Les météorites qui ont contribué à la formation de la Terre pourraient provenir du système solaire externe.

Illustration d'artiste de la ceinture d'astéroïdes

Illustration d’artiste de la ceinture d’astéroïdes. Crédit : NASA/JPL-Caltech

Des preuves suggèrent que les minéraux de surface des astéroïdes de la ceinture principale, qui auraient été à l’origine des éléments constitutifs de l’eau et de la vie sur Terre, ne sont stables qu’à basse température. Ces astéroïdes se sont formés sur des orbites lointaines et pourraient contribuer à expliquer la composition de la Terre.

On pense que notre système solaire s’est formé à partir d’un nuage de gaz et de poussière, appelé nébuleuse solaire, qui a commencé à se condenser sur lui-même par gravitation il y a environ 4,6 milliards d’années. En se contractant, ce nuage s’est mis à tourner et à prendre la forme d’un disque tournant autour de la masse de gravité la plus élevée en son centre, qui allait devenir notre Soleil.

Notre système solaire a hérité de toute sa composition chimique d’une ou plusieurs étoiles antérieures qui ont explosé en supernovae. Notre Soleil a récupéré un échantillon général de cette matière lors de sa formation, mais la matière résiduelle du disque a commencé à migrer en fonction de sa propension à geler à une température donnée. Lorsque le Soleil est devenu suffisamment dense pour initier des réactions de fusion nucléaire et devenir une étoile, il a récupéré un échantillon général de ce matériau au cours de sa formation, mais les résidus du disque ont formé des matériaux solides pour former des corps planétaires en fonction de leur propension à geler à une température donnée.

Lorsque le Soleil a irradié le disque environnant, il a créé un gradient de chaleur dans le système solaire primitif. Pour cette raison, les planètes intérieures, Mercure, Venus, Earth, and Mars, are mostly rock (mostly composed of heavier elements, such as iron, magnesium, and silicon), while the outer planets are largely composed of lighter elements, especially hydrogen, helium, carbon, nitrogen, and oxygen.

Surface Analysis of Asteroids

(a) 3.1 µm absorption depth (horizontal axis) indicating the presence of ammoniated phyllosilicates. Black: asteroids observed by AKARI. Orange: meteorites derived from C-type asteroids. Blue: theoretical calculation results for the initial composition, including ammonia ice (the number is the ratio of water to rocks and corresponds to the horizontal axis in Figure 3b). (b) Black lines: reflectance of the asteroids showing 3.1 µm absorption. Blue line: reflectance of mineral combinations containing ammoniated phyllosilicates obtained from theoretical calculations. Purple line: reflectance of an asteroid covered with water ice, obtained from theoretical calculations. The locations where three major absorption features appear are indicated by colored areas. Red area at around 2.7µm: hydrous minerals. Blue area at around 3.1 µm: ammoniated phyllosilicates or water ice. Green areas at around 3.4 µm and 4.0 µm: carbonates. Credit: Reproduced from Kurokawa et al. 2022 AGU Advances

Earth is believed to have formed partly from carbonaceous meteorites, which are thought to come from outer main-belt asteroids. Telescopic observations of outer main-belt asteroids reveal a common 3.1 µm reflectance feature that suggests their outer layers host either water ices or ammoniated clays, or both, which are only stable at very low temperatures. Interestingly, though several lines of evidence suggest carbonaceous meteorites are derived from such asteroids, the meteorites recovered on Earth generally lack this feature. The asteroid belt thus poses many questions for astronomers and planetary scientists.

A new study led by researchers at the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology suggests these asteroidal materials may have formed very far out in the early Solar System then been transported into the inner Solar System by chaotic mixing processes. In this study, a combination of asteroid observations using the Japanese AKARI space telescope and theoretical modeling of chemical reactions in asteroids suggests that the surface minerals present on outer main-belt asteroids, especially ammonia (NH3)-bearing clays, form from starting materials containing NH3 and CO2 ice that are stable only at very low temperature, and under water-rich conditions. Based on these results, this new study proposes that outer main-belt asteroids formed in distant orbits and differentiated to form different minerals in water-rich mantles and rock-dominated cores.

Mineral Compositions From Theoretical Calculations of Chemical Reactions Between Water and Rocks

Mineral compositions obtained from theoretical calculations of chemical reactions between water and rocks. (a) the starting materials only contain water and rocks. No ammoniated phyllosilicates are formed in any conditions. (b) the starting materials are water with ammonia ice and dry ice, and rocks. When the water/rock ratio (mass ratio) is high, namely, the proportion of water is large, ammoniated phyllosilicates are formed (light blue dotted line). Credit: Reproduced from Kurokawa et al. 2022 AGU Advances

To understand the source of the discrepancies in the measured spectra of carbonaceous meteorites and asteroids, using computer simulations, the team modeled the chemical evolution of several plausible primitive mixtures designed to simulate primitive asteroidal materials. They then used these computer models to produce simulated reflectance spectra for comparison to the telescopically obtained ones.

Their models indicated that in order to match the asteroid spectra, the starting material had to contain a significant amount of water and ammonia, a relatively low abundance of CO2, and react at temperatures below 70?, suggesting the asteroids formed much further out than their present locations in the early solar system. In contrast, the lack of the 3.1 mm feature in meteorites can be attributed to reaction possibly deeper inside asteroids where temperatures reached higher values thus, recovered meteorites may sample deeper portions of asteroids.

Formation and Evolution of C-Type Asteroids

A scenario for the formation and evolution of C-type asteroids derived from this study. Credit: Reproduced from Kurokawa et al. 2022 AGU Advances

If true, this study suggests that Earth’s formation and unique properties result from peculiar aspects of the Solar System’s formation. There will be several opportunities to test this model, for example, this study provides predictions for what the analysis of Hayabusa 2 returned samples will find. This distant origin of asteroids, if correct, predicts that there will be ammoniated salts and minerals in Hayabusa 2’s returned samples. A further check on this model will be provided by the analyses of returned materials from NASA’s OSIRIS-Rex mission.

This study also examined whether the physical and chemical conditions in outer main-belt asteroids should be able to form the observed minerals. The cold and distant origin of asteroids proposed suggests there should be a significant similarity between asteroids and comets and raises questions about how each of these types of bodies formed.

This study suggests the materials that formed the Earth may have formed very far out in the early Solar System and then been brought in during the especially turbulent early history of the solar system. Recent observations of protoplanetary disks by the Atacama Large Millimeter/submillimeter Array (ALMA) have found many ringed structures, which are believed to be direct observations of planetesimal formation.

As lead author Hiroyuki Kurokawa summarizes the work, “Whether our solar system’s formation is a typical outcome remains to be determined, but numerous measurements suggest we may be able to place our cosmic history in context soon.”

Reference: “Distant Formation and Differentiation of Outer Main Belt Asteroids and Carbonaceous Chondrite Parent Bodies” by H. Kurokawa, T. Shibuya, Y. Sekine, B. L. Ehlmann, F. Usui, S. Kikuchi and M. Yoda, 16 December 2021, AGU Advances.
DOI: 10.1029/2021AV000568

Leave a Comment