17722( 2) - 25-Aug-2025 16:30:48 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17722 Version: 2 Check-in Time: 25-Aug-2025 16:30:48 STScI Edit Number: 0 Title Seasonal and latitudinal variability of C2H2 in Uranus' stratosphere ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Rohini S Giles Southwest Research Institute Y CoI: Dr. Pippa Molyneux Southwest Research Institute N CoI: Dr. Thomas Kirk Greathouse Southwest Research Institute N CoI: Dr. Joshua Kammer Southwest Research Institute N CoI: Dr. Tracy M Becker Southwest Research Institute N ------------------------------------------------------------------------------------ Abstract Acetylene (C2H2) is formed in Uranus' stratosphere from the solar photolysis of methane, which means that its abundance is highly dependent on both season and latitude. Photochemical models have been developed to predict the spatial and temporal variability of C2H2, but there have been limited observations to date that can be used to test these models. Spatially-resolved mid-infrared observations have recently been used to study the C2H2 distribution, but at those wavelengths there is a degeneracy between chemical abundances and stratospheric temperatures. Far-ultraviolet spectroscopic observations can be used to retrieve the disk-averaged C2H2 abundance and are not sensitive to the atmospheric temperature, providing complementary information to the infrared observations. In 2014, Uranus was observed by HST during its northern spring using the COS G140L grating. These observations show the clear spectral signature of C2H2 and have not yet been published. We request one HST orbit in order to repeat this observation during the 2024/2025 cycle, as Uranus approaches northern summer solstice. We will compare the C2H2 abundances obtained from these HST COS spectra, and from Voyager UVS spectra obtained during southern summer, with predictions from seasonal photochemical models. A difference between the model predictions and the observations could suggest that atmospheric circulation plays a significant role. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to use COS to observe Uranus for one orbit. We will use TIME-TAG spectroscopy with the FUV G140L gratings. The goal of this observations is to measure atmosphere absorption by C2H2. The proposed observations follow the same method as Visit 4 from Proposal 14036, which was executed in November 2014. Using a consistent observing setup will allow to directly compare the new observations with the previous observations, in order to search for seasonal variability in the C2H2 abundance. ------------------------------------------------------------------------------------ 17722( 2) - 25-Aug-2025 16:30:48 - [ 2] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: URANUS ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: PLANET Planet Uranus ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=URANUS ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 5.6 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 17722( 2) - 25-Aug-2025 16:30:48 - [ 3] Visit: 01 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 URANUS COS/NUV ACQ/IMA BOA MIRRORA 1 30 S (30 SEQ 1-2 NON-INT GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 URANUS COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=6000, FP-POS=ALL 1 336 S G (1344 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 | ------------------------------------------------------------------------------------------------------------------------------------ URANUS 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE URANUS 2 COS/FUV TIME-TA PSA G140L 1105 none none none 1 1 N/A G URANUS 2 COS/FUV TIME-TA PSA G140L 1105 none none none 2 1 N/A G URANUS 2 COS/FUV TIME-TA PSA G140L 1105 none none none 3 1 N/A G URANUS 2 COS/FUV TIME-TA PSA G140L 1105 none none none 4 1 N/A G 17722( 2) - 25-Aug-2025 16:30:48 - [ 4] Summary Form for Proposal 17722 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures BOA, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=6000, FP-POS=ALL ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 1-2 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names URANUS ------------------------------------------------------------------------------------------------------------------------------------