Cassini UV Imaging Spectrograph
&
Saturn’s Wandering Shepherds
Larry W. Esposito
University of Potsdam and
University of Colorado
Cassini UV Imaging
Spectrograph
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Spectra and images from 550 -1900A
Hydrogen-Deuterium cell measures D/H
High speed photometer has 20m resolution
Chemistry of Saturn, Titan clouds
Exospheres of moons
Saturn’s magnetosphere neutrals
Ring origin and evolution
Summary: Cassini
• Joint USA and European mission to Saturn
and Titan
• Titan probe and 4 year orbiter mission
• Colorado UVIS will observe Saturn, Titan,
moons and rings
JUPITER’S IO TORUS
• A DONUT OF GLOWING GAS ORBITING JUPITER
• MOSTLY SULFUR AND OXYGEN GASES FROM IO’S
VOLCANIC ERUPTIONS
• THE LIGHT FROM THE GASES IS MOSTLY
INVISIBLE TO THE HUMAN EYE, BUT CAN BE
SEEN BY CASSINI’S UVIS IN THE EXTREME
ULTRAVIOLET
• THESE GASES HAVE LOST ELECTRONS AND ARE
HELD IN PLACE BY JUPITER’S MAGNETIC FIELD,
WHICH SPINS ONCE EACH JUPITER DAY
COMPUTER ANIMATION OF IO TORUS
• PRODUCED FROM THEORETICAL MODELS
• JUPITER IN CENTER, ARROWS SHOW
GEOGRAPHIC AND MAGNETIC POLE
• OFFSET OF MAGNETIC POLE CAUSES THE IO
TORUS TO WOBBLE AS JUPITER ROTATES
QuickTime™ and a
Video decompressor
are needed to see this picture.
CASSINI UVIS TORUS MOVIE
• OBSERVATIONS FROM 11 NOVEMBER 2000
• CONTINUOUS 27 HOUR COVERAGE (ABOUT
THREE JUPITER DAYS)
• JUPITER ITSELF WAS EDITED OUT
• MOVIE IS THE SUM OF THE LIGHT FROM THE
FOUR BRIGHTEST EMISSION LINES
• THE BRIGHTEST PARTS ARE OVER-EXPOSED TO
BRING OUT THE DETAILS
• NOTE THAT THE REAL TORUS IS MUCH MORE
PATCHY THAN THE THEORETICAL MODEL
WHAT TO WATCH FOR
• TORUS WOBBLES AND ROTATES
• DUSK (RIGHT SIDE) IS BRIGHTER THAN DAWN
(LEFT) SIDE
• TORUS BRIGHTENS NEAR LONGITUDE 210
(WATCH AS THE MOVING DOT REACHES LEFT
EDGE). THE DOT ROTATES WITH JUPITER
QuickTime™ and a
Video decompressor
are needed to see this picture.
Summary: Cassini at Jupiter
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Joint observations with Galileo
Jupiter movies show atmosphere dynamics
Ring movies complement Voyager, Galileo
UVIS movies shows Io torus behavior,
composition, temperature
Table 2
Ion Name Minimum Value Nominal Value Maximum Value Formal Error
S II
97.5
106
115
4.72
S III
412
442
460
16.8
S IV
63.1
68.9
74.8
3.15
SV
1.62
7.83
14.2
2.63
O II
429
500
570
41.6
O II I
0
38.1
76.4
17.4
Te
5.32
5.48
5.66
0.11
A series of EUV/FUV spectra of the Io plasma taken by the UV IS instrument
onboa rd the Cassini spacecraft has been ana lyzed . Fits of a spectral m odel, based on the
atomi c phys ics of the CHIANTI database, to the data have yielded radial profil es of the
electron temperature, ele ctron dens it y and ion dens iti es in the torus as a func tion of
radius.
The e le ctron temperature profil e of the UVI S data is found to closely match that of the
Voyage r-based model of Bagen al (1994), wit h the excep tion that Te out side of 7.5 R J
appears to increase more slowly in the UVIS data. The dens iti es of S II and O II are
consistent with the Voyage r model. Howev er, densiti es for S III, S IV and O III can be up
to a factor of 2 highe r in the UVIS model. The ov erall sulf ur to oxygen ratio is increased
from 0.75:1 dur ing the Voy ager e ra to 1.25:1 for UVIS.
SATURN’S F RING
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Discovered in 1979 by Pioneer 11
Braids and kinks seen by Voyager
Multiple strands, core and broader ring seen by VGR PPS
Radio and optical imply large dust fraction
Cuzzi and Burns (1988) proposed the current ring is just
the latest manifestation of collisions in a moonlet belt
• Voyager saw bright clumps; RPX saw transients: evidence
for moonlet collisions
F RING SHEPHERDS
• Pandora (70 km, outside) and Prometheus
(50 km, inside) straddle the ring
• Initially interpreted as ‘shepherds,’ but F
ring not in position to balance the torques
• Perturbations from these two moons can
explain much F ring structure
VOYAGER & HST
• Ring plane crossing (RPX) observations in 1995/96
showed significant deviations (about 20 degrees) from
predicted Voyager longitudes
• Pandora librates about a nearby Mimas 3:2 corotation
resonance
• Corrected orbits and HST show Prometheus 0.3 km
further, and Pandora 0.2 km closer to Saturn: both are
approaching F ring (opposite to and slower than the
expected shepherd drift)
• 2001/02 observations: each moon has now moved
(Pandora 0.4, Pro 0.3 km) closer to the F ring
POSSIBLE EXPLANATIONS
• Since their motions seem coordinated,
perhaps there is some coupled interaction
between the shepherds (2001 DPS Dones)
• Shepherds have random interaction with
small moonlets in a belt around the F ring
(1999 DPS Esposito)
RANDOM WALK MODEL
• A belt of N moonlets, radius R, with
eccentricity e in a region of width W, are
gravitationally scattered (and thus perturb
the shepherd orbits)
• This is a kind of Brownian motion! Treat
this as a discrete symmetric random walk in
semi- major axis, with constant step size
and interval
MODEL PARAMETERS
• Symmetric p=q
• Time interval T = 7 years. During this
period HST observed one jump for each
shepherd. To get from Voyager to HST
requires at least two transitions, so we have
7<T<11 years.
