Proposal_Information Title: Ultraviolet Emissions from Saturn's Rings and Ring Atmosphere Proposal_Category: GO Scientific_Category: Solar System Cycle: 5 Investigators PI_name: Doyle Hall PI_Institution: The Johns Hopkins University CoI_Name: Paul Feldman CoI_Institution: The Johns Hopkins University Contact: Y CoI_Name: Jay Holberg CoI_Institution: The University of Arizona CoI_Name: Melissa McGrath CoI_Institution: The Space Telescope Science Institute Abstract: We propose observations of Saturn's neutral ring atmosphere using the Faint Object Spectrograph to accomplish the following scientific objective: to define the density and vertical extent of the neutral gas above the densest part of the ring system. Faint Object Spectrograph observations have already proven effective in detecting neutrals in Saturn's magnetosphere, and the remarkably favorable observational geometry of Saturn's rings during Cycle 5 offers HST a rare opportunity to define the vertical extent of the neutral ring atmosphere. The proposed observations will answer fundamental questions on the nature and distribution of matter in Saturn's ring environment and are directly relevant to NASA's upcoming Cassini Mission. Questions ! Free format text (please update) Observing_Description: The geometry of the Saturn system near the time of a ring- plane crossing is ideal for observing emissions from the ring atmosphere (see Figure 2 of phase I proposal). The crosses (x) on the left show the approximate locations of proposed FOS/Red G270H observations designed to define the vertical extent of the neutral ring atmosphere. The equatorial diameter of Saturn corresponds to ~19 arcsec. We request a series of three FOS/Red G270H observations with the 4.3-aperture oriented above the densest part of the ring (1.8 Rs in the equatorial plane). The observations of Shemansky et al. (1993) provide the best basis for estimating exposure times. In the region near the OH 3085A feature, FOS spectra acquired near Saturn are contaminated by a relatively large continuum of scattered light, most likely originating from the planet and/or rings. This signal (in addition to instrument dark-counts) must be included as a background in the calculation to determine exposure times. Using the Shemansky et al. (1993) observations, we estimate this component to be sim 0.2 count s^-1 diode^-1, much larger than the nominal dark count signal of sim 0.01 count s^-1 diode^-1. From the formulae in the FOS Instrument Handbook (5.0) and the known distribution of the lines in the fluorescent OH 3085A feature, we calculate that to detect 30 Rayleighs of OH 3085A at the 5-Sigma confidence level requires a total exposure time of 45 minutes. During the same exposure time, a 17 Rayleigh line could be detected with 3-Sigma confidence. Our scientific objectives require defining the brightness, and if possible, the vertical distribution of OH 3085A emission above the ring at the 15--30 Rayleigh level, so we request 45 minute FOS G270H 4.3-Aperture exposures at each of the points marked with a cross (x) in Figure 2 (left panel). With acquisition overhead, we estimate that each exposure will require two orbits. Real_Time_Justification: The FOS/Red G270H observations need to be conducted during a ring-plane crossing period when the line-of-sight is nearly parallel to the ring plane. This geometry is optimal to define the presence and (possibly) the vertical extent of the hydroxyl (OH) component of the ring atmosphere. Because these kind of observations of Saturn's ring atmosphere in the FOS bandpass have not been performed before we do not know the signal levels to expect in our FOS spectra. Performing the ring-atmosphere observations in two parts allows for the implementation of the best observing strategy. In the case that the ring-atmosphere directly above the densest part of the ring system is an extremely tenuous gas, the OH 3085A FOS signal may be completely overwhelmed by the signal induced by off-axis scattered light from Saturn. In this case, the most sound observational strategy is to acquire some exposure time with the FOS aperture very close to the ring plane, above the densest part of the ring, but moving outward from Saturn to reduce the scattered light signal. However, if the OH 3085A signals are easily detected, then the optimum