A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Near-infrared spectral variation in Ryugu particles and implication for rapid space weathering by solar UV radiation

The 2.7 µm absorption band, represented by OH groups, in the near-infrared reflectance spectra of primitive Solar System bodies could be an indicator of the cause and degree of space weathering. We compared the absorption bands of particles sampled from the surface (chamber A) and the excavated subsurface (chamber C) on the C-type asteroid Ryugu by the JAXA Hayabusa2 mission. We developed a fitting method using multiple Gaussians to precisely calculate the band's peak position and depth, providing more reliable information on space weathering effects. We found that the chamber A particles were divided into groups Aα with shorter peak position and deeper band depth (non or poorly space weathered) and Aβ with longer peak position and shallower band depth (rich in space weathering). Chamber C particles are partly show intermediate characteristics between Aα and Aβ, which might imply weak space weathering by solar UV radiation during three months between the excavation by impact experiment and sampling by Hayabusa2. Supplementally, our preliminary experiment of UV irradiation to Ivuna meteorite showed decrease of 2.7 µm band depth and longward shift of the peak position, and further study is needed to investigate the effect of short-term UV space weathering on carbonaceous asteroids.

Enhanced 2.7 µm wavelength sub-80 ns pulse generation via iron-chromium co-doped ZnS polycrystalline

Zinc-sulfide (ZnS) polycrystalline co-doped with Cr2+ and Fe2+ ions, when used as a saturable absorber (SA), exhibits a longer recovery time and a larger excited state absorption cross-section compared to Fe:ZnS polycrystalline, due to the sensitizing effect of co-doping. The energy level structure and energy transfer process of Fe,Cr:ZnS polycrystalline were analyzed. A 2.7 µm wavelength passively Q-switched (PQS) pulsed laser was simulated and experimentally tested. The experiment achieved a narrower pulse width of 78 ns with a repetition rate of 44 kHz, compared to the Fe:ZnS SA Q-switched laser. The experimental results closely matched the simulation in terms of pulse profile, repetition rate, and pulse width. To the best of our knowledge, this is the first time that Fe,Cr:ZnS polycrystalline is used as a SA to realize PQS lasing. The superior characteristics of co-doped crystals as SAs have also been highlighted.

Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu

We report primordial aqueous alteration signatures in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 spacecraft of JAXA. Newly identified low-molecular-weight hydroxy acids (HO-R-COOH) and dicarboxylic acids (HOOC-R-COOH), such as glycolic acid, lactic acid, glyceric acid, oxalic acid, and succinic acid, are predominant in samples from the two touchdown locations at Ryugu. The quantitative and qualitative profiles for the hydrophilic molecules between the two sampling locations shows similar trends within the order of ppb (parts per billion) to ppm (parts per million). A wide variety of structural isomers, including α- and β-hydroxy acids, are observed among the hydrophilic molecules. We also identify pyruvic acid and dihydroxy and tricarboxylic acids, which are biochemically important intermediates relevant to molecular evolution, such as the primordial TCA (tricarboxylic acid) cycle. Here, we find evidence that the asteroid Ryugu samples underwent substantial aqueous alteration, as revealed by the presence of malonic acid during keto-enol tautomerism in the dicarboxylic acid profile. The comprehensive data suggest the presence of a series for water-soluble organic molecules in the regolith of Ryugu and evidence of signatures in coevolutionary aqueous alteration between water and organics in this carbonaceous asteroid.

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s-1) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

Microstructural and chemical features of impact melts on Ryugu particle surfaces: Records of interplanetary dust hit on asteroid Ryugu

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

Hydrogen-bearing vesicles in space weathered lunar calcium-phosphates

Water on the surface of the Moon is a potentially vital resource for future lunar bases and longer-range space exploration. Effective use of the resource depends on developing an understanding of where and how within the regolith the water is formed and retained. Solar wind hydrogen, which can form molecular hydrogen, water and/or hydroxyl on the lunar surface, reacts and is retained differently depending on regolith mineral content, thermal history, and other variables. Here we present transmission electron microscopy analyses of Apollo lunar soil 79221 that reveal solar-wind hydrogen concentrated in vesicles as molecular hydrogen in the calcium-phosphates apatite and merrillite. The location of the vesicles in the space weathered grain rims offers a clear link between the vesicle contents and solar wind irradiation, as well as individual grain thermal histories. Hydrogen stored in grain rims is a source for volatiles released in the exosphere during impacts.