0 Assigned ID: 5183 ----- Attempting Submission 1 (Mon Mar 25 15:21:00 GMT 2024) ----- COS FUV PSA science exposures with extended targets have special calibration limitations. See "Errors and Warnings" for more details. 0 ----- Attempting Submission 1 (Thu Aug 01 20:57:32 GMT 2024) ----- Abolute calibration is not critical to this project - we are primarily interested in the depth of the absorption line at 150 nm relative to the adjacent continuum. 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. 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. This team proposing these observations has significant experience in HST UV observations and UV studies of planetary atmospheres. PI Giles has led two studies analyzing C2H2 absorption in far-UV spectra of Jupiter (Giles et al., 2021; Giles et al., 2023). Co-Is Molyneux and Becker both have extensive HST/COS experience. Between them, they have led five previous HST/COS programs (14609, 15094, 15868, 16211 and 16652) and both have specific experience in processing and analyzing COS G140L reflectance observations (Molyneux et al., 2018; Molyneux et al., 2020). Co-Is Greathouse and Kammer are both experts in the UV observations of planetary atmospheres. All five team members are members of the Ultraviolet Spectrograph (UVS) instrument teams for the Juno, Europa Clipper and/or JUICE missions. Solar System Astronomy 6 false false true false false false false false SUPPORTED SOLAR SYSTEM YES PLANET Planet Uranus STD=URANUS EARTH COS/FUV Spectroscopic No No G140L 1105 1722353915997 /Users/rgiles/Documents/Telescope Proposals/HST/2024/OP-cache/hst_phase2-v01.xml Using ephemeris for target 17722-01-URANUS The scheduling data was generated by CASM 32.1.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17722 :visit-id "01" :proposal-version -1 :trans-version "20240604" :date "30-Jul-2024 10:38:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 52416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17722 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17722 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2263 :al-overhead-at-end 0 :internal-dur-at-end 350 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2645 :pcs-gap 0 ) true 1 0