• Step size 1/2 km. This is the size of the
observed change in semi-major axis in 2001
• Equal masses of Pandora and Prometheus
MODEL RESULTS
• NR < 500km: a range of solutions are possible
with smaller moons having smaller eccentricities,
or fewer larger moons with larger eccentricities
• Possible solutions: about 50 moonlets of radius 10
km or 25 moonlets with R=20km
• Random walks simulate longitude excursions
resembling the observations: more complex model
unnecessary
CONFIDENCE LEVELS
• How unlikely are the observed longitudes to
arise from a purely random process?
• Both moons continuing to converge on the
F ring: Prob = pq = 1/4
• Both independent transitions of Pandora and
Prometheus in the same year:
Prob = 1/T = 1/10 - 1/7
F RING MODEL
• Barbara and Esposito (2002) find about 50
moonlets with R = 8km in their model
• The moonlet belt model thus explains
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F ring origin (Cuzzi and Burns)
Transient features seen at RPX (Poulet)
Voyager clumps (Barbara and Esposito)
Wandering shepherds as a Brownian motion
Summary: F ring shepherds
• Although low in intrinsic likelihood, the moonlet belt
model explains the positions of Pandora and Prometheus as
purely random outcomes
• Cassini will seek moonlets in images and indirectly
through stellar occultations
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Imaging resolution better than HST after -45days
80 SATORB searches in first 10 revs
F ring movies
Up to 100 star occultations exceeding VGR resolution
UVIS OBSERVATIONS
• SATURN SYSTEM
• SATELLITES
• ATMOSPHERE
• RINGS
• MAGNETOSPERE
SYSCANS
DISTANT OCCULTATIONS
LONGITUDE COVERAGE
OCCULTATIONS
STARE
LIMB SKIM
AURORAL MAP
SPECTRAL IMAGES
OCCULTATIONS
SPECTROSCOPY
SURVEY & AURORA
UVIS system scans
• EUV and FUV low resolution spectra of
magnetosphere neutral and ion emissions.
• System scans at every apoapsis.
• Typical observation periods: 12 h per day
for 100 – 150 days. Data volume: 21Mb/day
• Some observations also required in inner
magnetosphere.
SATELLITES
• LATITUDE, LONGITUDE AND PHASE COVERAGE
COORDINATED WITH CAMERAS
• CLOSE-UP OBSERVATIONS WITH ISS, VIMS
• DISTANT STELLAR OCCULTATIONS TO DETERMINE
SATELLITE ORBITS AND SATURN REFERENCE FRAME.
DURATION: 1.5 HOUR, 10-25 PER MOON.
ENCELADUS, DIONE, MIMAS, TETHYS
ATMOSPHERE
• SOLAR OCC : VERTICAL PROFILES OF H, H2,
HYDROCARBONS, TEMP IN EXO, THERMOSPHERE
• STAR OCC: SAME FOR UPPER ATMOSPHERE
• UVIS STARE: LONG INTEGRATIONS MAP
HYDROCARBONS, AIRGLOW
• LIMB SKIM: MAP EMISSONS WITH HIGHEST
RESOLUTION AT THE LIMB
• AURORAL MAP: H&H2 EMISSIONS OVER SEVERAL
ROTATIONS
• UVIS EUVFUV SPECTRAL IMAGES: MAP
HYDROCARBON, AIRGLOW, AEROSOLS
FREQUENCY, DURATION, VOLUME
• SUNOCC’S TO COVER RANGE OF LATITUDES:
40MIN @ 32 KBITS = 77Mbit EACH.
• STAROCC’S 19 PROPOSED: 58 Mbit EACH
• UVIS STARE: ONCE PER ORBIT, 1-11 HOURS, 18-198
Mbit EACH
• LIMB SKIM: ABOUT ONE/ORBIT, 1-2 HOURS @
1kbit= 4-7Mbit EACH
• AURORAL MAP: SEVERAL TIMES DURING TOUR,
11-33 HOURS @2Kbit= 79-238Mbit EACH.
• SPECTRAL IMAGES: 532 HOURS REQUESTED @
5Kbit. 76, 152Mbit/ IMAGE CUBE.
Ring Stellar Occultation Objectives
• Highest radial resolution (20 m) structure of rings.
Full
radial scans at high and low incidence angles.
• Discovery and precise characterization of dynamical
features generated by ring-satellite interactions. Multiple
radial scans.
– Density waves and bending waves.
– Edge waves and ring shepherding.
– Embedded moonlets and discovery of new moons from dynamical
response in rings.
• Discovery and precise characterization of azimuthal
structure in rings. Multiple radial scans and azimuthal scans.
– Eccentric rings.
– Density waves and edge waves.
– Small-scale self-gravitational clumping in rings.
Ring Stellar Occultation Objectives (2)
• Measure temporal variability in ring structure.
Occultations early and late
in tour.
• Simultaneously measure UV reflectance spectrum of rings.
– Determine microstructure on particle surfaces.
– Compositional information on ring particles.
• Measure size distribution of large particles through occultation
statistics. Occultations at large and small distances from rings.
• Measure dust abundance in diffraction aureole.
• Simultaneously search for flashes from 0.1 m - 1.0 m meteoroid
impacts.