In the upper atmosphere of a planet, the lighter gases tend to diffuse, rising above their heavier neighbors, and unusual chemical reactions take place as the action of sunlight breaks some chemical bonds and stimulates the formation of others. The study of these tenuous regions of planetary atmospheres is called aeronomy. Many of the exotic chemical processes that occur can best be studied by examining radiation of very short wavelength, and it is to these problems that much of the work of the UVS team is directed. The observations are also sensitive to special processes in the Jovian atmosphere resulting from the magnetosphere. At the very top of the atmosphere, energetic charged particles interacting with the atmospheric molecules produce ultraviolet aurorae that indicate the location and nature of the bombarding electrons and ions. Emissions from atoms in the extended gas clouds around Io and possibly around other satellites were also expected to be observable. The ultraviolet instrument is a relatively straightforward optical spectrometer, adapted for use in space. A diffraction grating disperses the light into a spectrum, and the other optical elements are gold-coated mirrors. The field of view is rectangular, about 0.1 degrees × 0.9 degrees. An array of sensitive electronic detectors provides simultaneous measurements in 128 wavelength channels over a wavelength range from 50 to 170 nanometers. Photopolarimeter The fourth scan-platform instrument is designed to measure the brightness and polarization of light with high precision. Unlike the imaging system, however, it can look at only a single point (one pixel) at a time; thus it sacrifices spatial capability in favor of precision of measurement. The prime objectives of this photopolarimeter are related to study of the clouds of Jupiter. During the development stage for this instrument, the Principal Investigator was Charles F. Lillie of the Laboratory for Atmospheric and Space Physics of the University of Colorado at Boulder. Later, the Principal Investigator’s role was assumed by Lillie’s colleague, Charles W. Hord, a physicist with considerable experience in space missions, primarily with ultraviolet instruments. Four other scientists are Co-Investigators on the photopolarimeter. The instrument itself is essentially a small Cassegrain reflecting telescope of 15-centimeter aperture. Two consecutive filter wheels select the color and polarization of the light, which is then detected and measured by a photomultiplier tube. The entire photopolarimeter weighs just 2.5 kilograms.