Ryugu owes its existence to a previous, larger body in the main asteroid belt, with its formation resulting from the reassembly of fragments from that entity. The Hayabusa2 mission from the Japan Aerospace Exploration Agency (JAXA) brought back 5.4 grams of material from Ryugu in late 2020. Notably, a large part of these samples comprises hydrated minerals. They emerged within Ryugu's forebearer through the aqueous alteration of original anhydrous grains, which offer crucial insights into the composition of the protoplanetary dust that led to the formation of Ryugu's antecedent.
A team of researchers from the Institute of Astrophysics of Space (IAS) in Orsay, France, Tohoku University in Japan, and the SMIS beamline of the French synchrotron SOLEIL embarked on an in-depth study using infrared hyperspectral imaging. Their focus was two millimeter-sized "stones" of Ryugu, a choice that aimed to reveal the mineral composition of the fragments and compare Ryugu with other extraterrestrial materials.
The team's infrared analysis unearthed a fascinating characteristic of Ryugu's anhydrous grains: some of them are abundant in amorphous silicates, exhibiting a mineral composition that mirrors that of certain anhydrous primitive asteroids, comets, and interplanetary dust particles of cometary origin. These grains rich in amorphous content are thought to have formed via pre-accretional processes within the protoplanetary disk that birthed the Solar System.
The findings suggest that the parent of Ryugu was a sizable planetesimal originating in the outer solar system, close to the accretion region of comets. As part of planetary migrations, the progenitor would later have been relocated to the main asteroid belt. Following this, aqueous alteration seemingly played a key role in shaping the spectral diversity observed among today's "primitive" asteroid classes.
The understanding of Ryugu's lineage and the composition of its anhydrous grains present vital information about the protoplanetary environment from which our solar system was born. Furthermore, it affords a more granular perspective on the processes that led to the formation and diversity of primitive asteroids. The ongoing analysis of the Ryugu samples will undoubtedly yield more revelations about the asteroid itself and the broader history of our solar system.
Research Report:Ryugu's Anhydrous Ingredients and Their Spectral Link to Primitive Dust from the Outer Solar System
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