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Initial Sample Return Analysis from Asteroid Ryugu

The Japanese Space Agency (JAXA) launched a spacecraft named Hayabusa2 (Hayabusa means Falcon) to study asteroid 162173 Ryugu on December 3, 2014. Ryugu is classified as a C-type carbonaceous asteroid. (See figure 1) Hayabusa2 arrived at Ryugu on June 27, 2018 then successfully orbited and mapped the asteroid. The spacecraft soon deployed a series of two rovers and a small lander on the surface. Then, on Feb. 22, 2019, Hayabusa2 fired an impactor into the asteroid to create an artificial crater thus allowing the spacecraft to retrieve a sample from beneath the surface of the asteroid. On Dec. 6, 2020, the Hayabusa2 returned to earth and released a capsule containing a sample of the asteroid. The capsule made a fiery entry through our planet’s atmosphere and parachuted to a landing site inside the Woomera Range Complex in the South Australian Outback. (I personally visited Woomera, Australia on December 4, 2002 during a total solar eclipse expedition).

The Hayabusa2 mission is similar to NASA’s OSIRIS-REx mission to the carbonaceous asteroid Bennu. OSIRIS-REx successfully collected a sample from Bennu on October 20, 2020 and is currently headed back to earth and scheduled to arrive on September 24, 2023. Thus we will ultimately have samples from two different carbonaceous asteroids for analysis back on Earth.

The initial analysis of these samples are published online in the December 20, 2021 issue of Nature Astronomy (See Ref 1 &2). A total of 5.424 ±0.217 g was collected from Ryugu and kept as physically and chemically pristine as possible, handled only in a vacuum of pure nitrogen. The samples were divided into two main Chambers labeled A and C. Each chamber was further divided into three subunits Aa, Ab, Ac, Cd, Ce, Cf. (See Figure 2).

Chamber A contains a total of 3.237 ±0.002 g collected during the first touch-down sampling at the equatorial ridge region of Ryugu. These samples represent the surface materials of Ryugu at the uppermost centimeter-scale layer, and that this layer was influenced by radiation, temperature cycling and micro-meteoritic impacts.

Chamber C contains a total of 2.025 ±0.003 g collected during the second touch-down sampling at a site proximal to the artificial crater excavated by the Small Carry-on Impactor. The samples in Chamber C represent subsurface materials excavated by the impact experiments, and that these samples have not experienced long-term exposure to space.

The average bulk density of Ryugu particles in both chambers A and C is 1,282 ±231 kgm−3 . This value is lower than the average bulk density of CI chondrites which is 2,110 kgm−3. The Tagish Lake meteorite has the closest bulk density value at 1,660 ±80 kgm−3 making it the most porous meteorite. (See Photo 1) Assuming that millimeter-sized sample grains have the same grain density as CI chondrites, the microporosity of Ryugu samples is estimated to be 46%. This value is consistent with the porosity determined by instruments onboard the Hyabusa2 spacecraft. Thus the microscopic observations and weight measurements for the Ryugu samples imply low density and a high microporosity.

Samples from Chambers A and C both show a very dark albedo of approximately 0.02 or 2% which is in agreement with remote-sensing data of Ryugu’s surface taken by the spacecraft Hyabusa2. The chambers are spectroscopically homogeneous and featureless without high-temperature components such as chondrules or Calcium–Aluminum-rich inclusions but have many bright and patchy fine inclusions. Optical and infrared microscopic images show most of these bright spots disappear in different viewing angles and do not exhibit distinctive spectral signatures. These bright spots are not intrinsic, such as Calcium Aluminum Inclusions and chondrules, but caused by different photometric conditions.

Chamber A and C samples both exhibit clear absorption features at 2.7 µm and 3.4 µm. (See Graphs 1 & 2). The narrow and relatively deep absorption feature at 2.715 ±0.005 µm indicates the presence of hydroxyl (OH) in the samples, which is comparable to the 2.72 µm absorption feature detected across the surface of Ryugu.

The absorption centered at 3.4 µm, corresponds to both carbonates and Carbon-Hydrogen (CH)-rich phases. The 3.4 µm feature points more toward long chain aliphatic compounds (CH2/CH3). The absorption centered at 3.1 µm is interpreted to be nitrogen-rich compounds that may be hydroxylated. Candidates include ammoniated phyllosilicates, ammonium hydrated salts or nitrogen-rich organics.

The spectrum in the 1.9–2.55 µm range has weak structures at ~2.32 µm, ~2.13µm and ~2.5µm that can be attributed to magnesium hydroxide (Mg–OH) bearing minerals. In addition, a deep reddish slope at wavelengths < 1.6 µm uniquely points to an absorption by Fe2+ as a key component, potentially coupled to other ions of calcium or magnesium.

