The thermal infrared imager TIR onboard Hayabusa2 has been developed to investigate thermo-physical properties of C-type, near-Earth asteroid 162173 Ryugu. TIR is one of the remote science ...instruments on Hayabusa2 designed to understand the nature of a volatile-rich solar system small body, but it also has significant mission objectives to provide information on surface physical properties and conditions for sampling site selection as well as the assessment of safe landing operations. TIR is based on a two-dimensional uncooled micro-bolometer array inherited from the Longwave Infrared Camera LIR on Akatsuki (Fukuhara et al.,
2011
). TIR takes images of thermal infrared emission in 8 to 12 μm with a field of view of
16
×
12
∘
and a spatial resolution of
0.05
∘
per pixel. TIR covers the temperature range from 150 to 460 K, including the well calibrated range from 230 to 420 K. Temperature accuracy is within 2 K or better for summed images, and the relative accuracy or noise equivalent temperature difference (NETD) at each of pixels is 0.4 K or lower for the well-calibrated temperature range. TIR takes a couple of images with shutter open and closed, the corresponding dark frame, and provides a true thermal image by dark frame subtraction. Data processing involves summation of multiple images, image processing including the StarPixel compression (Hihara et al.,
2014
), and transfer to the data recorder in the spacecraft digital electronics (DE). We report the scientific and mission objectives of TIR, the requirements and constraints for the instrument specifications, the designed instrumentation and the pre-flight and in-flight performances of TIR, as well as its observation plan during the Hayabusa2 mission.
Snow ice (SI) forms from freezing wet snow, known as slush, and contributes to the thickness of level and brash ice. However, the mechanism of snow-slush-snow ice transformation has not been ...extensively investigated to date, despite the difference in the freezing rate of slush in comparison with water is important for estimating the ice thickness. In this study, we examined the growth of initial congelation ice (CI) and snow ice (SI) in a fresh water tank exposed to outdoor weather conditions in Luleå, northern Sweden. The tank of size 1.8 × 0.65 × 1.2 m in length, width and height was divided into two compartments to facilitate the simultaneous growth of CI and SI. A total of 12 experiments were conducted in the years 2021 and 2022. The transformation from slush to snow ice was achieved by submerging various amounts of snow in the compartments. It was observed that approximately 35% of the initial snow transformed into SI. Snow ice grew 4 mm°C−0.5 d−0.5 faster than congelation ice. The CI growth under SI was 1 mm°C−0.5 d−0.5 slower than the CI growth under CI. This study provides valuable insights for modelling snow-slush-snow ice transformation and designing future laboratory-scale experiments.
The surfaces of all solid bodies in the solar system, planets, moons, comets and asteroids, experience short-term temporal variations of solar irradiation which depend on their respective spin rates. ...These so-called insolation cycles affect temperature variations, climate, photosynthesis in plants, etc. Hence, experimental reproduction of these cycles is important for space analogue simulations. In this short note we describe a simple, low-cost method to simulate diurnal cycles in the laboratory using a type of commercial solar simulator commonly used for experimental investigation in planetary science.
The material strength of meteorites provides useful information on the make-up and history of asteroids. However, the unique determination of the material strength of a meteorite is difficult because ...of the wide range of strengths many meteorites exhibit. Even within a single sample, complicated textures and mineral granular compositions make measurements difficult. Michikami et al. (2019) investigated the impact-induced crack growth in ordinary (L5) chondrites and indicated that crack growth is largely affected by the strength of individual mineral grains (and/or chondrules). In this study, we examine the strengths of mineral grains in carbonaceous meteorites qualitatively. To this end, we use X-ray microtomography to investigate how chondrules are affected by impact-induced crack growth in carbonaceous meteorites. Spherical alumina projectiles with a diameter of 1.0 mm were fired into the surfaces of seven Allende (CV) meteorite target samples with sizes of ∼1 to 2 cm at a nominal impact velocity of 2.0 km/s. In addition, spherical glass projectiles with a diameter 0.8 mm were fired into the target surfaces of two Murchison (CM) and two Aguas Zarcas (CM) meteorite target samples with sizes of ∼2 cm at a nominal impact velocity of 4.0 km/s. The results show that most cracks in CV chondrites tend to grow along the boundary surfaces of the chondrules, while most chondrule-related cracks in CM samples grow regardless of the boundary surfaces of the chondrules. This suggests that crack growth is largely affected by the chondrules' strength as indicated by Michikami et al. (2019). The weaker the strength of chondrules, the more likely crack growth tends to occur regardless of chondrule boundaries. We found that the mesostasis of chondrules in CM meteorite Murchison (and likely Aguas Zarcas) has experienced aqueous alteration and the chondrules have become structurally weak as a whole. This indicates that impact-induced crack propagation in CM chondrites differs from thermal-fatigue induced crack propagation inferred from previous studies. As the sample material to be returned from asteroid Bennu is considered to be related to CM chondrites, we propose that observation of the cracks in chondrules in Bennu samples might tell us whether those cracks are impact- or thermal-fatigue-induced.
