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The Hayabusa2 mission journeys to C-type near-Earth asteroid (162173) Ryugu (1999 JU 3 ) to observe and explore the 900 m-sized object, as well as return samples collected from the surface layer. The Haybusa2 spacecraft developed by Japan Aerospace Exploration Agency (JAXA) was successfully launched on December 3, 2014 by an H-IIA launch vehicle and performed an Earth swing-by on December 3, 2015 to set it on a course toward its target Ryugu. Hayabusa2 aims at increasing our knowledge of the early history and transfer processes of the solar system through deciphering memories recorded on Ryugu, especially about the origin of water and organic materials transferred to the Earth’s region. Hayabusa2 carries four remote-sensing instruments, a telescopic optical camera with seven colors (ONC-T), a laser altimeter (LIDAR), a near-infrared spectrometer covering the 3-μm absorption band (NIRS3), and a thermal infrared imager (TIR). It also has three small rovers of MINERVA-II and a small lander MASCOT (Mobile Asteroid Surface Scout) developed by German Aerospace Center (DLR) in cooperation with French space agency CNES. MASCOT has a wide angle imager (MasCam), a 6-band thermal radiator (MARA), a 3-axis magnetometer (MasMag), and a hyperspectral infrared microscope (MicrOmega). Further, Hayabusa2 has a sampling device (SMP), and impact experiment devices which consist of a small carry-on impactor (SCI) and a deployable camera (DCAM3). The interdisciplinary research using the data from these onboard and lander’s instruments and the analyses of returned samples are the key to success of the mission.

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history. Both poles of asteroid Ryugu, the target of space mission Hayabusa2, preserve the least processed material by space weathering. Here, the authors show detection of 700 nm absorption band in the polar spectra of Ryugu, that allows to constrain the hydrothermal history of its spectrally blue parent body.

The pristine sample from the near-Earth carbonaceous asteroid (162173) Ryugu collected by the Hayabusa2 spacecraft enabled us to analyze the pristine extraterrestrial material without uncontrolled exposure to the Earth’s atmosphere and biosphere. The initial analysis team for the soluble organic matter reported the detection of wide variety of organic molecules including racemic amino acids in the Ryugu samples. Here we report the detection of uracil, one of the four nucleobases in ribonucleic acid, in aqueous extracts from Ryugu samples. In addition, nicotinic acid (niacin, a B 3 vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules. The observed difference in the concentration of uracil between A0106 and C0107 may be related to the possible differences in the degree of alteration induced by energetic particles such as ultraviolet photons and cosmic rays. The present study strongly suggests that such molecules of prebiotic interest commonly formed in carbonaceous asteroids including Ryugu and were delivered to the early Earth. Uracil was identified in the sample returned from the asteroid Ryugu. Having been provided to the early Earth as a component in such asteroidal materials, these molecules might have played a role for prebiotic chemical evolution on the early Earth

Understanding movements for spawning of tunas is essential for gaining insight into population dynamics and can provide information on the reproductive characteristics of tuna stocks. Skipjack tuna spawn in tropical and subtropical areas and southward movement of presumed adult fish from the northern extents of their range may be related to spawning. We investigated southward movement patterns of skipjack tuna from the northern habitat limit of the northwestern Pacific Ocean using plastic dart tags and archival tag data. Observations uncovered a new alternative movement group that resides around the northern habitat at least for 9 months (residence group), confirming spawning potential movement toward tropical and subtropical areas (spawning potential group). The spawning potential group, that spent the majority of time in water temperature environments above 24°C in spawning grounds, was considered to be undertaking movement for spawning. Extraordinarily high body temperatures reaching 31°C were found only in the spawning potential group, indicative of spawning activity. The frequency and timing of this high body temperature were also consistent with the current reproductive traits of the skipjack tuna, strengthening this group as potential spawners. Conversely, gonadal immaturity of skipjack tuna in the residence group lasting above 8 months after release in environments unsuitable for spawning (<24°C) suggests motivation for movement was not driven by spawning, but by feeding and avoiding unfavorable environments. Remaining in the northern habitat with short-distance daily movement in a rich prey environment indicates engaging in feeding. However, seasonal cooling in this area pushed the temperature of the skipjack tuna thermal limit southward, leading to this group moving slowly southward to avoid exposure to the lower limit. The southward movement of this group was motivated by thermal limit avoidance. Physiological changes demonstrate the importance of describing the two movement patterns and underlying motivations for patterns from a physiological thermal tolerance and reproductive ecology perspective.

All life on Earth contains amino acids and carbonaceous chondrite meteorites have been suggested as their source at the origin of life on Earth. While many meteoritic amino acids are considered indigenous, deciphering the extent of terrestrial contamination remains an issue. The Ryugu asteroid fragments (JAXA Hayabusa2 mission), represent the most uncontaminated primitive extraterrestrial material available. Here, the concentrations of amino acids from two particles from different touchdown sites (TD1 and TD2) are reported. The concentrations show that N,N-dimethylglycine (DMG) is the most abundant amino acid in the TD1 particle, but below detection limit in the other. The TD1 particle mineral components indicate it experienced more aqueous alteration. Furthermore, the relationships between the amino acids and the geochemistry suggest that DMG formed on the Ryugu progenitor body during aqueous alteration. The findings highlight the importance of aqueous chemistry for defining the ultimate concentrations of amino acids in primitive extraterrestrial samples. Amino acid concentrations from 2 particles returned from different touchdown sites on the surface of Ryugu are reported. Differences in chemistry suggest different levels of aqueous alteration are recorded at the 2 sampled locations.

Samples from the carbonaceous asteroid (162173) Ryugu provide information on the chemical evolution of organic molecules in the early solar system. Here we show the element partitioning of the major component ions by sequential extractions of salts, carbonates, and phyllosilicate-bearing fractions to reveal primordial brine composition of the primitive asteroid. Sodium is the dominant electrolyte of the salt fraction extract. Anions and NH 4 + are more abundant in the salt fraction than in the carbonate and phyllosilicate fractions, with molar concentrations in the order SO 4 2−  > Cl −  > S 2 O 3 2−  > NO 3 −  > NH 4 + . The salt fraction extracts contain anionic soluble sulfur-bearing species such as S n -polythionic acids ( n  < 6), C n -alkylsulfonates, alkylthiosulfonates, hydroxyalkylsulfonates, and hydroxyalkylthiosulfonates ( n  < 7). The sulfur-bearing soluble compounds may have driven the molecular evolution of prebiotic organic material transforming simple organic molecules into hydrophilic, amphiphilic, and refractory S allotropes. The asteroid Ryugu samples are by far the freshest extraterrestrial carbonaceous material. The authors report soluble ions and organic sulfur molecules linked with primordial brine and prebiotic organic evolution of the primitive asteroid.

The dynamical environment around the asteroid (162173) Ryugu is analyzed in detail using a constant-density polyhedron model based on the measurements from the Hayabusa2 mission. Six exterior equilibrium points (EPs) are identified along the ridge line of Ryugu, and their topological classifications fall into two distinctive categories. The initial periodic orbit (PO) families are computed and analyzed, including distant retrograde/prograde orbit (DRO/DPO) families and fifteen PO families emanating from the exterior EPs. The fifteen PO families are further divided into three categories: seven converge to an EP, seven reach Ryugu’s surface, and one exhibits cyclic behavior during its progression. The existence of initial PO families converging to an EP is analyzed using the bifurcation of a degenerate EP. Connection between these families and similar ones in the circular restricted three-body problem (CRTBP) is also examined. Bifurcated PO families are identified and computed from the initial PO families, including ten families from the DROs, fifteen from the DPOs, and twenty-five associated with the EPs. The bifurcated families are separately analyzed and categorized in terms of their corresponding initial families. Connections established by the same bifurcation points between different bifurcated families are identified. A comparison is made for the dynamical environments of Ryugu and Bennu to evaluate the similarities and differences in the evolution of EPs and the progression of PO families in top-shaped asteroids.

A new barbeled dragonfish, Eustomias shunyo sp. nov., is described based on three specimens [219.1–256.4 mm in standard length (SL)] collected at depths of 30–180 m from the Shatsky Rise in the western North Pacific. The new species is included in the subgenus Urostomias Regan and Trewavas 1930 and distinguishable from all congeners by the following combination of characters: 30 dorsal-fin rays; 48–49 anal-fin rays; 9–10 pectoral-fin rays; 7 pelvic-fin rays; 29–31 PV (photophores in ventral series between pectoral- and pelvic-fin insertions); 26–27 AC (photophores from dorsal to anal fin to end of row); 69–72 vertebrae; preanal length 63.7–65.3% SL; head length 9.7–10.1% SL; barbel length 12.2–13.7% SL; single luminous gland on upper margin of caudal fin; stem of barbel lacking external pigmentation except for its proximal portion, and melanophores sparsely scattered around small photophores on ventral side of stem; pale yellow patches arranged in 1 or 2 rows on dorsal and ventral sides of barbel stem; single or pared filaments on dorsal side of distal portion of barbel stem just before terminal bulb; single leaf-shaped dark patch on ventral side of barbel bulb. The revised diagnosis and a key to species of the subgenus Urostomias are presented.

One of the primary goals of Hayabusa2 is to land on the asteroid Ryugu to collect its surface materials. The key for a successful touchdown is to find a promising landing site that meets both scientific and engineering requirements. Due to the limited availability of pre-arrival information about Ryugu, the landing site selection (LSS) must be conducted based on proximity observations over a limited length of time. In addition, Ryugu was discovered to possess an unexpectedly high abundance of boulders with an absence of wide and flat areas, further complicating the LSS. To resolve these problems, we developed a systematic and stepwise LSS process with a focus on the surface topography of Ryugu and the associated touchdown safety. The proposed LSS scheme consists of two phases: Phase-I LSS, a comprehensive survey of potential landing areas at the 100-m scale based on the global mapping of Ryugu, and Phase-II LSS, a narrowing-down process of the candidate landing sites at the 10-m scale using high-resolution images and a local terrain model. To verify the feasibility of a precision landing at the target site, we also investigated the landing dispersion via a Monte Carlo simulation, which incorporates the effect of the irregular surface gravity field. One of the major characteristics of the Hayabusa2 LSS developed in this study is the iterative feedback between LSS analyses on the ground and actual spacecraft operations near the target asteroid. Using the newly developed method, we chose a landing site with a radius of 3 m, and Hayabusa2 successfully conducted its first touchdown on February 21, 2019. This paper reports the methodology and results of the stepwise iterative LSS for the first Hayabusa2 touchdown. The touchdown operation results reconstructed from flight data are also provided, demonstrating the validity of the adopted LSS strategy.

Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are 16 O-rich and -poor with Δ 17 O (=δ 17 O – 0.52 × δ 18 O) values of ~ –23‰ and ~ –3‰, resembling what has been proposed as early generations of chondrules. The 16 O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into 16 O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are 16 O-rich with Δ 17 O of ~ –23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body. Chondrule-like objects and CAIs in the Ryugu samples are early generations of chondrules and possibly oldest solids in the Solar System. They were transported from the inner solar nebula to the formation location of the Ryugu original parent body.