9112( 3) - 04/17/01 16:46 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9112 Version: 3 Check-in Date: 17-Apr-2001 16:45:48 1.Proposal Title: Composition of Saturn's rings, and its variation with location ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dale Cruikshank NASA Ames Research Center CoI: Dr. Jeff Cuzzi Ames Research Center N CoI: Dr. Richard French Wellesley College N CoI: Dr. Ted Roush Ames Research Center N CoI: Dr. Keith Noll Space Telescope Science Institute N ------------------------------------------------------------------------------------ 5. Abstract Saturn's rings have been a playground for dynamicists during the two decades since Voyager, and a variety of fascinating processes and structures have been identified and studied. One surprise has been that the ring system seems to be evolving dynamically and compositionally on geologically short timescales, and might be much younger than the solar system. However, there has been very little progress in understanding the composition of the rings during this time. While water ice is known to be the dominant constituent, the reddish color of the rings requires there to be other material there. Furthermore, studies of Voyager and HST-WFPC2 color images indicate that the ring color (and presumably therefore composition) varies dramatically from region to region, and has complex correlations with ring structure. Unfortunately, these multicolor observations are incapable of identifying specific constituents spectrally. The only existing moderate spectral resolution observations, on the other hand, have been ring-averages and smear all the diverse spectral contributions together. With STIS, we propose to separate out the known spatial-spectral variability with sufficient spectral resolution to make identifications of specific constituents. This will allow us to truly study the processes responsible for compositional evolution. It will also provide an excellent preparation and foundation for planning Cassini observations of the rings. ------------------------------------------------------------------------------------ 9112( 3) - 04/17/01 16:46 - [ 2] Observations Description ------------------------ We will obtain a series of spectra of the rings of Saturn with STIS, using three gratings to cover the spectral region 2000 - 10, 000 Angstrom. It is desired to place the 52 by 2 arcsec slit across the major axis of the rings (which have a subtended diameter of 50 arcsec), as closely as is feasible. The slit will thus simultaneously cross both ansae and the planet, giving us 90-180 resolution elements across each ring ansa, at 700-350km each respectively, depending on wavelength. A rotational tolerance of 5 degrees is acceptable. We will set on Saturn by first centering on a nearby satellite and making the offset to the center of the planet; available satellite ephemerides make this offset reliable to about 0.1 arcsec. Exposure times for three gratings, G230LB, G430L, and G750L, were calculated with the STIS ETC, using fluxes of the rings (per square arcsec) calculated from I/F values for each of the ring regions (A, B, and C) obtained from our most recent WFPC2 images, and corrected slightly to the expected opening angle of 24 degrees. At the longer wavelengths, the surface brightness of Saturn is about 20\ than that of the brightest ring (B), and the planet is therefore the limiting source in terms of saturation of the CCD. For exposures in G750L and G430L we will use exposures that bring Saturn's spectrum close to the full-well capacity of the CCD. In grating G230LB we use an exposure of 1200 sec, which is a practical maximum for a moving target. centerline Table 1. Predicted S/N for A, B, and C rings with three STIS gratings centerline\begintabularlcllll Grating& Exposure & Lambda (Angstrom)& Ring A & Ring B & Ring C, G750L & 45 sec & 5300--8000 & 800 & 900 & 300, & & 8000--10000& 550 & 600 & 200, , G430L & 70 sec & 3000--4000 & 200--400& 250-- 500 & 50--160, & & 4000--5500 & 600--800& 600--1000& 260--320, , G230LB &1200 sec & 2200--2600 & 70 & 80 & 30, & & 2600--3000 & 140--420& 150--500 & 50--200, endtabular Because of the symmetry in the ring system, and because each exposure captures both the east and west ansae, we can coadd the spectra spatially to increase S/N. With two independent UV exposures we can thus double the S/N for an average of the ansae. Furthermore, some degree of spectral averaging is feasible since the expected spectral features are ~ 10^2 Angstrom wide. With the time required for target acquisition and other overhead liens, in two orbits we can make two exposures of 1200 sec with grating 230LB, and four each of the other exposures with gratings G750L (45 sec) and G430L (70 sec), plus an image with STIS that will show the orientation of the slit relative to the rings. The time and overhead budget was determined by Co-I K. Noll using an RPS2 simulation. Real Time Justification ----------------------- N/A N/A Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ 9112( 3) - 04/17/01 16:46 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: SATELLITE-DIONE ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Dione for offset to Saturn ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=SATURN | STD=DIONE | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 10.4 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: SATURN-RING ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: RING STIS slit along major axis of rings, centered on Saturn. ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=SATURN | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | F(5500)=5.23e-12 | F(3000)=5.88e-13 | | Ephemeris Uncertainty: 0.01 S | Acquisition Uncertainty: 0.1 " | ------------------------------------------------------------------------------------------------------------------------------------ 9112( 3) - 04/17/01 16:46 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 127D TO 131D ORIENT 307D TO 311D SCHED 100% ON HOLD On Hold Comments: An issue in terms of schedulability of our observations near the opposition of Saturn (which occurs on 2001 December 3), is the servicing mission by the Space Shuttle, scheduled for approximately that time. If the servicing mission schedule interferes with our optimum observing window, we wish to have the option to open the range of acceptable slit orientation angles, so that we can obtain the data in Cycle 10. It is highly desirable to schedule French's WFPC2 observations (proposal 8802) contiguous with the STIS observations in this program. The series of images obtained at several wavelengths in that program will greatly help in the photometric calibration of our spectroscopic results. Dr. French has expressed his similar desire to have the observations done contiguously, or as nearly as possible. Additional Comments: This program seeks to obtain the spectrum of Saturn's rings from 2000 to 10, 000 A, with the maximum spatial resolution that can be obtained with the STIS. The spatial resolution and the evaluation of scattering of Saturn light are best done if the STIS slit is exactly along the major axis of the rings. The rings have a major axis length of about 44 arcsec, and the STIS slit is 52 arcsec in length, providing an excellent match. If the slit is oriented 2-3 degrees off the major axis, the science loss will be minimal, but if it is oriented more than about 14 degrees off, we lose a significant amount of blank sky between the inner edge of Ring C and the limb of Saturn, as well as spatial resolution in the ring system. We are specifying a range of slit orientation angles of +/- 2 degrees in order to achieve some degree of schedulabilty, but it should be understood that our science quality will be best from data taken with the slit as close to the major axis o f the rings as possible. An issue in terms of schedulability of our observations near the opposition of Saturn (which occurs on 2001 December 3), is the servicing mission by the Space Shuttle, scheduled for approximately that time. If the servicing mission schedule interferes with our optimum observing window, we wish to have the option to open the range of acceptable slit orientation angles, so that we can obtain the data in Cycle 10. It is highly desirable to schedule French's WFPC2 observations (proposal 8802) contiguous with the STIS observations in this program. The series of images obtained at several wavelengths in that program will greatly help in the photometric calibration of our spectroscopic results. Dr. French has expressed his similar desire to have the observations done contiguously, or as nearly as possible. STIS exposures of Saturn's rings were obtained by Dr. Amanda Bosh in Cycle 7, in a program similar to that conducted here. Her data were taken at a t im e when the rings were less open than they will be at the opposition of 2001, when our data will be obtained. More important, her exposures in the G750L grating have fringing problems that cannot be removed because the appropriate flat observation was not made. Further, her ultraviolet exposures are too short to give the required signal/noise at wavelengths shorter than about 2500 A. We are mitigating the problems that Dr. Bosh encountered by using a wider slit (higher S/N in the UV) and by taking the proper flat fields at the time of the G750L exposures. We are in contact with Dr. Bosh about the possibility of joining her data with our expected results. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 9112( 3) - 04/17/01 16:46 - [ 5] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SATELLITE-D STIS/C ACQ F25ND3 MIRROR 1 2 S IONE CD ------------------------------------------------------------------------------------------------------------------------------------ 3 SATURN-RING STIS/C ACCUM F25ND3 MIRROR 1 4 S MAX DUR 100% CD ------------------------------------------------------------------------------------------------------------------------------------ 10 SATURN-RING STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=4 1 280 S MAX DUR 100% CD ------------------------------------------------------------------------------------------------------------------------------------ 20 SATURN-RING STIS/C ACCUM 52X2 G750L 7751 CR-SPLIT=5 1 225 S MAX DUR 100% CD ------------------------------------------------------------------------------------------------------------------------------------ 30 CCDFLAT STIS/C ACCUM 52X2 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 50 SATURN-RING STIS/C ACCUM 52X2 G230LB 2375 CR-SPLIT=2 1 1200 S MAX DUR 100% CD ------------------------------------------------------------------------------------------------------------------------------------ 51 SATURN-RING STIS/C ACCUM 52X2 G230LB 2375 CR-SPLIT=2 1 1200 S MAX DUR 100% CD ------------------------------------------------------------------------------------------------------------------------------------ 9112( 3) - 04/17/01 16:46 - [ 6] Summary Form for Proposal 9112 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F25ND3 52X2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=4 CR-SPLIT=5 CR-SPLIT=2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 127D TO 131D ORIENT 307D TO 311D SCHED 100% ON HOLD MAX DUR 100% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G430L G750L G230LB ------------------------------------------------------------------------------------------------------------------------------------ Target Names SATELLITE-DIONE SATURN-RING CCDFLAT ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 4300 7751 2375 ------------------------------------------------------------------------------------------------------------------------------------