Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images

Abstract

Since insertion into orbit on December 7, 2015, the Akatsuki orbiter has returned global images of Venus from its four imaging cameras at eleven discrete wavelengths from ultraviolet (283 and 365 nm) and near infrared (0.9-2.3 µm), to the thermal infrared (8-12 µm) from a near-equatorial orbit. The Venus Express and Pioneer Venus Orbiter missions have also monitored the planet for long periods but from polar or near-polar orbits. The wavelength coverage and views of the planet also differ for all three missions. In reflected light, the images reveal features seen near the cloud tops (~ 70 km altitude), whereas in the near-infrared images of the nightside, features seen are at mid-to lower cloud levels (~ 48-60 km altitude). The dayside cloud cover imaged at the ultraviolet wavelengths shows morphologies similar to what was observed from Mariner 10, Pioneer Venus, Galileo, Venus Express and MESSENGER. The daytime images at 0.9 and 2.02 µm also reveal some interesting features which bear similarity to the ultraviolet images. The nighttime images at 1.74, 2.26 and 2.32 µm and at 8-12 µm reveal features not seen before and show new details of the nightside including narrow wavy ribbons, curved string-like features, long-scale waves, long dark streaks, isolated bright spots, sharp boundaries and even mesoscale vortices. Some features previously seen such as circum-equatorial belts (CEBs) and occasional areal brightenings at ultraviolet (seen in Venus Express observations) of the cloud cover at ultraviolet wavelengths have not been observed thus far. Evidence for the hemispheric vortex organization of the global circulation can be seen at all wavelengths on the day-and nightsides. Akatsuki images reveal new and puzzling morphology of the complex nightside cloud cover. The cloud morphologies provide some clues to the processes occurring in the atmosphere and are thus, a key diagnostic tool when quantitative dynamical analysis is not feasible due to insufficient information.

Morphology and Dynamics of Venus's Middle Clouds With Akatsuki/IR1

“…We studied the correlation between the cloud patterns on UltraViolet Imager (UVI) 365‐nm (cloud tops) and IR1 900‐nm (middle clouds) radiance using cylindrical projections of Akatsuki Level‐3 data (Ogohara et al, ). During the MESSENGER's flyby and the Akatsuki mission (Limaye et al, ; Peralta et al, ), NIR and UV images seem uncorrelated in most of the cases. For instance, UV planetary‐scale patterns like the Y feature are absent in NIR images, while the sharp discontinuities in NIR (Figure c) are missing in UV images.…”

“…At the upper and middle clouds of Venus, the zonal superrotation exhibits the largest speeds and vertical shear (Sánchez‐Lavega et al, ) and is also where most of the solar energy is deposited (Titov et al, ). The upper clouds' top at ∼70 km (Ignatiev et al, ) can be observed with ultraviolet (UV) and violet wavelengths (∼360–480 nm), with their morphology and dynamics been extensively studied for decades (Belton et al, ; Horinouchi et al, ; Khatuntsev et al, ; Limaye et al, ; Rossow et al, , ; Titov et al, ), thanks to the strong contrasts caused by an unknown absorber (Lee et al, ; Pérez‐Hoyos et al, ).…”

“…Wind measurements during the Venus Express (VEx) mission and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging's (MESSENGER) flyby confirmed these results and also showed that middle clouds' winds can be rather variable (Hueso et al, , ; Khatuntsev et al, ; Peralta et al, ; Sánchez‐Lavega et al, ). More recently, the middle clouds have been observed by amateur astronomers (Kardasis, ; Mousis et al., ; Sánchez‐Lavega et al., ) and with higher detail with Japan Aerospace Exploration Agency's (JAXA) Akatsuki mission (Iwagami et al, ; Limaye et al, ).…”

Ultraviolet imager on Venus orbiter Akatsuki and its initial results

“…The significant feature is that there are a large dark area in 283-nm image and a fine cloud complex in 365-nm image. The examples of cloud tracking results are described by Horinouchi et al (2018), and Limaye et al (2018) display the simultaneous images with UVI and other cameras.…”

Venus Atmospheric Thermal Structure and Radiative Balance

“…From its low inclination orbit, Akatsuki provides more low latitude radio occultation profiles (but fewer due to its 10.5-day orbit) compared to the high latitude coverage from Venus Express from its 24 hour polar orbit. Akatsuki results on the Venus cloud cover have been presented by Limaye et al (2018a), the thermal infrared results by and initial thermal structure (35-90 km) results from radio occultations are described by .…”