6029( 0) - 07/18/95 19:26 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6029 Version: ********** Check-in Date: ********** 1.Proposal Title: DISTRIBUTION OF O2 ON GANYMEDE ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Solar System GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: John Spencer Lowell Observatory CoI: Wendy Calvin United States Geological Survey Y CoI: Robert Johnson University of Virginia N ------------------------------------------------------------------------------------ 6. Abstract We propose to follow up on our discovery of molecular oxygen on Ganymede's surface (probably magnetospherically generated), by mapping its distribution as a function of latitude and terrain type. This will provide valuable constraints on the formation, destruction, and migration mechanism of the O_2, and thus on the processes that modify Ganymede's surface. We will do this using a N/S scan across Ganymede's disk with the FOS. We will also perform a similar N/S scan across Callisto primarily to provide a comparison spectrum for Fraunhofer line removal from the Ganymede spectrum, but also to look for possible local concentrations of O_2 on Callisto. Finally, we will obtain very high S/N images of Ganymede in the 6275 Angstrom O_2 band and nearby continuum, to do complementary direct mapping of the spatial variations in O_2 distribution over the whole disk. ------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 2] Observations Description ------------------------ We wish to investigate the spatial distribution of molecular oxygen on Ganymede by performing a pole-to-pole scan across the trailing side of the satellite (where the features are strongest in ground-based disk-integrated spectra) with the FOS, and by narrowband imaging of the 6275 Angstrom feature with WFPC2. For the FOS, we require one orbit for on board acquisition of Ganymede using the HRS using the LOCATE=EXTENDED mode designed for the Galilean satellites. A subsequent image of the acquisition aperture will assist in determining pointing of the scan. (If suitable guide stars can not be found we will return to an interactive acquisition strategy using FOS and G270H). A second orbit is designed for a continuous scan over a nominal 2.5 arcsec from north to south, parallel to Ganymede's spin axis, passing as close to center of the disk as pointing accuracy (c. 0.2 arcsec) will allow. Scan length allows 0.45 arcsec margin of error on either side of Ganymede's 1.6 arcsec disk. If centering using the HRS acquistion mode is substantially improved the error margin can be decreased leaving more time of the scan on the satellite. A minimum of 2.0 arcsec is recommeded. The scan would be smooth and continuous. Moving target pointing limits the scan to 26 minutes for a rate of 0.096 arcsec per min. The FOS will be read out in ``rapid'' mode with a time resolution of 30 seconds (= 0.048 arcsec). We will perform an essentially identical acquisition and scan of Callisto, for the primary purpose of obtaining an O_2 - free comparison spectrum under identical conditions which we can use to remove the Fraunhofer lines from the Ganymede spectrum, though it is perhaps possible that we will find small local enrichments of O_2 on Callisto as well. On a fifth orbit, we will obtain four PC images of Ganymede at each wavelength through the the F588N, F631N, and F658N filters, to build up the high S/N required to map the variations in depth of the 1 percent deep 6275 Angstrom O_2 feature on Ganymede. We request dithering the images so that 2 in each filter are acquired at one point on the CCD and the other two are offset approximately 55 pixels. This is to assist in removing small flat field and dark current variations. 55 pixels should maintain both the WF and PC pointing. S/N Estimation: At 6 degree phase angle, R = 5.3 AU, and Delta = 4.5 AU, Ganymede's V magnitude is 4.92 (Morrison and Morrison 1977), with an angular diameter of 1.61 arcsec. We estimate the number of counts using Figure 1.2.3 of the FOS Handbook. Fg/Fstd = 10^{Vg-Vstd/-2.5}. This gives the total flux from Ganymede as Fg/Fstd * N(counts)= 47,000 counts per diode per second. The 0.09 arcsec aperture intercepts only 0.004 of the total Ganymede flux for a count rate of 190 counts per diode per second. Reading out every 30 secs gives a S/N per readout of 77. At our planned scan rate we will cover an aperture width in 60 seconds, giving 11,400 counts and a S/N of 110 per diode per aperture width. The asymmetric 5770 Angstrom feature covers approximately 34 diodes so that summation in wavelength every 4th diode results in a S/N of 220, more than sufficient to detect the 2 percent feature. Callisto has similar size to Ganymede but has about half the albedo, so each aperture position on Callisto will have approximately SQRT(2) less S/N than on Ganymede. However, as spatial resolution on Callisto is a secondary goal, we can average the approx. 14 independent Callisto spectra together to get better S/N than each individual Ganymede spectrum, so that in the Ganymede/Callisto ratio spectrum required to see the O_2 feature the Callisto spectrum does not contribute appreciably to the noise. For the PC imaging, we estimate from Cycle 4 Io imaging results and the filter sensitivities given in the WFPC2 Handbook that exposures of 10 seconds will yield about 45,000 counts per pixel on Ganymede in with exposures of about 5 seconds through F588N, 8 seconds through F631N and 10 seconds through F658N, giving a S/N of 210. By combining four images at each wavelength a S/N of 420 per pixel will be acheived, and the c. 1 percent reflectivity variations in the depth of the 6275 Angstrom band will be visible in ratios between the wavelengths. Comparison of the F588N and F658N filters will allow correction of band depth for the continuum slope. With these short exposures and 3 minutes overhead per