This Voyager 2 high resolution color image, taken 2 hours before closest approach, provides obvious evidence of vertical relief in Neptune's bright cloud streaks.
Date: 25 Aug 1989
These clouds were observed at a latitude of 29 degrees north near Neptune's east terminator. The linear cloud forms are stretched approximately along lines of constant latitude and the Sun is toward the lower left. The bright sides of the clouds which face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun. These shadows are less distinct at short wavelengths (violet filter) and more distinct at long wavelengths (orange filter). This can be understood if the underlying cloud deck on which the shadow is cast is at a relatively great depth, in which case scattering by molecules in the overlying atmosphere will diffuse light into the shadow.
Because molecules scatter blue light much more efficiently than red light, the shadows will be darkest at the longest (reddest) wavelengths, and will appear blue under white light illumination.
The resolution of this image is 11 kilometers (6.8 miles per pixel) and the range is only 157,000 kilometers (98,000 miles). The width of the cloud streaks range from 50 to 200 kilometers (31 to 124 miles), and their shadow widths range from 30 to 50 kilometers (18 to 31 miles). Cloud heights appear to be of the order of 50 kilometers (31 miles).
Image Credit: NASA Jet Propulsion Laboratory
Credit: NASA Planetary Photojournal