•We have investigated impact-induced crack growth in CV and CM chondrites using 3D microtomography.•In CV chondrites, most cracks grow along chondrule boundaries.•In CM chondrites, most cracks grow regardless of chondrule boundaries.•We conclude that chondrules in CM chondrites that have been structurally weakened by aqueous alteration.
In order to understand the geological evolution of asteroids Eros, Itokawa and Ryugu and their collisional history, previous studies investigated boulder size distributions on their surfaces. ...However, quantitative comparison of these size distributions is hampered by numerous differences between these studies regarding the definition of a boulder's size, measuring technique and the fitting method to determine the power-index of the boulder size distributions. We provide a consistent and coherent model of boulder size distributions by remeasuring the boulders on the entire surfaces of Eros and Itokawa using the Small Body Mapping Tool (SBMT) and combining our observations with the Ryugu data of Michikami et al. (2019). We derived power-indices of the boulder size distributions of −3.25 ± 0.14 for Eros, −3.05 ± 0.14 for Itokawa and −2.65 ± 0.05 for Ryugu. The asteroid with the highest number density of boulders ≥ 5 m turns out to be Ryugu, not Itokawa, as suggested by an earlier study. We show that the appearance of the boulders tends towards more elongated shapes as the size of an asteroid decreases, which can be explained by differences in asteroid gravity and boulder friction angles. Our quantitative observational results indicate that boulder migration preferentially affects smaller boulders, and tends to occur on larger asteroids.
•First comparative survey free from observer bias of boulders on Eros, Itokawa and Ryugu.•Boulders appear to be more elongated as the size of an asteroid decreases.•Boulder migration preferentially affects smaller boulders and larger asteroids.
Over a broad size range, the shapes of impact fragments from catastrophic disruptions are distributed around the mean axial ratio 2: √2: 1, irrespective of experimental conditions and target ...materials. Although most blocks on asteroids are likely to be impact fragments, there is not enough quantitative data for reliable statistics on their three-axial lengths and/or ratios because it is difficult to precisely estimate the heights of the blocks. In this study, we evaluate the heights of blocks on asteroid Ryugu by measuring their shadows. The three-axial ratios of ~4100 small blocks with diameters from 5.0 cm to 7.6 m in Ryugu's equatorial region are investigated using eight close-up images of narrower localities taken at altitudes below 500 m, i.e. at <5.4 cm/pixel resolution, obtained immediately before the second touch-down of the Hayabusa2 spacecraft. The purpose of this study is to investigate the block shape distribution, which is important for understanding the geological history of asteroid Ryugu. Specifically, the shape distribution is compared to laboratory impact fragments. Our observations indicate that the shape distributions of blocks smaller than 1 m on Ryugu are consistent with laboratory impact fragment shape distributions, implying that the dominant shape-determining process for blocks on Ryugu was impact fragmentation. Blocks several meters in size in the equatorial region seem to be slightly flatter than the rest, suggesting that some blocks are partly buried in a bed of regolith. In conclusion, the shape distributions of blocks from several-cm to several-m in the equatorial region of asteroid Ryugu suggest that these are mainly fragments originating from the catastrophic disruption of their parent body and/or from a later impact.
•Blocks <1 m on Ryugu have similar three-axial shape distributions to laboratory impact fragments.•Blocks several meters in size appear to be slightly flatter.•Some blocks several meters in size are partly buried in a bed of regolith.•The dominant shape-determining process for blocks may be impact fragmentation.