strategy will be to scan the FOS aperture above the ring plane to search for vertical stucture in the distribution of the ring-atmosphere gas. To facilitate this conditional observational strategy, we request the use of a BDUMPSTR, as well as the REUSE TARGET OFFSET capability. The observations will be performed as follows: We request that our 6 allocated orbits be used in two groups separated by several orbits. After the first group, we request that a tape dump of the FOS data (i.e., a BDUMPSTR) be performed, allowing the PI to inspect the spectra, and decide on one of two observing strategies to use for the remaining orbits allocated to this program. The of this decision should be communited ti HST via REUSE TARGET OFFSET prior to the last orbits of observation. Solar_System_Targets Target_Number: 1 Target_Name: RING-ATMOSPHERE-1 Description: OFFSET SATURN Level_1: STD = SATURN Level_2: TYPE = TORUS, LONG = 90.0, LAT = +8.413, RAD = 1.154E+05, O_LONG = 0, O_LAT = +0, O_RAD = 0, POLE_LAT = +90, POLE_LONG = 0 Flux: F(3085) = 5 +/- 3 E-17 SURF-CONT-BKG(3085) = 6 +/- 2 E-17 Comments: This target specifies a point in Saturn's ring atmosphere by using the TORUS option. The observation poisition is offset 1.9 Rs to the dusk side of the planet (along the equatorial plane) and 0.281 Rs above the sunlit ring (i.e., at latitudes north of the equator). Here 1 Rs = 60100 km. Target_Number: 2 Target_Name: RING-ATMOSPHERE-2 Description: OFFSET SATURN Level_1: STD = SATURN Level_2: TYPE = TORUS, LONG = 90.0, LAT = +11.89, RAD = 1.167E+05, O_LONG = 0, O_LAT = +0, O_RAD = 0, POLE_LAT = +90, POLE_LONG = 0 Flux: F(3085) = 5 +/- 3 E-17 SURF-CONT-BKG(3085) = 6 +/- 2 E-17 Comments: This target specifies a point in Saturn's ring atmosphere by using the TORUS option. The observation poisition is offset 1.9 Rs to the dusk side of the planet along the equatorial plane and 0.4 Rs above the sunlit ring. Target_Number: 3 Target_Name: RING-ATMOSPHERE-3 Description: OFFSET SATURN Level_1: STD = SATURN Level_2: TYPE = TORUS, LONG = 90, LAT = +17.53, RAD = 1.198E+05, O_LONG = 0, O_LAT = +0, O_RAD = 0, POLE_LAT = +90, POLE_LONG = 0 Flux: F(3085) = 5 +/- 3 E-17 SURF-CONT-BKG(3085) = 6 +/- 2 E-17 Comments: This target specifies a point in Saturn's ring atmosphere by using the TORUS option. The observation poisition is offset 1.9 Rs to the dusk side of the planet along the equatorial plane and 0.6 Rs above the sunlit ring. Target_Number: 4 Target_Name: RING-ATMOSPHERE-4 Description: OFFSET SATURN Level_1: STD = SATURN Level_2: TYPE = TORUS, LONG = 90, LAT = +7.623, RAD = 1.273E+05, O_LONG = 0, O_LAT = +0, O_RAD = 0, POLE_LAT = +90, POLE_LONG = 0 Flux: F(3085) = 5 +/- 3 E-17 SURF-CONT-BKG(3085) = 6 +/- 2 E-17 Comments: This target specifies a point in Saturn's ring atmosphere by using the TORUS option. The observation poisition is offset 2.1 Rs to the dusk side of the planet along the equatorial plane and 0.28 Rs above the sunlit ring. Target_Number: 5 Target_Name: RING-ATMOSPHERE-5 Description: OFFSET SATURN Level_1: STD = SATURN Level_2: TYPE = TORUS, LONG = 90, LAT = +6.968, RAD = 1.392E+05, O_LONG = 0, O_LAT = +0, O_RAD = 0, POLE_LAT = +90, POLE_LONG = 0 Flux: F(3085) = 5 +/- 3 E-17 SURF-CONT-BKG(3085) = 6 +/- 2 E-17 Comments: This target specifies a point in Saturn's ring atmosphere by using the TORUS option. The observation poisition is offset 2.3 Rs to the dusk side of the planet along the equatorial plane and 0.28 Rs above the sunlit ring. Visits Visit_Number: 1 Visit_Requirements: SEQ 1-2 WITHIN 75M Exposure_Number: 10 Target_Name: DARK Config: S/C Opmode: DATA Aperture: NONE Number_of_Iterations: 1 Time_Per_Exposure: 20M Special_Requirements: RT ANALYSIS Comments: This "exposure" forces a dump of the currently active science tape recorder. This dump is needed in order to be sure that all of the science data from visit 2 will fit on one tape recorder, so that that data can be dumped at the end of visit 2. Visit_Number: 2 Visit_Requirements: BETWEEN 9-AUG-95:11:00 AND 9-AUG-95:12:00 Visit_Comments: Visit should be completed in four consecutive orbits. Exposure_Number: 11 Target_Name: RING-ATMOSPHERE-1 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 820S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 11-12 NON-INT Exposure_Number: 12 Target_Name: RING-ATMOSPHERE-1 