During the week of May 4 - 10, SwRI investigators Mark Bullock and Eliot Young observed Venus from NASA's Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Their goal: to observe the clouds of Venus's lower cloud deck and measure wind speeds.
Thermal radiation from Venus's hot surface and lower atmosphere wells up through spectral windows in Venus's thick atmosphere. Clouds in the lower cloud deck (around 48 - 51 km) block the thermal radiation and show up as dark silhouettes on the night side of Venus.
On ongoing mystery is the speed at which the venusian atmosphere rotates. Venus's solid surface takes 243 earth-days to complete a rotation, but the bulk of the atmosphere whips around in only 6 days. A crucial question is how angular momentum is transferred to the atmosphere. Observations of cloud motions will provide wind vector fields and help constrain models of the super-rotating atmsophere.
Venus is typically observable for an hour or two during morning or evening twilight. To get a longer time baseline, Young and Bullock enlisted the help of observers at sites around the world, including Drs. Nancy Chanover (Apache Point Observatory, New Mexico), Johan Warell (Nordic Optical Telescope, Canary Islands), and Sanjay Limaye (Mt. Abu, India) and Gary Emerson (Himalayan Chandra Telescope, India). All provided 2.3 micron images of Venus that clearly show the motion of cloud features. In addition, Young and Bullock scanned a spectral slit slowly across the disk of Venus to construct image cubes of Venus spanning wavelengths from 0.8 to 2.5 microns.
In the image above, the spectrometer slit is visible over Venus's night hemisphere. The sunlit crescent appears dark becuase it saturates the detector. The bright features in the night hemisphere are actually the regions devoid of clouds, where radiation from the lower atmosphere passes through without being blocked by clouds.