Development and in-flight calibration of IR2: 2-μm camera onboard Japan’s Venus orbiter, Akatsuki

Abstract

IR2, a near-infrared camera in the 2-μm region onboard Akatsuki has been developed to primarily study the middleto-lower atmospheric dynamics of Venus as probed in the 1.74-and 2.3-μm "windows" of the CO 2 atmosphere on the night side. The spatial and temporal variability of CO below the clouds is also studied by differentiating 2.32-μm CO-band images from simultaneous 2.26-μm images. Images of the night-side disk in these wavelengths will enable us to determine the zonal and meridional winds near the cloud-base altitudes. IR2 also images at 2.02 μm, the center of a CO 2 absorption band. Such images can visualize the variation of the cloud-top altitudes as contrast features due to different absorption path lengths of the reflected sunlight. Tracking of the 2.02-μm features will also enable us to obtain wind information at the cloud-top level. Together with the other cameras and the radio science equipment on Akatsuki, IR2 will contribute to understanding of the production and maintenance mechanism of super-rotation in the Venusian atmosphere. During cruise, IR2 observed zodiacal light with a broad-band H filter (1.65 μm), imaged the Earth-moon remotely from a distance of ~30 million km, and determined Venus's phase curves at small phase angles. We have just started the early phase operation check of IR2 at Venus, as the orbit insertion in December 2015 was successful.

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

“…There are five cameras on Akatsuki to image Venus at the ultraviolet to thermal infrared wavelengths-UVI (Iwagami et al 2018, submitted to EPS Akatsuki special issue), IR2 (Satoh et al 2016), LIR (Fukuhara et al 2011(Fukuhara et al , 2017b) and the Lightning and Airglow Camera (LAC) which does not produce an image (Takahashi et al 2008), and is used when the nightside of Venus is visible to Akatsuki to detect lightening and airglow. Table 1 provides the characteristics of the four imaging cameras.…”

Enormous Cloud Cover on Venus observed by Akatsuki’s IR2 data recovered by Restoration-by-Deconvolution (RD) method

“…The IR2 camera onboard Akatsuki utilizes narrow band filters in the atmospheric windows to probe through the middle and lower clouds of Venus atmospheric windows (Satoh et al, 2016).…”

“…The IR2 camera is operated under cooling temperature of the platinum silicide (PtSi) detector regulated by the cryocooler to be below 70K. This is crucial to Venus nightside observations because the thermally induced electrons saturate the detector within few tens of seconds when the detector temperature is 70K (Satoh et al, 2016).…”

“…The lower clouds can be probed using the CO2 atmospheric window in near-infrared (NIR) regime (Allen & Crawford, 1984). Akatsuki's IR2 camera has three filters (1.735µm, 2.26µm, and 2.32µm) to observe the lower clouds in Venus's nightside (Satoh et al, 2016). Contrast features emerging in these filters are due to spatially inhomogeneous scattering and absorption of the radiation from the lower atmosphere by cloud particles.…”

Performance of Akatsuki/IR2 in Venus orbit: the first year

“…On IR2, these observations are made possible by a set of narrow-band filters (1.735, 2.26, and 2.32 μm) installed in a 6-position filter wheel (Satoh et al 2016; hereafter referred to as Paper I). The scientific objectives with these filters include studying the atmospheric dynamics by measuring the cloud motion vectors (Hueso et al 2012), morphology of clouds (McGouldrick et al 2008), aerosol properties (Carlson et al 1993;Haus et al 2015) by combining 1.735-and 2.26-μm images, and mapping the sub-cloud CO (at ~ 35 km altitudes) by differentiating the 2.26-and 2.32-μm quasi-simultaneous image pair.…”