18389( 2) - 18-Aug-2026 14:53:13 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18389 Version: 2 Check-in Time: 18-Aug-2026 14:53:13 STScI Edit Number: 0 Title NUV Exospheric Magnesium as a Constraint on Europa's Ocean Salinity ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Emma Lovett Boston University Y CoI: Dr. Richard Cartwright The Johns Hopkins University Applied N Physics Laboratory CoI: Dr. Carl Schmidt Boston University N CoI: Prof. John T. Clarke Boston University N ------------------------------------------------------------------------------------ Abstract The characteristics of Europa's subsurface water depend on its salinity, which remains poorly constrained. While there is good evidence for NaCl salt and Na gas present at Europa's surface and exosphere, respectively, it is challenging to quantify how much of this material is actually sourced from Europa's saltwater, as opposed to being painted on by the ton/s of material escaping the adjacent moon, Io. Mg-based salts on the other hand, which have long been suspected at Europa, cannot derive from Io. The mixture of these dissolved salts in Europa's oceans has critical influence over conductivity and freezing points, but still remains unclear: NaCl and Mg-based salts have overlapping features in near-IR reflectance spectroscopy, and in situ mass spectrometer and plasma wave measurements have insufficient resolution to distinguish the Na and Mg mass-to-charge. The proposed observations address this issue by constraining near-UV Mg emissions in Europa's exosphere. A careful search for these emissions has yet to be undertaken. A small program of 4 orbits with STIS that would offer insightful new constraints on the composition of Europa's hydrated salts is proposed. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our program aims to detect or place upper limits of both neutral and ionized magnesium in Europa's exosphere. This will be achieved by sampling Europa's exosphere at both western and eastern elongations from Jupiter, when the moon is close to or in Jupiter's plasma equator to maximize sputtering yields, and by orienting the slit such that it passes through Europa's orbital plane where most gas is confined. We will use the STIS G230MB CCD grating mode with the c2836 center wavelength in two visits with two orbits each. One visit will view Europa's trailing hemisphere at western elongation from Jupiter. The first orbit will be used to acquire Europa, take an on-disk spectrum with the 52x0.5 arcsec slit that will act as a fiducial sunlight spectrum due to the high surface reflectivity, and then offset 1.2 arcseconds north to remove Europa's bright limb from the slit. The second orbit of this visit will then offset 1.2 arcseconds south of Europa and take another long exposure. This process will be repeated for a second visit that views Europa's leading hemisphere at eastern elongation from Jupiter. ------------------------------------------------------------------------------------ 18389( 2) - 18-Aug-2026 14:53:13 - [ 2] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 TARGET NAME: EUROPA-TRAILING ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE center of Europa's disk ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=JUPITER STD=EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF EUROPA-TRAILING BY JUPITER FROM EARTH, SEP OF EUROPA-TRAILING IO FROM EARTH GT 10", SEP OF EUROPA-TRAILING GANYMEDE FROM EARTH GT 10", SEP OF EUROPA-TRAILING CALLISTO FROM EARTH GT 10", OLG OF EUROPA FROM EARTH BETWEEN 250 290 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 5.46 +/- .5 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 4 TARGET NAME: EUROPA-LEADING ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE center of Europa's disk ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 STD=JUPITER STD=EUROPA ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS NOT OCC OF EUROPA-LEADING BY JUPITER FROM EARTH, SEP OF EUROPA-LEADING IO FROM EARTH GT 10", SEP OF EUROPA-LEADING GANYMEDE FROM EARTH GT 10", SEP OF EUROPA-LEADING CALLISTO FROM EARTH GT 10", OLG OF EUROPA BETWEEN 70 110 ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 5.46 +/- .5 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 18389( 2) - 18-Aug-2026 14:53:13 - [ 3] Visit: 01 (EUROPA-TRAILING-VISIT) 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 EUROPA-TRAI STIS/CCD ACQ F28X50LP MIRROR DIFFUSE-CENTER=GEOMETRIC-CENTE 1 0.1 S (0.1 LING R, S) ACQTYPE=DIFFUSE, CHECKBOX=21 ------------------------------------------------------------------------------------------------------------------------------------ 2 EUROPA-TRAI STIS/CCD ACCUM 52X0.5 G230MB 2836 CR-SPLIT=NO 