Proposal_Information Title: DISTRIBUTION OF O2 ON GANYMEDE Proposal_Category: GO Scientific_Category: Solar System Cycle: 5 Investigators PI_name: John Spencer PI_Institution: Lowell Observatory CoI_Name: Wendy Calvin CoI_Institution: United States Geological Survey Contact: Y CoI_Name: Robert Johnson CoI_Institution: University of Virginia Contact: N Abstract: ! Free format text (please update) 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. Questions ! Free format text (please update) Observing_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: ! The following is only in case adequate guide stars not ! be available so that we must use interactive acquistion. !FOS standard acquisition modes are strictly for point !sources and we require a scan as near to the center of !Ganymede as possible so that we may maximize the latitude !coverage and get the best possible constraints on O_2 !abundance. Therefore we require real-time contact for the !image mode acquisition of Ganymede and Callisto !with the FOS, to point as closely as possible to the north !pole at the start of the scan. 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. Solar_System_Targets Target_Number: 1 Target_Name: GANYMEDE Description: SATELLITE GANYMEDE Level_1: STD = JUPITER Level_2: STD = GANYMEDE Window: OLG OF GANYMEDE BETWEEN 260 280 RANGE JUPITER EARTH LT 4.7 Flux: V=4.61, B-V=0.83, U-B=0.50 Comments: Acquire center of Ganymede, then perform north-south scan. Also do WFPC2 images. Target_Number: 2 Target_Name: CALLISTO Description: SATELLITE CALLISTO Level_1: STD = JUPITER Level_2: STD = CALLISTO Window: OLG OF CALLISTO BETWEEN 260 280 RANGE JUPITER EARTH LT 4.7 Flux: V=5.65, B-V=0.86, U-B=0.55 Comments: Acquire center of Callisto then perform north-south scan. Scan_Data ! This is approximate - to be completed by PC. Scan_Number: 1 Cont_or_Dwell: C Scan_Width: 0 Scan_Length: 2.496 arcsec ! nominal, can go to 2.0 if pointing ! accuracy is better than estimated. Sides_Angle: 0 Number_Lines: 1 Scan_Rate: 0.0016 arcsec/sec ! based on 2.5 arcsec in 1560 sec ! moving target limit. ! Will change if scan length changes. First_Line_PA: 189.76 Scan_Frame: CEL Length_Offset: 1.25 arcsec !will change if scan length changes Width_Offset: 0.0 arcsec Visits Visit_Number: 1 Visit_Requirements: GROUP 1-3 WITHIN 14D Exposure_Number: 1 Target_Name: GANYMEDE Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-N2 Optional_Parameters: BRIGHT=RETURN LOCATE=EXTENDED SEARCH-SIZE=3 Number_of_Iterations: 1 Time_Per_Exposure: 1.8S ! 0.2 step time, no map Special_Requirements: ONBOARD ACQ FOR 2-3 SEQ 1-2 NON-INT Exposure_Number: 2 Target_Name: GANYMEDE ! map target in aperture Config: HRS Opmode: IMAGE Aperture: 2.0 Sp_Element: MIRROR-N2 Number_of_Iterations: 1 Time_Per_Exposure: 52S ! 16 x 16 x 0.2 Exposure_Number: 3 Target_Name: GANYMEDE Config: FOS/RD Opmode: RAPID Aperture: 0.1-PAIR-A Sp_Element: G570H Optional_Parameters: READ-TIME=30.0 Number_of_Iterations: 1 Time_Per_Exposure: 1560S Special_Requirements: NO SPLIT SPATIAL SCAN 1 SINGLE-EXP Visit_Number: 2 Exposure_Number: 1 Target_Name: CALLISTO Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-N2 Optional_Parameters: BRIGHT=RETURN LOCATE=EXTENDED SEARCH-SIZE=3 Number_of_Iterations: 1 Time_Per_Exposure: 1.8S ! 0.2 step time, no map Special_Requirements: ONBOARD ACQ FOR 2-3 SEQ 1-2 NON-INT Exposure_Number: 2 Target_Name: CALLISTO Config: HRS Opmode: IMAGE Aperture: 2.0 Sp_Element: MIRROR-N2 Number_of_Iterations: 1 Time_Per_Exposure: 52S Exposure_Number: 3 Target_Name: CALLISTO Config: FOS/RD Opmode: RAPID Aperture: 0.1-PAIR-A Sp_Element: G570H Optional_Parameters: READ-TIME=30.0 Number_of_Iterations: 1 Time_Per_Exposure: 1560S Special_Requirements: NO SPLIT SPATIAL SCAN 1 SINGLE-EXP Visit_Number: 3 Exposure_Number: 1 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F588N Number_of_Iterations: 2 Time_Per_Exposure: 5S Special_Requirements: SEQ 1-6 NON-INT Exposure_Number: 2 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F631N Number_of_Iterations: 3 Time_Per_Exposure: 8S Exposure_Number: 3 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F658N Number_of_Iterations: 2 Time_Per_Exposure: 10S Exposure_Number: 4 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F588N Number_of_Iterations: 2 Time_Per_Exposure: 5S Special_Requirements: POS TARG 2.40, 2.40 Exposure_Number: 5 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F631N Number_of_Iterations: 3 Time_Per_Exposure: 8S Special_Requirements: POS TARG 2.40, 2.40 Exposure_Number: 6 Target_Name: GANYMEDE Config: WFPC2 Opmode: IMAGE Aperture: PC1 Sp_Element: F658N Number_of_Iterations: 2 Time_Per_Exposure: 10S Special_Requirements: POS TARG 2.40, 2.40 Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS