VIRTIS Observations of Venus' Lower Atmosphere
Since the successful orbit insertion of Venus Express in April 2006, the VIRTIS instrument has gathered numerous high-resolution spectra of Venus' atmosphere in the wavelength range 2-5 micron. Taking advantage of a broad spectral window around 2.3 micron, the data has allowed for mapping of minor constituents in the deep atmosphere below the planet's thick cloud deck.
VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer, comprises two channels:
Using the high-resolution VIRTIS-H channel, a team of astronomers led by Emmanuel Marcq has measured the abundances of several minor components in the lower atmosphere between 30-40 km altitude. Thermal emission from these hot (200 °C at 35 km) layers of the lower atmosphere is able to escape through the huge amount of CO2 and the thick overlying cloud layers through a spectral window in a broad band around 2.3 μm. On the nightside of Venus this emission can be observed unhindered by the more intense reflected sunlight.
Figure 1 is an overlay of two example nightside spectra of Venus' lower atmosphere around 2.35 μm taken with the two different channels of the VIRTIS instrument. The dark red line shows a lower resolution spectrum obtained with the M-channel during the Venus orbit insertion in April 2006. The black line shows a higher resolution spectrum obtained with the H-channel during orbit #277.
Both spectra clearly show the broad absorption feature identified as a CO spectral band. The finer resolution of the H-channel allows for additional molecule species to be identified from their less pronounced spectral features also indicated in the figure, which include water vapour (H2O) and carbonyl sulphide (OCS).
The relative depth of the spectral features is directly related to the abundance of the corresponding absorbing constituent in the lower atmosphere. Using a radiative transfer computer model, Marcq et al. were able to derive constraints on the vertical profiles of CO, OCS and H2O in the 30-40 km altitude range. The model goes up to latitudes unreacheable with ground-based instruments (that cover only up to 60° latitude) as the quasi-polar orbit of Venus Express allows for VIRTIS to observe the lower atmosphere emission also from polar latitudes. Also the VIRTIS observations provide more accurate measurements of the abundances.
Distribution of CO, OCS and H2O - Atmospheric circulation
The observed distribution of CO, OCS and H2O in the lower atmosphere below the cloud deck as a function of latitude agrees with trends already known from earlier space-based observations (Galileo/NIMS during a Venus flyby) and also from ground-based observations:
Overall, in Venus' atmosphere CO is more abundant above the clouds in the upper atmosphere since it is produced by the intense UV light from the Sun that dissociates CO2 molecules. In contrast, OCS is mainly found close to the surface, where it is formed by chemical reactions involving the sulphur-rich minerals.
The increase in CO and decrease of OCS in the lower atmosphere at high latitudes observed by VIRTIS therefore point to downward motions in these places. The situation is reversed around the equator where upward atmospheric motion is seen to dominate resulting in an decrease in CO and increase in OCS.
This circulation pattern of up- and downward motion connected through horizontal winds is called Hadley cell circulation and was expected by various circulation computer models of Venus. A similar circulation pattern is also present on Earth and Mars.
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