6219( 0) - 07/18/95 21:11 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6219 Version: ********** Check-in Date: ********** 1.Proposal Title: THE VERTICAL STRUCTURE OF THE ATMOSPHERE OF NEPTUNE ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM,OUTER PL GTO/WF2 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: John Trauger Jet Propulsion Laboratory CoI: Jon Holtzman Lowell Observatory N CoI: Jeremy Mould Mt. Stromlo and Siding Spring Observatory N CoI: James Westphal California Institute of Technology N CoI: Christopher Burrows Space Telescope Science Institute N CoI: John Clarke University of Michigan N CoI: David Crisp Jet Propulsion Laboratory N CoI: John Hoessel University of Wisconsin-Madison N CoI: John Gallagher University of Wisconsin-Madison N CoI: Richard Griffiths Johns Hopkins University N CoI: Jeff Hester Arizona State University N ------------------------------------------------------------------------------------ 6. Abstract We propose to use WFPC2 to make synoptic-scale observations of the atmosphere of Neptune. These observations will allow us to follow up and expand on our successful Cycle 4 WFPC2 Neptune program. The new observations will place more stringent constraints on the composition and the vertical structure of the clouds and hazes above the main cloud deck. They will also help to resolve the nature, longevity, and dynamics of the bright streaks, dark spots, and other large-scale features observed by during the Voyager Neptune encounter and our Cycle 4 program. ------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 2] Observations Description ------------------------ Neptune is currently the most distant planet from the sun, with an orbital radius of 4.5 billion kilometers (30 Astronomical Units). Even though its diameter is about four times that of the Earth (49,420 vs. 12,742 km), ground- based telescopes reveal a tiny blue disk that subtends less than 2.3 arc- seconds. Neptune has therefore been a particularly challenging object to study from the ground because its disk is badly blurred by atmospheric seeing. In spite of this, ground-based astronomers had learned a great deal about this planet since its position was first predicted by John C. Adams and Urbain Leverrier in 1845. For example, they had determined that Neptune was composed primarily of hydrogen and helium gas, and that its blue color caused by the presence of trace amounts of the gas methane, which absorbs red light. They had also detected bright cloud features whose brightness changed with time, and tracked these clouds to infer rotation periods between 17 and 22 hours. When the Voyager-2 spacecraft flew past the Neptune in 1989, its instruments revealed a surprising array of meteorological phenomena, including strong winds, bright, high-altitude clouds, and two large dark spots attributed to long-lived giant storm systems. These bright clouds and dark spots were tracked as they moved across the planet's disk, revealing wind speeds as large as 325 meters per second (730 miles per hour). The largest of the giant, dark storm systems, called the Great Dark Spot (GDS), received special attention because it resembled Jupiter's Great Red Spot, a storm that has persisted for more than three centuries. The lifetime of the GDS could not be determined from the Voyager data alone, however, because the encounter was too brief. Its nature and evolution has been impossible to monitor with ground-based telescopes because it could not be resolved on Neptune's tiny disk, and its contribution to the disk-integrated brightness of Neptune confused by the presence of a rapidly-varying bright cloud feature, called the "Bright Companion" that usually accompanied the Great Dark spot. The Voyager observations also provided inadequate constraints on the composition and vertical distribution of the aerosols that produce principal atmospheric features, including the main cloud deck, the discrete dark spots (GDS and the smaller spot, called D2), and the bright cloud bands. Uncertainties in the altitudes of these features have frustrated attempts to determine their compositions because these aerosols have no distinctive spectral features. Plausible candidate compositions are therefore derived by identifying the materials that could condense at the altitude (and temperatures) where the clouds are observed. For example, the main cloud deck, which forms the base of the visible atmosphere at about 3 bars, is thought to be composed of hydrogen sulfide (H2S) ice, since this material condenses these