Primitive chondrites and cosmic dust derived from asteroids and comets represent the oldest solid material in the Solar System. Although they are precious sources of information about the processes in the Protoplanetary Disk and on planetesimals, they lack “geological” information, i.e., from what parts of what bodies they originate, which have been an obstacle for scientists to deduce persuasive interpretations. Sample return missions including the Stardust and Hayabusa explorers are intended to solve this problem. This article presents the results of detailed analyses of the samples obtained by the Stardust and Hayabusa Missions. Both missions have yielded many revolutionary findings on the early and present Solar System despite the small amounts of recovered samples. Greater successes are expected from the next sample return missions currently performed and planned.
Cosmic dust measurement has been one of the fundamental investigations of the Solar System exploration and sample returns from low Earth orbit have substituted knowledge gaps of the terrestrial collection of astromaterials. Exo-planetary system studies now demand deeper understanding of the Solar System dust structure and compositions as templates for all other planetary systems in the Universe. Sample return technologies and operations have been extended to asteroids and comets, cosmic dust parent bodies, and advanced toward more challenging destinations such as Jupiter Trojans and icy satellite plumes. These achievements inevitably created the new research field of microgravity geology. After a quarter century of scientific and engineering advancements, Japan now has original dust detectors, collectors, and sample return instruments supported by ground hypervelocity impact and microgravity experimental facilities.
In this paper I discuss scattering theories applicable to non-spherical particles such as ice crystals in clouds, terrestrial dust and interplanetary dust particles. Here I consider two scattering regimes; (1) particle size smaller than or comparable to and (2) particle size much larger than the wavelength of interest. The following two methods have been applied to examine the scattering properties of non-spherical particles; the discrete dipole approximation (DDA) and physical optics (PO) for the regimes (1) and (2), respectively. The DDA is applicable for the size parameter smaller than about 50, whereas the PO may be valid for the size parameter above 100. I first introduce the retrieval of size and ice water content of ice clouds by space-borne 94 GHz cloud radar and lidar on the basis of the DDA and PO calculations. Then lidar remote sensing of dust particles in the terrestrial atmosphere is also presented. Analysis of scattering properties of interplanetary dust particles is also described by taking into account its fractal shape. Finally future direction of scattering theory and observations are suggested.
Organic aerosol layers have been observed and suggested to form in CH4-containing reducing planetary atmospheres. In Titan’s atmosphere, composed mainly of N2 and CH4, organic aerosols are produced through photochemical reactions driven by solar ultraviolet light. Such organic aerosols may have existed in the CH4-CO2 atmosphere of early Earth and exoplanets. Since the aerosol layers in CH4-containing atmospheres control the atmospheric structure and surface temperature, their formation mechanisms are important for the evolution of planetary atmospheres. Although the Cassini spacecraft has revealed that aerosol particles are initially formed in Titan’s upper atmosphere by irradiation of extreme ultraviolet light, the growth mechanisms of aerosol particles during precipitation in the middle atmosphere remain poorly constrained. Here, we conduct laboratory experiments on photochemical and heterogeneous reactions to constrain the aerosol growth mechanisms in CH4-containing atmospheres. Our results of the experiments and photochemical modeling suggest that the addition reaction of CH3 radicals formed by CH4 photolysis is the major process for the growth of aerosol particles in the middle atmosphere of Titan. This reaction would affect the carbon budget and cycles on Titan, through increasing the amounts of organic aerosols and reducing the amounts of C2H6.
Ground-based measurement for black carbon (BC) using a laser-induced incandescence technique was performed at NOTO Ground-based Research Observatory (NOTOGRO) in Suzu City, Ishikawa, Japan (37.45°N, 137.36°E) from April 17 to May 14 in 2013. Mass concentration of BC ranged 1.5–823 ng/m3 in 1-hour average. In the observed period, we captured East Asian outflow, the domestic and clean air masses by the analysis of backward trajectories. Temporal variations of BC number and mass distributions indicated that the size of BC particles in the East Asian outflow was the largest, followed by the domestic and clean air masses. The analysis of mixing state of BC indicated that the ratio of internally mixed BC particles in which diameter is 200±10 nm were 65%, 54%, 23% in the East Asian outflow, domestic air mass, and clean air mass, respectively. This result qualitatively supported that the degree of enhancement in light absorption observed by a photoacoustic soot spectrometer.
Simultaneous observations of ionic constituents for size-segregated aerosol particles were conducted at Chikushi Campus of Kyusyu University by using a Continuous Dichotomous Aerosol Chemical Speciation Analyzer (ACSA-12) and a denuder-filter (DF) method from September to December, 2014. For SO42- and NO3-, the sum of fine (＜2.5 μm) and coarse (2.5–10 μm) mode concentrations of ACSA-12 correlated well with the DF data. Concentration of ΔH＋ also showed a good correlation with fine mode H＋ from the DF method but insignificant correlation for coarse mode data during the study period. Temporal and seasonal variations of fine mode WSOC concentrations from ACSA-12 were similar to that of fine mode C2O42- concentrations from the DF method. Tendency of covariation for fine mode NO3- concentrations was also found for fine mode WSOC or C2O42- concentrations. Further experiments for WSOC including more realistic calibration is strongly recommended for data validation of ACSA-12.