佐藤 毅彦

Abstract We performed a unique Venus observation campaign to measure the disk brightness of Venus over a broad range of wavelengths in 2020 August and September. The primary goal of the campaign was to investigate the absorption properties of the unknown absorber in the clouds. The secondary goal was to extract a disk mean SO₂ gas abundance, whose absorption spectral feature is entangled with that of the unknown absorber at ultraviolet wavelengths. A total of three spacecraft and six ground-based telescopes participated in this campaign, covering the 52–1700 nm wavelength range. After careful evaluation of the observational data, we focused on the data sets acquired by four facilities. We accomplished our primary goal by analyzing the reflectivity spectrum of the Venus disk over the 283–800 nm wavelengths. Considerable absorption is present in the 350–450 nm range, for which we retrieved the corresponding optical depth of the unknown absorber. The result shows the consistent wavelength dependence of the relative optical depth with that at low latitudes, during the Venus flyby by MESSENGER in 2007, which was expected because the overall disk reflectivity is dominated by low latitudes. Last, we summarize the experience that we obtained during this first campaign, which should enable us to accomplish our second goal in future campaigns.

The 2-μm near-infrared (NIR) camera, IR2, onboard Japan's Venus orbiter Akatsuki acquired Venus images after the successful orbit insertion in December 2015. IR2 utilizes a platinum silicide (PtSi) Schottky-barrier array sensor (1040x1040 pixels) in which photon is detected by the photo-electric effect. This is by nature not a very high efficiency mechanism therefore unused light is subjected to multiple reflection within the silicon substrate (400-μm thick in IR2). Because very intense day crescent (some ~3 orders of magnitudes brighter) of Venus exists in the same field of view when the night-side disk is imaged, light spread from the former significantly affects the photometry of the latter. To restore the night-side features to a level that can be measured photometrically, we have developed a simulation to model the point-spread function (PSF) of IR2 in which effect of multiple light reflection is accounted for. Different elements in the array sensor (the NIR-sensitive PtSi pixels, the vertical scanning lines, and the charge-sweep device area) are considered and the light reflection is traced until the beam becomes weaker than a threshold. While the multiple rings (the innermost one corresponds to the critical angle of total internal reflection) are successfully reproduced, the cross pattern did not show up from this simulation and we had to artificially add it. The concept of simulation may be useful for other sensors of which substrate is relatively transparent for the wavelengths of interest while the target objects contain large dynamic range.

Although Venus is a terrestrial planet similar to Earth, its atmospheric circulation is much different and poorly characterized1. Winds at the cloud top have been measured predominantly on the dayside. Prominent poleward drifts have been observed with dayside cloud tracking and interpreted to be caused by thermal tides and a Hadley circulation2–4; however, the lack of nightside measurements over broad latitudes has prevented the unambiguous characterization of these components. Here we obtain cloud-tracked winds at all local times using thermal infrared images taken by the Venus orbiter Akatsuki, which is sensitive to an altitude of about 65 kilometres5. Prominent equatorward flows are found on the nightside, resulting in null meridional velocities when these are zonally averaged. The velocity structure of the thermal tides was determined without the influence of the Hadley circulation. The semidiurnal tide was found to have an amplitude large enough to contribute to the maintenance of the atmospheric superrotation. The weakness of the mean meridional flow at the cloud top implies that the poleward branch of the Hadley circulation exists above the cloud top and that the equatorward branch exists in the clouds. Our results should shed light on atmospheric superrotation in other celestial bodies.

We have developed a novel method called Restoration by Simple Subtraction (RSS) to clean and restore the Venus night-side disk in “contaminated” Akatsuki/IR2 images at 2.26 and 1.735 μm. Light spread from the intense day crescent is cancelled by subtracting a near-simultaneous 2.32-μm image (scaled) from a 2.26-μm image. Net-contamination over the night-side disk is effectively separated and subsequently used to clean a 1.735-μm image. For the first time ever, the IR2 night-side data have become of photometric quality. Ten data sets from Orbits 24 (August 16–20, 2016) and 25 (August 26–30, 2016) were processed by the RSS method and analyzed to study the aerosol properties and variations of low-altitude carbon monoxide (CO). A new coordinate system called “M3L” is introduced to interpret the data in terms of mixtures of different mode aerosols. No simultaneous increases/decreases in mode 3 and other modes were found, but they are rather anti-correlated, indicating interchanges between them in the Venus atmosphere. A prominent feature called clouds aligned linearly in M3L coordinates (CALM) is interpreted as an indicator of the most quiescent regions with gentle convection, which maintains the cloud microphysics (interchange between mode 3 and other modes plus sulfuric acid vapor) in an organized manner. The mid-latitude bright streaks are found to deviate from CALM, suggesting the existence of forced downwelling which was previously reported based on high-resolution numerical simulations. Latitudinal variations of mode mixtures as interpreted by the M3L coordinates were compared with those in the Venus Express observation-based studies. A common trend, more mode 3 particles near the equator, which decreases toward the middle latitudes, is observed despite a decade of difference in the observation periods. CO abundance in low latitude regions was found to be characterized either by a constant or with a possible local-time variation. Although statistically favored, controversial local-time variations of CO require further investigation.