pressures (if the H2S concentration is comparable to that seen on other Jovian planets). Similarly, the bright cloud bands are thought to be composed of methane (CH4) ice, since this gas is thought to been near saturation in the upper troposphere and lower stratosphere. Once again, these phenomena cannot be studied with ground-based telescopes because individual these different clouds cannot be resolved by ground-based telescopes. HST provides new opportunities to study these and other phenomena in Neptune's atmosphere. Images taken with WFPC2's Planetary Camera (PC) can resolve Neptune's disk as well as most ground-based telescopes can resolve the disk of Jupiter. The spatial resolution of these images is not as high as that obtained by the Voyager-2 Narrow-Angle Camera during that spacecraft's closest approach to Neptune, but they have a number of other assets that enhance their scientific value, including improved ultra-violet and infrared sensitivity, better signal-to-noise, and, and greater photometric accuracy. The images of Neptune acquired by the WFPC2 Science team in late June clearly demonstrate these capabilities. The side of the planet facing the Earth at the start of the program (11:36 Universal Time on July 27) was imaged in filters spanning the ultraviolet (255 and 300-nm), visible (467, 588, 620, and 673- nm), and near-infrared (890-nm). The planet then rotated 180 degrees in longitude, and the opposite hemisphere was imaged in a subset of these filters (300, 467, 588, 620, and 673-nm). The ultraviolet images show an almost featureless disk that is slightly darker near the limb. The observed contrast increases in the blue, green, red, and near-infrared images, which reveal many of the features seen by Voyager 2, including the dark band near 60 S latitude and several distinct bright cloud features. The bright cloud features are most obvious in the red and infrared parts of the spectrum where methane gas absorbs most strongly (619 and 890 nm). These bright clouds thought to be high above the main cloud deck, and above much of the absorbing methane gas. The edge of the planet's disk also appears somewhat bright in these filters, indicating the presence of a ubiquitous, high-altitude haze layer. The northern hemisphere is occupied by a single prominent cloud band centered near 30 N latitude. This planet-encircling feature may be the same bright cloud discovered in the fall of 1993 by ground-based observers. Northern hemisphere clouds were much less obvious at the time of the Voyager-2 encounter. The tropics are about 20 % darker than the disk average in the 890-nm images, and one of these images reveals a discrete bright cloud on the equator, near the limb. The southern hemisphere includes two broken bright bands. The largest and brightest is centered at 30 S latitude, and extends for least 40 degrees of longitude, like the Bright Companion to the Great Dark Spot. There is also a thin cloud band at 45 S latitude, which almost encircles the planet. One feature that is conspicuously absent is the storm system known as the Great Dark Spot. The second smaller dark spot, DS2, that was seen during the Voyager-2 encounter was also missing. The absence of these dark spots was one of the biggest surprises of this program. The WFPC2 Science team initially assumed that the two storm systems might be near the limb, where they would not be particularly obvious in these images. An analysis of our longitude coverage revealed that less than 20 degrees of longitude had been missed at wavelengths where these spots had their greatest contrast (467 and 588 nm). The Great Dark Spot covered almost 40 degrees of longitude at the time of the Voyager-2 fly-by. Even if it were on the limb, it would appear as a dark "bite" out of the limb. Because no such feature was detected, we concluded that these features had vanished. This conclusion was reinforced by the more recent WFPC2 observations by Hammel and Lockwood, which also show no evidence of high contrast discrete dark spots. We are also analyzing these data with sophisticated radiative transfer models to place new constraints on the vertical structure of the aerosols in Neptune's atmosphere. This effort is still in progress, but has produced several intersting preliminary results. For example, we find no compelling evidence of the H2S cloud deck at pressures near 3-5 bars, that has been assumed in virtually every published study of the vertical structure of Neptune's atmosphere. The WFPC2 data can best be fit by a rayleigh-scattering