Cassini UV Imaging Spectrograph&

Saturn’s Wandering Shepherds

Larry W. Esposito

University of Potsdam and University of Colorado

Cassini UV Imaging Spectrograph

• 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

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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

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Summary: Cassini at Jupiter

• Joint observations with Galileo

• Jupiter movies show atmosphere dynamics

• Ring movies complement Voyager, Galileo

• UVIS movies shows Io torus behavior, composition, temperature

Table 2Ion NameMinimum Value Nominal ValueMaximum Value Formal ErrorS II 97.5 106 115 4.72S III 412 442 460 16.8S IV 63.1 68.9 74.8 3.15S V 1.62 7.83 14.2 2.63O II 429 500 570 41.6O III 0 38.1 76.4 17.4Te 5.32 5.48 5.66 0.11

A series of EUV/FUV spectra of the Io plasma taken by the UVIS instrument

onboard the Cassini spacecraft has been analyzed. Fits of a spectral model, based on the

atomic physics of the CHIANTI database, to the data have yielded radial profiles of the

electron temperature, electron density and ion densities in the torus as a function of

radius.

The electron temperature profile of the UVIS data is found to closely match that of theVoyager-based model of Bagenal (1994), with the exception that Te outside of 7.5 RJ

appears to increase more slowly in the UVIS data. The densities of S II and O II areconsistent with the Voyager model. However, densities for S III, S IV and O III can be upto a factor of 2 higher in the UVIS model. The overall sulfur to oxygen ratio is increasedfrom 0.75:1 during the Voy ager era to 1.25:1 for UVIS.

SATURN’S F RING

• 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– 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– 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 SYSCANS• SATELLITES DISTANT OCCULTATIONS

LONGITUDE COVERAGE• ATMOSPHERE OCCULTATIONS

STARELIMB SKIM

AURORAL MAPSPECTRAL

IMAGES• RINGS OCCULTATIONS

SPECTROSCOPY• MAGNETOSPERE 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.