<title>Abstract</title>Terrestrial exoplanets orbiting within or near their host stars’ habitable zone are potentially apt for life. It has been proposed that time-series measurements of reflected starlight from such planets will reveal their rotational period, main surface features and some atmospheric information. From imagery obtained with the Akatsuki spacecraft, here we show that Venus’ brightness at 283, 365, and 2020 nm is modulated by one or both of two periods of 3.7 and 4.6 days, and typical amplitudes  &lt;10% but occasional events of 20–40%. The modulations are unrelated to the solid-body rotation; they are caused by planetary-scale waves superimposed on the super-rotating winds. Here we propose that two modulation periods whose ratio of large-to-small values is not an integer number imply the existence of an atmosphere if detected at an exoplanet, but it remains ambiguous whether the atmosphere is optically thin or thick, as for Earth or Venus respectively. Multi-wavelength and long temporal baseline observations may be required to decide between these scenarios. Ultimately, Venus represents a false positive for interpretations of brightness modulations of terrestrial exoplanets in terms of surface features.

© 2020 Elsevier Inc. We describe the dayside cloud top structure of Venus as retrieved from 93 images acquired at a wide variety of solar phase angles (0–120°) using the 2.02-μm channel of the 2-μm camera (IR2) onboard the Venus orbiter, Akatsuki, from April 4 to May 25, 2016. Since the 2.02-μm channel is located in a CO2 absorption band, the sunlight reflected from Venus allowed us to determine the cloud top altitude corresponding to unit aerosol optical depth at 2.02 μm. First, the observed solar phase angle dependence and the center-to-limb variation of the reflected sunlight in the region equatorward of 30° were used to construct a spatially averaged cloud top structure characterized by cloud top altitude zc, Mode 2 modal radius rg,2, and cloud scale height H, which were 70.4 km, 1.06 μm, and 5.3 km, respectively. Second, cloud top altitudes at individual locations were retrieved on a pixel-by-pixel basis with an assumption that rg,2 and H were uniform for the entire planet. The latitudinal structure of the cloud top altitude was symmetric with respect to the equator. The average cloud top altitude was 70.5 km in the equatorial region and showed a gradual decrease of ~2 km by the 45° latitude. It rapidly dropped at latitudes of 50–60° and reached 61 km in latitudes of 70–75°. The average cloud top altitude in the region equatorward of 30° showed negligible local time dependence, with changes up to 1 km at most. Local variations in cloud top altitude, including stationary gravity wave features, occurred within several hundreds of meters. Although long zonal or tilted streaky features poleward of ~45° were clearly identifiable, features in the low and middle latitudes were usually subtle. These did not necessarily appear as local variations at the cloud top level, where mottled and patchy UV patterns were observed, suggestive of convection and turbulence at the cloud top level.

Venus has a thick atmosphere that rotates 60 times as fast as the surface, a phenomenon known as super-rotation. We use data obtained from the orbiting Akatsuki spacecraft to investigate how the super-rotation is maintained in the cloud layer, where the rotation speed is highest. A thermally induced latitudinal-vertical circulation acts to homogenize the distribution of the angular momentum around the rotational axis. Maintaining the super-rotation requires this to be counteracted by atmospheric waves and turbulence. Among those effects, thermal tides transport the angular momentum, which maintains the rotation peak, near the cloud top at low latitudes. Other planetary-scale waves and large-scale turbulence act in the opposite direction. We suggest that hydrodynamic instabilities adjust the angular-momentum distribution at mid-latitudes.

The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the 283 and 365 nm wavelengths. There are absorption bands of SO2 and unknown absorbers in these wavelength regions. The UV images provide the spatial distribution of SO2 and the unknown absorber around cloud top altitudes. The images also allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with 2-h intervals are used to understand the dynamics of the Venusian atmosphere by estimating the wind vectors at the cloud top altitude, as well as the mass transportation of UV absorbers. The UVI is equipped with off-axial catadioptric optics, two bandpass filters, a diffuser installed in a filter wheel moving with a step motor, and a high sensitivity charge-coupled device with UV coating. The UVI images have spatial resolutions ranging from 200 m to 86 km at sub-spacecraft points. The UVI has been kept in good condition during the extended interplanetary cruise by carefully designed operations that have maintained its temperature maintenance and avoided solar radiation damage. The images have signal-to-noise ratios of over 100 after onboard desmear processing.

