The planet Uranus demonstrated increased atmospheric activity around the time of the 2007 equinox, likely in response to extreme insolation change (Sromovsky et al. 2009, Icarus 203, 265). Convective sites in the planet's southern hemisphere reached unprecedented altitudes in 2003 (Hammel et al. 2005, Icarus 175, 284); a bright northern feature showed the highest contrast yet detected in an outer planet atmosphere (Sromovsky et al. 2007, Icarus 192, 558); and a dark atmospheric feature was detected by Hubble (Hammel et al. 2009, Icarus 201, 257). The historical record makes references to discrete structures (both bright and dark) on Uranus during previous equinoctial apparitions (the last equinox occurred in 1965). The best amateur facilities are now just able to resolve the disk of Uranus and detect such activity if it is very large or has very high contrast. Amateurs also have access to a great many nights of telescope time. If a discrete cloud feature on Uranus is reported through the amateur network, we propose to obtain follow-up images with Hubble's WFC3. The proposed TOO images will permit determination of detailed structure of the feature at visible wavelengths, and will provide vertical and horizontal constraints on the feature's scattering properties. Hubble is the only facility that can provide such information at visible wavelengths. The proposal was accepted in earlier cycles, but not executed because the defined criteria for activation were not met. Because the timescales of change on Uranus are unknown, we resubmit this for consideration in the current cycle.

Proposed Observations: We request three orbits of WFC3 imaging in Cycle 18 to obtain disk-resolved images of an anomalous feature on Uranus. In the first and third orbits, we use the following filters: most of a filter set designed for outer planet observations near the methane absorption band near 889 nm as well as a matched methane-continuum pair near 727 nm; filters corresponding to Strömgren b and y filters used for long-term brightness monitoring; and the F658N filter similar to one that showed maximum contrast for the Uranus dark spot in C15. In the second orbit, we sample slightly different wavelengths: some of the same 889-nm methane filters; a matched methane-continuum pair near 619 nm; and filters with good contrast for discrete features. We will not dither due to short exposure times (dithering imposes a 2.5-min penalty for splitting the exposure, and dithering several exposures could require a 5.6-min buffer readout). We will use sub-arrays to avoid the need for buffer readouts during our planned visits. Comparisons with images at nearby wavelengths will permit identification of artifacts and cosmic rays. Filter Specifics: The methane sets are specifically crafted to probe the vertical aerosol distribution of cloud features as a function of altitude. In addition, our filter choices provide continuity both with those used for past HST observations and with past ground-based observations (e.g., Lockwood’s long-term photometry program). The exposures are based on prior HST images made with similar filters, or ETC calculations using ground-based spectroscopy as input. These times are optimized for the “typical” Uranus, and are appropriate for an anomalously dark feature. If an anomalously bright feature is seen, then the times will be reduced further, and perhaps one additional filter might be included. Timings for each orbit will be optimized in Phase 2 sequencing. To compute total times, we assumed a 6-minute guide-star acquisition, 2.6 minutes for each filter change, and an addition 2 minutes for each quad “Q” filter change. Timing: The first orbit should be as soon as possible after discovery, preferably within a few days but this is not critical. The convective episode reported by Hammel et al. (2005a) lasted at least 4 days, perhaps as long as a month. The bright feature tracked by Sromovsky et al. (2007) lasted for at least a year and perhaps significantly longer. The Uranus Dark Spot and its companion material lasted for at least four months (Hammel et al. 2008). Since the triggering feature for this TOO will necessarily have higher contrast than any feature observed by Keck, we estimate the TOO feature would have a similar or even longer lifetime, i.e., many months. Each orbit should have the feature as close as possible to the central meridian to permit optimal imaging of detailed structure. The TOO trigger conditions require that the feature’s rotation period be known prior to TOO execution; thus the location of the feature relative to the central meridian will be known as well. The feature will likely have a rotation period of order 16-17.5 hours, based on past HST and Keck imaging (Karkoschka 1998; de Pater et al. 2002; Hammel et al. 2001, 2005b; Sromovsky et al. 2007; Hammel et al. 2008). The second orbit should occur about 35 hours (two rotations of Uranus, or 22 HST orbits) after the first orbit. The third should follow about 35 hours after that. The function of the two additional orbits is to characterize the horizontal and vertical evolution of the feature in much the same manner as was done for Saturn’s Great White Spot (Barnet et al. 1992; Beebe et al. 1992) and for the Uranus dark spot and its companions (Hammel et al. 2008). Evolution of smaller, lower-contrast Uranus features is seen on this time scale with Keck and HST (Hammel et al. 2005a, 2005b; Sromovsky et al. 2007; Hammel et al. 2008). Proprietary period: We are not requesting any proprietary data period for this proposal. SOLAR SYSTEM 0 false 0.0 0.0 false false false false SUPPORTED SOLAR SYSTEM PLANET planet uranus STD=URANUS EARTH WFC3/UVIS Imaging No No F467M WFC3/UVIS Imaging No No F547M WFC3/UVIS Imaging No No F658N WFC3/UVIS Imaging No No FQ727N WFC3/UVIS Imaging No No FQ750N WFC3/UVIS Imaging No No FQ889N WFC3/UVIS Imaging No No FQ924N WFC3/UVIS Imaging No No FQ937N WFC3/UVIS Imaging No No F467M WFC3/UVIS Imaging No No F547M WFC3/UVIS Imaging No No F658N WFC3/UVIS Imaging No No F775W WFC3/UVIS Imaging No No F845M WFC3/UVIS Imaging No No FQ619N WFC3/UVIS Imaging No No FQ889N WFC3/UVIS Imaging No No FQ906N WFC3/UVIS Imaging No No FQ937N WFC3/UVIS Imaging No No F467M WFC3/UVIS Imaging No No F547M WFC3/UVIS Imaging No No F658N WFC3/UVIS Imaging No No FQ727N WFC3/UVIS Imaging No No FQ750N WFC3/UVIS Imaging No No FQ889N WFC3/UVIS Imaging No No FQ924N WFC3/UVIS Imaging No No FQ937N POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 2 false (Tic-Visit :proposal-id 12463 :visit-id "01" :proposal-version -1 :trans-version "20110825" :date "07-Dec-2011 12:30:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 373776 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2699 :al-overhead-at-end 0 :internal-dur-at-end 447 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 1 0 1324818362000 66.59649989441269 (Tic-Visit :proposal-id 12463 :visit-id "02" :proposal-version -1 :trans-version "20110825" :date "07-Dec-2011 12:27:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 316496 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2753 :al-overhead-at-end 0 :internal-dur-at-end 384 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 12463 :visit-id "03" :proposal-version -1 :trans-version "20110825" :date "07-Dec-2011 12:30:39.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 373776 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2699 :al-overhead-at-end 0 :internal-dur-at-end 447 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 1 0 1324393026000 45.22104692032909 (Tic-Visit :proposal-id 12463 :visit-id "04" :proposal-version -1 :trans-version "20110825" :date "12-Dec-2011 06:48:52.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 373776 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2642 :al-overhead-at-end 0 :internal-dur-at-end 447 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 12463 :visit-id "05" :proposal-version -1 :trans-version "20110825" :date "12-Dec-2011 06:48:54.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 316496 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2776 :al-overhead-at-end 0 :internal-dur-at-end 384 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 12463 :visit-id "06" :proposal-version -1 :trans-version "20110825" :date "14-Dec-2011 12:06:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 373776 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12463 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12463 :visit-id "06" :orbit-number 1 :pcs-time 241 :si-vis-utilization 2657 :al-overhead-at-end 0 :internal-dur-at-end 447 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) true 1 0