17739( 7) - 04-Nov-2025 14:29:23 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17739 Version: 7 Check-in Time: 04-Nov-2025 14:29:23 STScI Edit Number: 5 Title A UV eclipse test for Callisto's atmosphere, plasma interaction and water ocean ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Lorenz Roth Royal Institute of Technology Y CoI: Dr. John R. Spencer Southwest Research Institute N CoI: Dr. Richard Cartwright The Johns Hopkins University Applied N Physics Laboratory CoI: Dr. Kurt David Retherford Southwest Research Institute N CoI: Dr. Joachim Saur Universitat zu Koln N CoI: Dr. Carl Schmidt Boston University N CoI: Dr. Darrell F. Strobel The Johns Hopkins University N CoI: Dr. Nathaniel J. Cunningham Nebraska Wesleyan University N CoI: Dr. Shane Carberry Mogan University of California - Berkeley N CoI: Dr. Andrew Poppe University of California - Berkeley N ------------------------------------------------------------------------------------ Abstract Jupiter moon Callisto might have a subsurface ocean as well as the fourth densest atmosphere of all solar system moons. The primary O2 atmosphere was inferred indirectly through sensitive HST/COS observations of electron-excited UV oxygen emissions. Because the interaction of Callisto's atmosphere-ionosphere leads to a strong diversion of the magnetospheric electrons, the UV emissions were interpreted to be from photo-electrons. However, optical oxygen emissions were detected in 2021 when Callisto was in eclipse of Jupiter and thus must be excited by magnetospheric electrons and not photo-electrons. Archival COS observation of Callisto in sunlight can not resolve the question of the excitation, but interestingly they suggest O or H2O must be abundant in addition to O2. After a 3-year period without eclipses, Callisto will be again eclipsed by Jupiter in the coming HST cycle. We propose sensitive COS observations of the diagnostic 1304 A and at 1356 A emissions from before ingress, through the eclipse passage until after egress. The changes in UV emissions over eclipse will reveal the excitation mechanism and provide constraints on the composition of the atmosphere. If the UV emissions disappear in eclipse, a strong plasma diversion and dense ionosphere are confirmed, which challenges the necessity of a subsurface ocean to explain magnetic induction. ESA's JUpiter Icy Moon Explorer and NASA's Europa Clipper mission will carry out several flybys at Callisto. An improved understanding of the moon's atmosphere and ocean delivers important information for optimal science planning and addressing the habitability goals for these milestone missions. ------------------------------------------------------------------------------------ Observations Description ------------------------ We observe Callist with 7 orbits to cover the complete transition from before ingress to after egress from eclipse. 1 visit with 2 orbits around ingress into eclipse, 17739( 7) - 04-Nov-2025 14:29:23 - [ 2] 1 visit with 3 orbits starting in eclipse and continuing to 1 hour after egress. 2 orbits for reference spectra at an offset from the target for background correction. Crucially, the moment of going into and emerging from total umbral shadow should be within an exposure. For the egress visit, the first orbit should be at the egress and the second orbit/exposure will be ~1 hour after egress, as the time scale for recovery of a sublimated H2O component are on this order. After a season without eclipses, Callisto is eclipsed again from August 2025. Optimally, the visits are scheduled as close in time as possible (same or consecutive eclipse passages). For the offset, the aperture is placed 5" away along the detector y axis, identical for the large offsets used in GO 14755. The offset exposures can be taken when Callisto is eclipsed or in sunlight. We do not set parameter to constrain exact geometries (like ingress) but will look into possible scheduling options case by case. ------------------------------------------------------------------------------------ 17739( 7) - 04-Nov-2025 14:29:23 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: CALLISTO-SUBJOVIAN ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Jupiter moon Callisto ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=JUPITER STD=CALLISTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF CALLISTO-SUBJOVIAN BY JUPITER FROM EARTH, SEP OF CALLISTO-SUBJOVIAN IO FROM EARTH GT 10", SEP OF CALLISTO-SUBJOVIAN EUROPA FROM EARTH GT 10", SEP OF CALLISTO-SUBJOVIAN GANYMEDE FROM EARTH GT 10", CML OF CALLISTO-SUBJOVIAN FROM EARTH BETWEEN 330 30 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 5.65 +/- 0.1 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: CALLISTO-OFFSET ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Offset from Callisto for reference ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=JUPITER STD=CALLISTO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF