Surface physical condition of asteroid Ryugu using close-up optical and thermal images

In 2018, the Hayabusa2 spacecraft [1] successfully conducted some descend operations toward Ryugu’s surface. They included MINERVA rover release in September, MASCOT lander release and two touchdown rehearsals in October. During these operations, we acquired high-resolved optical and thermal images from altitudes below 100 m, us- ing Optical Navigation Camera (ONC-T) and Ther- mal Infrared Imager (TIR), respectively. Close-up optical images by ONC-T show detailed physical conditions of the surface materials, such as particle size distribution of pebbles, surface morphol- ogy of small boulders and craters. Moreover, close-up thermal images by TIR indicate thermophysical prop- erties of the surface materials and its regional differ- ence, which cannot be resolved by higher altitude observations (e.g., home-position observations from 20 km altitude). Combination between optical and thermal observations is of great importance to under- stand the nature of the asteroid surface materials. In this study, we investigate the surface particle size from close-up ONC images. Thermophysical property of the surface component materials inferred from TIR images is also discussed, especially for the range observed for the thermal inertia of boulders

Transcriptome analysis of haploid male gametophyte development in Arabidopsis

BACKGROUND: The haploid male gametophyte generation of flowering plants consists of two- or three-celled pollen grains. This functional specialization is thought to be a key factor in the evolutionary success of flowering plants. Moreover, pollen ontogeny is also an attractive model in which to dissect cellular networks that control cell growth, asymmetric cell division and cellular differentiation. Our objective, and an essential step towards the detailed understanding of these processes, was to comprehensively define the male haploid transcriptome throughout development. RESULTS: We have developed staged spore isolation procedures for Arabidopsis and used Affymetrix ATH1 genome arrays to identify a total of 13,977 male gametophyte-expressed mRNAs, 9.7% of which were male-gametophyte-specific. The transition from bicellular to tricellular pollen was accompanied by a decline in the number of diverse mRNA species and an increase in the proportion of male gametophyte-specific transcripts. Expression profiles of regulatory proteins and distinct clusters of coexpressed genes were identified that could correspond to components of gametophytic regulatory networks. Moreover, integration of transcriptome and experimental data revealed the early synthesis of translation factors and their requirement to support pollen tube growth. CONCLUSIONS: The progression from proliferating microspores to terminally differentiated pollen is characterized by large-scale repression of early program genes and the activation of a unique late gene-expression program in maturing pollen. These data provide a quantum increase in knowledge concerning gametophytic transcription and lay the foundations for new genomic-led studies of the regulatory networks and cellular functions that operate to specify male gametophyte development

Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth

International audienceThe Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid

Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body

Planetesimals – the initial stage of the planetary formation process – are considered to be initially very porous aggregates of dusts, and subsequent thermal and compaction processes reduce their porosity. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30-50%, higher than meteorites but lower than cometary nuclei, which are considered to be remnants of the original planetesimals. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations