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著者人数: 16名, Accepted: 2019-04-12, 資料番号: SA1190014000

著者人数: 13名, Accepted: 2018-11-28, 資料番号: SA1180242000

著者人数: 19名, Accepted: 2016-12-08, 資料番号: SA1160259000

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.

The oxygen isotope ratios 17O/16O and 18O/16O in Venus׳s atmosphere were measured simultaneously by ground-based IR spectroscopy. The CO2 absorption lines in the 2648 cm−1 (for 17O/18O) and 4582 cm−1 (for 18O/16O) regions were observed using the IRTF/CSHELL spectrometer. The deviations of the isotope fractions are found to be δ17O=+92±158‰ and δ18O=−42±85‰ as compared to the terrestrial standard (HITRAN 2012) where the uncertainties include both random and systematic errors. Such combination agrees with the Earth–Moon fractionation line within the errors. This is consistent to the fact that the proto-Venus matter was also well mixed with the proto-Earth–Moon matter.

著者人数: 15名, Accepted: 2014-10-19, 資料番号: SA1004980000

著者人数: 22名, Accepted: 2013-07-13, 資料番号: SA1004507000

著者人数: 11名, Accepted: 2012-01-30, 資料番号: SA1003638000

The abundance of HDO above the clouds in the dayside atmosphere of Venus was measured by ground-based 2.3 μm spectroscopy over 4 days. This is the first HDO observation above the clouds in this wavelength region corresponding to a new height region. The latitudinal distributions found show no clearly defined structure. The disk-averaged mixing ratio is 0.22 ± 0.03 ppm for a representative height region of 62–67 km. This is consistent with measurements found in previous studies. Based on previous H2O measurements, the HDO/H2O ratio is found to be 140 ± 20 times larger than the telluric ratio. This lies between the ratios of 120 ± 40 and 240 ± 25, respectively, reported for the 30–40 km region by ground-based nightside spectroscopy and for the 80–100 km region by solar occultation measurement on board the Venus Express.

著者人数: 14名, Accepted: 2011-06-09, 資料番号: SA1003050000

The abundance of carbon monoxide in the Venus’ dayside atmosphere above the clouds was measured by ground-based 2.3 μm spectroscopy for 4 days. The hemispherical distributions found show no significant latitudinal or longitudinal structure. The disc-averaged mixing ratio of 58 ± 17 ppm found at a representative height of 62–67 km is consistent with previous measurements. Such a flat distribution of CO abundance above the clouds seems to be controlled by an efficient horizontal eddy diffusion with a time scale of 30 days or shorter although the CO distribution below the clouds seems to be controlled by the meridional circulation. The pole-ward wind speed of the meridional circulation above the clouds is estimated to be 0.2 m s −1 or less based on the difference between the CO mixing ratios above and below the clouds.

The abundance of hydrogen chloride (HCl) in the Venus atmosphere was measured by ground-based IR spectroscopy. The dayside measurements were performed in May 2007 with a resolution of 40,000, and the nightside measurements in October 1999 with a resolution of 1000. The hemispheric distributions of the HCl mixing ratio measured above the Venus’ clouds show no significant structure with a disc-averaged value of 0.74±0.06 ppm which is in the similar range as the previous report of 0.6±0.2 ppm. The representative height for the dayside measurements is estimated to be 60–66 km. Recent results by Venus Express/SPICAV/SOIR show much smaller values of 0.1–0.2 ppm at 64–94 km; however the direct comparison is difficult due to the different spatial conditions. The hemispheric distributions of the 35Cl/37Cl isotope ratio are also found to show no significant structure with a disc-averaged value of 3.1±0.4 which coincides with the terrestrial value of 3.1. The HCl mixing ratios below the clouds are also found to show no significant structure with a disc-averaged value of 0.40±0.05 ppm, which is similar to the previous reports of 0.4–0.5 ppm. The larger HCl mixing ratio above the clouds than below suggests the production of HCl in the cloud region or above. Also, a uniform hemispherical distribution of H2O is found below the clouds with a disc-averaged mixing ratio of 25±5 ppm; this is in the same range as the previous measurements. Those uniform distributions of HCl and H2O support the fact that their chemical lifetimes are much longer than that of mixing as has been discussed so far.

著者人数：21名, Accepted: 2006-04-10, 資料番号: SA1000187000

Accepted: 2007-03-30, 資料番号: SA1000233000

We review two models describing the Venus climate system: the carbonate and pyrite models. It has been argued carbonate and pyrite are potentially important minerals controlling the climate of Venus, though existence of either minerals has not been confirmed. Although it used to be proposed that carbonation reaction might explain the Venus’ atmospheric CO 2 abundance, it is unlikely Venus’ surface is reactive enough to control the Venus’ massive CO 2 atmosphere. Venus’ surface carbonate is also able to affect the climate through the reaction with atmospheric SO 2 to form anhydrite. Under the carbonate model the climate state is not in equilibrium and would be unstable due to the reaction between carbonate and SO 2 . On the other hand, pyrite–magnetite reaction is proposed to explain the Venus’ atmospheric SO 2 abundance. Under pyrite–magnetite reaction, however, the climate would be stabilized such that the existing climate state is maintained over a geological timescale, while some observational facts such as atmospheric abundance of SO 2 and surface temperature could also be reasonably explained.

A two-dimensional model study revealed that the meridional circulation can have a large influence on the distributions of the cloud and condensable gases on Venus. In our model, the generally observed depression of near-infrared optical thickness at midlatitude is reproduced as a result of the enhancement of cloud thickness at low and high latitudes. At high latitudes, the modeled poleward transport of H2SO4-H2O droplets in the upper atmosphere makes a thick cloud, since most of the H2SO4-H2O droplets photochemically produced are transported poleward by the meridional circulation. At low latitudes, the modeled large-scale ascent of the concentrated H2SO4 vapor forms a dense lower cloud in accord with observations by entry probes. The equatorward transport of H2SO4 vapor below the cloud by the modeled meridional circulation and the sedimentation of droplets in the lower cloud leads to the accumulation of H2SO4 vapor around the cloud base at low latitudes. The calculated H2SO4 vapor distribution agrees with radio occultation observations. The middle cloud appears to be caused by the condensation of H2SO4 vapor transported upward from below by convective mixing.

Accepted: 2008-12-10, 資料番号: SA1000963000

Received 2 March 2008; revised 29 July 2008; accepted 18 September 2008; published 31 December 2008. [1] We evaluated the spatial variation of Venusian surface emissivity at 1.18 mm wavelength and that of near-surface atmospheric temperature using multispectral images obtained by the Near-Infrared Mapping Spectrometer (NIMS) on board the Galileo spacecraft. The Galileo NIMS observed the nightside thermal emission from the surface and the deep atmosphere of Venus, which is attenuated by scattering from the overlying clouds. To analyze the NIMS data, we used a radiative transfer model based on the adding method. Although there is still an uncertainty in the results owing to the not well known parameters of the atmosphere, our analysis revealed that the horizontal temperature variation in the near-surface atmosphere is no more than ±2 K on the Venusian nightside and also suggests that the majority of lowlands likely has higher emissivity compared to the majority of highlands. One interpretation for the latter result is that highland materials are generally composed of felsic rocks. Since formation of a large body of granitic magmas requires water, the presence of granitic terrains would imply that Venus may have had an ocean and a mechanism to recycle water into the mantle in the past.

International audience; The Venus Express spacecraft images the nightside thermal emissions using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS). At 1.02, 1.10, and 1.18 mum, thermal emission from the surface is observed. The signal is attenuated by scattering and absorption in the dense atmosphere. The measured flux at the top of the atmosphere is positively correlated with surface temperature and surface emissivity. The surface temperature of Venus is relatively well constrained as being mainly a function of altitude with a gradient lesser or equal to the adiabatic lapse rate. This study examines the correlation of VIRTIS images showing a signal of the surface at 1.02 mum with viewing geometry, stray sunlight, cloud opacity, and topography and applies semiempirical relations to remove their influence. The remaining contrast can be either ascribed to surface emissivity or unexpected temperature variations. Temperature variations due to active volcanism are unlikely to be persistent over the time of observations; therefore, the mosaic of all processed images is here interpreted in terms of surface emissivity variation. The emissivity variation found is correlated with geomorphological features established from Magellan synthetic aperture radar images. It is generally lower at tessera terrain. Some, but not all, volcanic edifices show increased emissivity. Large lava flows in the Lada terra-Lavinia planitia region also show an increased thermal emission. This might indicate a more felsic surface composition of tessera highlands and large-scale extrusive volcanism of ultramafic composition.

Accepted: 2011-03-20, 資料番号: SA1002864000

著者人数: 31名, Accepted: 2011-02-15, 資料番号: SA1002853000