Observations of the phase variation of Venus CO 2 equivalent widths

Abstract

We present equivalent widths of Venus CO 2 scans of the P branch ( P8– P32) of the 5ν 3 band at 8689 Å, the P16 line of the 5ν 3 band, and the P14 line of the ν 1 + 5 ν 3 band at 7820 Å covering phase angles between 5°.1 and 170°. The equivalent widths reach a minimum at 10°, in agreement with a phase function with a backward lobe at 160° which is caused by a single internal reflection within the cloud particles. This is evidence that Venus cloud particles are composed of liquid droplets. Maximum equivalent widths are observed at ∼60°, a value which is closer to the maximum of single-layer Mie scattering models than to that of two-layer models. At high phase angles we observe equivalent widths greater than those computed from homogeneous scattering models, indicating that at high altitudes the mixing ratio of scattering particles to CO 2 increases with depth. At all phase angles, particularly at large phase angles, the temporal and spatial variations in the observed equivalent widths confuse the phase variation.