17150( 5) - 04-Apr-2023 20:03:08 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17150 Version: 5 Check-in Time: 04-Apr-2023 20:03:08 STScI Edit Number: 0 Title Targeted Observations of Ceres' Occator Crater with HST/STIS ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Samantha Trumbo Cornell University Y ------------------------------------------------------------------------------------ Abstract We propose targeted single-orbit STIS observations of Ceres' Occator crater in order to unambiguously confirm or reject the possible identification of sodium chloride color centers in Dawn visible-channel spectra of Occator's bright spots Cerealia and Vinalia Faculae. The Dawn visible channel was affected by perplexing artifacts resulting in anomalous spectral shapes that required improvised, empirical calibration steps, and the published visible-channel spectra of Cerealia Facula across different image cubes and reasonable reduction methods appear to disagree. Though some exhibit features potentially consistent with sodium chloride, others show a complete lack of visible spectral features altogether. As abundant sodium chloride is expected for faculae formation mechanisms involving a relict ocean brine reservoir, resolving this ambiguity would be an important step toward understanding not just the formation and evolution of Ceres' enigmatic bright spots, but also its potential status as a relict ocean world. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to obtain spectra in the G430L and G750L first-order spectroscopy modes using a single 0.05" slit position centered on Occator crater. We take the most conservative approach in estimating our required signal-to-noise ratio (SNR) and corresponding exposure times, based on the weaker M-center feature potentially seen in Dawn spectra. Despite the objective weakness of this potential feature, a 5-sigma detection is possible with an SNR of 185 per pixel, given the 0.09 micron FWHM of the feature and the 4.92 Angstrom/pixel dispersion in G750L. We then estimate the necessary exposure time in two ways. First, we scale from the SNR of 73 achieved by 6-second archival G750L exposures of Io with the 0.1" slit. Accounting for the slit change and the difference in brightness between Io and the expected brightness of an Occator pixel (given the relative sizes and albedos of the faculae and background), we estimate an exposure time of 180 seconds to achieve an SNR of 185 at 0.72 microns. Second, we independently estimate the time needed for our calculated SNR using the STIS exposure time calculator (using a 0.83" extended source and solar spectral shape normalized to the appropriate V-magnitude), which gives an exposure time of 209 seconds (with a time to saturation of 337 seconds) in very good agreement. As the possible F-center signatures on Ceres are at least 4.5 times as strong as their M-center counterparts, the same exposure time in G430L will provide an even stronger F-center detection, despite the slightly lower SNR (156 per pixel for a 180-second exposure) in this setting. A single orbit will allow for multiple such exposures, and therefore a higher SNR and even 17150( 5) - 04-Apr-2023 20:03:08 - [ 2] better detection/rejection limits. We propose to place the 0.05" slit across Occator crater for the duration of one orbit to obtain the highest possible SNR spectra in G430L and G750L. Though the features we aim to constrain may appear quite weak by our conservative estimates, the combination of such high SNR and the knowledge of exactly in which pixel the features should be strongest will allow for strong constraints via the calculations laid out above and careful comparison to pixels away from the crater. Indeed, we anticipate that considered estimation of the background Ceres signal and ratios to off-crater pixels will allow for more robust detections and removal of any unanticipated systematics. In the event of reduced-gyro operations, the main impacts to this program are expected to be reduced schedulability, as these observations are already time-constrained, and increased overheads for target acquisition, resulting in reduced science exposure times. ------------------------------------------------------------------------------------ 17150( 5) - 04-Apr-2023 20:03:08 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: CERES-OCCATOR ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: FEATURE FEATURE CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=CERES TYPE=PCENTRIC, LONG=239.33, LAT=19.82, RAD=477.4 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF CERES-OCCATOR BY CERES FROM EARTH, CML OF CERES FROM EARTH BETWEEN 96 146 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 6.37 +/- .13 This is the science target, Occator crater on Ceres. Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: CERES-ACQ ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: DWARF-PLANET DWARF-PLANET CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 6.37 +/- .13 This is the acquisition target, Ceres. Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 17150( 5) - 04-Apr-2023 20:03:08 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-PERP-TO-SLIT Pattern_Purpose MOSAIC Number_Of_Points 3 Point_Spacing 0.05 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 0.0 Angle_Between_Sides Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 17150( 5) - 04-Apr-2023 20:03:08 - [ 5] Visit: 01 (CERES-OCCATOR) Visit Requirements: PCS MODE FINE; BETWEEN 20-FEB-2023:00:00:00 AND 10-MAY-2023:00:00:00; VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET On Hold Comments: Additional Comments: Timing windows are selected such that Ceres is near its maximum diameter of ~0.8" on the sky. 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-ACQ STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, 1 0.5 S (0.5 SEQ 1-4 NON-INT DIFFUSE-CENTER=GEOMETRIC-CENTE S) R, CHECKBOX=19 ------------------------------------------------------------------------------------------------------------------------------------ 2 WAVE STIS/CCD ACCUM 52X0.05 G430L 4300 1 DEF (0 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CERES-OCCAT STIS/CCD ACCUM 52X0.05 G430L 4300 WAVECAL=NO, SIZEAXIS2=80, 1 190 S (570 PATTERN 1 3 OR CR-SPLIT=2 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CERES-OCCAT STIS/CCD ACCUM 52X0.05 G750L 7751 WAVECAL=NO, SIZEAXIS2=80, 1 190 S (570 PATTERN 1 4 OR CR-SPLIT=2 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 WAVE STIS/CCD ACCUM 52X0.05 G750L 7751 1 DEF (0 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 NONE STIS/CCD ACCUM 52X0.05 G750L 7751 LAMP=TUNGSTEN, GAIN=4 4 100 S (400 S) Comments: More iterations and longer exposure times for increased S/N. ------------------------------------------------------------------------------------------------------------------------------------ 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-ACQ 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WAVE 2 STIS/CCD ACCUM 52X0.05 G430L 4300 none none none none 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 1 none none 1 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 1 none none 2 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 2 none none 1 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 2 none none 2 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 3 none none 1 1 N/A CERES-OCCATOR 3 STIS/CCD ACCUM 52X0.05 G430L 4300 3 none none 2 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 1 none none 1 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 1 none none 2 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 2 none none 1 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 2 none none 2 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 3 none none 1 1 N/A CERES-OCCATOR 4 STIS/CCD ACCUM 52X0.05 G750L 7751 3 none none 2 1 N/A 17150( 5) - 04-Apr-2023 20:03:08 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ WAVE 5 STIS/CCD ACCUM 52X0.05 G750L 7751 none none none none 1 N/A NONE 6 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 1 none 1 N/A NONE 6 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 2 none 1 N/A NONE 6 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 3 none 1 N/A NONE 6 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 4 none 1 N/A 17150( 5) - 04-Apr-2023 20:03:08 - [ 7] Summary Form for Proposal 17150 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.05, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, CHECKBOX=19, CR-SPLIT=2, DIFFUSE-CENTER=GEOMETRIC-CENTER, GAIN=4, LAMP=TUNGSTEN, SIZEAXIS2=80, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 20-FEB-2023:00:00:00 AND 10-MAY-2023:00:00:00, PATTERN 1 3, PATTERN 1 4, PCS MODE FINE, SEQ 1-4 NON-INT, VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G430L, G750L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names CERES-ACQ, CERES-OCCATOR, NONE, WAVE ------------------------------------------------------------------------------------------------------------------------------------