6513( 4) - 07/23/96 16:57 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6513 Version: 4 Check-in Date: 23-Jul-1996 16:52:45 1.Proposal Title: A Targeted HST Search for New Species in the Lunar Atmosphere ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Alan Stern Southwest Research Institute CoI: Tom Morgan Southwest Research Institute N CoI: Ann Sprague University of Arizona N CoI: Michel Festou Observatoire Midi-Pyrenees N CoI: Brian Flynn Southwest Research Institute N CoI: Donald Hunten University of Arizona N CoI: Michael Mendillo Boston University N ------------------------------------------------------------------------------------ 6. Abstract The study of the lunar atmosphere was revolutionized in the late 1980s when Potter & Morgan (1988a) first detected neutral Na and K at emission brightnesses up to a few kilorayleighs. Because the atmospheric Na and K are derived from the surface, it has been suspected that other metal species also exist in the lunar atmosphere. Indeed, straightforward stoichiometry arguments, which correctly predict the observed Na/K ratio, suggest that more abundant surface species, such as Si, Al, and Mg, should also populate the lunar atmosphere. These three species emit most strongly at mid-UV wavelengths longward of the Apollo 17 UV spectrometer red cutoff (Feldman & Morrison 1991), but too far in the UV to be studied from the ground. Detections of one or all of these species would provide important new data about the composition of the lunar atmosphere, and open a new window into the processes that eject atoms from the lunar surface into the lunar atmosphere. We request 2-orbits for HRS and spectroscopy to search for Al, Si, and Mg atoms in the lunar atmosphere. The observations will be made guiding on gyros; FHSTs are not required. We do not need to point at or near the lunar surface; instead, we will take advantage of the 1000-1500 km lunar scale height to place the apertures relatively far (i.e., 0.3-0.7 deg) from the Moon. No previous UV observations of the lunar atmosphere have been made by HST. The anticipated results should also bear on studies of the exospheres at Mercury and Io. ------------------------------------------------------------------------------------ 6513( 4) - 07/23/96 16:57 - [ 2] Observations Description ------------------------ This section discusses (i) how the lunar atmosphere spectroscopy can be most easily carried out, and (ii) the feasibility of detections. Concerning the operational aspects of this program, we first note that we have discussed and developed our plans via discussions with K. Noll of STScI. Our requirements are not stringent for the following reasons: (i) we can observe at any time during Cycle 6 that the Moon is available; (ii) the HRS will be used in echelle mode, so that scattered moonlight will be highly dispersed (S. Hulbert, personal communication); (iii) we will place the slit relatively far, 0.3-0.7 deg from the Moon, to further minimize scattered light effects; (iv) we do not need to guide on a spot as we integrate--- that is, HST can hold on some spot in the sky near the Moon and let the lunar atmosphere drift during the integration (because the scale of the atmosphere is so large; as such, the observation itself can be done on gyros). We additionally note that we can observe at most lunar position angles (cf., Mendillo & Baumgardner 1995). Concerning scattered light, the Cycle 6 FOC handbook (cf., Figure 30) states that the scattered light intensity from the lunar disk takes on the constant value of ~eq18 mag arcsec^-2 inward of 5 degrees from the center of the Moon. This corresponds to count rates of ~eq2 to 10times10^-4 counts diode^-1 sec^-1, which is small compared to even the 10 times-depleted Mg, Si, and Al count rates for the HRS predicted below. A second check on the impact of scattered light is available from studies of scattered light near planetary satellites. In such a study, Shemansky & Maheson (1995) give a scattered light fraction 10^-7 times that of the primary in FOS exposures 100 arsec from Galilean satellites. Although we plan to put the LSA 2-3 times this far away from the Moon, even this level of scatter would produce a count rate of only ~eq 1-3times10^-3 counts diode^- 1 sec^-1. These estimates show that the HRS count rate would not be strongly influenced by the scattered light background unless the species of interest were as much as 100-times depleted. Thus, these two results compare well, indicating that scattered light should not significantly limit these observations, which will be made >1000-3000 arcsec from the lunar limb. We now turn to the issue of species detectability. The observed sodium coma surrounding the Moon exhibits narrow emission lines (~20 mAngstrom FWHM); K is similar. We therefore expect the Mg, Si, and Al lines to have widths like these (varying somewhat of course, depending on their atomic weights). Therefore, the high-resolution HRS Echelle-B (i.e., R~80000) capability