I propose to observe a disappearance of Ganymede behind the dark limb of Jupiter with five filters of the ACS/HRC camera. Two exposures in each filter can be taken during such an event. The images will provide the spectral variation of the altitude of the apparent limb of Jupiter. The altitude of the apparent limb is dependent on the presence of hazes in Jupiter's stratosphere. Hazes of vertical optical depths below 0.001 could be detected with these observations, providing an extremely sensitive probe of high hazes. The observations probe altitudes levels near the 1-mb pressure level, for which we have very limited data. The creation of aerosols, their growth, and their transport by winds is currently a mostly theoretical study. It would significantly benefit from constraints derived from the proposed observations. ACS/HRC is the only instrument capable of the required spatial resolution in the ultraviolet. Furthermore, a favorable geometry of Ganymede's orbit occurs only once every six years. This proposal achieves unique results with a minimum of HST time.

I propose to image the disappearance or reappearance of Ganymede behind Jupiter's dark limb from 16 minutes before the midpoint (when half of Ganymede is covered by Jupiter) to 16 minutes past the midpoint. The ACS/HRC camera is used with five filters, F250W, F330W, F435W, F658N, and F892N. Exposure times are 3 seconds for filter F435W, 20 seconds for F250W, and 10 seconds otherwise. HRC will record more than a quarter of Jupiter's disk together with Ganymede. The filters are cycled four times. One cycle of five exposures (each filter once) takes 8 minutes including all overheads. Additionally, the same part of Jupiter is imaged once with ACS/HRC and filters F220W (at least 200 seconds exposure time), F550M (3 s), and three wavelength settings of FR656N (10 s each). F220W probes the highest altitudes in Jupiter's atmosphere, closest to the levels probed by Ganymede. F550M fills in between F435W and F658N. FR656N probes the ammonia absorption at 647 nm. For each filter, the first or last Ganymede exposure will show the whole disk of Ganymede unaffected by Jupiter's atmosphere, while the last or first exposure records the background level. The two intermediate exposures record the location of Jupiter's limb for all filters except F250W with only one exposure due to the limited duration of the event. The exposures on Ganymede in filters F330W, F658N, and F892N are expected to reach signals of about 30,000 electrons per pixel, corresponding to signal-to-noise ratios of almost 200. Filters F435W and F250W will give signals of 80,000 and 8,000electrons per pixel, respectively. The Jupiter exposures with F220W, F550M, and FR656N are expected to reach signals of 8,000, 90,000, and 30,000 electrons/pixel, respectively. HST will be tracking Jupiter. This will cause a smear of up to one HRC-pixel of Ganymede due to its orbital motion (except two pixels for F250W). This does not significantly affect the observations. The observations need to be scheduled around an occultation disappearance or reappearance of Ganymede behind Jupiter's dark limb, so that the 32-minute time interval centered on the mid-event (half of Ganymede behind Jupiter) can be used for imaging. SOLAR SYSTEM 12 false 0.0 false false false false false false SUPPORTED SOLAR SYSTEM Southern part of Jupiter including Ganymede. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=10.0,ANG=200.0,REF=NORTH SOLAR SYSTEM Southwestern part of Jupiter including Ganymede during disappearance. Only one of visits 11-14 meeds to be executed. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=14.0,ANG=239.0,REF=NORTH SOLAR SYSTEM Southeastern part of Jupiter including Ganymede during reappearance. Only one of visits 11-14 needs to be executed. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=15.0,ANG=150.0,REF=NORTH SOLAR SYSTEM Northeastern part of Jupiter. Only one of visits 11-14 needs to be executed. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=15.0,ANG=64.0,REF=NORTH SOLAR SYSTEM Northwestern part of Jupiter. Only one of visits 11-14 needs to be executed. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=14.0,ANG=335.0,REF=NORTH SOLAR SYSTEM Southern part of Jupiter including Ganymede. PLANET OFFSET PLANET STD=JUPITER TYPE=POS_ANGLE,RAD=14.0,ANG=135.0,REF=NORTH ACS/HRC Imaging No No F250W ACS/HRC Imaging No No F330W ACS/HRC Imaging No No F435W ACS/HRC Imaging No No F658N ACS/HRC Imaging No No F892N ACS/HRC Imaging No No F220W ACS/HRC Imaging No No F550M Must include an occultation disappearance of Ganymede at Jupiter's dark limb centered on exposure B550 or an occultation reappearance of Ganymede at Jupiter's dark limb centered on exposure A550. (Tic-Visit :proposal-id 10468 :visit-id "13" :proposal-version -1 :trans-version "20050323" :date "01-May-2005 22:00:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 441632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "13" :obset-id "13" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 10468 :visit-id "13" :obset-id "A3" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 10468 :visit-id "13" :obset-id "B3" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 10468 :visit-id "13" :obset-id "C3" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "13" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2772 :al-overhead-at-end 0 :internal-dur-at-end 280 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) false 1 0 The middle of the disappearance of Ganymede should happen between exposures A953 and C953. (Tic-Visit :proposal-id 10468 :visit-id "21" :proposal-version -1 :trans-version "20051113" :date "22-Mar-2007 16:33:19.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 627424 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "21" :obset-id "21" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "21" :orbit-number 1 :pcs-time 339 :si-vis-utilization 2709 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) false 1 0 The middle of the disappearance of Ganymede should happen between exposures A502 and C673. (Tic-Visit :proposal-id 10468 :visit-id "22" :proposal-version -1 :trans-version "20070302" :date "29-Mar-2007 13:32:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 627424 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "22" :obset-id "22" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "22" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2709 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) true 1 0 The middle of the disappearance of Ganymede should happen between exposures A502 and C673. (Tic-Visit :proposal-id 10468 :visit-id "22" :proposal-version -1 :trans-version "20070302" :date "29-Mar-2007 13:32:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 627424 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "22" :obset-id "22" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "22" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2709 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) false 1 0 The middle of the disappearance of Ganymede should happen between exposures A953 and B953. (Tic-Visit :proposal-id 10468 :visit-id "31" :proposal-version -1 :trans-version "20051113" :date "22-Mar-2007 17:18:55.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 403344 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "31" :obset-id "31" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "31" :orbit-number 1 :pcs-time 339 :si-vis-utilization 1809 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) false 1 0 The middle of the disappearance of Ganymede should happen between exposures A502 and B410. (Tic-Visit :proposal-id 10468 :visit-id "32" :proposal-version -1 :trans-version "20051113" :date "22-Mar-2007 17:22:37.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 403344 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "32" :obset-id "32" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "32" :orbit-number 1 :pcs-time 339 :si-vis-utilization 1809 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) false 1 0 The middle of the reappearance of Ganymede should happen between exposures B300 and C953. (Tic-Visit :proposal-id 10468 :visit-id "41" :proposal-version -1 :trans-version "20051113" :date "22-Mar-2007 20:16:19.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 627424 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "41" :obset-id "41" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "41" :orbit-number 1 :pcs-time 339 :si-vis-utilization 2709 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) false 1 0 The middle of the reappearance of Ganymede should happen between exposures A502 and C673. (Tic-Visit :proposal-id 10468 :visit-id "42" :proposal-version -1 :trans-version "20051113" :date "22-Mar-2007 20:22:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 627424 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10468 :visit-id "42" :obset-id "42" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10468 :visit-id "42" :orbit-number 1 :pcs-time 339 :si-vis-utilization 2709 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3079 :pcs-gap 0 ) true 1 0