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Bulk chemical composition of samples recovered from asteroid Ryugu

Hisashi Homma and Kazuko Motomura

Summer 2023 Volume 39, No. 2 , 01-07

Spacecraft HAYABUSA2 successfully collected a 5.4 g sample from the surface of asteroid Ryugu that was returned to Earth on Dec. 6, 2020. Analysis of the asteroid Ryugu sample was performed using a ZSX Primus IV wavelength dispersive X-ray spectrometer and a Thermo plus EVO2 TG-DTA8122 thermogravimetric differential thermal analyzer coupled with GC-MS (TG-MS).

A very small (24 mg) Ryugu sample (C0108) was analyzed by XRF in powder form without any pelletization or thin film covering. Analytical results by the fundamental parameter (FP) method for 23 elements including carbon and oxygen were consistent with the values from other analytical methods. Elemental abundance in Ryugu shows close similarity with the abundance determined for the CI chondrite meteorite, whose composition is the most primitive and similar to solar system elemental abundance.

About 1 mg of Ryugu sample grain A0040 was used for the TG-MS measurement. Total H₂O and CO₂ content of the Ryugu sample were 6.8 and 5.5 mass%, respectively. The Ryugu sample contains less H₂O than CI chondrite does. The TG-MS measurement reveals differences in H₂O release behavior at low temperature (< 300°C) between Ryugu and CI chondrite.

Highlights

  • WDXRF accurately quantified 23 elements, including carbon and oxygen, in just 24 mg of unpelletized extraterrestrial powder using the fundamental parameter (FP) method, while preserving the sample for additional analyses.
  • The elemental composition closely matches that of primitive CI carbonaceous chondrites, indicating a chemically primitive material with an elemental distribution similar to that of the early solar system.
  • Combined WDXRF and TG-MS analysis revealed significantly lower water content and different low-temperature water-release behavior than CI chondrites, providing evidence for distinct hydration history and mineralogy.

Summary

Accurate bulk elemental characterization of extremely limited geological samples requires analytical methods that maximize information while minimizing sample consumption and preparation. Wavelength-dispersive X-ray fluorescence (WDXRF) demonstrated that reliable quantitative analysis can be achieved using only 24 mg of loose powder without pelletizing or protective films, preserving valuable material for subsequent investigations. A specialized vacuum and sample-handling system prevented particle loss during measurement while allowing direct determination of carbon and oxygen.

Because no suitable matrix-matched reference materials exist for this type of sample, quantification was performed using the fundamental parameter (FP) method rather than conventional empirical calibration. The method successfully quantified 23 elements ranging from major constituents to trace elements. Validation with meteorites, geological standards, and other reference materials showed excellent agreement with known compositions, demonstrating that total oxygen can be determined accurately even when samples contain significant amounts of water.

The resulting elemental composition closely resembles that of primitive CI carbonaceous chondrites across both major and trace elements, supporting the interpretation that the material represents chemically primitive solar system matter. However, oxygen abundance was somewhat lower than expected, corresponding to reduced water content.

Thermogravimetric analysis coupled with mass spectrometry (TG-MS) complemented the elemental measurements by quantifying evolved gases during heating. The material contained approximately 6.8 mass% water and 5.5 mass% carbon dioxide, substantially less water than typical CI chondrites. The thermal profiles also showed markedly different water-release behavior below 300°C, suggesting differences in hydration state or mineral composition. Together, WDXRF and TG-MS provide a powerful, minimally destructive workflow for characterizing rare geological and extraterrestrial samples when only milligram-scale quantities are available.

Frequently asked questions

WDXRF is inherently non-destructive because it measures fluorescent X-rays emitted from the sample without chemically altering it. For extremely limited materials, loose powder can be analyzed directly without pelletizing, allowing the entire sample to be recovered afterward for complementary techniques. Specialized vacuum control and sample handling minimize particle movement and prevent material loss during analysis.

Traditional XRF calibration depends on reference materials that closely match the composition of unknown samples. For unusual materials with no suitable standards, such as primitive extraterrestrial matter, this approach is impractical. The FP method uses theoretical X-ray physics together with calibration standards covering a broad compositional range, enabling accurate quantification even when matrix-matched reference materials are unavailable.

Oxygen is a major constituent of silicates, oxides, and hydrated minerals. Accurate oxygen quantification improves overall elemental balance and helps estimate mineral composition. It also provides insight into hydration levels because reduced oxygen abundance can reflect lower water content within the sample.

While XRF measures elemental composition, TG-MS identifies and quantifies volatile compounds released as a sample is heated. This allows direct measurement of water, carbon dioxide, sulfur-containing gases, and their release temperatures. Combining both techniques provides a more complete understanding of elemental composition, volatile content, and mineralogical characteristics.

The temperatures at which water is released indicate how water is bound within the material. Low-temperature release often reflects weakly bound or adsorbed water, whereas higher-temperature release is associated with structural hydroxyl groups or hydrated minerals. Different thermal release profiles therefore provide insight into mineralogy, alteration history, and hydration processes.

CI carbonaceous chondrites are among the most chemically primitive materials known and closely approximate the elemental composition of the early solar system, excluding highly volatile elements. A similar elemental distribution suggests that the analyzed material has experienced relatively little chemical differentiation and preserves information about the earliest stages of solar system formation.

Yes. With appropriate instrument configuration and optimized analytical conditions, WDXRF can accurately quantify light elements including carbon and oxygen from milligram-scale powder samples. Validation against independent analytical methods demonstrates that reliable results can be achieved without requiring destructive sample preparation or prior measurement of water content.

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