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The ultraviolet spectrometer (see figure) onboard the Galileo Orbiter will study properties of Jupiter's high atmosphere; the Galilean satellites Io, Europa, Ganymede, and Callisto; and the doughnut-shaped cloud of ionized plasma that surrounds Io.
Observations in the ultraviolet range of the spectrum yield composition information that cannot be obtained any other way.
The UVS will be mounted on the movable scan platform of the Orbiter, its aperture aligned in the same direction as the other remote-sensing instruments (the imaging cameras, photopolarimeter radiometer, and near infrared mass spectrometer). Their data, taken together, will provide a comprehensive picture of many aspects of the Jovian system.
The UVS instrument's wavelength range of 115 to 430 nanometers (1150 to 4300 Angstroms) overlaps and extends the range available on the Voyager spacecraft and heightens the probability of discovering new ultraviolet phenomena.
Voyager measured a northern polar aurora on Jupiter that extended 30,000 kilometers (18,000 miles). These auroras occur when electrons and ions spiral into the atmosphere along the planet's magnetic field lines that also intercept Io. The impact causes the dominant gases in the atmosphere--atomic hydrogen, molecular hydrogen, and helium--to light up. The intensity of the ultraviolet emissions is a measure of the vertical distribution of the gases in the atmosphere.
Emissions over the planet as a whole occur because of sunlight and electron impacts. These "airglow" emissions can tell much about the structure of the upper atmosphere.
Galileo's UVS will look for complex molecules in Jupiter's atmosphere. Such complex molecules are destroyed by solar ultraviolet light, and the spectrum of scattered ultraviolet light is imprinted with a distinctive "fingerprint" of the molecule that was destroyed. Acetylene has been identified on Jupiter in this manner. It is important to identify such complex molecules to understand if complex hydrocarbons might be present on Jupiter. These hydrocarbons are the "building blocks" for life on Earth, and confirmation of their presence on another body in the solar system would have far-reaching implications.
During the Orbiter's 20-month, 12-orbit tour of the Jupiter system, it will map the Galilean satellites extensively. The UVS will look for evidence of atmospheres--an indication that volatiles are escaping from the moons and that their compositions are still evolving. The UVS will also search reflections from the satellite surfaces for evidence of ammonia, ozone, and sulfur dioxide. The UVS can measure hydrogen, oxygen, nitrogen, carbon, sulfur, calcium, lithium, magnesium, molecular nitrogen, nitric oxide, hydroxyl, carbon monoxide, cyanogen, and sulfur dioxide, as well as ions of molecular nitrogen, carbon monoxide, carbon dioxide, and magnesium.
Volcanic eruptions on Io are believed to be the source of a large doughnut-shaped cloud of ionized sulfur and oxygen that encircles Jupiter along the orbital path of Io. Temperatures of the sulfur and oxygen ions in this plasma torus can be more than ten times the temperatures at the surface of the Sun. These ultraviolet observations, along with the direct measurements of the ions and electrons in the Io Torus by the fields and particles instruments and observations of Io's volcanic activity by the imaging system, will provide a comprehensive picture of Io's evolution and relationship with Jupiter's magnetic field.
The ultraviolet spectrometer, developed and built by the Laboratory for Atmospheric and Space Physics at the University of Colorado, consists of a 250-mm Cassegrain telescope, a 125-mm (5-inch) monochromator, three photomultipliers, and control logic within an onboard computer. The instrument weighs about 4.2 kilograms (9.2 lbs) and uses 4.5 watts of power. It scans one of three channels in the wavelength range of 115 to 430 nanometers in 4-1/3 seconds, and its counters are read and data are transmitted to Earth at 1000 bps.
Principal investigator is C. W. Hord of the University of Colorado. Five coinvestigators are members of the UVS team.
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