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Hubble Space Telescope Observation Plans


WFPC-2 Observations of Saturn's Ring Atmosphere

Approximately six hours after Bosh's team's May observations, the WFPC2 team, led by John Trauger of the Jet Propulsion Laboratory (JPL) and John Clarke of the University of Michigan, will attempt to measure the vertical extent of the Saturninan rings' extended atmosphere. Using WFPC2, Trauger and Clarke's team will discriminate between the elevated dust and hydroxyl (OH) emission.


In Voyager images of Saturn dark bands were occasionally seen in the rings, resembling dark spokes in a wheel. These may be produced when small clouds of dust are knocked loose from Saturn's rings, perhaps by the impact of a meteorite, extending radially from Saturn and absorbing some of the reflected sunlight from the rings. These appear most commonly as the rings leave Saturn's shadow. In addition, there is evidence for water molecules released from the icy covering of the ring particles, from Voyager ultraviolet spectra of hydrogen atom emission surrounding Saturn and from HST / FOS spectra of the OH (hydroxyl) molecule just beyond the extent of the rings. H and OH are the dissociation products of water molecules from the rings, and they are believed to be one of the principal sources of neutral material and plasma in Saturn's magnetosphere.

OH emissions should be detectable from a spectral emission near 3100 Angstroms and extend vertically above and below the ring plane. To see the faint emission above the bright reflected sunlight from the rings, Saturn's rings must be close to edge-on so that they appear dark. When the rings are nearly edge-on it may also be possible to measure the vertical extent of any dust or OH gas above and below the rings. Images will be taken through red, blue, and near-UV filters to try to discriminate the different sources of light. Scattered light from Saturn will have a nearly solar color, while suspended dust should appear somewhat bluer and OH emission should appear only in the near-UV filter.

Using the Planetary Camera (PC) on the rings, Saturn will be deliberately shifted out of the PC aperture. The team will experiment with filter combinations and exposure times during the May event in order to prepare for, and optimize, data collection for the August event. The August observations will be made roughly six hours before the ring-plane crossing and are coordinated around Nicholson's observations. The OH images will also be compared with the FOS spectra taken under Hall's program to deduce the amount and distribution of OH around the rings.

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