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 830S Special_Requirements: MIN DUR 99.9% MAX DUR 100% END ORBIT Exposure_Number: 13 Target_Name: RING-ATMOSPHERE-1 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 975S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 13-14 NON-INT Exposure_Number: 14 Target_Name: RING-ATMOSPHERE-1 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 980S Special_Requirements: MIN DUR 99.9% MAX DUR 100% Exposure_Number: 21 Target_Name: RING-ATMOSPHERE-2 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 829S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 21-22 NON-INT Exposure_Number: 22 Target_Name: RING-ATMOSPHERE-2 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 834S Special_Requirements: MIN DUR 99.9% MAX DUR 100% Exposure_Number: 23 Target_Name: RING-ATMOSPHERE-3 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 1015S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 23-24 NON-INT Exposure_Number: 24 Target_Name: RING-ATMOSPHERE-3 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 1020S Special_Requirements: MIN DUR 99.9% MAX DUR 100% Comments: Exposures 11-24 acquire 4 orbits worth of FOS/RD G270H spectra of the Saturn ring atmosphere, offset 1.9 Rs on the dusk side of the planet (along the equatorial plane) and vertically spaced above the ring-plane. These orbits should be several orbits before Visit 3 of this program is performed. This allows for the spectra from Visit 2 to be read-down (via a BDUMPSTR). During the inter- val between Visits 2 and 3, the PI will inspect the data. Based on the spectral content, one of two observing strategies will be followed. If the spectra show a strong OH 3085A feature, well above the scattered light background, then in Visit 3 the Targets RING-ATMOSPHERE-2 and RING-ATMOSPHERE-3 will be observed again. However, if the spectra show very little OH 3085A emission above the background, then the exposures during Visit 3 will be on Targets RING-ATMOSPHERE-4 and/or RING-ATMOSPHERE-5. To facilitate this conditional observing strategy for Visit 3, we will use the RE-USE TARGET OFFSET capability. Exposure_Number: 30 Target_Name: DARK Config: S/C Opmode: DATA Aperture: NONE Number_of_Iterations: 1 Time_Per_Exposure: 20M Special_Requirements: RT ANALYSIS Comments: This "exposure" forces a dump of the currently active science tape recorder, so that the data from this visit can be analysed before the next visit. Visit_Number: 3 Visit_Requirements: BETWEEN 10-AUG-95:09:30 AND 10-AUG-95:10:30 Visit_Comments: Visit should be completed in two consecutive orbits. Exposure_Number: 25 Target_Name: RING-ATMOSPHERE-2 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 1015S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 25-26 NON-INT Exposure_Number: 26 Target_Name: RING-ATMOSPHERE-2 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 900S Special_Requirements: MIN DUR 99.9% MAX DUR 100% Exposure_Number: 27 Target_Name: RING-ATMOSPHERE-3 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 975S Special_Requirements: MIN DUR 99.9% MAX DUR 100% SEQ 27-28 NON-INT Exposure_Number: 28 Target_Name: RING-ATMOSPHERE-3 Config: FOS/RD Opmode: ACCUM Aperture: 4.3 Sp_Element: G270H Wavelength: 2223-3278 Number_of_Iterations: 1 Time_Per_Exposure: 920S Special_Requirements: MIN DUR 99.9% MAX DUR 100% Comments: Exposures 25-28 are currently specified for Targets RING-ATMOSPHERE-2 and 3. However, as explained in the comment for Exposures 11-24, this should be changed to RING-ATMOSPHERE-4 and/or RING-ATMOSPHERE-5 if the OH 3085A signal level detected in Visit Number 2 is inadequate. To facilitate this conditional observing strategy, we use the RE-USE TARGET OFFSET command. In the latter case, it is very likely that both orbtis will be used to observe Target RING-ATMOSPHERE-5, because the scattered light should be lowest in this location. However, an observation at Target RING-ATMOSPHERE-4 might be warranted for moderate scattered light levels. If scattered light levels are large, then the specific combination of RING-ATMOSPHERE-4 vs. RING-ATMOSPHERE-5 to be obtained will be determined during the break between Visits 2 and 3. Data_Distribution Medium: 8MM ! 8MM or 6250BPI or 1600BPI Blocking_Factor: 10 ! 10 or 1 Ship_To: PI_Address ! STSCI or PI_Address or Ship_Via: UPS ! UPS (2-day) or OVERNIGHT