6715( 4) - 09/03/96 13:31 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6715 Version: 4 Check-in Date: 03-Sep-1996 13:24:14 1.Proposal Title: Ultraviolet Spectra of Saturn's Satellites, Ion Modification of Surface Ice ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: KEITH NOLL SPACE TELESCOPE SCIENCE INSTITUTE CoI: Dale Cruikshank NASA Ames Research Center N CoI: Yvonne Pendleton NASA Ames Research Center N CoI: Ted Roush San Francisco State University N CoI: Robert Johnson University of Virginia N ------------------------------------------------------------------------------------ 6. Abstract Saturn's medium-sized icy satellites display a diverse range of surface alteration caused by one or more external processes. We will investigate the role of magnetospheric charged particle interactions with the surfaces of these satellites by searching for spectral signatures of ion- irradiated ice. To do this, we will obtain the first high S/N, ultraviolet wavelength spectra of Enceladus, Tethys, Dione, and the leading hemisphere of Rhea to wavelengths as short as 2200 Angstrom . We will search in particular for the strong Hartley band of ozone at 2600 Angstrom ; O_3 is formed by the impact of O^+ ions in ice. We have recently identified this band in Ganymede, which resides in a plasma environment similar to that of Saturn's satellites. A spectrum of Rhea's trailing hemisphere obtained by us also shows evidence of possible O_3 absorption that can be confirmed by observing the spectrum of the opposite hemisphere. The range of hemispheric albedo differences from Enceladus to Rhea and the differences in ion density and energy will allow us to determine the relative importance of plasma bombardment in the long-term alteration of these satellite surfaces. In addition to O_3, other possible UV absorbers incude peroxide, H_2O_2, the expected photolysis product of O_3, SO_2, observed in Europa and apparently the preferred reaction product when S is present, and simple hydrocarbons which show distinct UV structure. No other telescope or instrument is capable of these measurements. ------------------------------------------------------------------------------------ Observations Description ------------------------ We are proposing to obtain the first ultraviolet spectra of Saturn's mid-sized icy satellites. The HST is the only instrument capable of measuring the ultraviolet spectra of Rhea, Dione, Tethys, and Enceladus. The IUE did measure several of these satellites, but never produced an albedo spectrum for any. The existing ratio spectra of trailing/leading hemispheres have low S/N (~1-3) and extend only to 260 nm. The HST FOS is well able to produce high S/N spectra for these objects as can be seen in Figure 2, a 190 second integration in the G270H grating on Rhea. For each satellite hemisphere observed we will execute a 2 orbit sequence. The first orbit is required for accurate target acquisition. In the second orbit we will obtain our science exposures in the G270H and G400H gratings. The sequence has been tested using RPS2. We plan to use the FOS/RD detector because of its higher sensitivity. All four proposed targets have a V magnitude greater than the limit for observations with the FOS MIRROR, ruling out use of ACQ/BINARY. It is now possible to use the FOS ACQ/PEAK onboard acquisition for moving target observations. We will use the scan sequence A+B1+C1+F1 (FOS Instrument Handbook v6.0, Table 2-2) to obtain a final pointing accuracy of 0.12 arcsec. Good centering in the aperture is required to avoid loss of signal at some wavelengths. Guide star acquisition and the ACQ/PEAK sequence uses all but 4 minutes of the first orbit. In the second orbit we will obtain two science exposures, one each in the G270H and the G400H gratings. We plan integrations of approximately 200 seconds in the G400H and 1600 seconds in the G270H gratings. A summary of expected S/N per diode is listed in Table 2 below. The count rates in Table 2 were calculated using Table 1-9 in the FOS handbook, the mean visible magnitudes of the satellites, and a correction factor, f, which is the ratio of the UV/visible albedo measured for these objects. This calculation yields good agreement with the observed count rate for Rhea's trailing hemisphere observed by us in cycle 4. It is very important to obtain high S/N because of the very complex solar spectrum that must be removed to create an albedo spectrum. All of the high frequency structure in figure 2 can be attributed to slight mismatches