exposure, the twelve images will fit into a single orbit. Real Time Justification ----------------------- Calibration Justification ------------------------- No special requirements requested. Additional Comments ------------------- We require observations of the trailing side of Ganymede where the O_2 band is strongest: orbital longitudes from 260 to 280 or an observing window of about 9 hours every 7 days. The observations for the latter Galileo Spacecraft orbits will be defined in August of 1995. In two of these orbits targeted observations of the trailing hemisphere of Ganymede using the Near Infrared Mapping Spectrometer are currently planned. If we obtain data during the 1995 Jupiter opposition we will have the opportunity to guide the selection of Galileo/NIMS target areas. ------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: GANYMEDE ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE GANYMEDE ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = GANYMEDE | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | OLG OF GANYMEDE BETWEEN 260 280 RANGE | JUPITER EARTH LT 4.7 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V=4.61 | Acquire center of Ganymede, then B-V=0.83 | perform north-south scan. Also do U-B=0.50 | WFPC2 images. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: CALLISTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE CALLISTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = CALLISTO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | OLG OF CALLISTO BETWEEN 260 280 RANGE | JUPITER EARTH LT 4.7 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V=5.65 | Acquire center of Callisto then B-V=0.86 | perform north-south scan. U-B=0.55 | | ------------------------------------------------------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 5] Visit: 01 Visit Requirements: GROUP 1-3 WITHIN 14D 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 GANYMEDE HRS ACQ 2.0 MIRROR-N2 BRIGHT=RETURN 1 1.8S ONBOARD ACQ FOR 2-3 SEQ 1-2 NON-INT LOCATE=EXTENDED SEARCH-SIZE=3 ------------------------------------------------------------------------------------------------------------------------------------ 2 GANYMEDE HRS IMAGE 2.0 MIRROR-N2 1 52S ------------------------------------------------------------------------------------------------------------------------------------ 3 GANYMEDE FOS/RD RAPID 0.1- G570H READ-TIME=30.0 1 1560S NO SPLIT SPATIAL SCAN 1 SINGLE-EXP PAIR-A ------------------------------------------------------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 6] Visit: 02 Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 CALLISTO HRS ACQ 2.0 MIRROR-N2 BRIGHT=RETURN 1 1.8S ONBOARD ACQ FOR 2-3 SEQ 1-2 NON-INT LOCATE=EXTENDED SEARCH-SIZE=3 ------------------------------------------------------------------------------------------------------------------------------------ 2 CALLISTO HRS IMAGE 2.0 MIRROR-N2 1 52S ------------------------------------------------------------------------------------------------------------------------------------ 3 CALLISTO FOS/RD RAPID 0.1- G570H READ-TIME=30.0 1 1560S NO SPLIT SPATIAL SCAN 1 SINGLE-EXP PAIR-A ------------------------------------------------------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 7] Visit: 03 Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 GANYMEDE WFPC2 IMAGE PC1 F588N 2 5S SEQ 1-6 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 GANYMEDE WFPC2 IMAGE PC1 F631N 3 8S ------------------------------------------------------------------------------------------------------------------------------------ 3 GANYMEDE WFPC2 IMAGE PC1 F658N 2 10S ------------------------------------------------------------------------------------------------------------------------------------ 4 GANYMEDE WFPC2 IMAGE PC1 F588N 2 5S POS TARG 2.40, 2.40 ------------------------------------------------------------------------------------------------------------------------------------ 5 GANYMEDE WFPC2 IMAGE PC1 F631N 3 8S POS TARG 2.40, 2.40 ------------------------------------------------------------------------------------------------------------------------------------ 6 GANYMEDE WFPC2 IMAGE PC1 F658N 2 10S POS TARG 2.40, 2.40 ------------------------------------------------------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 8] Scan Paramters Form ------------------------------------------------------------------------------------------------------------------------------------ Data ID: 1 Exposure Logsheet lines: FGS Scan: C Dwell Only: dwell points/line: 0 Seconds per dwell: 0.00 Scan width (arc-secs): 0.0000 Scan length (arc-secs): 2.4960 Angle between sides (degrees) 0.00 Number of lines: 1 Scan rate (arc-sec/sec): 0.0016 PA of first scan line (degrees) 189.760 Scan frame (CEL or S/C): CEL Length Offset (arc-sec): 1.2500 Width Offset (arc-sec): 0.0000 ------------------------------------------------------------------------------------------------------------------------------------ 6029( 0) - 07/18/95 19:26 - [ 9] Summary Form for Proposal 6029 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS FOS/RD WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ IMAGE RAPID ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BRIGHT=RETURN LOCATE=EXTENDED SEARCH-SIZE=3 READ-TIME=30.0 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Solar System ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 1-3 WITHIN 14D ONBOARD ACQ FOR 2-3 SEQ 1-2 NON-INT NO SPLIT SPATIAL SCAN 1 SINGLE-EXP SEQ 1-6 NON-INT POS TARG 2.40, 2.40 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 G570H F588N F631N F658N ------------------------------------------------------------------------------------------------------------------------------------ Target Names GANYMEDE CALLISTO ------------------------------------------------------------------------------------------------------------------------------------