Abstract
IR2, a near-infrared camera in the 2-μm region onboard Akatsuki has been developed to primarily study the middle-to-lower atmospheric dynamics of Venus as probed in the 1.74- and 2.3-μm “windows” of the $$\hbox {CO}_2$$ 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 $$\hbox {CO}_2$$ 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.
Highlights
After nine orbital revolutions around the sun in five Earth years (Nakamura et al 2011), Japan’s Venus orbiter arrived at Venus on December 7, 2015 (Nakamura et al 2016)
Contrast features at 1.74 and 2.3 μm on the night-side disk are the silhouette of spatially inhomogeneous scatterers/absorbers, back-illuminated by the glow of the hot (300–500 K) lower atmosphere (Kamp and Taylor 1990)
Resolved maps in these windows were acquired by space instruments, such as Near-Infrared Mapping Spectrometer (NIMS) on Galileo (February 1990 flyby) (Carlson et al 1993) and Visible, Infrared and Thermal Imaging Spectrometer (VIRTIS) (Drossart 2007; Piccioni 2007) on Venus Express (Svedhem et al 2009; Titov et al 2009)
Summary
After nine orbital revolutions around the sun in five Earth years (Nakamura et al 2011), Japan’s Venus orbiter arrived at Venus on December 7, 2015 (Nakamura et al 2016). Cloud‐top altimetry Haus et al (2014) performed a comprehensive study of mesospheric temperature and cloud parameters while demonstrating usefulness of analyzing multi-window (1.74-, 2.3-, and 4.3-μm) spectra of the Venus night-side disk (VEX/VIRTIS-M). If the cloud albedo and the cloud-top structure are spatially uniform, contrast features at 2.02 μm reflect the undulation of the cloud top Such cloud-top altimetry has been performed using VEX/VIRTIS-M data (Ignatiev et al 2009) in a different CO2 band (1.6 μm), revealing stable cloud-top altitudes of 74 km at latitudes up to ∼ 50◦ and lower (63–69 km) in the polar regions. Tests with a prototype model have proved that this mechanism would maintain relative positions of lens elements at an accuracy of 10–20 μm By statistically adding such errors to ray tracing simulations, the probability of achieving MTF >0.5 is reasonably high. In‐flight calibration While in space, we have acquired three datasets that were used for the photometric calibration of IR2:
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