atmosphere, with very little aerosol scattering below the 1.5 bar level. This result is consistent with our recent IRTF/CSHELL observations of Neptune, which show no evidence for H2S. In addition, with the recently revised, and dramatically-lower stratospheric methane abundances, our models indicate the bright cloud streaks are near the 1--1.5 bar level, where methane gas is near saturation. These dramatic changes in the large-scale storm systems and planet- encircling clouds bands on Neptune are not yet completely understood, but they emphasize the dynamic nature of this planet's atmosphere, and the need for new observations. Additional spectral coverage is also needed to confirm the intriguing vertical structure results that we have derived from the Cycle 4 data. our Cycle program is designed to address these needs. Unlike our Cycle 4 program, which acquired images of only two sides of the planet, with central meridians separated by ~180 degrees of longitude, this program will provide images of the planet at 3 rotation angles, separated by ~120 degrees of rotation. This approach will provide improved coverage of all longitudes, and minimize foreshortening near the planet's limb. Each visit will consist of a single orbit. During each of these orbits, we will acquire images using F300W, F467M, 588N, FQCH4N15 (619nm), and FQCH4P15 (890nm). This range of spectral filters will allow us to map all of the atmospheric phenomena mentioned above, and provide additional constraints on the vertical structure of the aerosols that produce these phenomena. Our Cycle 5 program include two additional orbits to provide improved spectral constraints on the aerosols in the lower troposphere (> 1 bar). These measurements will confirm or refute our preliminary findings concerning the absence of the lower cloud layer. Our modeling studies show that Neptune's albedo is particularly sensitive to the abundance of lower tropospheric aerosols at wavelengths between the strong methane bands at 619, 727, and 890 nm. At shorter wavelengths, the albedo is dominated by Rayleigh scattering. At wavelengths within these methane bands, it is dominated by methane absorption. We have therefore added Ramp filter exposures at the continuum wavelengths near 750, 826, and 833nm. The 750 and 833 nm exposures must be taken on the WFC's, but the 826 nm exposure can be taken on the PC. Results obtained at these wavelengths will be compared to exposures taken in the strong methane bands at 619 and 890 nm. Observations will also be taken in the strong methane band at 727nm (FQCH4N, WFC-4). Real Time Justification ----------------------- None Calibration Justification ------------------------- This program requires calibration data for the Ramp filters. This calibration includes flat fields, wavelength calibration (done with Argon lines from the calchannel D2 lamp) and photometric calibration with standard stars. Photometric calibration can best be achieved by placing standard stars at the positions on the ramp where Neptune will be centered. Additional Comments ------------------- Orbits 4 and 5 each include three ramp filter exposures. The spectral calibration of the Ramp filters by the WFPC2 Science Team indicates that the 826nm exposure (3rd exposure in orbits 4 and 5) can best be acquired with Ramp filter FR868N by placing the object (Neptune) on the PC, and using this filter in the nominal (zero-degree) rotation. This pointing is not currently accepted by the RPS2 software, however. ------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: NEPTUNE ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: PLANET NEPTUNE ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = NEPTUNE | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1 = 16-MAY-1995 TO 16-SEP-1995 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | SURF(V) = 7.9 | OBSERVATIONS TAKEN AS NEAR POSSIBLE F(3000.0) = 0.0 | TO JULY 16, 1995 OPPOSITION. F(4670.0) = 0.0 | F(5880.0) = 0.0 | F(6265.0) = 0.0 | F(7270.0) = 0.0 | F(7500.0) = 0.0 | F(8260.0) = 0.0 | F(8330.0) = 0.0 | F(8940.0) = 0.0 | SIZE = 2.3 arcsec | | ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 5] Visit: 01 Visit Requirements: BETWEEN 16-MAY-1995:00:00:00 AND 17-Sep-1995:23:30:21; PCS MODE F 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 ------------------------------------------------------------------------------------------------------------------------------------ 11 NEPTUNE WFPC2 IMAGE PC1 F300W CLOCKS=NO, CR- 1 180S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 12 