5842( 11) - 11/27/95 13:51 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5842 Version: 11 Check-in Date: ********** 1.Proposal Title: A Search for Ice at the Poles of Ceres ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Solar System GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Alan Stern Southwest Research Institute CoI: Rick Binzel MIT N CoI: Michel Festou Southwest Research Institute N CoI: Brian Flynn Southwest Research Institute N CoI: Larry Lebofsky University of Arizona N ------------------------------------------------------------------------------------ 6. Abstract One of the most intriguing suggestions about the largest asteroid, 1 Ceres, is that it may display water-ice on its surface. IR evidence for this hypothesis was presented over 12 years ago in the form of H_2O-ice absorption on Ceres' surface. Recently, however, King et al., have suggested the IR absorption attributed to water -ice may instead be due to NH_4- bearing minerals. IUE searches for OH emission resulting from the dissociation of water vapor above Ceres have yielded contradictory results. We propose to address this long-standing controversy using a new approach. We will obtain maps of Ceres in NUV, MUV, and FUV filters that together will discriminate for the presence or absence of significant quantities of surface ice. As a byproduct, these images will also provide the first-ever global maps of the albedo distribution on Ceres. ------------------------------------------------------------------------------------ 5842( 11) - 11/27/95 13:51 - [ 2] Observations Description ------------------------ \medskip We are proposing to map Ceres using the FOC/f96 chain in the near-UV (F342W), mid-UV (F275W+F278M), and far- UV (F152M+F175W). The selection of two in-series filters in the mid- and far-UV was motivated to effectively eliminate red leaks. We have tentatively selected the FOC 512x512 format, with a FOV of 7.2x7.2", 25x25Mum 0.028x0.028") square pixels, a 16 bit word length, and N_ max=0.12 Hz pixel^-1. We plan to use a standard, FOC Mode III (Blind Pointing) Acquisition, which is clearly capable of putting Ceres in this aperture. \medskip For the June 1995 visit, Ceres spans a diameter near 0.55", or many FOC/96 pixels. Although the FOC's imaging resolution is best in the UV, the OTA+COSTAR+FOC will obtain an effective resolution that is somewhat coarser than a pixel, or about 0.04" (FOC V5.0 Handbook, 1994). This corresponds to a linear resolution of ~70 km. It also means the images will be oversampled, which is advantageous for building S/N. This effective resolution will put ~12 resolution elements across Ceres' diameter, and ~240 resolution elements on the disk. \medskip Our nominal observation program involves obtaining images of two opposite hemispheres of Ceres at all three program wavelengths. S/N calculations reveal that 90 minutes of exposure time are required to obtain a S/N of 12 for an FOC resolution element with an albedo of 0.20 in F152M+F175W images (this is albedo one might see in an icy patch). By comparison, Ceres' mean visible albedo is 0.10; the lunar albedo at 1400 Angstrom\ is 0.05. Our S/N estimate assumes a standard UV solar spectrum (Mount & Rottman 1983), the instrument and filter parameters presented in the V5.0 FOC Handbook (1994), and a background count (i.e., noise) rate of 0.002 Hz pixel^-1. \medskip The longer-wavelength F275W+F278M and F342W images will be count rate (i.e., N max) limited, and therefore will require the use of neutral density filters. At N_ max=0.12 Hz, attaining S/N=25 per resolution element will require about 1000 seconds (17 minutes) each. We can obtain such well-exposed images of Ceres in both the F342W and F275W+F278M bands in a single orbit (since ~45 minutes of integration time are available on these orbits). However, three F152M+F175W images taken over 2 consecutive orbits will be required to obtain the desired far -UV S/N. Since 3 HST orbits take 4.8 hours and Ceres rotates in 9.1 hours, in some circumstances it may be necessary to hemispherically-average images of Ceres. However, small far-UV features with albedo~0.20 (twice the Ceres mean) can be detected in a single orbit at S/N=7 with F152M+F175W. The total, 4-orbit FOC program we have initially designed to cover a single hemisphere on Ceres is summarized as follows: orbits 1 & 3: 1 image each in F275W+F278M and F342W+F346M; orbits 2 & 4 2 images each orbit in F152M+F175W. Real Time Justification ----------------------- \medskip There are no special dark time, time-critical, continuous-viewing zone, or TOO observations required for this program. No special calibrations are needed. There are no supporting observations required for this program, however, we wish to note that Michel Festou will be leaving Observatoire Midi- Pyrenees for a year to be a visiting scientist at Southwest Research Institute in 1995- 1996. As such, he will be as full a participant as the other CoIs in the analysis of these HST data. THE MAGNTIUDE OF CERES VARIES AS IT MOVES AROUND THE SUN, PRIMARILY DUE TO HELIOCENTRIC AND GEOCENTRIC DISTANCE EFFECTS. THE SUB-EARTH LATITUDE IS ~30 DEG NORTH DURING VISIT 1, SO THE SOUTH POLE WILL NOT BE VISIBLE. A SECOND VISIT IN 96 IS REQUIRED FOR THAT OBJECTIVE, AND DD TIME HAS BEEN REQUESTED. THE ONLY TIME IN CY 5 WHEN BOTH POLES CAN BE SEEN WITH AN EQUATORIAL SUB-EARTH LAT IS DURING SOLAR CONJUNCTION. THE WHOLE VISIT 1 IMAGING SEQUENCE MUST GROUP WITHIN 6 HOURS. Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ 5842( 11) - 11/27/95 13:51 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5842( 11) - 11/27/95 13:51 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: 1-CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID CERES, ASTEROID 1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=1 (CERES) | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V=8.7 | B-V=+0.72 | | ------------------------------------------------------------------------------------------------------------------------------------ 5842( 11) - 11/27/95 13:51 - [ 5] Visit: 01 Visit Requirements: PCS MODE Fine BETWEEN 08-JUN-95 AND 03-JUL-95 GROUP 1 WITHIN 6H 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 1-CERES FOC/96 IMAGE 512x512 F278M,F27 Check- 1 720S SEQuence 10-20 NON-INTerruptible 5W,F4ND Filter=Yes Comments: COUNT RATES CALCULATED BY ANTONELLA NOTA USING FOCSIM ------------------------------------------------------------------------------------------------------------------------------------ 20 1-CERES FOC/96 IMAGE 512x512 F342W, Check- 1 1020S END ORBIT MAXimum DURation 220% MINimum F372M, Filter=Yes DURation 70% SEQuence 10-20 NON- F1ND, INTerruptible F6ND ------------------------------------------------------------------------------------------------------------------------------------ 30 1-CERES FOC/96 IMAGE 512x512 F152M,F17 Check- 1 900S MAXimum DURation 225% MINimum DURation 80% 5W Filter=Yes NO SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 35 1-CERES FOC/96 IMAGE 512x512 F152M,F17 Check-Filter=No 1 900S MAXimum DURation 225% MINimum DURation 80% 5W NO SPLIT SEQuence 30-35 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 40 1-CERES FOC/96 IMAGE 512x512 F278M,F27 Check- 1 1020S MAXimum DURation 150% MINimum DURation 80% 5W,F4ND Filter=Yes ------------------------------------------------------------------------------------------------------------------------------------ 50 1-CERES FOC/96 IMAGE 512x512 F342W, Check- 1 1020S END ORBIT MAXimum DURation 220% MINimum F372M, Filter=Yes DURation 80% SEQuence 40-50 NON- F1ND, INTerruptible F6ND ------------------------------------------------------------------------------------------------------------------------------------ 60 1-CERES FOC/96 IMAGE 512x512 F152M,F17 Check- 1 900S MAXimum DURation 225% MINimum DURation 80% 5W Filter=Yes NO SPLIT Comments: ALL EXPOSURE TIMES PREDICTED BY ANONELLA NOTA FOR V=8.75, color=G2V, ASSUMING DIAMETER=0.55 arcsec ------------------------------------------------------------------------------------------------------------------------------------ 65 1-CERES FOC/96 IMAGE 512x512 F152M,F17 Check-Filter=No 1 900S MAXimum DURation 225% MINimum DURation 80% 5W NO SPLIT SEQuence 60-65 NON-INTerruptible Comments: ALL EXPOSURE TIMES PREDICTED BY ANONELLA NOTA FOR V=8.75, color=G2V, ASSUMING DIAMETER=0.55 arcsec ------------------------------------------------------------------------------------------------------------------------------------ 5842( 11) - 11/27/95 13:51 - [ 6] Summary Form for Proposal 5842 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters Check-Filter=Yes Check-Filter=No ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Solar System ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE Fine BETWEEN 08-JUN-95 AND 03-JUL-95 GROUP 1 WITHIN 6H SEQuence 10-20 NON-INTerruptible END ORBIT MAXimum DURation 220% MINimum DURation 70% SEQuence 10-20 NON-INTerruptible MAXimum DURation 225% MINimum DURation 80% NO SPLIT MAXimum DURation 225% MINimum DURation 80% NO SPLIT SEQuence 30-35 NON-INTerruptible MAXimum DURation 150% MINimum DURation 80% END ORBIT MAXimum DURation 220% MINimum DURation 80% SEQuence 40-50 NON-INTerruptible MAXimum DURation 225% MINimum DURation 80% NO SPLIT SEQuence 60-65 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F278M,F275W,F4ND F342W, F372M, F1ND, F6ND F152M,F175W ------------------------------------------------------------------------------------------------------------------------------------ Target Names 1-CERES ------------------------------------------------------------------------------------------------------------------------------------