6706( 3) - 02/15/96 09:27 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6706 Version: 3 Check-in Date: 15-Feb-1996 09:08:44 1.Proposal Title: DEUTERIUM ABUNDANCE IN THE UPPER ATMOSPHERE OF VENUS ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: JEAN-LOUP BERTAUX SERVICE D AERONOMIE DU CNRS CoI: John T. CLARKE University of MICHIGAN N CoI: Mike J. MUMMA NASA GSFC N CoI: Tobias OWEN University of Hawaii N ------------------------------------------------------------------------------------ 6. Abstract It is proposed to measure with GHRS the Deuterium and Hydrogen Ly-alpha intensities from the sunlit disk of Venus at maximum elongation, in order to determine the abundance of Deuterium in the upper atmosphere (z =100 km). It is a well recognized fact that the D/H ratio in the lower atmosphere is 1.5 x 10^- 2, showing an enrichment of Deuterium with respect to the Earth of a factor more than 100. However, previous IUE observations have failed to show any Deuterium Ly-alpha emission, implying a D/H ratio of a few 10-3 only in the upper atmosphere. The purpose of the present proposal is to detect the presence of D atoms in the upper atmosphere of Venus, or to place a more stringent upper limit. Indeed, the abundance of D atoms in the upper atmosphere is a crucial parameter to estimate the present escape flux of D atoms from the planet, which in turn is very important to interpret the high D/H ratio in the lower atmosphere in terms of the actual history of H2O in the plan. et Venus. The two lines D and H are separated by 0.33Angstrom and well resolved with HRS Echelle A. The D/H ratio in the bulk lower atmosphere transfers into a different D/H Ly-alpha emission ratio because of atmospheric processes, different solar excitation rates, and radiative transfer. All these effects require modellings which are well mastered by the proposers, with computer codes used in particular for the interpretation of the only positive planetary D Ly-alpha measurement performed up to now (Earth from Atlas-1). ------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 2] Observations Description ------------------------ During Cycle 6 time frame, there is only one opportunity to observe Venus at maximum elongation near 20 August 96. This would be done with HRS, Echelle A grating, with the LSA as were done our previous observations of Mars D/H ratio (Bertaux et al., 1992). In principle, the STIS instrument will replace HRS after January 97, and the next opportunity with STIS would have to wait March 98. We discuss below the compared merits of the use of the two instruments for the proposed observation. We have investigated the anticipated capabilities of the STIS for observing the H Ly alpha line profile from an extended source of emission.The efficiency of the STIS with echelle grating is expected to be similar to that of GHRS with Echelle A. However, the STIS has different effective resolution and sensitivity when observing diffuse emissions. In this case the sensitivity is determined by the product of the efficiency and the area of the aperture, and the resolution is degraded by the width of the aperture. The candidate STIS apertures are the planetary 45 deg. apertures (0.6 arc sec by fraction of 35.7 arc sec falling on detector) and the sky aperture (0.5 or 0.2 arc sec by 52 arc sec). The resolution is somewhat degraded for the planetary aperture also by the 45 deg. rotation with respect to the dispersion direction, and the area of the planetary apertures which will fall onto the MAMA detector is as yet undetermined. Following a discussion with Randy Kimble of th. e STIS team, we derive the followi Aperture Dimension Area Resolution (arc sec) (sq. arc sec) (Angstroms) GHRS Ech A LSA--------1.74 x 1.74--------- 3.0--- ------------- 0.1 STIS Plan 45 deg.--less than 35.7 x 0.6--- less than 21-------------- .13 STIS Sky 1---------------52 x 0.5-------------15 --------- -------.08 STIS Sky 2------------ ---52 x 0.2---------------6---------------.03 For observations of Venus to separate the H and D Ly alpha lines (0.33 Angstrom separation), the preferred STIS mode will be the planetary apertures assuming at least the diameter of Venus fits onto the detector. Venus near elongation on 20 August 1996 will have a diameter of 23 arc sec, giving a sensitivity (23 x 0.6) / (3.0) = 4.6 times that of the GHRS with Echelle A. In addition, we will have coverage of the geocoronal emission off the Venus disk for scaling and subtraction, and we will have spatial resolution across the Venus disk to look for the expected limb brightening of the optically thin D Ly alpha emission. The slant column near the limb increases as sqrt(2piR/H), which may lead to a limb enhancement of the D line of 10-30 times