Ozone in the lowermost troposphere is one of the most significant air pollutants and the amount has been increasing over the past few decades in the northern hemisphere. Ozone in the Earth’s atmosphere has been measured in several wavelength regions including the ultra-violet (UV), visible (VIS), thermal infrared (TIR) and microwave (MW) regions. The tropospheric ozone estimate was improved by combining radiances measured in several wavelength regions or measured with several geometries. This paper reviews estimates of the tropospheric ozone profile using a combination technique involving several space-based measurements. The combination technique was applied to estimate the tropospheric ozone column for the first time in the late 1980 s. The stratospheric ozone column derived from the SAGE-I limb measurement was subtracted from the total ozone column derived from the TOMS nadir measurement. The tropospheric ozone residual method was also applied to TOMS and SBUV, TOMS and UARS/MLS, and Aura/OMI and Aura/MLS measurements. Feasibility studies to estimate the tropospheric ozone profile using the UV, VIS, and TIR wavelength regions were performed based on the maximum a posteriori retrieval method in recent decades. The combination of UV and TIR nadir measurements showed the highest sensitivity to the lowermost tropospheric ozone. This combination was applied to the satellite measurements of Aura/OMI and Aura/TES, and of MetOp/GOME-2 and MetOp/IASI. In the case of GOME-2 and IASI measurements, the degree of freedom for signal (DFS) value in the lowermost troposphere was increased approximately 40% by combining the two measurements. We also report a feasibility study of estimating the tropospheric ozone by combining the MW limb and TIR nadir measurements assuming Aura/MLS and Aura/TES. The DFS value of the lowermost tropospheric ozone increased approximately 6% as a result of adding the MW measurement to the TIR measurement, although the MW measurement alone has no sensitivity in the lowermost troposphere.
Terahertz waves have been utilized for new and/or better understandings of the earth and planetary sciences through observations of atmospheric molecules and the physical properties of surfaces, but applications in the field of planetary science using observations from terahertz instruments on board spacecraft going into deep space are still at the initiation stage. In this review paper, we describe the scientific achievements of Rosetta/MIRO, which was the first terahertz instrument on board a deep-space spacecraft for the observation of the nucleus of comet 67P/Churyumov-Gerasimenko, the expected scientific achievements of JUICE/SWI, which will be launched to observe Jupiter and its icy satellites, and expectations for a future terahertz instrument on a Mars orbiter. Keeping in mind these achievements and expectations, we discuss the expansion of planetary science allowed by the observations made by terahertz instruments on board deep-space spacecraft.
The moon and asteroids are known to be covered with dust grains. Since dust grains are thought to travel over the surface of these bodies, they are potentially hazardous for spacecraft, and especially for sample-return missions. However, dust grains detached from the surface are not easy to detect. We have provided the light detection and ranging (LIDAR) system on board Hayabusa2 with a new operational mode, the “dust count mode,” in which LIDAR evaluates whether or not each return pulse from 50 continuous observational ranges exceeds a threshold value. Once Hayabusa2 arrives at its target body of Ryugu, we will make observations with various threshold values in order to detect dust distribution.