6762( 18) - 07/09/96 10:30 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6762 Version: 18 Check-in Date: 09-Jul-1996 10:13:29 1.Proposal Title: Ultraviolet Spectra of Uranian Satellites ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: TED ROUSH NASA AMES RESEARCH CTR. CoI: Dale Cruikshank NASA Ames Research Center N CoI: Keith Noll Space Telescope Science Institute N CoI: Yvonne Pendleton NASA Ames Research Center N ------------------------------------------------------------------------------------ 6. Abstract The ultraviolet reflectance spectra of the icy satellites of Uranus are largely unknown. We propose to use the Hubble Space Telescope Faint Object Spectrograph in order to obtain the first high S/N UV spectra of Ariel, Titania, and Oberon. Because of our innovative targeting approach, we have also been able to include Umbriel in our observational plans. These satellites sample almost the full range of UV albedos and UV/VIS colors exhibited by the large Uranian satellites. The spectral resolution and range will overlap with earth- based telescopic and spacecraft observations of these objects allowing for comparisons of the UV data with existing visual and near- infrared spectra of these objects. These comparisons will ultimately provide greater constraints on the relatively low albedo spectrally neutral non-ice component on the Uranian satellites. The existance of UV spectral features due to species such as O_3, H_2O_2 or carbon-rich macromolecules (e.g. polycyclic aromatic hydrocarbons) can provide evidence for modification of the surfaces via plasma or meteoritic bombardment, alteration by high-energy ultraviolet radiation, or accretion of particles from nearby sources such as planetary rings or dust bands. ------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 2] Observations Description ------------------------ We are proposing to obtain the first ultraviolet spectra of four large Uranian satellites. The HST is the only instrument capable of measuring the ultraviolet spectra of Ariel, Umbriel, Titania, and Oberon. The International Ultraviolet Explorer did not measure these satellites and the Voyager 2 data consist of a single broad-band data point. The HST FOS can produce high S/N spectra of these objects. We intend to use the FOS/RD detector because of its higher sensitivity. All proposed targets have V magnitudes less than the upper limit for obserations with FOS MIRROR. Our intial scan sequence is Z+E2 (FOS Instrument Handbook v6.0, Table 2-2) which yields a final point accuracy of 0.08 arc sec. Good centering on the aperture is required to avoid loss of signal at some wavelengths. Because the ephemeris positions of the Uranian satellites are well known, after our initial guide star and target acquisition we intend to use the relative positions of the satellites to acquire each subsequent satellite. This strategy has allowed us to include Umbriel for observations without requiring additional spacecraft orbits. We have calculated the anticipated signal-to-ratio (SNR) for each satellite in both grating settings based upon our planned observing sequence. In our calculations we have used the older more conservative visual magnitudes given by Cruikshank (1980) and Nicholson and Jones (1980), colors provided in the 1995 Astronomical Almanac, and for the U fluxes of Ariel and Umbriel, we assumed the ratio of B fluxes to Titania was the same at U. We calculated the number of counts per second per diode (N) using the equation and Tables 1-7 and 1-8 in the FOS handbook. These values were used to calculate the SNR using the allocated observing time (t) (e.g. SNR = (t*N)^0.5). The anticipated SNR is listed for the two grating settings for each object in Table 1. Table 1. Anticipated Signal-to-Noise Ratio Satellite B (G400H) U (G270H) --------------------------- ---------------------------------- Ariel 63 61 Umbriel 59 47 Titania 72 69 Oberon 65 66 ------------------------------------------------------------- It is worth noting that even though a portion of the spectrum sampled by G400H grating observations can be obtained from ground-based telescopes, we believe it is essential to obtain high quality spectra in the interval from 3300-4000 Angstrom\\\\\\\\\\\\\\\\ in order to allow for good overlap with planned CCD observations (see below). Also previous FOS spectra of Jovian and Saturnian satellites exhibit a flattening of the spectrum near the long wavelength limit of the FOS data near 3250 Angstrom. The reality of this feature can be more thoroughly assessed by obtaining the overlapping G400H spectrum. Real Time Justification ----------------------- We request observations as close as possible to Uranus' opposition (25 July 1996 or 30 July 1997) in order to benefit from maximum brightness of the satellites. The Uranian satellites exhibhit a very strong surge in brightness at opposition (solar phase angle, g <<1 deg). The