The primary goals of the infrared spectrometer investigation are directed toward analysis of the composition and structure of the atmosphere of Jupiter. Among the molecules to be searched for on Jupiter were hydrogen (H₂), helium (He), methane (CH₄), ammonia (NH₃), phosphine (PH₃), water (H₂O), carbon monoxide (CO), simple compounds of silicon and sulfur, and a variety of organic compounds consisting of atoms of carbon and hydrogen (e.g., C₂H₂, C₂H₄, C₂H₆). In addition to indicating the abundance of these constituents of the atmosphere, the infrared spectra also contain information on atmospheric structure, that is, on the variation of temperature and pressure with altitude. The presence of clouds or dust layers can also be inferred from the shapes of spectral lines.
In addition to its spectroscopy of Jupiter, the Voyager instrument could be used as a heat-measuring device to map the temperatures of both the satellites and the atmosphere of Jupiter. Particularly interesting for the satellites are measurements of the surface cooling and heating rates, since these rates reveal the physical compactness of the surface, easily distinguishing between rock and sand or dust.
The infrared instrument is a Michelson interferometer at the focus of a gold-plated telescope of 51-centimeter aperture. The spectrum is not obtained directly, by moving a prism or grating, but indirectly through the interference effects of light of different wavelengths: hence its name, an interferometer. The complex interference pattern generated by the motion of one of the mirrors in the light path is transmitted to Earth, where computer analysis is required to transform it into a recognizable spectrum. The entire instrument, which has a mass of 20 kilograms, is called IRIS, for infrared interferometer spectrometer.
The development of the infrared system for Voyager posed many problems. IRIS was designed to cover the optimum spectral region for studies of the atmospheres of Jupiter and Saturn. However, when it was decided in 1974 that an extended Voyager mission might also allow a visit to Uranus, Hanel and his colleagues realized that their instrument had serious deficiencies for investigation of that colder and more distant planet, and they proposed that a modified IRIS (MIRIS) be substituted for the original design. A crash program was authorized to develop MIRIS in parallel with IRIS, and in early 1977, as launch approached, it appeared that the improved instrument would be ready. Problems occurred during testing, however, and for several weeks in June and July the decision hung in the balance. Unfortunately, there simply was not enough time to solve all the problems; both Voyagers were launched carrying the original IRIS, and the final flight qualification of MIRIS came in October, about six weeks after launch. With no other missions planned beyond Saturn, no alternate use for MIRIS has been found; it remains “on the shelf,” one of the rare cases where a technological gamble by NASA did not pay off.
Lyle Broadfoot, ultraviolet spectrometer Principal Investigator
Ultraviolet Spectrometer
A second spectroscopic instrument on the Voyager scan platform examines short-wave, or ultraviolet, radiation. The Principal Investigator for the ultraviolet spectrometer (UVS) is A. Lyle Broadfoot of Kitt Peak National Observatory in Tucson, Arizona. Broadfoot’s expertise lies in instrumentation for atmospheric research, and he was previously the Principal Investigator for a similar ultraviolet instrument on the Mariner 10 mission to Venus and Mercury. Associated with Broadfoot in the Voyager investigation are fourteen other scientists from the United States, Canada, and France.
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.