NEPTUNE WFPC2 IMAGE PC1 F467M CLOCKS=NO, CR- 1 40S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 13 NEPTUNE WFPC2 IMAGE PC1 F588N CLOCKS=NO, CR- 1 70S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 14 NEPTUNE WFPC2 IMAGE FQCH4N1 FQCH4N15 6193.0 CLOCKS=NO, CR- 1 350S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 15 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 260S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 16 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 140S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 6] Visit: 02 Visit Requirements: AFTER 01 BY 325M TO 21300S; On Hold Comments: Additional Comments: Timed for 1/3 of a Neptune rotation after the First Visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 NEPTUNE WFPC2 IMAGE PC1 F300W CLOCKS=NO, CR- 1 180S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 22 NEPTUNE WFPC2 IMAGE PC1 F467M CLOCKS=NO, CR- 1 40S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 23 NEPTUNE WFPC2 IMAGE PC1 F588N CLOCKS=NO, CR- 1 70S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 24 NEPTUNE WFPC2 IMAGE FQCH4N1 FQCH4N15 6193.0 CLOCKS=NO, CR- 1 350S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 25 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 260S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 26 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 140S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 7] Visit: 03 Visit Requirements: AFTER 01 BY 665M TO 41700S On Hold Comments: Additional Comments: Timed for 2/3 of a Neptune rotation after the First Visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 NEPTUNE WFPC2 IMAGE PC1 F300W CLOCKS=NO, CR- 1 180S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 32 NEPTUNE WFPC2 IMAGE PC1 F467M CLOCKS=NO, CR- 1 40S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 33 NEPTUNE WFPC2 IMAGE PC1 F588N CLOCKS=NO, CR- 1 70S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 34 NEPTUNE WFPC2 IMAGE FQCH4N1 FQCH4N15 6193.0 CLOCKS=NO, CR- 1 350S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 35 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 260S 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 36 NEPTUNE WFPC2 IMAGE FQCH4P1 FQCH4P15 8922.0 CLOCKS=NO, CR- 1 140S END ORBIT 5 SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 8] Visit: 04 Visit Requirements: AFTER 01 BY 1020M TO 306000S On Hold Comments: Additional Comments: Should be taken as soon as possible after 3rd visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 41 NEPTUNE WFPC2 IMAGE FQCH4W4 FQCH4N 7274.0 CLOCKS=NO, CR- 1 260S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 42 NEPTUNE WFPC2 IMAGE LRF LRF 7500.0 CLOCKS=NO, CR- 1 100S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 43 NEPTUNE WFPC2 IMAGE LRF LRF 8260.0 CLOCKS=NO, CR- 1 350S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 44 NEPTUNE WFPC2 IMAGE LRF LRF 8330.0 CLOCKS=NO, CR- 1 230S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 9] Visit: 05 Visit Requirements: AFTER 04 BY 510M TO 33854S; On Hold Comments: Additional Comments: Should be taken 8-10 hours after 4th visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 51 NEPTUNE WFPC2 IMAGE FQCH4W4 FQCH4N 7274.0 CLOCKS=NO, CR- 1 260S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 52 NEPTUNE WFPC2 IMAGE LRF LRF 7500.0 CLOCKS=NO, CR- 1 100S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 53 NEPTUNE WFPC2 IMAGE LRF LRF 8260.0 CLOCKS=NO, CR- 1 350S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 54 NEPTUNE WFPC2 IMAGE LRF LRF 8330.0 CLOCKS=NO, CR- 1 230S SPLIT=NO, ATD- GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6219( 0) - 07/18/95 21:11 - [ 10] Summary Form for Proposal 6219 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=NO CR-SPLIT=NO ATD-GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GTO/WF2 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM,OUTER PLANETS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 16-MAY-1995 00 00 00 AND 17-Sep-1995 23 30 21; PCS MODE F AFTER 01 BY 325M TO 21300S; AFTER 01 BY 665M TO 41700S END ORBIT AFTER 01 BY 1020M TO 306000S AFTER 04 BY 510M TO 33854S; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300W F467M F588N FQCH4N15 FQCH4P15 FQCH4N LRF ------------------------------------------------------------------------------------------------------------------------------------ Target Names NEPTUNE ------------------------------------------------------------------------------------------------------------------------------------