The status and initial products of the 1-mu m camera onboard the Akatsuki mission to Venus are presented. After the successful retrial of Venus' orbit insertion on Dec. 2015 (5 years after the failure in Dec. 2010), and after a long cruise under intense radiation, damage in the detector seems small and fortunately insignificant in the final quality of the images. More than 600 dayside images have been obtained since the beginning of regular operation on Apr. 2016 although nightside images are less numerous (about 150 in total at three wavelengths) due to the light scattered from the bright dayside. However, data acquisition stopped after December 07, 2016, due to malfunction of the electronics and has not been resumed since then. The 0.90-mu m dayside images are of sufficient quality for the cloud-tracking procedure to retrieve wind field in the cloud region. The results appear to be similar to those reported by previous 1-mu m imaging by Galileo and Venus Express. The representative altitude sampled for such dayside images is estimated to be 51-55 km. Also, the quality of the nightside 1.01-mu m images is sufficient for a search for active volcanism, since interference due to cloud inhomogeneity appears to be insignificant. The quality of the 0.97-mu m images may be insufficient to achieve the expected spatial resolution for the near-surface H2O mixing ratio retrievals.

The existence of large stationary gravity waves was discovered during Akatsuki's first observation sequence in 2015. In this study, the further detection of large stationary gravity waves in brightness temperature images over a 1.5 year period is reported. The waves periodically appeared mostly above four specific highland regions in the low latitudes when these regions were in the local afternoon. The wave amplitudes attenuated after the wave locations passed beyond the evening terminator, and the locations of the waves tended to slowly drift eastward over their lifetimes. The appearances of stationary waves depend not only on surface topography but also on latitude and local time, suggesting that solar heating during the daytime and atmospheric structure affected by solar heating may control the excitation and propagation of stationary waves.

After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to similar to 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48-70 km altitude. The H2SO4 vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing.

We provide an overview of data products from observations by the Japanese Venus Climate Orbiter, Akatsuki, and describe the definition and content of each data-processing level. Levels 1 and 2 consist of non-calibrated and calibrated radiance (or brightness temperature), respectively, as well as geometry information (e.g., illumination angles). Level 3 data are global-grid data in the regular longitude-latitude coordinate system, produced from the contents of Level 2. Non-negligible errors in navigational data and instrumental alignment can result in serious errors in the geometry calculations. Such errors cause mismapping of the data and lead to inconsistencies between radiances and illumination angles, along with errors in cloud-motion vectors. Thus, we carefully correct the boresight pointing of each camera by fitting an ellipse to the observed Venusian limb to provide improved longitude-latitude maps for Level 3 products, if possible. The accuracy of the pointing correction is also estimated statistically by simulating observed limb distributions. The results show that our algorithm successfully corrects instrumental pointing and will enable a variety of studies on the Venusian atmosphere using Akatsuki data.

Copyright © 2016 by the International Astronautical Federation (IAF). All rights reserved. Japan&#039;s Venus Climate Orbiter Akatsuki was proposed to ISAS (Institute of Space and Astronautical Science) in 2001 as an interplanetary mission. We made 5 cameras with narrow-band filters to image Venus at different wavelengths to track the cloud and minor components distribution at different heights to study the Venusian atmospheric dynamics in 3 dimension. It was launched on May 21st, 2010 and reached Venus on December 7th, 2010. With the thrust by the orbital maneuver engine, Akatsuki tried to go into the westward equatorial orbit around Venus with the 30 hours&#039; orbital period, however it failed by the malfunction of the propulsion system. Later the spacecraft has been orbiting the sun for 5 years. On December 7th, 2015 Akatsuki met Venus again after the orbit control and Akatsuki was put into the westward equatorial orbit whose apoapsis is about 0.44 million km and orbital period of 14 days. Its main target is to shed light on the mechanism of the fast atmospheric circulation of Venus. The systematic imaging sequence by Akatsuki is advantageous for detecting meteorological phenomena with various temporal and spatial scales. We have five photometric sensors as mission instruments for imaging, which are 1 m-infrared camera (IR1), 2 m-infrared camera (IR2), ultra-violet imager (UVI), long-wave infrared camera (LIR), and lightning and airglow camera (LAC). These photometers except LIR have changeable filters in the optics to image in certain wavelengths. Akatsuki&#039;s long elliptical orbit around Venus is suitable for obtaining cloud-tracked wind vectors over a wide area continuously from high altitudes. With the observation, the characterizations of the meridional circulation, mid-latitude jets, and various wave activities are anticipated. The technical issues of Venus orbit insertion in 2015 and the scientific new results will be given in this paper.