CALLISTO-OFFSET BY JUPITER FROM EARTH, SEP OF CALLISTO-OFFSET IO FROM EARTH GT 10", SEP OF CALLISTO-OFFSET EUROPA FROM EARTH GT 10", SEP OF CALLISTO-OFFSET GANYMEDE FROM EARTH GT 10", CML OF CALLISTO-OFFSET FROM EARTH BETWEEN 330 30 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 0 +/- 0 Ephemeris Center: EARTH Extended: YES 17739( 7) - 04-Nov-2025 14:29:23 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 TARGET NAME: EUROPA-ACQ ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Europa for acquisition ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=JUPITER STD=EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF EUROPA-ACQ BY JUPITER FROM EARTH, SEP OF EUROPA-ACQ IO FROM EARTH GT 10", SEP OF EUROPA-ACQ GANYMEDE FROM EARTH GT 10", SEP OF EUROPA-ACQ CALLISTO FROM EARTH GT 10" ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 5.3 +/- 0.3 Europa for acquistion for visit when Callisto is in eclipse at start. Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 17739( 7) - 04-Nov-2025 14:29:23 - [ 5] Visit: 01 (Ingress) Visit Requirements: BETWEEN 2025.316:12:18:00 AND 2025.316:13:18:00 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 CALLISTO-SU COS/NUV ACQ/SEA PSA G230L 3000 SCAN-SIZE=3, 1 10 S (10 BJOVIAN RCH CENTER=FLUX-WT-FLR S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CALLISTO-SU COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, 1 1200 S BJOVIAN G BUFFER-TIME=1400, FP-POS=3 see below (1388 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CALLISTO-SU COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, 1 2000 S BJOVIAN G BUFFER-TIME=1400, FP-POS=4 see below (2163 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CALLISTO-SU COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, 1 1200 S BJOVIAN G BUFFER-TIME=1400, FP-POS=3 see below (2096 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CALLISTO-SU COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, 1 2000 S BJOVIAN G BUFFER-TIME=1400, FP-POS=4 see below (2163 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 CALLISTO-SU COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, 1 2000 S BJOVIAN G BUFFER-TIME=1400, FP-POS=3 see below (2096 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 | ------------------------------------------------------------------------------------------------------------------------------------ CALLISTO-SUBJOVIAN 1 COS/NUV ACQ/SEA PSA G230L 3000 none none none none 1 N/A RCH CALLISTO-SUBJOVIAN 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 1388.0 S G CALLISTO-SUBJOVIAN 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2163.0 S G CALLISTO-SUBJOVIAN 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 3 2096.0 S G CALLISTO-SUBJOVIAN 5 COS/FUV TIME-TA PSA G130M 1291 none none none none 4 2163.0 S G CALLISTO-SUBJOVIAN 6 COS/FUV TIME-TA PSA G130M 1291 none none none none 5 2096.0 S G 17739( 7) - 04-Nov-2025 14:29:23 - [ 6] Visit: 03 (Offset 5.0 #1) Visit Requirements: On Hold Comments: Additional Comments: Offset exposure using POS TARG (x=0, y=5) for background measurement. Offset setup identical to large offset in GO 14755 for comparability. 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 CALLISTO-OF COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, FP-POS=3, 1 2000 S POS TARG null, 5 FSET G BUFFER-TIME=2500 see below (2192 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 | ------------------------------------------------------------------------------------------------------------------------------------ CALLISTO-OFFSET 1 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2192.0 S G 17739( 7) - 04-Nov-2025 14:29:23 - [ 7] Visit: 04 (Offset 5.0 #2) Visit Requirements: On Hold Comments: Additional Comments: Offset exposure using POS TARG (x=0, y=5) for background measurement. Offset setup identical to large offset in GO 14755 for comparability. 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 CALLISTO-OF COS/FUV TIME-TA PSA G130M 1291 SEGMENT=BOTH, FP-POS=4, 1 2000 S POS TARG null, 5 FSET G BUFFER-TIME=2500 see below (2189 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 | ------------------------------------------------------------------------------------------------------------------------------------ CALLISTO-OFFSET 1 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2189.0 S G 17739( 7) - 04-Nov-2025 14:29:23 - [ 8] Summary Form for Proposal 17739 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/SEARCH, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1400, BUFFER-TIME=2500, CENTER=FLUX-WT-FLR, FP-POS=3, FP-POS=4, SCAN-SIZE=3, SEGMENT=BOTH ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 2025.316:12:18:00 AND 2025.316:13:18:00, POS TARG null, 5 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names CALLISTO-OFFSET, CALLISTO-SUBJOVIAN ------------------------------------------------------------------------------------------------------------------------------------