These detections are evidence of aqueous alteration of Ryugu’s parent body and consistent with the lack of high-temperature calcium aluminum inclusions and chondrules. Evidence thus far indicates that asteroid Ryugu is most similar to CI chondrites than to any other type of meteorite found on Earth. (See Table 1). Current evidence suggests that asteroid Ryugu is dominated by hydrous carbonaceous chondrite-like materials, similar to CI chondrites with a darker, more porous and fragile nature.

These first three research papers performed a non-destructive compositional and physical analysis of the returned samples from asteroid Ryugu. These samples appear to be among the most primordial material available for study in the laboratory. The color, shape, surface morphology, and structure of the returned pebbles and powder in Chambers A and C match those of Ryugu’s surface material observed from the Hyabusa2 spacecraft. (See references 1,2,3)

Figure 1: Asteroid 162173 photographed from a distance of approximately 12 miles (19 kilometers). Ryugu is a rather small asteroid measuring 0.87 km (0.54 miles) x 0.92 km (0.57miles) x 1.13 km (0.70 miles). The mass of Ryugu is estimated to be about 450 million tons with a volume of 0.377 ± 0.005 km3 and density of 1.19 ± 0.03 g/cm3 . Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST
Figure 2: Optical microscopic images of bulk samples from Chambers A and C. Subunit samples (a) (b) (c) are those from the Chamber A while (d) (e) (f) are those from the Chamber C. The specimen weights for Chamber Aa are 0.79g, Ab 1.15g and Ac 1.16g. Weights for Chamber Cd are 0.56g, Ce 0.44g and Cf 0.51g. Container inner diameters are 21mm (0.827 inches). Both chamber samples are an aggregation of black millimeter-sized pebbles and sub-millimeter fine powder similar in size and composition. In total, more than five thousand particles of >100 µm2 are recognized in these images. (Photo from Reference 1 article and JAXA website)
Graph 1: Bulk A and C reflectance spectra from MicrOmega, a hyperspectral microscope operating in the near-infrared compared to normalized spectra at the surface of Ryugu. Note the 2.7 µm absorption line indicates the presence of hydroxyl (OH). Water (H2O) dissociates into H+ and OH . (From Reference 2)
Graph 2: The 3.4 µm feature is characteristic of both Carbon-Hydrogen (CH) rich spectrum at a localized spot- graph (a) while graph (b) is typical of a carbonate-rich spot. (From Reference 2)
Table 1: Ryugu samples compared with various meteorites. The Ryugu samples did not contain any chondrules or calcium aluminum inclusions. The bulk density was also lighter than collected meteorites. The closest match to the Ryugu samples is the Tagish Lake carbonaceous chondrite. (From Reference 1)
Photo 1: Tagish Lake meteorite from the authors personal collection. Tagish Lake is a classified as a CI2 (ungrouped) carbonaceous chondrite which fell in British Columbia, Canada (59°42’15.7″N., 134°12’4.9″ W) on January 18, 2000. Two hundred individuals were collected totaling 5-10 kg (11- 22 lbs). Specimen is a fragment weighing 0.284 grams and measuring 10mm x 7mm x 4mm. Photo by the author. Tagish lake is the meteorite that is the closest match to the asteroid Ryugu samples.

Addendum: Amino acids have just been confirmed in the Asteroid Ryugu samples. More on this important finding in the May issue of Meteorite-Times magazine.

References:
1) Preliminary analysis of the Hayabusa2 samples returned from C-type asteroid Ryugu
Toru Yada , Masanao Abe, Tatsuaki Okada et. al. Nature Astronomy Letters 20 Dec 2021 open source https://doi.org/10.1038/s41550-021-01550-6

2) First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope Pilorge C.  , Okada T., Hamm V. et. al. https://doi.org/10.1038/s41550-021-01549-z

3) Pebbles and sand on asteroid 162173 Ryugu: In situ observation and particles returned to Earth Tachibana S., Sawada H., Okazaki. et. al. https://www.science.org/doi/epdf/10.1126/science.abj8624. Science Feb 10, 2022.

4) Hyabusa2 website (NASA): In Depth | Hayabusa 2 – NASA Solar System Exploration

5) Hyabusa2 website (JAXA) https://www.hayabusa2.jaxa.jp/en/

6) Hyabusa2 Data Sheet: https://global.jaxa.jp/projects/sas/hayabusa2/pdf/sat33_fs_23_en.pdf

7) 162173 Ryugu https://en.wikipedia.org/wiki/162173_Ryugu#:~:text=M%C3%BCller%20et%20al.,albedo%20of%200.044%20to%200.050.

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