The asteroids keep intact dust from the formation of the solar system.

Here, the asteroid Ida and its moon, Dactyl -

© NASA / JPL / USGS

  • The Hayabusa-2 probe brought back to Earth the very first samples from a carbonaceous asteroid, according to a study published by our partner The Conversation.

  • The study of the samples collected should make it possible to reconstitute part of the missing link in the stages of formation of our solar system.

  • The analysis of this phenomenon was carried out by Hugues Leroux and Damien Jacob, professors at the University of Lille.

After a 6-year journey in space aiming to meet a small carbonaceous asteroid, the Hayabusa-2 probe is back on Earth.

In her luggage, she brings back precious samples collected on the surface of the asteroid and which will be studied in detail by powerful instruments for characterizing matter.

Bring dust from the asteroid Ryugu to Lille

The Hayabusa-2 mission, from the Japanese space agency JAXA, sent a probe into orbit around the asteroid Ryugu.

She mapped it in detail and took some samples of its surface to bring them back to Earth.

The return to Earth took place on the night of Saturday 5 to Sunday 6 December 2020. The samples will be available for laboratory analyzes during 2021. This will be a first phase of study by teams already formed (analyzes preliminary), which is part of the University of Lille team.

Then the samples will be available to the entire scientific community on the basis of calls for projects.

Asteroid Ruygu photographed by the Japanese space probe Hayabusa 2 © JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST, CC BY-SA

Ryugu is a carbonaceous asteroid, that is, it contains carbonaceous matter.

It is also expected to contain phyllosilicates (silicates that contain water, either in molecular form or as OH hydroxyls).

The study of samples should therefore allow major advances on fundamental questions - on the one hand, the origin of water on Earth, and on the other hand, the nature of the primitive carbonaceous matter that potentially had a role on the emergence of life.

This is the first return of samples from a carbonaceous asteroid - the first Hayabusa probe brought back samples from the small non-carbonaceous asteroid Itokawa in 2010.

Look for clues to the formation of the solar system in the "freezer" that is space

To understand why we are looking for clues to the formation of the solar system on asteroids, we need to talk a little about this formation.

Within our galaxy, a cloud of gas and dust began to collapse under its own weight about 4.5 billion years ago - it was the start of the birth of the solar system.

The rotation of the entire cloud leads to the formation of a “protoplanetary” disc, at the center of which is our rising sun.

It is a tumultuous time, when the dust constituting the elementary bricks of the solar system is transformed deeply by agglomeration (one speaks of "accretion") and collisions.

Gradually, the protoplanetary disk was transformed to tend towards the configuration that we know today: the Sun and its planets.

Currently, we can observe protoplanetary disks in our galaxy: we have to look in regions of star formation, for example the famous Orion nebula.

Protoplanetary disk around the young star HL Tau, located about 450 light years away © ALMA (ESO / NAOJ / NRAO), CC BY-SA

But, paradoxically, the dust forming these protoplanetary discs is better detected when they are small, and it is very difficult to see bodies larger than about 1 mm, which completely absorb the infrared radiation used for their observation.

As a result, when accretion begins in the protoplanetary disc, the objects formed become larger and larger and are no longer detected.

This is why the stage of formation of the massive bodies of the solar system is still largely unknown.

In search of the building blocks of the solar system

The accretion stage leads to the formation of objects of varying sizes, including comets, asteroids and planets (asteroids formed in regions closer to the Sun than comets).

Some asteroids are a mile or less.

They are particularly interesting because they have preserved within them the elementary bricks of the dust disk.

Indeed, the matter which composes them has not, or little, evolved since the accretion, because the zones of the space which they inhabit are too cold to allow for example the growth of the grains or the chemical reactions inducing of new mineral species.

These objects are, in a way, freezers of the nascent solar system.

To study them is to go back in time 4.5 billion years.

These small objects usually orbit the sun in stable orbits, called asteroid or comet belts.

However, it happens that certain orbits are destabilized.

The trajectory of the bodies is then deviated and they can dive into the internal solar system, where the telluric planets are located, including the Earth.

This proximity is then a magnificent opportunity to study them more closely.

Porosity, a clue that Ryugu dust will tell us more than meteorites found on Earth

Illustration of the Hayabusa-2 probe with Earth in the background © Yarnalgo / Flickr, CC BY-SA 2.0

The spectroscopic and thermometric studies which were carried out while the Hayabusa-2 probe was in orbit show that the asteroid Ryugu is very porous, even at very fine scales.

This porosity gives the asteroid a low density and low mechanical cohesion.

Until now, we have only been able to study carbon-rich extraterrestrial matter from meteorites.

These meteorites are fragments of asteroids that violently crossed the atmosphere before hitting the ground.

It seems unlikely that very porous and mechanically fragile material, like that of Ryugu, can survive atmospheric entry.

The samples brought from this asteroid are therefore scientifically unique, unparalleled in meteorite collections, and certainly quite close to a weakly modified primitive dust assemblage on the asteroid.

It is an ideal setup for studying the transition stage of the dust cycle between the protoplanetary disk and the first bodies of the solar system.

In our laboratory at the University of Lille, we will study the Ryugu samples with advanced imaging and chemical analysis techniques, down to the atomic scale, using transmission electron microscopes to obtain a electron beam smaller than angstrom, that is, smaller than the size of atoms.

This work is part of a large international consortium, with around 200 mobilized researchers who will study these samples very carefully.

Decipher the infinitely small to understand the infinitely large

One of the technical challenges is not to damage the samples during the preparation and analysis steps.

For example, during an electron microscopy study, matter is bombarded with high-energy electrons.

The interaction between the electron beam and the sample can unfortunately induce irreversible modifications of the material studied.

Simulation of the surface gravel of the asteroid Ruygu © Kestrel / Wikimedia, CC BY-SA 4.0

The carbonaceous matter and the hydrated phases of the samples from the asteroid Ruygu are particularly sensitive to this degradation under the electron beam.

To study these fragile samples, we will therefore use a new generation of detectors, which will make it possible to minimize the quantity of electrons necessary for a good characterization of the material and will significantly reduce its degradation under the electron beam.

This problem of degradation during the observation of a sample by electrons is a major and long known problem.

It is particularly marked for biological samples - this is why the method of “cold” observation of fragile samples, called cryo-electron microscopy was developed, and awarded the Nobel Prize in chemistry in 2017.

Our "Asteroids" file

In a transmission electron microscope, a beam of electrons accelerated at high speed passes through the sample and a series of detectors collect the signals emitted after their interactions with matter.

These interactions with matter make it possible to characterize it on an atomic scale, because, on the most recent devices, the size of the electronic probe used is of the order of magnitude of the size of the atoms.

We will thus be able to determine the nature and organization of the atoms that make up Ruygu's dust: certain detectors will make it possible to study the organization of matter, to identify the solid phases that make up the sample, to decipher the microstructures and their training;

other detectors will be dedicated to chemical analysis, including the study of the valence of elements.

Ultimately, these studies at the atomic scale should make it possible to reconstruct part of the missing link in the stages of formation of our solar system.

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This analysis was written by Hugues Leroux and Damien Jacob, professors at the University of Lille.

The original article was published on The Conversation website.

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