Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 5660 Report Date: 18-Jul-95:17:49 Version: ********** Check-in Date: ********** 1.Proposal Title: SEARCH FOR EVIDENCE OF ACTIVE ICE VOLCANISM ON EUROPA CYCLE3 - HIGH, CARRYOVER TO CYCLE 4 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID SOLAR SYSTEM GO/CAR Sub Category SATELLITES ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Dr. Keith S. Noll STSCI USA 410-338-1828 ASST. ASTRONOMER ------------------------------------------------------------------------------------ 7. Abstract Evidence from a variety of observational sources indicates that Europa's surface may be very young. Active resurfacing may be continuous, and the most likely mode is venting of gas from cracks in an ice crust overlying a layer of liquid water. Inconclusive evidence from Voyager images and infrared spectra and photometry suggests that transient eruptive events may occur. The primary constituent of such eruptions would be H2O. The weak gravitational field of Europa and the complicating presence of Jupiter's magnetosphere could allow a substantial fraction of the H2O, or its photolytic daughter, OH, to escape refreezing on the surface. We propose to search for emission from the very strong 3090 A OH A-X (0,0) at two locations near Europa as an indicator of the current level of geologic activity on its surface. The sensitivity and small aperture of the HST are essential to the success of this program. With one hour of integration per position we can reach a sensitivity of 1 Rayleigh, or a column abundance of less than 3.E+10 molecules/cm2, sufficient to constrain plausible models of resurfacing. ------------------------------------------------------------------------------------ 8. Scientific Key Words: SATELLITES, EUROPA, SURFACE ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 4.00 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: FOS ------------------------------------------------------------------------------------ 12. Special sched req: Real Time obs. ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 5660 PI: Dr. Keith S. Noll Proposal Title: SEARCH FOR EVIDENCE OF ACTIVE ICE VOLCANISM ON EUROPA CYCLE3 - HIGH, CARRYOVER TO CYCLE 4 ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Dr Keith S Noll STSCI USA Dr Hal A Weaver STSCI USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. We have two visits of two exposures each with the FOS using the G270H grating; The first with one exposure of Europa for target acquisition and one long exposure at a position off Europa's limb where we will search for emission from OH. The second will use blind pointing for one long exposure. We expect the entire program to require 5 orbits of spacecraft time. The wavelength coverage of the FOS with the G270H grating is from 2222 - 3301 A, well suited for detection of OH emission which will be stongest from 3080 - 3100 A. The spectral resolution of the G270H is 2.3 A at 3000 A. Though individual lines of OH will be much narrower than this (FWHM ~ 0.0002 A), the grouping of the strongest emission lines of the A-X (0,0) band in the interval from 3080 - 3100 A (Schleicher 1988) enhances the detectability of OH with an instrument configuration that spans this interval. In addition, it will be necessary to discriminate line emission from scattered light that may contaminate the spectrum. The baseline we will establish from both longer and shorter wavelength intervals where no OH emission is expected will allow us to subtract scattered radiation by fitting a scaled spectrum of Europa to the scattered light continuum. We have modeled the expected scattered light profile based on azimuthally average point-spread functions and the angular diameter of Europa. Scattered light remains significant out to a distance of 4 arcsec from the center of Europa and then drops rapidly. Based on this we plan to place one long exposure at a distance of 5 arcsec from Europa allowing some room for error in the pointing. The spectrum at this position should have no detectable scattered radiation from Europa. The two models of surface brightness distribution that we have produced predict surface brightnesses of 2.5 R and 70 R for the "daughter" and "parent" models respectively. After 1 hour of integration we can expect to detect as little as 1 R with S/N = 3 using the 1 arcsec aperture and the FOS Blue digicon. The models of OH distribution and the possible production rate of H2O are sufficiently speculative that we feel a reasonable margin is warranted. It is evident, however, that the observations will be sufficiently sensitive to obtain meaningful upper limits in the event OH emission is not observed. Because the distribution of OH is unknown we also want to obtain a spectrum of a position closer to Europa. At 2.5 arcsec from the center of Europa the scattered light has dropped to 1% the intrinsic surface brightness of Europa, or 3000 Rayleighs/A. While this significantly exceeds the expected brightness of the OH emission, we believe that we will be able to subtract off the scattered component by fitting the observed Europa spectrum to the portions of the off-disk spectrum that do not contain OH emission. The acquisition sequence will consist of a five row aperture scan of the 4.3" aperture with the G270H grating in place after a blind pointing (1.2" uncertainty) maneuver to place Europa in the 4.3" square aperture. We will be able to determine the y position of Europa by directly observing the counts in each row and calculate any y-offset that may be required. In the x direction we will have precalculated spectra with solar spectral lines vs. diode position for a centered target. Any deviation can be easily converted into an x-offset. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. No observation of this type has ever been attempted for any solar system satellite that we are aware of. The closest analog we have found is the recent possible detection of OH near the asteroid Ceres (A'Hearn and Feldman 1992) with the IUE. However, it is clear that the HST is far superior in sensitivity and spectral resolution to the IUE for a search of this type. The IUE was able to detect OH with a surface brightness of 11 R with a S/N of 3 after 450 minutes of integration; this is about a factor of 100 less sensitive than the HST will achieve. Observations suggesting the possibility of active H2O resurfacing are desribed in detail in the scientific justification. The FOS Blue digicon and a 1.3 by 4.3 arcsec aperture can reach a sensitivity of 0.12 R (3-sigma) after 1.5 hour of integration. This is calculated for a wavelength of 309 nm and assumed the FOS + HST efficiency of 0.7 and an aperture throughput of 0.45. Models of possible OH surface brightness can differ by at least an order of magnitude depending on assumptions about the distribution of OH. ------------------------------------------------------------------------------------ 9. Description of previous HST work. 2432 Exceptional Solar System Objects ... not related 2481 HST Observations of Periodic Comets ... not related 2483 The Volatile Composition of New Comets ... not related 3064 HST Observations of Comet Levy ... not related 3744 Spectrophotometry of Phobos and Deimos ... not related Although none of the programs are scientifically related to this proposal, the observing techniques used and experience gained in the comet observations are very relevant to our proposed search for extended OH emission from Europa. Inner coma imaging of comet Levy (1990c) with the HST. H. Weaver + 7 coauthors Icarus 97, 85-98 (1992) Titan: Evidence for seasonal change - a comparison of Hubble Space Telescope and Voyager images. J. Caldwell + 9 including K. Noll and H. Weaver Icarus 97, 1-9 (1992) ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). The Space Telescope Science Institute provides salary support for both the PI and the Co-I. Workstation hardware and software are also partially supoorted by STScI. ------------------------------------------------------------------------------------ 11. Address Information Name: KEITH S NOLL Category: PI Institution: STSCI Address: 3700 SAN MARTIN DRIVE City: BALTIMORE State: MD Zip Code: 21218 Country: USA Telephone: 410-338-1828 Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 5660 [ 4] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = EUROPA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO EUROPA FROM EARTH GT 20", | SEP OF GANYMEDE EUROPA | FROM EARTH GT 20", | SEP OF CALLISTO EUROPA | FROM EARTH GT 20", | SEP OF JUPITER EUROPA | FROM EARTH GT 60", | OLG OF EUROPA BETWEEN 220 320 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |SURF(V) = 5.40 +/- 0.33 | FOR TARGET ACQUISITION 2 |SURF-CONT(3100) = 2.7 +/- 0.7 E-12 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: EUROPA+2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = EUROPA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | TYPE = POS_ANGLE, | SEP OF IO EUROPA FROM EARTH GT 20", RAD = 3.85, | SEP OF GANYMEDE EUROPA ANG = 171.81, | FROM EARTH GT 20", REF = SUN | SEP OF CALLISTO EUROPA | FROM EARTH GT 20", | SEP OF JUPITER EUROPA | FROM EARTH GT 60", | OLG OF EUROPA BETWEEN 220 320 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |SURF(V) = 5.40 +/- 0.33 | FIRST SCIENCE TARGET 2.6 ARCSEC FROM 2 |SURF-CONT(3100) = 2.7 +/- 0.7 E-12 | CENTER OF EUROPA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 5660 [ 5] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: EUROPA+5 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = EUROPA | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | TYPE = POS_ANGLE, | SEP OF IO EUROPA FROM EARTH GT 20", RAD = 5.86, | SEP OF GANYMEDE EUROPA ANG = 171.81, | FROM EARTH GT 20", REF = SUN | SEP OF CALLISTO EUROPA | FROM EARTH GT 20", | SEP OF JUPITER EUROPA | FROM EARTH GT 60", | OLG OF EUROPA BETWEEN 220 320 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |SURF(V) = 5.40 +/- 0.33 | SECOND SCIENCE TARGET 5.0 ARCSEC 2 |SURF-CONT(3100) = 2.7 +/- 0.7 E-12 | FROM CENTER OF EUROPA | ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5660 [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 EUROPA FOS/BL IMAGE 4.3 G270H 3090 Y-SIZE=5 1 35S 60 2 2 SEQ 1-2 NOGAP Y-SPACE=128 CYCLE 4 COMB=YES ORIENT 0D +/- 15D SUB-STEP=4 FROM NOMINAL Comments: FIVE FOS SPECTRA AT DIFFERENT Y DEFLECTIONS WILL BE OBTAINED FOR TARGET POSITION INFORMATION. ORIENT CAN BE EXPANDED IF NECESSARY. ------------------------------------------------------------------------------------------------------------------------------------ 2 EUROPA+2 FOS/BL IMAGE 4.3 G270H 3090 Y-SIZE=1 1 77M 1 2 1 CYCLE 4 Y-OFFSET=-256 SAME ORIENT FOR 2 X=0 AS 1 COMB=YES GUID TOL 0.2" SUB-STEP=4 Comments: SINGLE GUIDE STAR OK ORIENT CAN BE EXPANDED IF NECESSARY. MUST BE SCHEDULED BEFORE SL9 IMPACT PLEASE EXPAND EXPOSURE TO FILL ORBIT MERGE TO PRODUCE EQUAL EXP TIMES ------------------------------------------------------------------------------------------------------------------------------------ 3 EUROPA+5 FOS/RD IMAGE 4.3 G270H 3090 Y-SIZE=1 1 55M 1 2 3 CYCLE 4 COMB=YES GUID TOL 0.2" SUB-STEP=4 Comments: SINGLE GUIDE STAR OK ORIENT CAN BE EXPANDED IF NECESSARY. SCHEDULE CONTIGUOUS TO ORBITS FROM PROGRAM 5385. BLIND POINTING ACQUISITION. ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 5660 [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters Y-SIZE Y-SPACE COMB SUB-STEP Y-OFFSET X ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO/CAR ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 4.00 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category SATELLITES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 1-2 NOGAP; CYCLE 4; ORIENT 0D +/- 15D FROM NOMINAL; SAME ORIENT FOR 2 AS 1; GUID TOL 0.2" ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G270H ------------------------------------------------------------------------------------------------------------------------------------ Target Names EUROPA EUROPA+2 EUROPA+5 ------------------------------------------------------------------------------------------------------------------------------------