Recently it was found that the low-latitude zonal wind and the amplitudes of Kelvin and Rossby waves at the cloud top of Venus show long-term variations in a synchronized manner. For the purpose of explaining this synchronization, we investigated the influence of the background zonal wind profile on the upward propagation of Kelvin and Rossby waves at altitudes 60-80 km. Results from a linearized primitive equation model suggests that Kelvin waves can reach the cloud top height when the background wind speed is slow, whereas Rossby waves can reach the cloud top when the background wind speed is fast. These features obtained from the model are consistent with the observations. Since the momentum deposition by these waves can accelerate or decelerate the mean flow, these waves may contribute to the variation of the background wind. The calculated spatial distributions of the momentum dissipation indicate that the Kelvin waves accelerate the low-latitude atmosphere, and thus they can act to induce transition from the slow wind period to the fast wind period. On the other hand, the Rossby waves decelerate mainly the mid-latitude atmosphere, so that additional mechanisms are required to decelerate the low-latitude atmosphere. A possible mechanism is momentum advection caused by the Rossby wave-induced meridional circulation. (C) 2014 Elsevier Inc. All rights reserved.

We present phase curves for Venus in the 1-2 mu m wavelength region, acquired with IR1 and IR2 on board Akatsuki (February-March 2011). A substantial discrepancy with the previously-published curves was found in the small phase angle range (0-30 degrees). Through analysis by radiative-transfer computation, it was found that the visibility of larger (similar to 1 mu m or larger) cloud particles was significantly higher than in the standard cloud model. Although the cause is unknown, this may be related to the recently reported increase in the abundance of SO2 in the upper atmosphere. It was also found that the cloud top is located at similar to 75 km and that 1-mu m particles exist above the cloud, both of these results being consistent with recent studies based on the Venus Express observations in 2006-2008. Further monitoring, including photometry for phase curves, polarimetry for aerosol properties, spectroscopy for SO2 abundance, and cloud opacity measurements in the near-infrared windows, is required in order to understand the mechanism of this large-scale change. (C) 2014 Elsevier Inc. All rights reserved.

A Martian rover mission is currently entertained in Japan Aerospace Exploration Agency (JAXA) for launch in 2020s. The primary scopes of this mission are to demonstrate the entry-descent-landing and surface exploration technologies on Mars, to search for signs of live organisms, and to determine when the ocean is lost in the past Martian history. In order to cut down the mission cost and the development risk, the spacecraft is designed as a compact medium-class spacecraft whose total wet mass at Earth departure is approximately 900 kg. The spacecraft system consists of the cruise module and the atmospheric entry module which is subdivided into a forebody and an aftbody aeroshell, a landing module, and a rover, in a typical reference mission design under consideration. The current status of research and development is briefly presented as well.

We have investigated the cloud top structure of Venus by analyzing ground-based images taken at the mid-infrared wavelengths of 8.66 mu m and 11.34 mu m. Venus at a solar phase angle of similar to 90 degrees, with the morning terminator in view, was observed by the Cooled Mid-Infrared Camera and Spectrometer (COMICS), mounted on the 8.2-m Subaru Telescope, during the period October 25-29, 2007. The disk-averaged brightness temperatures for the observation period are similar to 230 K and similar to 238 K at 8.66 mu m and 11.34 Inn, respectively. The obtained images with good signal-to-noise ratio and with high spatial resolution (similar to 200 km at the sub-observer point) provide several important findings. First, we present observational evidence, for the first time, of the possibility that the westward rotation of the polar features (the hot polar spots and the surrounding cold collars) is synchronized between the northern and southern hemispheres. Second, after high-pass filtering, the images reveal that streaks and mottled and patchy patterns are distributed over the entire disk, with typical amplitudes of similar to 0.5 K, and vary from day to day. The detected features, some of which are similar to those seen in past UV images, result from inhomogeneities of both the temperature and the cloud top altitude. Third, the equatorial center-to-limb variations of brightness temperatures have a systematic day-night asymmetry, except those on October 25, that the dayside brightness temperatures are higher than the nightside brightness temperatures by 0-4 K under the same viewing geometry. Such asymmetry would be caused by the propagation of the migrating semidiurnal tide. Finally, by applying the lapse rates deduced from previous studies, we demonstrate that the equatorial center-to-limb curves in the two spectral channels give access to two parameters: the cloud scale height Hand the cloud top altitude z(c), The acceptable models for data on October 25 are obtained at H=2.4-4.3 km and z(c)= 66-69 km; this supports previous results determined from spacecraft observations. (C) 2014 Elsevier Inc. All rights reserved.