Until recently, the influence of basal liquid water on the evolution of buried glaciers in Mars' mid latitudes was assumed to be negligible because the latter stages of Mars' Amazonian period (3 Ga ...to present) have long been thought to have been similarly cold and dry to today. Recent identifications of several landforms interpreted as eskers associated with these young (100s Ma) glaciers calls this assumption into doubt. They indicate basal melting (at least locally and transiently) of their parent glaciers. Although rare, they demonstrate a more complex mid-to-late Amazonian environment than was previously understood. Here, we discuss several open questions posed by the existence of glacier-linked eskers on Mars, including on their global-scale abundance and distribution, the drivers and dynamics of melting and drainage, and the fate of meltwater upon reaching the ice margin. Such questions provide rich opportunities for collaboration between the Mars and Earth cryosphere research communities.
Small asteroids such as Itokawa are covered with an unconsolidated regolith layer of centimeter-sized or smaller particles. There are two plausible formation mechanisms for regolith layers on a ...sub-kilometer-sized asteroid: (i) fragments produced by thermal fatigue by day-night temperature cycles on the asteroid surface and (ii) impact fragment. Previous studies suggest that thermal fatigue induces crack growth along the boundary surface of the mineral grain while impact phenomena may induce crack growth regardless of the boundary surface of the mineral grain. Therefore, it is possible that the crack growth within a mineral grain (and/or a chondrule) differs depending on the crack formation mechanism, be it thermal fatigue or an impact.
In order to investigate how mineral grains and chondrules are affected by impact-induced crack growth, we fired spherical alumina projectiles (diameter ~1 mm) into 9 mm side length cubic targets of L chondrites at a nominal impact velocity of 2.0 km/s. Before and after the six successful impact experiments, the cracks within mineral grains and chondrules in the respective targets are examined using X-ray microtomography at a resolution with the voxel size of 9.0 μm. The results show that most cracks within chondrules and troilite (FeS) grow regardless of the boundary surfaces of the grains while most cracks within ductile Fe-Ni metal grow along the boundary surfaces of the grains. This may indicate that crack growth is largely affected by the strength of mineral grains (and/or chondrules). From the experimental results and the fact that the shapes of polymineralic and monomineralic particles from Itokawa are similar, we conclude that the Itokawa particles have not been produced by thermal fatigue but instead are likely to be impact fragments, as described in previous papers (Tsuchiyama et al., 2011, 2014; Michikami et al., 2018).
•Most cracks within chondrules and troilite grow regardless of the boundary surfaces of the grains during impacts.•Most cracks within ductile Fe-Ni metal grow along the boundary surfaces of the grains during impacts.•The Itokawa particles are shown to be not the products of thermal fatigue but impact fragments on the asteroid surface.
Dust devils, small‐scale convective vortices found on Earth and on Mars, can transfer substantial quantities of dust from the ground into the atmosphere. It has been proposed that the low‐pressure ...region found at the center of dust devil vortices provides a lift (the ‘ΔP’ effect) that ‘sucks up’ material from the surface. Two simple models are compared to investigate the physics behind the ΔP effect and the relevance of competing processes. The first considers an impermeable bed of particles where lifting is by vertical pressure gradients, the second considers a permeable bed where lifting is by drag forces on the particles as gas is sucked from the bed. Pressure gradient lifting appears to be far more efficient than drag force lifting. We describe conditions that favor lifting by the ΔP effect and make qualitative predictions that might be tested in the laboratory, the field, or through observations from Mars Landers.
•In order to explore the physics of cometary nuclei, we have investigated the processing porous water ice mixed with some carbon dust when exposed to solar radiation.•We observe subsurface hardening, ...dust ejection and the build-up of a dust mantle.•The carbon agglomerates forming the dust mantle have some cohesive strength.
Our interpretation of the data returned by Rosetta and other cometary missions is based on the predictions of theoretical models and the results of laboratory experiments. For example, Kossacki et al. (2015) showed that 67P's surface hardness reported by Spohn et al. (2015) can be explained by sintering. The present work supports Rosetta's observations by investigating the hardening process of the near-surface layers and the change in surface morphology during insolation. In order to create as simple an analogue as possible our sample consists of pure, porous H2O ice and carbon black particles. The observations suggest that translucence of the near-surface ice is important for enabling subsurface hardening. As an end product of our experiments we also obtained carbon agglomerates with some residual strength.
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