one billionth of a meter — equivalent to dividing the thickness of a human hair by 100,000.
Studies of the Ryugu asteroid samples reveal that they are relatively dry, containing very little molecular water (H₂O). However, these samples are predominantly composed of clay minerals rich in magnesium and iron, making up more than 70% of the asteroid’s volume. These clays are minerals with a microscopic structure made up of stacked sheets (which can potentially house water molecules), known as « phyllosilicates. » They contain abundant hydroxyl groups (-OH), which are intrinsically the building blocks of their crystalline structure. These groups are made of oxygen and hydrogen, the same atoms that form water molecules.
One of the reasons for the presence of phyllosilicates in small asteroids traces back to the early days of the Solar System.
Around 4.57 billion years ago, during the formation of the Solar System, dust and ice gradually gathered together to form planets, asteroids, and other celestial objects (such as comets…). On small asteroids like Ryugu, dust made of anhydrous silicates (without water) coexisted with water ice. Shortly after the formation of the asteroid, it warmed up due to the spontaneous decay of radioactive elements, causing the water ice to melt. This led to a period of « aqueous alteration, » where this water interacted with the surrounding silicates to form the phyllosilicates that we observe today on Ryugu.
During the formation of Earth, and shortly after its creation, massive asteroid impacts similar to Ryugu could have delivered abundant quantities of phyllosilicates. Following the high-temperature events on Earth, hydroxyl groups (-OH) would have detached from the mineral structure of the clays, forming water molecules. The intense asteroid bombardment Earth experienced in its early years, including impacts from objects like Ryugu, may have significantly contributed to the progressive formation of the oceans. Therefore, while Ryugu is made up of more than 70% phyllosilicates, it also contains a few percent of organic material (around 5 to 7% by mass).
Source: Le point
