4/16/26, 04:00PM: Submitting corrected final final version of HST Cycle 34 Titan WFC3, STIS proposal. 4/16/26, 03:46PM: Submitting corrected final version of HST Cycle 34 Titan WFC3, STIS proposal. 4/16/26, 03:42PM: Submitting final version of HST Cycle 34 Titan WFC3, STIS proposal. 4/16/26, 11:47AM: Submitting version 1 of HST Cycle 34 Titan WFC3, STIS proposal. 402389729 Submission failed due to error communicating with the submission server. Please retry in a few minutes. If this error persists please contact the help desk at apt@stsci.edu. Assigned ID: 7428 ----- Attempting Submission 1 (Thu Apr 16 15:47:43 GMT 2026) ----- HST Phase I Proposal 7428 successfully submitted. Receipt: # 7428-1 ----- Attempting Submission 2 (Thu Apr 16 19:43:08 GMT 2026) ----- HST Phase I Proposal 7428 successfully submitted. Receipt: # 7428-2 ----- Attempting Submission 3 (Thu Apr 16 19:46:11 GMT 2026) ----- HST Phase I Proposal 7428 successfully submitted. Receipt: # 7428-3 ----- Attempting Submission 4 (Thu Apr 16 20:00:50 GMT 2026) ----- HST Phase I Proposal 7428 failed submission. ----- Attempting Submission 5 (Thu Apr 16 20:01:04 GMT 2026) ----- HST Phase I Proposal 7428 successfully submitted. Receipt: # 7428-5 Could not connect to APT Status Server 08/18/2026: Second submission - minor updates to the abstract; removed comments from the first visit, fixed the number for the second visit 08/14/2026: Initial submission 2092351318 ----- Attempting Submission 1 (Fri Aug 14 17:58:30 GMT 2026) ----- HST Phase II Proposal 18342 successfully submitted. Receipt: # 18342-1 ----- Attempting Submission 2 (Tue Aug 18 20:01:24 GMT 2026) ----- APT shows a warning about not being able to connect to the STScI server. Attempting submission anyway. 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. 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 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. Kristin S. Sotzen is a Titan scientist who has experience analyzing HST/WFC3 data and will analyze the data with guidance from CoIs Lorenz and Karkoschka Ralph Lorenz is a preeminent Titan expert with experience assessing Titan's haze distribution based on data from HST Erich Karkoschka has extensive experience analyzing STIS data for changes in Titan's haze distribution Solar System Astronomy 6 false false false false false false false SUPPORTED SOLAR SYSTEM SATELLITE SATELLITE TITAN STD=SATURN STD=TITAN NOT OCC OF TITAN BY SATURN FROM EARTH, SEP OF TITAN RHEA FROM EARTH GT 10" EARTH STIS/CCD Spectroscopic No No G750L 7751 WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No FQ889N WFC3/UVIS Imaging No No F673N WFC3/UVIS Imaging No No F547M WFC3/UVIS Imaging No No F953N POS-TARG MOSAIC DITHER 0.051 STIS-ALONG-SLIT false true 15 0.0 POS-TARG STIS-PERP-TO-SLIT 90.0 1 false /Users/student/Documents/HST_Titan/Phase_II/OP-cache/Titan_Haze_Distribution_C34-v01.xml 1786634964018 The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18342 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "13-Aug-2026 11:29:23.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 92960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18342 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18342 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2587 :al-overhead-at-end 0 :internal-dur-at-end 701 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 1787083182354 /private/var/folders/f2/l206wsps0r1_9x53_3sknw1w0000gn/T/OPVisit10398227971672235459.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18342 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 15:59:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 195696 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18342 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18342 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2562 :al-overhead-at-end 0 :internal-dur-at-end 250 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0