over a narrow spatial range at the limb. As a conclusion, it seems that STIS could have a higher sensitivity than GHRS for the detection of D in the upper atmosphere of Venus. However, we see one advantage to perform Venus observations during cycle 6 : it is because SOHO spacecraft (ESA/NASA) will be in flight at this time, and one will be able to measure with the SUMER instrument precisely the solar Ly-alpha profile, which center illuminates H, and which blue wing illuminates the D atoms at a rate which depends on the state of solar activity. Exposure time with HST/LSA/Echelle A: Our observations of Mars with HST/HRS have yielded an upper limit (or 2 sigma positive detection) of 30 Rayleigh for the D Ly-alpha emission of Mars with about 80 minutes of exposure time. For Venus, we would like to reach a level below our previous IUE lower limit of 300 R, say 100 R at 2 sigma level. Therefore, a total of 40 minutes of exposure time seems reasonable to shoot for. The observing strategy employed in the Esposito program was for HST to slew to the location of Venus while in the Earth's shadow, observe Venus from shadow as soon as it rises, then slew away before sunrise. Following discussion with the STScI staff we assume that blind pointing can be used with a few arc sec accuracy or, if necessary, we could offset from a nearby guide star more than 50 deg. from the Sun, we will be able to get 10 min. exposure time per orbit in this mode. Four orbits would then be required for 40 min. total exposure, plus a 5th orbit for the sky exposure. Real Time Justification ----------------------- The present proposal to observe Venus was made for cycle 1. At that time it was rejected because HST was not allowed to look at Venus (about 46 degrees max imum elongation). Since then, successfull operations were performed on Venus in January 95 for another programme (SO2, P.I. L. Esposito), proving that it is indeed possible. The LSA FOV should be pointed near the bright limb of Venus, but still well within the bright disc. One exposure of about 5-10 minutes should be carried out, depointed from Venus by about 5 arc min. in order to get the pure geocoronal signal to be subtracted from Venus observations and yield the H Lyman-alpha from Venus. If this proposal is accepted, we will organize with the PI of SUMER on SOHO a campaign to measure the actual solar La profile at the time of observations (taking into account the solar rotation). Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: SUNLIT-VENUS ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET VENUS ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=VENUS | TYPE=POS_ANGLE,RAD=5.,ANG=0.,REF=SUN | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | SURF(V) = 1.5 | Center GHRS LSA on sunlit Venus: SURF-LINE(1216)=3.+/-1.e-11 | offset 5 arc sec from Venus center W-LINE(1216)=0.08+/-0.04 | toward the Sun. | ------------------------------------------------------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 5] Visit: 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 101 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S SEQ 101-109 NON-INT VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON Comments: ECH-A SPECTRUM OF SUNLIT VENUS DISK - SERIES OF NINE ORBITS WITH 4m:32s INTEGRATION PER ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 102 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 103 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 104 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 105 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 6] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 106 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 107 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 108 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 109 SUNLIT- HRS ACCUM 2.0 ECH-A 1215.5 FP-SPLIT = 1 272S VENUS NO,STEP-PATT = 9,STEP-TIME = 1.0,DOPPLER = ON,SPYBAL = NO Comments: ECH-A SPECTRUM WITH NO SPYBAL BETWEEN SPECTRA FOR SAME WAVELENGTH SCALE ------------------------------------------------------------------------------------------------------------------------------------ 6706( 3) - 02/15/96 09:27 - [ 7] Summary Form for Proposal 6706 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FP-SPLIT = NO STEP-PATT = 9 STEP-TIME = 1.0 DOPPLER = ON SPYBAL = NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 101-109 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements ECH-A ------------------------------------------------------------------------------------------------------------------------------------ Target Names SUNLIT-VENUS ------------------------------------------------------------------------------------------------------------------------------------