Infrared Interferometer Spectrometer
The Mariner 9 infrared interferometer spectrometer (IRIS) experiment was designed to provide information on the vertical structure, composition, and dynamics of the atmosphere and on the emissive properties of the surface of Mars. Measurements were made in the region of thermal emission spectra from 6 to 50 micrometers, using a modified Michelson interferometer with a spectral resolution of 2.4 inverse cm (apodized) and 1.2 inverse cm (unapodized), to determine the vertical temperature profile, general atmospheric circulation, minor atmospheric constituents, and surface temperature, composition, and thermal properties as a function of latitude and local time for dark and bright areas and the polar cap region. The instrumentation, mounted on the bottom of the spacecraft on a multiple-pointing, motor-driven scan platform, consisted primarily of (1) a scan mirror, (2) a coated cesium iodide entrance window, (3) a cesium iodide beam splitter, (4) a fixed mirror, (5) a movable mirror with electromagnetic drive, (6) a condensing mirror, (7) a thermistor bolometer detector, (8) a reference interferometer, (9) an internal warm blackbody calibrator, and (10) a programmer. The scan mirror selected IR radiation from one of three directions; Mars, deep space, or the internal warm blackbody. From this mirror, the radiation was reflected to the interferometer through the entrance window, which acted as an IR filter and had an effective aperture area of 10 sq cm. The beam splitter then divided the incoming radiation into two approximately equal components. After reflections from the fixed and moving mirrors, respectively, the two beams interfered with each other and were focused by the condensing mirror onto the bolometric detector, which provided an electrical output proportional to the intensity as a function of the path length difference or phase difference between the IR radiation reflected or transmitted by the beam splitter. The electrical output, converted from analog to digital form, was called an interferogram and represented a circular fringe pattern that appeared at the focal plane of the condensing mirror. Each interferogram had a duration of 18.2 s and contained 4,096 samples. After seven interferograms were taken in the operating mode, one was taken of the internal warm (298 plus or minus 3 deg K) blackbody, followed by another set of seven Mars interferograms, and finally by an interferogram from the deep space background (4 deg). The IRIS, which had a field of view of 4.5 deg, viewed an area 116 km in diameter from an orbital altitude of 1,600 km. The instrument was identical in all critical areas to the interferometers designed for the Nimbus-B and -D meteorological satellites, except that the Mariner 9 IRIS had better spectral resolution. The experiment began collecting excellent data soon after orbital insertion. NSSDC has all the useful data that exist from this investigation.