Inorganic and organic assessments
The concentrations of total organics, major elements, and major ions monitored by the coupons at different locations, occasions, and/or durations (coupon set code; CPS-1 to CPS-12) are summarized in Figure 4. Concentrations of elements, ions, and total organics were less than or on the order of 103 nano-gram (i.e., 1 μg or less) for all the coupons (~30 mm in diameter). Raw concentration of the elements (Fe, Cu, Ni, Cr, Zn, Na, K, Ca, Mn, Al, Ti, Mg, Co, Sn, Pb, V, and P), ions (including Cl−, NO2−, Br−, NO3−, SO42−, PO43−, and F−), and organics are shown in Tables 2, 3, and 4, respectively. The concentrations normalized to surface area (i.e., ng cm−2) can be obtained using the monitor coupon size (Figure 1-b).
The major inorganic elemental and ion species detected were sodium (Na), calcium (Ca), and potassium (K), chloride (Cl−), sulfate (SO42−), and nitrate (NO3−). Their profiles are positively correlated with total organic content in the range of 101 to 103 ng per monitor coupon. Figure 5 shows the time-dependent static accumulation of ambient aerial deposits onto the monitor coupons throughout the procedure (CPS-1–12, including STA1, SFA2, and VAB). There are clear positive correlations between total sodium, potassium, and chloride, and total organic contents, which closely approximate the 1:1 line. Both major inorganic ions and total organic fluxes converged in the range of 101 to 103 ng per monitor coupon, resulting in a static deposition amount of <1 μg overall within ~7.0 cm2 and ~5.3 cm2 for aluminum schale and pyrex schale, respectively. Representative compositions of organic deposits are obtained by the gas chromatogram (Figure 6). A wide range of aliphatic hydrocarbons of straight-chain alkanes (<n-C19H40) were detected (e.g., extracted ion chromatogram, m/z = 57). The total amounts of other miscellaneous organics were quantified based on the corresponding response of the internal standard method (e.g., Takano et al., 2020). The exposure time-dependent accumulation profiles are similar to the results of the cleanliness test in the clean-room chamber at ISAS/JAXA (i.e., the nitrogen-circulation clean chamber; Sugahara et al., 2018).
Tables 5 and 6 summarize the basic statistics and the correlation matrix for the concentrations of major components in the static samples (i.e., CPS1–12 and CAS1–12; n = 10). Most profiles of the major components (e.g., Na, K, Cl−, F−, NO3−, Zn, SO42−, Mg, and P) positively correlate with each other, except for the profile of aluminum (Figures 5e and S2). We note that we used clean-room-type aluminum foil, to store and carry the monitor coupons, which could potentially introduce a certain amount of aluminum onto the coupon. We also conducted microscopic observations of the solid contaminants to evaluate their morphological characteristics (Figure 7), and found that globular shape particles were more common (70%) than fibrous material (13%). Further non-destructive analysis using micro-Raman and infrared spectroscopy (e.g., Kitajima et al., 2015) at ISAS/JAXA can be performed on request.
We stored potential Earth-derived artifacts obtained from each procedure (Table 1) (i.e., the category-3 particles; Uesugi et al., 2019). Therefore, if necessary, we can provide a careful description, on request, as suggested from the lessons learned from the previous Hayabusa mission with regard to high resolution small-scale mass spectrometry (e.g., NanoSIMS: Ito et al., 2014; ToF-SIMS: Naraoka et al., 2015) and spectroscopy (e.g., X-ray absorption: Yabuta et al., 2014). Interestingly, it should be noted that Chan et al. (2021) has reported a detailed description of carbonaceous and organic matters that are derived from Itokawa.
Safety declaration of the Ryugu sample processes
The Hayabusa2 mission succeeded in delivering its re-entry capsule to Earth in December 2020, and the total sample amount was confirmed to exceed 5 g (Tachibana et al., 2021; Yada et al., 2021a,b), including millimeter-sized grains to centimeter-sized pebbles. This study reports the pre-launch phase environmental assessment during the manufacturing of the sampler devices, their installation onto the spacecraft, and their transportation. We conclude that the exposure-time-dependent deposition profiles of potential contaminants were firmly underscored and similar to those obtained from the clean-room facility assessment (Sugahara et al., 2018). All the potential contaminants have been stored at the JAXA Extraterrestrial Sample Curation Center in a nitrogen-purged storage, and are available for future analysis on request.
The total amount of potential contaminants in the sampler system is estimated to be less than 100 ng cm-2, which is much smaller than the total amount of returned samples. Along with the fact that there was no off-nominal situation from the spacecraft launch to the capsule re-entry to Earth, we conclude that ambient background artifacts have been characterized and will not affect detailed sample analysis.
We expect that the lessons learned in the process from implementation to practical knowledge during the last eight years will also represent a valuable baseline for future sample-return missions (e.g., Dworkin et al., 2017; Usui et al., 2020; Anand et al., 2020; Chan et al., 2020; Neveu et al., 2020; Farley et al., 2020; Fujiya et al., 2021).