Sign error corrected in POS TARG values for all images using the WF3-FIX aperture. Change was already discussed with Tony Roman. ----- Attempting Submission 3 (Mon Nov 26 19:26:23 GMT 2007) ----- A Deep Search for Martian Dust Rings It has been long suspected that Mars is encircled by two faint rings of dust, one originating from each of its moons Phobos and Deimos. Similar dust rings are associated with many of the small, inner moons orbiting Jupiter, Saturn, Uranus and Neptune. On December 31, 2007, Earth will pass through Mars' equatorial plane just a week after its December 24 opposition, providing an exceedingly rare opportunity to image the rings under nearly ideal viewing geometry. The next equivalent viewing opportunity occurs in 2022. Using the Wide Fields of WFPC2 and a highly optimized observing plan, we expect to be able to detect rings with edge-on reflectivities of ~ 10^-8, which is at or below the level where most dynamicists expect rings to be visible. This is a factor of 10-30 more sensitive than the detection limit we achieved during a slightly inferior viewing opportunity in 2001. The rings have been predicted to show some interesting dynamical properties, including large asymmetries and inclinations. A positive detection will test these predictions, serving as an effective test of dynamical models developed to account for the properties of other faint planetary rings as well. With such a stringent limit, even a negative result will be of considerable interest, challenging dynamicists to explain the remarkably low density of dust within the Martian system.
We position the hypothetical ring vertically (parallel to the PC1 y-axis) in one field of view, while Mars is positioned in an adjacent frame and substantially over-exposed. We use broad filter F606W and 180-sec exposures; CLOCKS=YES is used to limit the danger of saturation. Visits 01 and 02 are identical: one orbit with Mars in WF3 and the ring in WF2, followed by another with Mars in PC1 and the ring in WF4. A third visit uses the same pointings but rotates the ring plane by 45 degrees; these images are used to model and subtract the background light gradient within the other images. Each visit includes a brief unsaturated image of Mars through filter F502N to provide pointing and photometric reference. SOLAR SYSTEM 12 false 0.0 false false false false false false SUPPORTED SOLAR SYSTEM PLANET Planet Mars STD=MARS WFPC2 Imaging No F606W WFPC2 Imaging No F502N true /private/tmp/OPVisit3388.xml One orbit spent imaging each side of the Martian rings. Rings oriented parallel to the PC y-axis. First orbit positions Mars in WF3 with the ring extending vertically into WF2. The second orbit positions Mars in the PC with the ring extending into WF4. In both cases, Mars's limb is positioned ~ 1'' off the edge of the frame targeting the rings. POS TARG values are derived as follows. Orbit 1: The WF3 pixel size is 0.1''. The edge of the WF2 overlaps with Y ~ 95 of the WF3. Allowing a 1'' or 10-pixel margin for pointing error, the limb of Mars should not extend below Y ~ 105 on WF3. Mars has radius 7.92'' = 79.2 pixels, so Mars should be centered at Y ~ 185. The center of the WF3-FIX aperture is (416.5,424.5), so the Y offset is -240 pixels in WF3 coordinates or POS TARG = (0,+24). Orbit 2: The PC pixel size is 0.0455''. The edge of the WF4 field overlaps with Y ~ 150 on PC1. Allowing for 1'' or ~ 25-pixel margin, the limb should be at Y ~ 175. We also need Mars entirely inside the PC, so its limb should not extend past X ~ 775, allowing for buffer. Mars has radius 7.92'' = 175 PC pixels. Thus the center of Mars should fall at (600,350). The center of the PC1-FIX aperture is (420,424.5) so the offset is (180,-75) in pixels or POS TARG = (8.2,-3.4). NOTE: These observations take advantage of a viewing geometry that does not recur until 2022. Timing and orientation constraints have very little flexibility. However, if absolutely necessary, Visits 01 and 02 can be split into single-orbit visits to enhance schedulability. (Tic-Visit :proposal-id 11187 :visit-id "01" :proposal-version -1 :trans-version "20070709" :date "26-Nov-2007 11:25:39.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 806688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11187 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 11187 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11187 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2649 :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 3028 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11187 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2649 :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 3028 :pcs-gap 0 ) true 2 0 true /Users/office/11187-revised-2007-11-26/OP-cache/11187-v02.xml One orbit spent imaging each side of the Martian rings. Rings oriented parallel to the PC y-axis. First orbit positions Mars in WF3 with the ring extending vertically into WF2. The second orbit positions Mars in the PC with the ring extending into WF4. In both cases, Mars's limb is positioned ~ 1'' off the edge of the frame targeting the rings. POS TARG values are the same as in Visit 01. (Tic-Visit :proposal-id 11187 :visit-id "02" :proposal-version -1 :trans-version "20070709" :date "26-Nov-2007 11:20:20.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 806688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11187 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 11187 :visit-id "02" :obset-id "A2" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11187 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2649 :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 3028 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11187 :visit-id "02" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2649 :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 3028 :pcs-gap 0 ) true 2 0 true /Users/office/11187-revised-2007-11-26/OP-cache/11187-v03.xml One orbit spent imaging the scattered light pattern from Mars with the rings rotated 45 degrees to fall atop the diffraction spikes. POS TARG values are the same as in Visits 01 and 02. (Tic-Visit :proposal-id 11187 :visit-id "03" :proposal-version -1 :trans-version "20070709" :date "26-Nov-2007 11:14:24.0" :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 11187 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11187 :visit-id "03" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2649 :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 3028 :pcs-gap 0 ) true 1 0