The Nature of the Near-Infrared Features on the Venus Night Side

Abstract

Near-infrared images of the Venus night side show bright contrast features that move from east to west, in the direction of the cloud-top atmospheric superrotation. Recently acquired images of the Venus night side along with earlier spectroscopic observations allow identification of the mechanisms that produce these features, their level of formation, and the wind velocities at those levels. The features are detectable only at wavelengths near 1.74 and 2.3 micrometers, in narrow atmospheric windows between the CO(2) and H(2)O bands. The brightest features have brightness temperatures near 480 Kelvin, whereas the darkest features are more than 50 Kelvin cooler. Several factors suggest that this radiation is emitted by hot gases at altitudes below 35 kilometers in the Venus atmosphere. The feature contrasts are produced as this thermal radiation passes through a higher, cooler, atmospheric layer that has horizontal variations in transparency. The 6.5-day east-west rotation period of the features indicates that equatorial wind speeds are near 70 meters per second in this upper layer. Similar wind speeds have been measured by entry probes and balloons at altitudes between 50 and 55 kilometers in the middle cloud layer. The bright features indicate that there are partial clearings in this cloud deck. The presence of these clearings could decrease the efficiency of the atmospheric greenhouse that maintains the high surface temperatures on Venus.

Spectroscopic Observations of Bright and Dark Emission Features on the Night Side of Venus

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Latitudinal variations of CO and OCS in the lower atmosphere of Venus from near-infrared nightside spectro-imaging

“…This emission is due to thermal radiation from the hot sub-cloud atmosphere leaking through the overlying sulfuric cloud layers. Its origin was established from various factors (Crisp et al, 1989;Krasnopolsky, 1986), including the absence of polarization, lack of systematic dependence with distance to the terminator, and the agreement with simulated thermal spectra (Kamp et al, 1988;Bézard et al, 1990). In particular, the 2.3-µm window shows signatures from various absorbers and provides a unique way of probing the lower atmosphere under the cloud deck in the altitude range 25-40 km above the surface.…”

Ground-Based Near-Infrared Imaging Observations of Venus During the Galileo Encounter

“…dark markings that rotate from east to west with periods of about 6 days (1,2). This infkred emission is produced by hot gases in the lower atmosphere (0 to 50 km).…”

“…It is most intense at wavelengths near 1.74 and 2.3 pm, in the relatively transparent "spectral windows" between strongly absorbing CO, and H,O bands in the Venus atmosphere. The planetwide s u h i c acid (H2S04) clouds provide the primary source of opacity in these windows (2)(3)(4)(5)(6). Kamp et al (4,5) found that 10 to 20% differences in the optical depths of the clouds could account for the contrast between bright and dark markings.…”