Jovian Stratospheric Hazes: The High Phase Angle View from Galileo

Abstract

High phase angle Galileo images of Jupiter's limb reveal the presence of stratospheric hazes in the equatorial region (latitude 9°N) and the transition zone at the southern edge of the north polar region (60°N). During orbit E4, images were taken in both violet and near-IR continuum filters, effectively probing Jupiter's stratosphere at two different altitudes. A discrete layer detached from the limb is present at 60°N, 315°W, but not 20° further east at 60°N, 295°W. Bright streaks running roughly north–south are also present on Jupiter's crescent at 60°N. No such discrete features have been seen previously in high phase angle images of the giant planets. Nor are such features seen at 9°N, where the haze appears to be more uniformly distributed in height and longitude. Radial intensity profiles across the limb are inverted to give vertical extinction profiles over ∼200 km in Jupiter's stratosphere. These show that extinction in the discrete haze layer is enhanced by a factor of ∼2 over its surroundings in the violet filter, with less pronounced variation in the near-IR filter. Haze distribution models were found for both latitudes in which the near-IR images constrain the haze properties near or above the 100-mbar pressure level, where the mean particle radius is about 0.45 μm and the haze number density is near 0.15 cm−3. In these models the violet images constrain the haze properties about 25 km higher in the atmosphere, near or above the 20-mbar pressure level, where the haze particle size is 0.32±0.01 μm at 9°N and 0.27±0.01 μm at 60°N. At this altitude, the haze number density increases by almost an order of magnitude between the equator and the polar transition region—from 0.1 cm−3 at 9°N to 0.7 cm−3 at 60°N. An alternative solution is possible at 60°N, in which the haze is placed near 1 mbar in both filters, with mean particle sizes of 0.6 μm in the violet and 1.3 μm in the near IR.