Galileo Orbiter Photopolarimeter/Radiometer
As was demonstrated by the GISS Orbital Cloud Photopolarimeter (OCPP) instrument on the Pioneer Venus mission, measurement of intensity and polarization of sunlight scattered by clouds and aerosols can be a very powerful tool for inferring cloud/aerosol structure and particle microphysical properties. That strategy is being continued with the Photopolarimeter/Radiometer (PPR) on the Orbiter spacecraft of the Galileo mission to Jupiter.
With its varied measurement capabilities, the intended mission objectives of the PPR experiment are to: (1) determine the vertical and horizontal distribution of cloud and haze particles in the Jovian atmosphere, including their size, shape, and refractive index; (2) determine the energy budget of Jupiter and the variation in amount and distribution of the sunlight reflected and the thermal radiation emitted by Jupiter and its satellites; and (3) measure and map the photometric, polarimetric, and thermal radiometric properties of the Galilean satellites.
Launched in October 1989, the Galileo Orbiter spacecraft arrived at Jupiter in December 1995, having released the Probe several months earlier. The Probe entered the Jovian atmosphere just prior to the Orbiter's orbital insertion (the maneuver that placed it in orbit about Jupiter) for a planned two-year mission. The six-year outbound cruise included flybys of Venus, Earth, and the asteroids Gaspra and Ida, providing opportunities for PPR observations. In addition, PPR participated with the other Galileo optical remote-sensing instruments in the only direct observations of the collisions with Jupiter by the Shoemaker-Levy 9 comet fragments.
The Great Red Spot
Of the various features seen in the atmosphere of Jupiter, the Great Red Spot (GRS) has usually provoked the greatest interest. Large enough to be easily discernible by an observer on Earth using a small telescope or binoculars, it was first seen over 300 years ago, and although it exhibits variations in its coloration, detailed morphology, and contrast with its surroundings, it remains a permanent feature. Yet based upon the present evidence, this enigma is best characterized as a giant, anticyclonic vortex in the upper troposphere of Jupiter's atmosphere.
With all the unanswered questions that attend a Jovian weather system that is so long-lived, the GRS was chosen to be the focus of a coordinated observing campaign early in the Galileo Orbiter mission involving the Near-Infrared Mapping Spectrometer (NIMS), the Solid-State Imaging (SSI) camera, the Ultraviolet Spectrometer (UVS), and the PPR. Thus, one of the primary objectives of the GISS PPR observations was to characterize the thermal structure of the upper atmosphere in the GRS and its nearby surroundings. This "temperature sounding" is performed using measurements of the radiation emitted by the atmosphere at four discrete wavelengths in the thermal infrared region of the spectrum. Because of the variation in atmospheric absorption of radiation at these wavelengths, each spectral band is sensitive to a different level in the upper troposphere and thus allows us to infer the temperature at several levels in the region of 200- to 700-mbar atmospheric pressure.
Figure 3 displays maps of the retrieved temperatures at 250 (middle panel) and 500 mbar (bottom) for the GRS and a "panhandle" region stretching from the northwest portion of the spot. For reference, the top panel of the figure shows a map of the same region made from a Hubble Space Telescope image at a blue visual wavelength along with a schematic representation of the raster scanning pattern of the PPR field of view employed for the observation.
As indicated by the darker colors, the interior of the GRS is systematically colder than the surroundings by 4 to 8 degrees Celsius at both the 250 and 500-mbar levels. These colder temperatures support the interpretation of the GRS as an anticyclonic vortex, characterized by counterclockwise circulation at its outer edge and upwelling vertical motions in the interior, with the associated adiabatic cooling of those rising parcels of atmosphere. Such upwelling in the middle of the GRS is also consistent with increased high-level clouds inferred from the SSI images also acquired during the coordinated Galileo Orbiter observations.
The PPR makes high-precision polarimetry observations at three wavelengths (410, 678, and 945 nm) in the visual and near-infrared portion of the spectrum, using a Wollaston prism and two silicon photodiode detectors to simultaneously measure the two orthogonal intensity components at each of three successive half-wave retarder orientations for each band. These six intensity measurements for a given spectral band determine the total intensity, degree of linear polarization, and the polarization direction, as well as provide a simultaneous cross-calibration of the two detectors. At seven additional spectral bands, measurements are made using the Wollaston and the two detectors but without the multiple retarder orientations; the emphasis is in this case on photometry at weak, medium, and strong methane absorption bands (840, 619, and 892 nm, respectively) in Jupiter's atmosphere, at weak and medium ammonia absorption bands (648 and 789 nm), and at two continuum wavelengths (633 and 829 nm).
Significant additional capability to the PPR is added by the inclusion of a lithium tantalate pyroelectric detector, a space view telescope, and a reflective chopper blade that alternates at 30 Hz between scene and space views being directed to the detector; these features permit the PPR to make thermal radiometry measurements. The same positionable wheel which holds the polarimetry and photometry filters includes five filters for thermal bands centered at 17, 21, 28, 36, and 70 µm (which were selected primarily for temperature sounding of the upper atmosphere of Jupiter) and two additional positions for measuring the entire spectrum and the solar-only portion in order to acquire radiation budget information.
Early in the main Jupiter orbital phase of the mission, the PPR exhibited anomalous behavior with the filter wheel stuck at the 36-µm thermal band position. If planned recovery attempts should prove unsuccessful, the overall objectives will of course be compromised by the limitation of observations to that single channel.
Martin, T.Z., G.S. Orton, L.D. Travis, L.K. Tamppari, and I. Claypool 1995. Observation of Shoemaker-Levy impacts by the Galileo Photopolarimeter Radiometer. Science 268, 1875-1879.
Orton, G.S., J.R. Spencer, L.D. Travis, T.Z. Martin, and L.K. Tamppari 1996. Galileo Photopolarimeter-Radiometer observations of Jupiter and the Galilean Satellites. Science 274, 389-391.
Please address all inquiries about this research to Dr. Larry Travis.