between strong lines in the solar spectrum and in the Rhea spectrum. It is worth noting that even though a portion of the G400H grating can be done from ground-based telescopes, we believe that it is essential to obtain high quality spectra in the interval from 330-400 nm in order to allow for good overlap with planned CCD observations. Several FOS spectra that we have obtained in past programs for both Saturn and Jupiter satellites show a flattening of the spectrum near the long wavelength limit of the FOS at 325 nm. The reality of this feature can be confirmed by obtaining the overlapping G400H spectrum. In addition, on Ganymede, the \\oz\\ absorption feature appears to extend beyond the end of the G270H range. It is critical to obtain a spectral range broad enough that a local continuum can be determined. Null .1in =1.5em =10000 vbox halign # & # & # & # & # multispan5 Table 2: Signal to Noise for Saturn Satellites noalign 12pt height 1pt 1pt height 1pt 8pt satellite & V_0 & f & G270H S/N diode^-1 & G400H S/N diode^-1 & (average) & (UV/vis) & t=1600 sec & t=200 sec & & & Lambda=265 nm & Lambda=360nm noalign 8pt height 1pt 8pt noalign Enceladus & 11.7 & 1.0? & 91 & 79 Tethys & 10.2 & 0.76 & 159 & 158 Dione & 10.4 & 0.68 & 137 & 144 Rhea & 9.7 & 0.30,0.6 & 126 & 154 noalign noalign 8pt height 1pt The quantity f in Table 2 was calculated using an average Band \\# 1 albedo (~ 260 nm) from Nelson and Lane (1987) for Tethys, Rhea, and Dione. The UV albedo of Enceladus has not been measured and we assume a value of f=1. For the G400H we assume the albedo is the same as in the V band, except for Rhea, where we use f=0.6. Visible-wavelength albedos were taken from Roush et al. (1995). Real Time Justification ----------------------- We will request observations near eastern and western elongations. This is only mildly time critical since there are many opportunities and the windows are several orbits each. D. Cruikshank is currently involved in a program of ground - based infrared spectroscopy of Saturn's satellites that will result in greatly improved spectra from 1-2.5 \\um . We also plan to propose new observations of the visible and near-IR spectra of the Saturn satellites using modern CCDs. We will obtain significantly higher resolution and S/N spectra from 0.4-1.0 \\um\\ that will overlap at the shorter wavelengths with the FOS spectra we will obtain. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 1] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: RHEA-L ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE RHEA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=SATURN | STD=RHEA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | OLG OF RHEA BETWEEN 80 100 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 9.7+/-0.1 | B-V = 0.78+/-0.2 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: DIONE-T ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE DIONE ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=SATURN | STD=DIONE | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | OLG OF DIONE BETWEEN 255 285 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 10.4+/-0.1 | B-V = 0.71+/-0.2 | | ------------------------------------------------------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 2] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: DIONE-L ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE DIONE ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=SATURN | STD=DIONE | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | OLG OF DIONE BETWEEN 75 105 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 10.4+/-0.1 | B-V = 0.71+/-0.2 | | ------------------------------------------------------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 3] Visit: 01 Visit Requirements: PCS MODE Fine On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 RHEA-L FOS/RD ACQ/PEA 4.3 G570H SEARCH-SIZE-X = 1,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 12 K 3,SCAN-STEP-Y = 1.23 ------------------------------------------------------------------------------------------------------------------------------------ 12 RHEA-L FOS/RD ACQ/PEA 1.0 G570H SEARCH-SIZE-X = 6,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 13 K 2,SCAN-STEP-X = 0.61,SCAN-STEP-Y = 0.61 ------------------------------------------------------------------------------------------------------------------------------------ 13 RHEA-L FOS/RD ACQ/PEA 0.5 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 14 K 3,SCAN-STEP-X = 0.29,SCAN-STEP-Y = 0.29 ------------------------------------------------------------------------------------------------------------------------------------ 14 RHEA-L FOS/RD ACQ/PEA 0.3 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 15-17 K 3,SCAN-STEP-X = 0.17,SCAN-STEP-Y = 0.17 ------------------------------------------------------------------------------------------------------------------------------------ 15 RHEA-L FOS/RD ACCUM 1.0 G400H 1 200 S ------------------------------------------------------------------------------------------------------------------------------------ 17 RHEA-L FOS/RD ACCUM 1.0 G270H 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 4] Visit: 02 Visit Requirements: PCS MODE Fine On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 DIONE-T FOS/RD ACQ/PEA 4.3 G570H SEARCH-SIZE-X = 1,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 22 K 3,SCAN-STEP-Y = 1.23 ------------------------------------------------------------------------------------------------------------------------------------ 22 DIONE-T FOS/RD ACQ/PEA 1.0 G570H SEARCH-SIZE-X = 6,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 23 K 2,SCAN-STEP-X = 0.61,SCAN-STEP-Y = 0.61 ------------------------------------------------------------------------------------------------------------------------------------ 23 DIONE-T FOS/RD ACQ/PEA 0.5 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 24 K 3,SCAN-STEP-X = 0.29,SCAN-STEP-Y = 0.29 ------------------------------------------------------------------------------------------------------------------------------------ 24 DIONE-T FOS/RD ACQ/PEA 0.3 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 25-27 K 3,SCAN-STEP-X = 0.17,SCAN-STEP-Y = 0.17 ------------------------------------------------------------------------------------------------------------------------------------ 25 DIONE-T FOS/RD ACCUM 1.0 G400H 1 200 S ------------------------------------------------------------------------------------------------------------------------------------ 27 DIONE-T FOS/RD ACCUM 1.0 G270H 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 5] Visit: 03 Visit Requirements: PCS MODE Fine On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 DIONE-L FOS/RD ACQ/PEA 4.3 G570H SEARCH-SIZE-X = 1,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 32 K 3,SCAN-STEP-Y = 1.23 ------------------------------------------------------------------------------------------------------------------------------------ 32 DIONE-L FOS/RD ACQ/PEA 1.0 G570H SEARCH-SIZE-X = 6,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 33 K 2,SCAN-STEP-X = 0.61,SCAN-STEP-Y = 0.61 ------------------------------------------------------------------------------------------------------------------------------------ 33 DIONE-L FOS/RD ACQ/PEA 0.5 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 34 K 3,SCAN-STEP-X = 0.29,SCAN-STEP-Y = 0.29 ------------------------------------------------------------------------------------------------------------------------------------ 34 DIONE-L FOS/RD ACQ/PEA 0.3 G570H SEARCH-SIZE-X = 3,SEARCH-SIZE-Y = 1 1 S ONBOARD ACQ FOR 35-37 K 3,SCAN-STEP-X = 0.17,SCAN-STEP-Y = 0.17 ------------------------------------------------------------------------------------------------------------------------------------ 35 DIONE-L FOS/RD ACCUM 1.0 G400H 1 200 S ------------------------------------------------------------------------------------------------------------------------------------ 37 DIONE-L FOS/RD ACCUM 1.0 G270H 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 6715( 4) - 09/03/96 13:31 - [ 6] Summary Form for Proposal 6715 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE-X = 1 SEARCH-SIZE-Y = 3 SCAN-STEP-Y = 1.23 SCAN-STEP-Y = 0.61 SCAN-STEP-Y = 0.29 SCAN-STEP-Y = 0.17 SCAN-STEP-X = 0.61 SCAN-STEP-X = 0.29 SCAN-STEP-X = 0.17 SEARCH-SIZE-Y = 2 SEARCH-SIZE-X = 3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE Fine ONBOARD ACQ FOR 12 ONBOARD ACQ FOR 13 ONBOARD ACQ FOR 14 ONBOARD ACQ FOR 15-17 ONBOARD ACQ FOR 22 ONBOARD ACQ FOR 23 ONBOARD ACQ FOR 24 ONBOARD ACQ FOR 25-27 ONBOARD ACQ FOR 32 ONBOARD ACQ FOR 33 ONBOARD ACQ FOR 34 ONBOARD ACQ FOR 35-37 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G570H G400H G270H ------------------------------------------------------------------------------------------------------------------------------------ Target Names RHEA-L DIONE-T DIONE-L ------------------------------------------------------------------------------------------------------------------------------------