1 70 S LING see below (70 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 EUROPA-TRAI STIS/CCD ACCUM 52X0.5 G230MB 2836 1 2100 S POS TARG 1.2, 1.2 LING see below (1676 S) Comments: POS-TARG X and Y coordinates will depend on spacecraft roll angle and cannot be determined until after observations are scheduled. The values input currently are to include an offset in the Orbit Planner. Once scheduled, the correct offset x- and y-coordinates will be provided. ------------------------------------------------------------------------------------------------------------------------------------ 4 EUROPA-TRAI STIS/CCD ACCUM 52X0.5 G230MB 2836 CR-SPLIT=3 1 2100 S POS TARG 1.2, 1.2 LING see below (2273 S) Comments: POS-TARG X and Y coordinates will depend on spacecraft roll angle and cannot be determined until after observations are scheduled. The values input currently are to include an offset in the Orbit Planner. Once scheduled, the correct offset x- and y-coordinates will be provided. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ EUROPA-TRAILING 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A EUROPA-TRAILING 2 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none none 1 70 S EUROPA-TRAILING 3 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 1 1 839.0 S EUROPA-TRAILING 3 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 2 1 837.0 S EUROPA-TRAILING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 1 2 806 S EUROPA-TRAILING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 2 2 806 S 18389( 2) - 18-Aug-2026 14:53:13 - [ 4] 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 | ------------------------------------------------------------------------------------------------------------------------------------ EUROPA-TRAILING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 3 2 661.0 S 18389( 2) - 18-Aug-2026 14:53:13 - [ 5] Visit: 02 (EUROPA-LEADING-VISIT) 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 EUROPA-LEAD STIS/CCD ACQ F28X50LP MIRROR DIFFUSE-CENTER=GEOMETRIC-CENTE 1 0.1 S (0.1 ING R, S) ACQTYPE=DIFFUSE, CHECKBOX=21 ------------------------------------------------------------------------------------------------------------------------------------ 2 EUROPA-LEAD STIS/CCD ACCUM 52X0.5 G230MB 2836 CR-SPLIT=NO 1 70 S ING see below (70 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 EUROPA-LEAD STIS/CCD ACCUM 52X0.5 G230MB 2836 1 2100 S POS TARG 1.2, 1.2 ING see below (1676 S) Comments: POS-TARG X and Y coordinates will depend on spacecraft roll angle and cannot be determined until after observations are scheduled. The values input currently are to include an offset in the Orbit Planner. Once scheduled, the correct offset x- and y-coordinates will be provided. ------------------------------------------------------------------------------------------------------------------------------------ 4 EUROPA-LEAD STIS/CCD ACCUM 52X0.5 G230MB 2836 CR-SPLIT=3 1 2100 S POS TARG 1.2, 1.2 ING see below (2273 S) Comments: POS-TARG X and Y coordinates will depend on spacecraft roll angle and cannot be determined until after observations are scheduled. The values input currently are to include an offset in the Orbit Planner. Once scheduled, the correct offset x- and y-coordinates will be provided. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ EUROPA-LEADING 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A EUROPA-LEADING 2 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none none 1 70 S EUROPA-LEADING 3 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 1 1 839.0 S EUROPA-LEADING 3 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 2 1 837.0 S EUROPA-LEADING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 1 2 806 S EUROPA-LEADING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 2 2 806 S 18389( 2) - 18-Aug-2026 14:53:13 - [ 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 | ------------------------------------------------------------------------------------------------------------------------------------ EUROPA-LEADING 4 STIS/CCD ACCUM 52X0.5 G230MB 2836 none none none 3 2 661.0 S 18389( 2) - 18-Aug-2026 14:53:13 - [ 7] Summary Form for Proposal 18389 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.5, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, CHECKBOX=21, CR-SPLIT=3, CR-SPLIT=NO, DIFFUSE-CENTER=GEOMETRIC-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG 1.2, 1.2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G230MB, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names EUROPA-LEADING, EUROPA-TRAILING ------------------------------------------------------------------------------------------------------------------------------------