Venus night-side photometry with “cleaned” Akatsuki/IR2 data: Aerosol properties and variations of carbon monoxide

Abstract

Abstract 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\u202fμm. 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 c louds a ligned l inearly in M 3L 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.