18342( 2) - 19-Aug-2026 13:57:31 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18342 Version: 2 Check-in Time: 19-Aug-2026 13:57:31 STScI Edit Number: 0 Title Observing Titan's Haze Distribution: One (Titan) Year Later ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kristin Showalter Sotzen The Johns Hopkins University Applied Y Physics Laboratory CoI: Dr. Ralph D. Lorenz The Johns Hopkins University Applied Y Physics Laboratory CoI: Dr. Erich Karkoschka University of Arizona Y ------------------------------------------------------------------------------------ Abstract Though Titan has been observed from Earth, Earth orbit, Cassini, and most recently JWST for the past several decades, with a year that spans just shy of 30 Earth years, there remain gaps in our observations of Titan's atmospheric variability over the course of its year. It is known that Titan displays interannual variability in seasonal albedo, haze distribution, and tropospheric activity. The time around equinox (i.e., now) when the subsolar point on Titan is changing latitude most quickly, sees the most rapid and profound changes as the asymmetric pole-to-pole Hadley circulation reverses, and substantial changes were observed from one Earth year to the next in previous equinoxes. We will observe Titan near equinox to provide additional data on Titan's seasonal and annual atmospheric variability. Our science objectives include: 1) STIS spectral imaging to build on previous data sets tracking Titan's north-south asymmetry over its year, the first HST observations of which occurred approximately one Titan year ago. 2) WFC3/UVIS imaging in blue and green wavelengths as well as the 775nm, 889 nm, 953 nm methane absorption band to observe the north-south asymmetry and methane distribution as well as measure Titan's albedo for comparison to previous photometric campaigns. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations in Cycles 34 will use two HST orbits per cycle, one for the STIS observations and one for WFC3/UVIS. As for previous successful STIS observations of Titan, the STIS orbit will consist of 15 exposures with the grating G750L for wavelengths 530-1000 nm at 0.5 nm/pixel, providing a spatially-resolved image cube. Each successive exposure is offset by 0.051 arc-sec perpendicular to the slit orientation from the previous one. The aperture is 52X0.05E1. The saved part of the image is 140 spatial rows and all 1024 spectral columns, which provides sufficient background sky wile avoiding buffer dumps during the observation. STIS calibration exposures are a CCDFLAT and WAVECAL taken at start of Earth occultation. Acquisition is accomplished with a 1 sec exposure with the MIRROR and a F28X50LP aperture using ACQTYPE=DIFFUSE, CHECKBOX=19, DIFFUSE-CENTER = FLUX-CENTROID. Exposure times will be the longest possible to fit all Titan 18342( 2) - 19-Aug-2026 13:57:31 - [ 2] exposures into the orbit before occultation starts, about 73 sec each, providing signal-to-noise ratios (SNRs) of ~50 per pixel in the red continuum and ~1000 for the spectrally averaged quantities. These sensitivities enable measurement of relative radiance differences at the <1% level, sufficient to quantify the NSA, center-to-limb variations, and wavelengthdependent haze properties. The wavelength range over a factor of 1.9 probes haze optical depths varying over a factor of 4, including: the visible where haze absorbs; near-infrared wavelengths where haze is bright against absorbing gas (methane); and methane bands with absorption strengths ranging over three orders of magnitude probing eight scale heights in Titan's atmosphere. The spatial sampling provides well-sampled center-to-limb profiles for 15 latitudes, enabling characterization of hemispheric gradients and detection of latitudinal variations in haze opacity and methane absorption at the few-percent level. The WFC3/UVIS orbit will consist of 2 exposures each of the F438W, F336W, FQ889N, F673N, F547M, F953N, and F775W filters. These filters will show the NSA peak in the blue wavelengths and the reversal in the 889 nm methane band. Using the WFC3/UVIS online Exposure Time Calculator (ETC), we find that exposure times for our filters of 11 seconds (F438W), 64 seconds (F336W), 305 seconds (FQ889N), 16 seconds (F673N), 5 seconds (F547M), 175 seconds (F953N), and 4 seconds (F775W) give us SNRs > 175 without saturating. These data enable measurement of hemispheric brightness contrasts to ~1-2% and disk-integrated photometry to <1% precision. We will use a post-flash for each of our WFC3/UVIS exposures to mitigate CTE effects, and we will use the UVIS2-C512C-SUB sub-array to minimize the time needed for buffer dumps. Additionally, we could lengthen the FQ889N exposure time, without saturating, to accommodate a buffer dump to increase science return relative to instrument overhead. ------------------------------------------------------------------------------------ 18342( 2) - 19-Aug-2026 13:57:31 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: TITAN ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE SATELLITE TITAN ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=SATURN STD=TITAN ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF TITAN BY SATURN FROM EARTH, SEP OF TITAN RHEA FROM EARTH GT 10" ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 8.6 +/- 0.4 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 18342( 2) - 19-Aug-2026 13:57:31 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-PERP-TO-SLIT Pattern_Purpose MOSAIC Number_Of_Points 15 Point_Spacing 0.051 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 0.0 Angle_Between_Sides Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 18342( 2) - 19-Aug-2026 13:57:31 - [ 5] Visit: 01 (C34-TITAN-STIS-MAP) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 TITAN STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, CHECKBOX=19, 1 1.0 S (1 DIFFUSE-CENTER=FLUX-CENTROID S) ------------------------------------------------------------------------------------------------------------------------------------ 2 TITAN STIS/CCD ACCUM 52X0.05E1 G750L 7751 WAVECAL=NO, CR-SPLIT=NO, 1 73 S (1095 PATTERN 1 2 GAIN=1, SIZEAXIS2=140 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/CCD ACCUM 52X0.05 G750L 7751 1 DEF (0 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CCDFLAT STIS/CCD ACCUM 52X0.05 G750L 7751 2 DEF (0 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ TITAN 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 1 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 2 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 3 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 4 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 5 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 6 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 7 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 8 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 9 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 10 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 11 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 12 none none none 1 N/A 1 18342( 2) - 19-Aug-2026 13:57:31 - [ 6] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 13 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 14 none none none 1 N/A 1 TITAN 2 STIS/CCD ACCUM 52X0.05E G750L 7751 15 none none none 1 N/A 1 WAVE 3 STIS/CCD ACCUM 52X0.05 G750L 7751 none none none none 1 N/A CCDFLAT 4 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 1 none 1 N/A CCDFLAT 4 STIS/CCD ACCUM 52X0.05 G750L 7751 none none 2 none 1 N/A 18342( 2) - 19-Aug-2026 13:57:31 - [ 7] Visit: 02 (C34-TITAN-WFC3-512) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 TITAN WFC3/UVI ACCUM UVIS2-C51 F438W FLASH=20 2 11 S (22 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 TITAN WFC3/UVI ACCUM UVIS2-C51 F336W FLASH=20 2 64 S (128 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 TITAN WFC3/UVI ACCUM UVIS2-C51 F673N FLASH=20 2 16 S (32 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 TITAN WFC3/UVI ACCUM UVIS2-C51 F547M FLASH=20 2 5 S (10 S) S 2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ 5 TITAN WFC3/UVI ACCUM UVIS-QUAD FQ889N FLASH=20 2 305 S (610 S -SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 TITAN WFC3/UVI ACCUM UVIS2-C51 F953N FLASH=20 2 175 S (350 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 7 TITAN WFC3/UVI ACCUM UVIS2-C51 F775W FLASH=20 2 4 S (8 S) S 2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ TITAN 1 WFC3/UVI ACCUM UVIS2-C5 F438W none none 1 none 1 N/A S 12C-SUB TITAN 1 WFC3/UVI ACCUM UVIS2-C5 F438W none none 2 none 1 N/A S 12C-SUB TITAN 2 WFC3/UVI ACCUM UVIS2-C5 F336W none none 1 none 1 N/A S 12C-SUB TITAN 2 WFC3/UVI ACCUM UVIS2-C5 F336W none none 2 none 1 N/A S 12C-SUB TITAN 3 WFC3/UVI ACCUM UVIS2-C5 F673N none none 1 none 1 N/A S 12C-SUB TITAN 3 WFC3/UVI ACCUM UVIS2-C5 F673N none none 2 none 1 N/A S 12C-SUB TITAN 4 WFC3/UVI ACCUM UVIS2-C5 F547M none none 1 none 1 N/A S 12C-SUB 18342( 2) - 19-Aug-2026 13:57:31 - [ 8] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ TITAN 4 WFC3/UVI ACCUM UVIS2-C5 F547M none none 2 none 1 N/A S 12C-SUB TITAN 5 WFC3/UVI ACCUM UVIS-QUA FQ889N none none 1 none 1 N/A S D-SUB TITAN 5 WFC3/UVI ACCUM UVIS-QUA FQ889N none none 2 none 1 N/A S D-SUB TITAN 6 WFC3/UVI ACCUM UVIS2-C5 F953N none none 1 none 1 N/A S 12C-SUB TITAN 6 WFC3/UVI ACCUM UVIS2-C5 F953N none none 2 none 1 N/A S 12C-SUB TITAN 7 WFC3/UVI ACCUM UVIS2-C5 F775W none none 1 none 1 N/A S 12C-SUB TITAN 7 WFC3/UVI ACCUM UVIS2-C5 F775W none none 2 none 1 N/A S 12C-SUB 18342( 2) - 19-Aug-2026 13:57:31 - [ 9] Summary Form for Proposal 18342 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.05, 52X0.05E1, F28X50LP, UVIS-QUAD-SUB, UVIS2-C512C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, CHECKBOX=19, CR-SPLIT=NO, DIFFUSE-CENTER=FLUX-CENTROID, FLASH=20, GAIN=1, SIZEAXIS2=140, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W, F438W, F547M, F673N, F775W, F953N, FQ889N, G750L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names CCDFLAT, TITAN, WAVE ------------------------------------------------------------------------------------------------------------------------------------