is needed to provide the maximum contrast between the emission features and residual scattered lunar continuum. In addition to using ECH-B, we will use the HRS in ACCUM mode and the LSA to maximize sensitivity. Figure 3 shows the expected count rates for the center of the Al, Si, and Mg resonance lines, as a function of distance from the center of the Moon above the subsolar point at quarter phase. We predict that at an altitude of 1.3 lunar radii (0.3 deg above the limb) the 2852 Angstrom Mg I line should yield a count rate of ~20 counts/sec. Similarly, the 2516 Angstrom Si line should yield a count rate of 1 count/sec. As such, a strong (10-sigma) detection of Mg I at the stoichiometrically- predicted level could occur in as little as ~5 sec; a 10-sigma detection of Si I could occur in ~100 seconds. Even if these species are depleted by factors of 50 in their abundance, relative to the stoichiometric model, 5-sigma detections will occur in 60 and 1250 sec, respectively. Our 2-orbit program will consist of three HRS ECH-B integrations targeted to specific lines, and one FOS/BL survey exposure. That is, we will obtain an integration time of ~1400 sec in 12-14 Angstrom-wide bands around each of the three wavelength regions of interest (i.e., order 22 for Si I 2516 Angstrom; order 20 for Mg I 2852 Angstrom; order 18 for Al I 3083, 3092 Angstrom), and a ~800 sec lower-dispersion FOS G270 expsoure to provide a survey of broader wavelength coverage. With the data from this small, 2-orbit program, we expect to explore the lunar atmosphere in key regions of the mid-UV, and make a major advance in our understanding of species abundances and source processes. Real Time Justification ----------------------- We do not require any real-time, dark-time, CVZ, or TOO scheduling. The observations proposed in this program will be coupled to a second International Lunar Atmosphere Week (ILAW 2). Each of the Investigators on this HST proposal were members of the ILAW 1 observing team. We ourselves will conduct reference Na/K imaging and spectroscopy from McDonald, Kitt Peak, and Mt. Lemmon. Other observers involved in ILAW 2 will also be observing the lunar atmosphere during this period, which we hope will form the most complete set of lunar atmosphere observations ever obtained. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6513( 4) - 07/23/96 16:57 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6513( 4) - 07/23/96 16:57 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: MOON-ATMOSPHERE ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET MOON ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=EARTH | STD=MOON | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | TYPE=POS_ANGLE,RAD=2664,ANG=281.5,REF=NO | RTH,R_RAD=0.000833,R_ANG=-15.5,EPOCH=06- | Oct-1996:12:00:00.0 | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | F-LINE(2516)=2.03e-17 | Any azumuth 0.3 to 0.7 deg from the F-LINE(2852)=1.79e-15 | moon (any position angle) can be F-LINE(3092)=6.60e-18 | used for this observation; tracking | a specific point relative to the | moon is not required. HST can be | pointed at any point in the sky | within 0.3 to 0.7 deg of the moon | and kept within that annulus during | integration. | ------------------------------------------------------------------------------------------------------------------------------------ 6513( 4) - 07/23/96 16:57 - [ 5] Visit: 01 Visit Requirements: PCS MODE GYRO 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 MOON-ATMOSP HRS ACCUM 2.0 ECH-B18 3036-32 FP-SPLIT=STD 1 1414.4 HERE 09 Comments: For Al line at 3092 A. ------------------------------------------------------------------------------------------------------------------------------------ 2 MOON-ATMOSP HRS ACCUM 2.0 ECH-B22 2497-26 FP-SPLIT=STD 1 1414.4 HERE 13 Comments: For Si line at 2516 A. ------------------------------------------------------------------------------------------------------------------------------------ 3 MOON-ATMOSP HRS ACCUM 2.0 ECH-B20 2740-28 FP-SPLIT=STD 1 544.0 S HERE 80 Comments: For Mg line at 2852 A. ------------------------------------------------------------------------------------------------------------------------------------ 10 MOON-ATMOSP FOS/BL ACCUM 4.3 G270H 2222-33 1 820 S HERE 01 Comments: Survey spectrum covering full range of mid-UV. ------------------------------------------------------------------------------------------------------------------------------------ 6513( 4) - 07/23/96 16:57 - [ 6] Summary Form for Proposal 6513 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE GYRO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements ECH-B18 ECH-B22 ECH-B20 G270H ------------------------------------------------------------------------------------------------------------------------------------ Target Names MOON-ATMOSPHERE ------------------------------------------------------------------------------------------------------------------------------------