13693( 10) - 17-Jul-2015 15:19:22 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13693 Version: 10 Check-in Time: 17-Jul-2015 15:19:22 STScI Edit Number: 5 Title The Ultraviolet Spectrum of Ceres ------------------------------------------------------------------------------------ Type Cycle GO 22 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Amanda R. Hendrix Planetary Science Institute Y CoI: Dr. Faith Vilas Planetary Science Institute N CoI: Dr. Jian-Yang Li Planetary Science Institute N ------------------------------------------------------------------------------------ Abstract We propose a focused, four-orbit program to study Ceres, the largest -- yet most enigmatic -- asteroid, using ultraviolet spectroscopy. While Ceres will soon be visited by the Dawn spacecraft, Dawn does not have the capability to study Ceres in the UV, the spectral region of a mysterious surface absorption. The UV spectrum of Ceres may provide an important link with the most primitive materials in the solar system, and could reveal clues about the water activity on Ceres -- and HST is our only opportunity to study this unique object. ------------------------------------------------------------------------------------ Observations Description ------------------------ The proposed observations will obtain excellent quality STIS-UV reflectivities in the 115- 318 nm spectral region (using G140L and G230L), and will also measure the reflectance in the 290-570 nm range (using G430L) to provide adequate overlap with Dawn instrumentation. There is simply no other way to study this critical spectral region, and whatever UV spectral shape we measure will be of unique importance in helping us understand the surface composition of Ceres. We will devote the bulk of our observing time to the G140L and G230L detectors, to maximize the signal-to-noise in the critical 115-318 nm region. We will use the 0.2" slit for the G230L exposures and the 0.5" slit for the G140L exposures to maximize SNR without saturating the detector. We are flexible regarding slit orientation. We plan to observe Ceres at two sub-Earth longitudes (ideally centered on 0degE and either 120deg or 240deg E), to understand any large-scale variability on the surface; 120deg E and 240deg E are the regions in which Herschel detected H2O (Kueppers et al., 2014). Furthermore, based on the limited data in the existing map, these hemispheres appear to be sufficiently distinct in UV albedo and we thus suggest these hemispheres as interesting comparison regions, whether H2O is detected or not. In order to thoroughly determine the spectral shape of Ceres in the far-UV -- and probe for the presence of H2O -- we have determined that we need to optimize SNR from both the G140L and the G230L detectors. We have estimated SNRs at several wavelengths by assuming an extended source (of 0.65") with a solar spectrum normalized to a 200-nm intensity of 3.1e-13 ergs/cm2-s-A-arcsec2, which is given by measured G230L fluxes from our Cycle 19 13693( 10) - 17-Jul-2015 15:19:22 - [ 2] HST program (in which we observed that Saturn's C ring has a geometric albedo similar to Ceres at 200 nm), and correcting to solar flux levels at 2.54AU, and scaling to other wavelengths by the assumed Ceres broadband spectrum. In addition, we will take a single 1-sec exposure on each hemisphere using G430L, giving us high SNR at longer wavelengths and avoiding saturation. This will provide more leverage on the overall spectral shape of Ceres, the shape of the UV downturn, and a cross-calibration with Dawn-VIR at visual wavelengths, with little observing time impact. We have determined that our planned exposure times and slit widths accommodate the MAMA count rate restrictions for bright objects, and do not saturate the CCD at the longer wavelengths in the passband. We have verified that the total time accommodates guide star and target acquisition, as well as STIS overhead times. Acquiring 3 700-sec G140L exposures, 3 700-sec G230L exposures and one short G430L exposure combination on each hemisphere will allow us to carefully and thoroughly measure two hemispheres of Ceres and look for large-scale variations in this part of the spectrum. Thus, over 4 orbits, we can (a) confirm the UV spectral shape suggested so far only in broad band passes, (b) test whether there is FUV variability across the surface, either in carbonaceous and/or H2O components, and (c) provide measurements that complement the approaching Dawn mission. ------------------------------------------------------------------------------------ 13693( 10) - 17-Jul-2015 15:19:22 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: CERES-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: DWARF-PLANET Ceres centered on 0 longitude ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS CML OF CERES-1 FROM EARTH BETWEEN 335 25 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 8 +/- 0.5 Ceres opposition is July 23, so close to that date is optimal. Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: CERES-2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: DWARF-PLANET Ceres at 2nd longitude ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS CML OF CERES-2 FROM EARTH BETWEEN 95 145 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 8 +/- 0.5 The second target is Ceres, either centered on 120 deg. E or centered on 240 deg. E . Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 13693( 10) - 17-Jul-2015 15:19:22 - [ 4] Visit: 01 Visit Requirements: AFTER 12-JUL-2015:00:00:00 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CERES-1 STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, 1 .5 S (0.5 GS ACQ SCENARIO BASE1B3 DIFFUSE-CENTER=GEOMETRIC-CENTE S) R, CHECKBOX=3.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 CERES-1 STIS/CCD ACCUM 52X0.5 G430L 4300 CR-SPLIT=NO 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CERES-1 STIS/NUV ACCUM 52X0.2 G230L 2376 1 700 S (700 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CERES-1 STIS/NUV ACCUM 52X0.2 G230L 2376 1 700 S (700 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CERES-1 STIS/CCD ACCUM 52X0.5 G430L 4300 CR-SPLIT=NO 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ CERES-1 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A CERES-1 2 STIS/CCD ACCUM 52X0.5 G430L 4300 none none none none 1 N/A CERES-1 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA CERES-1 4 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA CERES-1 5 STIS/CCD ACCUM 52X0.5 G430L 4300 none none none none 1 N/A 13693( 10) - 17-Jul-2015 15:19:22 - [ 5] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CERES-1 STIS/CCD ACQ F28X50LP MIRROR 1 0.5 S (0.5 GS ACQ SCENARIO BASE1B3 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CERES-1 STIS/FUV TIME-TA 52X0.5 G140L 1425 BUFFER-TIME=1131 1 2250 S -MAMA G (2250 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ CERES-1 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A CERES-1 2 STIS/FUV TIME-TA 52X0.5 G140L 1425 none none none none 1 N/A -MAMA G 13693( 10) - 17-Jul-2015 15:19:22 - [ 6] Visit: 03 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CERES-2 STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, 1 .5 S (0.5 GS ACQ SCENARIO BASE1B3 DIFFUSE-CENTER=GEOMETRIC-CENTE S) R, CHECKBOX=3.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 CERES-2 STIS/CCD ACCUM 52X0.5 G430L 4300 CR-SPLIT=NO 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CERES-2 STIS/NUV ACCUM 52X0.2 G230L 2376 1 700 S (700 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CERES-2 STIS/NUV ACCUM 52X0.2 G230L 2376 1 700 S (700 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CERES-2 STIS/CCD ACCUM 52X0.5 G430L 4300 CR-SPLIT=NO 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ CERES-2 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A CERES-2 2 STIS/CCD ACCUM 52X0.5 G430L 4300 none none none none 1 N/A CERES-2 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA CERES-2 4 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA CERES-2 5 STIS/CCD ACCUM 52X0.5 G430L 4300 none none none none 1 N/A 13693( 10) - 17-Jul-2015 15:19:22 - [ 7] Visit: 04 Visit Requirements: On Hold Comments: Additional Comments: Visit 4 should be 6N orbits after Visit 3, where N is an integer, to observe roughly the same longitude on Ceres given Ceres' 9 hour rotational period. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CERES-2 STIS/CCD ACQ F28X50LP MIRROR 1 0.5 S (0.5 GS ACQ SCENARIO BASE1B3 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CERES-2 STIS/FUV TIME-TA 52X0.5 G140L 1425 BUFFER-TIME=1131 1 2250 S -MAMA G (2250 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ CERES-2 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A CERES-2 2 STIS/FUV TIME-TA 52X0.5 G140L 1425 none none none none 1 N/A -MAMA G 13693( 10) - 17-Jul-2015 15:19:22 - [ 8] Summary Form for Proposal 13693 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, 52X0.5, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, BUFFER-TIME=1131, CHECKBOX=3.0, CR-SPLIT=NO, DIFFUSE-CENTER=GEOMETRIC-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 12-JUL-2015:00:00:00, GS ACQ SCENARIO BASE1B3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, G430L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names CERES-1, CERES-2 ------------------------------------------------------------------------------------------------------------------------------------