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Alteration processes on asteroid and comet surfaces, such as thermal fracturing, (micrometeorite) impacts or volatile outgassing, are complex mechanisms that form diverse surface morphologies and roughness on various scales. These mechanisms and their interaction may differ on the surfaces of different bodies. Asteroid Ryugu and comet 67P/Churyumov–Gerasimenko, both, have been visited by landers that imaged the surfaces in high spatial resolution. We investigate the surface morphology and roughness of Ryugu and 67P/Churyumov–Gerasimenko based on high-resolution in situ images of 0.2 and 0.8 mm pixel resolution over an approximately 25 and 80 cm wide scene, respectively. To maintain comparability and reproducibility, we introduce a method to extract surface roughness descriptors (fractal dimension, Hurst exponent, joint roughness coefficient, root-mean-square slope, hemispherical crater density, small-scale roughness parameter, and Hapke mean slope angle) from in situ planetary images illuminated by LEDs. We validate our method and choose adequate parameters for an analysis of the roughness of the surfaces. We also derive the roughness descriptors from 3D shape models of Ryugu and orbiter camera images and show that the higher spatially resolved images result in a higher roughness. We find that 67P/Churyumov–Gerasimenko is up to 6 per cent rougher than Ryugu depending on the descriptor used and attribute this difference to the different intrinsic properties of the materials imaged and the erosive processes altering them. On 67P/Churyumov–Gerasimenko sublimation appears to be the main cause for roughness, while on Ryugu micrometeoroid bombardment as well as thermal fatigue and solar weathering may play a significant role in shaping the surface.

Impact craters of different geological ages, sizes and morphologies are not only the most obvious surface features on Ceres’ surface. The investigation of their spectral properties in combination with Ceres’ geology and topography reveals not only lateral compositional variations in Ceres’ surface material but also possible stratigraphic differences within Ceres’ crust. Spectral properties of impact craters with different ages do show distinct trends implying variations with increasing exposure duration of the impact material onto Ceres’ surface. Local concentrations of H2O ice and carbonates are associated with the youngest, either recently emplaced or excavated, surface deposits. On the contrary, regionally higher amounts of ammoniated phyllosilicates originate from deeper regions of Ceres’ crust and strengthen the theory of ammonia being a primordial constituent of Ceres. The blue spectral slope, clearly associated with relatively weak absorptions of OH-bearing and/or ammoniated phyllosilicates, is limited to fresh impact material. Either, the blue spectral slope diminishes slowly with increasing geologic age due to space weathering processes, or shortly as a result of gravitation-induced slumping, forming a fine and loosely consolidated regolith.

著者人数: 37名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 岡田, 達明; 坂谷, 尚哉; 田中, 智; 矢野, 創), Number of authors: 37 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Okada, Tatsuaki; Sakatani, Naoya; Tanaka, Satoshi; Yano, Hajime）, Accepted: 2019-06-04, 資料番号: SA1190164000

Images from the Optical Navigation Camera system (ONC) onboard the Hayabusa2 spacecraft show the MASCOT lander during its descent to the surface of asteroid (162173) Ryugu. We used results from a previous stereo-photogrammetric analysis that provided precise ONC image orientation data (camera position and pointing), ONC orthoimages, and an ONC-based 3D surface model to combine them with the visibilities of MASCOT itself and its shadow on-ground within the ONC images. We integrated additional information from instruments onboard MASCOT (MASMag, MARA, MASCam) and derived MASCOT’s release position and modeled its free-fall descent path and its velocity over 350 s from its release at ∼41 m altitude above ground until its first contact with the surface of Ryugu. After first contact, MASCOT bounced over the surface of Ryugu for 663 s and came to rest at its first settlement point after four intermediate surface contacts. We again used ONC images that show MASCOT and partly its shadow and reconstructed the bouncing path and the respective velocities of MASCOT. The achieved accuracy for the entire descent and bouncing path is ∼0.1 m (1σ).

Mapping Ceres’ surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres’ crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres’ geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres’ surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres’ crust. Subsequent impacts in this basin reach far deeper into Ceres’ crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres’ deeper crust. Spectral differences with respect to the small fresh craters on Dantu's floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes.

Impact crater Dantu not only exhibits a very complex geological history but also shows an exceptional heterogeneity of its surface composition. Because of its location within a low-lying region named Vendimia Planitia, which has been proposed to represent an ancient impact basin, Dantu possibly offers a window into the composition of Ceres’s deeper crust, which apparently is enriched in ammonia. Local concentration of carbonates within Dantu or its ejecta blanket may be either exposed or their emplacement induced by the Dantu impact event. Because carbonates can be seen along Dantu’s crater walls, exposed due to recent slumping, but also as fresh spots or clusters of spots scattered across the surface, the deposition/formation of carbonates took place over a long time period. The association of several bright spots enriched in carbonates with sets of fractures on Dantu’s floor might be accidental. Nevertheless, its morphological and compositional similarity to the faculae in Ceres’s prominent impact crater Occator including its hydrated state does not exclude a cryo-volcanic origin, i.e., upwelling of carbonate-enriched brines influenced by H2O ice in the subsurface. Indeed, an isolated H2O ice spot can be identified near Dantu, which shows that ice still exists in Ceres’s subsurface at midlatitudes and that it can exist on the surface for a longer period of time.

The dwarf planet Ceres shows spatially well-defined regions, which exhibit a negative (blue) spectral slope between 0.5 and 2.5 µm. Comparisons with planetary bodies known to exhibit a blue slope and spectral properties of materials identified on Ceres’ surface based on infrared wavelength signatures indicate the spectral changes could be related to physical properties of the surface material rather than variations in its composition. The close association of bluish surface regions to fresh impact craters implies a possible relationship to an impact-triggered alteration and/or space weathering processes. The bluish regions could be linked with blankets of ultra-fine grains and partly amorphous phyllosilicates, which form larger agglomerates due to the sticky behavior of impact induced phyllosilicate dust and/or the amorphization of the ejecta material during the impact process. Space weathering processes (micro-meteoritic impacts, temperature changes) cause a reversal of the agglutination process and a re-crystallization of the surface material with time resulting in a reddening of the spectral slope.