First asteroid gas sample delivered by the Hayabusa2 mission: A treasure box from Ryugu

Abstract

The Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial ³He/⁴He and ²⁰Ne/²²Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth's atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.

Fig. 1.. Mass spectrum of the Hayabusa2 sample container gas with the QMS installed in the GAEA.

Prominent peaks from helium-4 [mass/charge ratio (m/z) 4], molecular nitrogen (m/z 28), and argon-40 (m/z 40) were observed in the container gas on site on 7 December 2020. The blue solid line represents the blank-corrected Ryugu gas data calculated by subtracting contributions from the instrumental blank gases, while the gray dotted line is the uncorrected (raw) measured data. The mass spectrum of the standard gas prepared from terrestrial atmosphere is also shown as red dots. The ion intensity of m/z 28 for the air was set at the same value as that of the container gas for comparison of the spectral pattern.

Fig. 2.. Isotopic compositions of He and Ne of the sample container.

Isotopic ratios of He and Ne of the sample container were determined using gas pipettes (NT1P2E, NT1P3B, NT1P4C, NT1P4Q1, NT1P5C, and NT1P5D) separated from a gas collection bottle of NT1 (see Materials and Methods). They can be explained by mixing of solar wind (SW) and terrestrial atmosphere. The mixing line in (A) is calculated using the ⁴He/²⁰Ne ratios of 13.1 and 8.1 for fractionated SW and terrestrial atmosphere, respectively. Neon isotopes plot on the mixing line between SW and terrestrial atmosphere (B). “P1 (or Q)” is a primordial gas trapped in an enigmatic (likely carbonaceous) carrier, phase Q (15). “P3” and “HL” are presolar gas components residing in presolar nanodiamond grains (15). A mixing line between SW and galactic cosmic ray (GCR)–produced Ne is also shown. Data sources are as follows: (–25) for terrestrial atmosphere; (26) for Jupiter’s atmosphere; (15) for P1, P3, and HL gases; and (16) for SW.

Fig. 3.. Elemental compositions of noble gases and nitrogen in the Hayabusa2 sample container normalized to Earth’s atmospheric composition and ³⁶Ar.

Mass spectrometry analyses for the gas collection bottle were performed at several laboratories. Uncertainties (1σ) are shown as the light gray bars. A clear excess in ⁴He was observed compared with Earth’s atmospheric composition (22). Doubly normalized elemental ratios, (X/³⁶Ar)_container/(X/³⁶Ar)_air, for N₂ or isotope X (X = ⁴He, ²⁰Ne, ³⁶Ar, ⁸⁴Kr, or ¹³²Xe).

Fig. 4.. Schematic diagram of the fragmentation and pulverization of the Ryugu grains.

The original grain surfaces are assumed to have been exposed to SW that is concentrated within the ~50-nm-thick uppermost layer (red line). Isotropic fragmentation (A) and surface pulverization (B) generate a fresh cross-section surface of the SW layer (yellow line). The surface pulverization seems consistent with the presence of many powdery samples sticking on the inner surface of the sample container (10, 32).