optimum window in 1996 is 15 July - 3 August (g <= 0.5 deg) but the window 5 July to 13 August (g <= 1.0 deg) is acceptable. The optimum window in 1997 is 20 July - 8 August (g <= 0.5 deg) but the window 10 July to 18 August (g <= 1.0 deg) is acceptable. In order to provide additional leverage for compositional information contained in the HST UV observations, we intend to undertake a separate observing program to obtain visual spectra (0.4-1 Mu) of the Uranian satellites using CCD's from Earth-based telescopes. This complementary spectral information will more readily provide the ability to absolutly compare the HST and ground-based observations in both the visual and near-infrared wavelength regions. New observational studies of the near-infrared spectra (1-2.5 Mum) of the Uranian satellites are already in progress. New results will be presented by Tryka Etal\\\\\\\\\\\\\\\\ at the 1995 Division of Planetary Sciences meeting. Additional observations have just been completed (in collaboration with T.C. Owen, T.R. Geballe, and C. de Bergh) at the UKIRT in Hawaii. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: ARIEL ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE ARIEL ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=URANUS | STD=ARIEL | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 14.4+/-0.2 | B-V = 0.65 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: TITANIA ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE TITANIA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=URANUS | STD=TITANIA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 14.0+/-0.06 | B-V = 0.70 | U-B = 0.28 | | ------------------------------------------------------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 5] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 4 | TARGET NAME: OBERON ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE OBERON ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=URANUS | STD=OBERON | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 14.3+/-0.03 | B-V = 0.68 | U-V = 0.20 | | ------------------------------------------------------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 6] Visit: 01 Visit Requirements: BETWEEN 05-Jul-1996 AND 13-Aug-1996 SCHED 90% 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 OBERON FOS/RD ACQ/BIN 4.3 MIRROR 1 1.32 S ONBOARD ACQ FOR 20 ARY ------------------------------------------------------------------------------------------------------------------------------------ 15 OBERON FOS/RD ACQ/PEA 0.3 G400H SCAN-STEP-X=0.11,SCAN-STEP-Y=0.11 1 0.30 S ONBOARD ACQ FOR 20-35 K ,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ 20 OBERON FOS/RD ACCUM 1.0 G400H 1 720 S ------------------------------------------------------------------------------------------------------------------------------------ 30 OBERON FOS/RD ACCUM 1.0 G270H 1 1350 S EXPAND MIN DUR Comments: USE SATELLITE EPHEMERIS FIND TITANIA ------------------------------------------------------------------------------------------------------------------------------------ 35 TITANIA FOS/RD ACQ/PEA 0.3 G270H SCAN-STEP-X=0.11,SCAN-STEP-Y=0.11 1 2.05 S ONBOARD ACQ FOR 40-55 K ,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ 40 TITANIA FOS/RD ACCUM 1.0 G270H 1 2000 S EXPAND MIN DUR ------------------------------------------------------------------------------------------------------------------------------------ 50 TITANIA FOS/RD ACCUM 1.0 G400H 1 720 S Comments: USE SATELLITE EPHEMERIS FIND ARIEL ------------------------------------------------------------------------------------------------------------------------------------ 55 ARIEL FOS/RD ACQ/PEA 0.3 G270H SCAN-STEP-X=0.11,SCAN-STEP-Y=0.11 1 2.91 S ONBOARD ACQ FOR 60-70 K ,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ 60 ARIEL FOS/RD ACCUM 1.0 G270H 1 1750 S EXPAND MIN DUR ------------------------------------------------------------------------------------------------------------------------------------ 70 ARIEL FOS/RD ACCUM 1.0 G400H 1 720 S EXPAND MIN DUR Comments: USE SATELLITE EPHEMERIS FIND UMBRIEL ------------------------------------------------------------------------------------------------------------------------------------ 6762( 18) - 07/09/96 10:30 - [ 7] Summary Form for Proposal 6762 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-X=0.11 SCAN-STEP-Y=0.11 SEARCH-SIZE-X=5 SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 05-Jul-1996 AND 13-Aug-1996 SCHED 90% ONBOARD ACQ FOR 20 ONBOARD ACQ FOR 20-35 EXPAND MIN DUR ONBOARD ACQ FOR 40-55 ONBOARD ACQ FOR 60-70 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G400H G270H ------------------------------------------------------------------------------------------------------------------------------------ Target Names OBERON TITANIA ARIEL ------------------------------------------------------------------------------------------------------------------------------------