Our project aims to search for methane-oxidizing microbes on the Mars surface. The project is in preparation under the scheme of the MELOS working group. Martian soil will be sampled from a depth of about 5 - 10 cm below the surface, where organisms are expected to be protected from the harsh hyper-oxidative environment of the Mars surface. The soils will be stained with a cocktail of fluorescent reagents, and examined by fluorescence microscopy. A combination of fluorescent dyes has been selected to identify life forms in samples. A combination of dyes will be used to detect membranes surrounding the "cell". A substrate dye that emits fluorescence upon cleavage by a catalytic reaction will be used to detect the catalytic activity of the "cell". This combination will also be useful for detecting pre-biotic organic material as well as remnants of ancient Martian life. Hydrolysis of the polymers in the "cell" followed by HPLC for amino acid analysis will be effective for examining whether Martian life is identical to or different from terrestrial life. The number and type of the amino acids as well as their chirality will be analyzed to distinguish whether the polymers are contaminants from Earth.

We have deduced the scattering properties of aerosols in the jovian upper troposphere by analyzing imaging data obtained at a wide variety of solar phase angles (4-140 degrees) by the Cassini Imaging Science Subsystem (ISS) Narrow Angle Camera (NAC) during its flyby of Jupiter. The limb-darkening curves along the South Tropical Zone (STrZ) are extracted from CB2 images (effective wavelength: 750 nm) to constrain the single scattering phase functions of aerosols. The best-fit Mie scattering phase function for cloud is obtained with the real part of the refractive index n(r,cloud) = 1.85 and the effective radius r(eff,cloud) = 0.3 mu m. The best-fit combination of n(r,cloud) and r(eff,cloud) would strongly suggest the idea that the abundant small particle population in the upper troposphere is not composed of pure NH3 ice. Although the optical properties of the stratospheric haze are not well constrained compared with those of cloud, the haze is found to be optically thin (&lt;0.06) and to be strongly forward scattering (effective radius r(eff,haze) = 0.5 mu m). We compare our results with the scattering phase function at red wavelength (640 nm) for the STrZ derived by Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558-592), which was deduced from analysis of the Pioneer 10 Imaging Photopolarimeter (IPP) data (12-150 degrees for solar phase angles). Our new Mie scattering phase function can reproduce the Pioneer 10 observations well. In contrast, their scattering phase function described by the double Henyey-Greenstein function does not reproduce the Cassini ISS observations. This is attributed to the fact that their scattering phase function is underconstrained, primarily due to a considerable gap in observations for an intermediate solar phase angle range (34-109 degrees). Our new Mie scattering phase function has advantages over that of Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558-592). (1) Since the Cassini ISS data do not have a large gap in solar phase angle, the new Mie scattering phase function is better constrained. (2) The Mie scattering phase function can be applied easily to different wavelengths. With such characteristics, we now have a reliable baseline scattering phase function that can be used to interpret the ever-changing appearance of jovian clouds as changes of the vertical cloud structure and/or distribution of chromophores in the atmosphere. (C) 2012 Elsevier Inc. All rights reserved.

Japanese Venus Climate Orbiter/AKATSUKI was proposed in 2001 with strong support by international Venus science community and approved as an ISAS (Institute of Space and Astronautical Science) mission soon after the proposal. The mission life we expected was more than two Earth years in Venus orbit. AKATSUKI was successfully launched at 06:58:22JST on May 21, 2010, by H-IIA F17. After the separation from H-IIA, the telemetry from AKATSUKI was normally detected by DSN Goldstone station (10:00JST) and the solar cell paddles' expansion was confirmed. The malfunction happened on the propulsion system during the Venus orbit insertion (VOI) on Dec 7, 2010. We failed to make the spacecraft become a Venus orbiter, and the spacecraft entered an orbit around the Sun with a period of 203 days. Most of the fuel still had remained, but the orbital maneuvering engine was found to be broken. We decided to use only the reaction control system (RCS) for orbital maneuver and three minor maneuvers in Nov 2011 were successfully done so that AKATSUKI will meet Venus in 2015. We are considering several scenarios for VOI using only RCS. Copyright © (2012) by the International Astronautical Federation.