0 Assigned ID: 7696 ----- Attempting Submission 1 (Thu Apr 16 21:53:26 GMT 2026) ----- HST Phase I Proposal 7696 successfully submitted. Receipt: # 7696-1 POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE NO ORIENT POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE POS TARG OUTSIDE OF APERTURE On both visits, several diagnostic warnings appear regarding the POS TARG being placed outside of the aperture. At the time of submission, the input POS TARG x- and y-coordinates represent the maximum distance the slit would move from the target. The actual offset can only be calculated after the spacecraft roll angle is determined. -1447231670 ----- Attempting Submission 1 (Mon Aug 17 20:34:38 GMT 2026) ----- On both visits, several diagnostic warnings appear regarding the POS TARG being placed outside of the aperture. At the time of submission, the input POS TARG x- and y-coordinates represent the maximum distance the slit would move from the target. The actual offset can only be calculated after the spacecraft roll angle is determined. NUV Exospheric Magnesium as a Constraint on Europa's Ocean Salinity 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. 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. The PI Emma Lovett is a 5th year graduate student at Boston University. Her PhD research focuses on mapping Europa's exosphere through ground-based observations using the high resolution spectrograph on the Keck 1 telescope. She has mapped Europa's exospheric Na and K to offer ground-based support for the Juno mission as it measures plasma parameters contemporaneously. Emma plans to simulate Europa's exosphere by adapting a pre-existing exosphere model developed by her PhD advisor, Carl Schmidt. Carl Schmidt has experience with HST through GO programs 15147, 17099 and 15638 all of which targeted Jupiter's moons and their extended gas and plasma clouds. He is PI of the Keck support program for the Juno mission, which offers coordinated observations of Europa's Na and K gases concurrent or near concurrent with this HST campaign. Carl has developed models of the Europa exosphere and simulated MESSENGER impact plume observations of magnesium 2853A emission in Mercury's exosphere. He is a research professor at Boston University where he serves as PI Emma Lovett's PhD advisor. Richard Cartwright is a Solar System astronomer who uses ground- and space-based telescopes to measure the spectral properties of icy moons, primarily in the near-infrared. He has been investigating the surface composition of Europa using IRTF/SpeX for the past few years, focusing on identification of non-H2O ice species such as chloride and sulfate salts. He is the PI of HST program 16704 that observed the four large Uranian moons with STIS (G230 and G430 gratings) in Cycle 29. John Clarke is an emeritus professor at Boston University with extensive experience in UV solar system observations with HST. He has been a PI or Co-I in nearly every cycle for 3 decades and has an extensive publication record on these observations with a focus on giant planet aurorae, satellite atmospheres, exospheres and Lyman alpha. Solar System Astronomy 0 false true false false false false true SUPPORTED SOLAR SYSTEM YES SATELLITE center of Europa's disk STD=JUPITER STD=EUROPA 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 EARTH SOLAR SYSTEM YES SATELLITE center of Europa's disk STD=JUPITER STD=EUROPA 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 EARTH STIS/CCD Spectroscopic No No G230MB 2836 C:\Users\elovett\EuropaResearch\Proposals\HST Mg cycle 34\OP-cache\HST_cycle34_proposal_Phase_II-v01.xml Using ephemeris for target 18389-01-EUROPA-TRAILING The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18389 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "17-Aug-2026 16:14:15.0" :trans-errors '(:WARNINGS 10 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 147808 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18389 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18389 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18389 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 340 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 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. 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. C:\Users\elovett\EuropaResearch\Proposals\HST Mg cycle 34\OP-cache\HST_cycle34_proposal_Phase_II-v02.xml Using ephemeris for target 18389-02-EUROPA-LEADING The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18389 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "17-Aug-2026 16:14:20.0" :trans-errors '(:WARNINGS 10 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 147808 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18389 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18389 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18389 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 340 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 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. 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.