Asteroid Ryugu is Made of the Most Primitive Material Ever Studied on Earth

A sample of black-colored pebbles and fine powder from the near-Earth asteroid Ryugu. Credit: Yada et al., 2021.

The Hayabusa2 mission was launched on December 3, 2014, with the goal of collecting samples from the asteroid (162173) Ryugu. The Hayabusa2 spacecraft made two landings on Ryugu and collected samples that were returned to Earth on December 6, 2020. These samples are made up of small rock fragments up to about 10 millimeters with a total mass of 5.4 grams. A research team led by Tetsuya Yokoyama of the Tokyo Institute of Technology examined 95 milligrams of the Ryugu samples consisting of powder and small particles from both landing sites. The research team investigated the mineralogy, chemistry, and isotopic compositions of Ryugu samples using electron microscopy, X-ray fluorescence, thermogravimetric analysis, and various mass spectrometry-based techniques.

The Ryugu samples are breccias consisting of clasts that contain various proportions of fine-grained, hydrous phyllosilicate minerals (i.e., clays) such as serpentine and saponite, plus coarser grains of carbonates, magnetite, and sulfides. The mineralogy and petrology of the Ryugu samples closely resemble those of carbonaceous chondritic meteorites belonging to the Ivuna (CI) group, which have experienced extensive aqueous alteration. Based on oxygen isotope analysis and 53Mn-53Cr dating, the primary minerals on Ryugu were altered by aqueous fluids at a temperature of 37 ± 10°C between 3.1 and 6.8 million years after the formation of the calcium-aluminum-rich inclusions, which are generally considered to represent the earliest solids formed in the solar system. The Ryugu samples were probably never heated above 100°C after aqueous alteration because the interlayer water in their phyllosilicate minerals was found to release water at 90°C. These research findings suggest that Ryugu samples may be the most primitive solar system materials ever investigated on Earth. The next step in studying these priceless returned samples is to precisely determine their chemical and isotopic compositions, which will provide a new standard for research on the early solar system. READ MORE