The Presence of Ice in the Venus Atmosphere as Inferred from a Halo Effect

Abstract

view Abstract Citations (9) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Presence of Ice in the Venus Atmosphere as Inferred from a Halo Effect O'Leary, Brian T. Abstract The most common terrestrial halo phenomenon is a luminous ring located 22 from the Sun or Moon, and is due to the presence of hexagonal ice crystals in the atmosphere. Photometric profiles of halos show a brightness maximum and minimum at certain scattering angles which can be predicted by refraction theory. Photometry of the integrated light of Venus was performed at Kitt Peak National Observatory in three colors (B, V, and R) between phase angles 153 and 165 before and after inferior conjunction of 1966. These results along with an interpretive study of the photometric, colorimetric, and polarimetric characteristics of Venus as it passes through these phase angles suggest a halo effect of brightness dispersions of about 0.05 mag. brighter than the background phase-curve at predicted halo maximum and about 0.04 mag. fainter than the background phase-curve at predicted halo minimum. The corresponding polarizations of Venus observed near the halo maximum and minimum agree both in quantity and in direction with the predictions of Fresnel's laws of reflection as applied to the 22 halo. No photometric or polarimetric observations to date are inconsistent with the presence of a Venus halo effect. But since all observations are fairly close to error scatter, the existence of a Venus halo effect cannot now be definitely established. A halo effect would indicate definite proof of ice as a constituent of the Venus cloud layer because of the uniqueness of the angle and the curves of brightness, color, and polarization corresponding to an index of refraction of 1.31 and a hexagonal shape. The findings then tentatively indicate confirmation of H2O in the Venus atmosphere and a temperature and pressure environment at the cloud tops conducive to the formation of ice. The amplitude of the effect shows that, at most, a small fraction (a few per cent) of the tops of the Venus clouds would consist of these halo-producing crystals, i.e., hexagonal ice with sizes somewhat greater than a wavelength of light. There is also slight evidence of the secondary halo in the Venus atmosphere; this would be due to right-angle interfaces in ice crystals. It is suggested that the Venus halo eflect should be sought again at the next opportunity in summer, 1967, with as much accuracy as possible. The Kitt Peak observations also indicated that Venus was a tenth of a magnitude redder in B - R color index after inferior conjunction than before inferior conjunction at phase angles around 160 . The interpretation of this phenomenon might depend on a change in the scattering properties of the cloud tops from the Venus morning to afternoon or on local differences between the clouds near each limb during the times of observation. Venus was found to become redder at phase angles greater than 1560, contrary to the trend of previous observations. Publication: The Astrophysical Journal Pub Date: December 1966 DOI: 10.1086/148954 Bibcode: 1966ApJ...146..754O full text sources ADS |