A recent analysis of grains collected from asteroid Ryugu has shed light on the mysterious journey of this carbon-rich rock throughout the Solar System. Contrary to previous assumptions, it has been discovered that a significant portion of Ryugu originated far from the Sun before finding its way to the asteroid belt, eventually settling at a distance similar to Earth’s from the Sun. These findings not only reveal the intricate histories of asteroids but also highlight their potential as valuable repositories of the Solar System’s diverse past.
Ryugu, classified as a C-type asteroid, falls within the most common group of asteroids in the Main Belt, an area situated between Mars and Jupiter. However, unlike its counterparts, Ryugu does not reside in the Main Belt. Its close proximity to Earth’s orbit suggests that it experienced a disruptive event that propelled it out of its original location in the asteroid belt. The Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 probe, launched in 2014 as a sample return mission, extensively examined Ryugu from 2018 to 2019 and eventually brought back samples to Earth in 2020. The mission not only collected valuable samples but also discovered that Ryugu is not a single large chunk of rock. Instead, it is composed of smaller rocks loosely bound together, forming what scientists refer to as a “rubble pile.”
While Ryugu shares similarities with asteroids found in the asteroid belt, such as the Polana and Eulalia families, recent studies conducted on the samples obtained from Ryugu suggest a different origin for some of its minerals. One notable difference is the presence of organic matter, commonly associated with comets originating from the outer Solar System. Additionally, the isotopes of oxygen found on Ryugu align with those typically found in the outer regions of our celestial neighborhood.
Led by astrophysicist Rosario Brunetto from the University of Paris-Sarclay in France, an international team conducted an in-depth analysis of grains rich in olivine, pyroxene, and amorphous silicates. These grains, least affected by the water present on the asteroid, were studied using infrared spectrometry. The team discovered that the infrared light reflected by these grains closely resembled those observed in objects from the outer Solar System. This resemblance included objects such as asteroid Hektor, a Trojan in the swarms sharing Jupiter’s orbit, Comet Hale-Bopp, and interplanetary dust that likely originated from comets.
Based on their research, Brunetto and her team propose that the primary parent body of Ryugu was a planetesimal – a celestial object capable of developing into a planet – that initially formed in the outer reaches of the Solar System, assimilating ingredients from that region. However, at some point, its growth was disrupted, possibly due to an external force, causing it to break apart and journey to the Main Belt. This migration process subjected Ryugu to water alteration, leading to its eventual transformation.
A Time Capsule of the Solar System
In light of these findings, the researchers emphasize the significant role that asteroid Ryugu plays as a crucial archival record of the Solar System’s evolution. The pristine lithologies present in the Ryugu samples contain anhydrous grains that closely resemble those of outer Solar System primitive bodies. Consequently, Ryugu presents numerous reservoirs of anhydrous primitive dust, carrying invaluable information about the formation and progression of planetesimals within the protoplanetary disk.
The exploration of asteroid Ryugu has unveiled remarkable insights into the complex journeys and diverse origins of asteroids within the Solar System. By analyzing ancient grains, scientists have deciphered the unique story of Ryugu, from its distant formation in the outer Solar System to its final resting place within the Main Belt. These discoveries further solidify the importance of asteroids as windows into the history and development of our cosmic neighborhood.