Cassini UVIS Results from Saturn, Titan,
Icy Satellites and Rings
AOGS
22 June 2005
L. Esposito, A. Hendrix, J. Colwell, C. J. Hansen, J. Hallett, K.
Larsen, W. McClintock, W. R. Pryor, D. Shemansky, A. I. F.
Stewart, R. West
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Outline
UVIS has returned significant new results on
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Saturn’s aurora
Oxygen in the Saturnian system
Saturn’s rings
Titan
Icy satellites
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UVIS Characteristics
UVIS has 4 separate channels:
Far UltraViolet (FUV)
• 110 to 190 nm
• 3 slit widths => 2.8, 4.8, 24.9 nm spectral resolution
• 2D detector: 1024 spectral x 64 one-mrad spatial pixels
Extreme UltraViolet (EUV)
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55 to 110 nm
3 slit widths => 2.8, 4.8, 19.4 nm spectral resolution
2D detector: 1024 spectral x 64 one-mrad spatial pixels
Solar occultation port
High Speed Photometer (HSP)
• 2 or 8 msec time resolution
Hydrogen – Deuterium Absorption Cell (HDAC)
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Cassini UVIS
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Saturn Aurora
H Lyman-a
H2 bands
July 13, 2004
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UVIS auroral trend & Cassini RPWS SKR signal
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Saturn Auroral Spectrum
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Aurora -- Summary
Auroral spectrum similar to Jupiter’s, with CH4 absorption
Auroral brightness varies by a factor of 3 or more
Daily-averaged auroral emissions vary with SKR and solar wind ion fluxes
Saturn has an extended H cloud (and a ring-related O cloud)
Reflected sunlight spectrum maps acetylene distribution.
Acetylene bands are stronger at Saturn than Jupiter.
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Saturn System Oxygen
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Saturn System Oxygen
Brightness of atomic oxygen in Saturn system is variable;
displayed a prominent peak early in observations
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Saturn System Oxygen: Implications
Neutral oxygen dominates Saturn’s inner magnetosphere.
The rise and fall in abundance of OI between Dec 25, 2003 and May 13,
2004 amounts to ~500 Mkg of mass of water apparently lost from the
system in this period. Mean inferred loss rate of bulge in mass during 2
months is ~4 X1027 atoms s-1.
Total mass of OI + OH in system is ~2200. Mkg.
The estimated micron sized particles in the E-ring involved in the Mimas
– Tethys region is 600 Mkg.
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Rings
3 Types of Ring Observation with UVIS
1. Spectra and images from 550 -1900A.
2. Stellar and solar occultations with spatial resolution of 10 m.
3. Meteoroid impact detection.
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Cassini Division
Approach picture from
Cassini:
F
May 10, 2004
Dist: 27 million km.
Pixel: 161 km.
C
B
A
Moon: Prometheus
Cassini ISS image:
Space Science
Institute (Boulder),
NASA/JPL.
Encke Gap
W~350 km
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Saturn Orbit Insertion
We summed all SOI spectra from same distance to produce radial profile and color rendition
UV spectra show water abundance increasing outward to a peak in outer A ring
Rings A and B more icy than ring C and Cassini Division, consistent with VIMS
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UVIS Stellar Occultations
UVIS observes a star with its HSP as the rings pass between Cassini and the star
Star brightness is measured 500 times each sec.
Measured brightness indicates how much ring material is blocking the star
Occultations provide high resolution (~20 m) profiles of ring structure and information
(at scales of ~10-100 m) on:
ring mass
ring particles
ring thickness
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Stellar Occultation Raw Data: Xi Ceti, October 2004
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Azimuthal Structure
Cassini Xi Ceti occ, October 2004 compared with Voyager
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Encke Gap
Pan Wakes
Density Waves
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Rings Results
Sharp edges seen at highest resolution yet (~10 m)
Mass and thickness of Cassini Division: low surface mass density suggests short ring
darkening timescale from meteoroid impacts
Particle size information at highest resolution yet:
occultation statistics provide information on sizes of clumps of particles across the
rings.
Measurement of orientation of clumps and wakes in the rings form comparison with
Voyager optical depth: multiple occultation geometries will enable construction of
detailed 3-D picture of rings.
Structure, composition, mass, thickness, evolution
consistent with recycling of rings - continuing regeneration of
old material in the ring system, particularly in A ring
Transient clumpiness in A ring suggests gravitational wakes
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Titan
UVIS studies Titan’s
atmosphere by observing
solar and stellar occultations,
reflected and emitted light
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Tb Stellar Occultations
UVIS observed two stellar occultations on the second close Titan
flyby (Tb, December 2004): Lambda Sco and Spica
Advantages of occultation measurements:
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Detailed vertical profiles of constituents
Accurate knowledge of the altitude ~ 1 km
– Impact parameter does not depend on instrument pointing although
the signal will drop if the pointing wanders off the star (this
happened with the Spica occultation)
Self calibrating
– The key measurement is the ratio of signal of the occulted star
divided by the unocculted signal measured by the same instrument a
few minutes later/earlier.
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Stellar spectrum
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Distribution of Species with Altitude
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Methane distribution with altitude
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Summary of First Results from the
TB Lambda Sco Occultation
UVIS measured properties of the atmosphere between 450 and 1600 km
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Overlaps at low altitudes with CIRS, and at high altitudes with
INMS
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Region between ~500 and 1050 km only measured by UVIS
(Measurements confirmed the Huygens engineering model of the atmosphere and
confirmed the project decision to go ahead with the release of the probe)
Species identified:
CH4
C2H2
C2H4
C2H6
C4H2
HCN
methane
acetylene
ethylene
ethane
diacetylene
hydrogen cyanide
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Limb Haze Images from T3
Titan hazes
T-3 Titan FUV Observation
February 15, 2005
Start Time: 10:26:52
End Time: 11:34:52
Lyman Alpha Image: 1146 –1271 Å
LBH Image:
1286 – 1427 Å
Solar Image:
1661 – 1913 Å
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Icy Satellites
UVIS Icy Satellite Science Objectives are to Investigate:
Surface age and evolution
• UV albedo maps
Surface composition
• Reflection spectra
Tenuous atmospheres / exospheres
• Emission spectra, occultations
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Example: Phoebe UV Albedo Map
Similar geometry
Time: C/A-01:22
Range: 31,300 km
Phase angle: 83°
Lat/Long: 21°S, 349°W
Blue/green=reflected solar
Red=background Ly-a (IPH)
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ISS_000PH_HIRESC
Note brightness
difference between
high and low latitudes
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Composition: Phoebe’s Disk-Integrated FUV reflectance
spectrum
Upturn due to H2O ice - present in all icy moons’ reflectance spectra
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Phoebe’s spectral variations with latitude
High latitude region is more H2O-ice rich; low latitude region has more non-ice material
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No Emissions Detected above Background
A gas (oxygen) detection threshold of <2 x 1013 cm-2 is based on a minimum 2-sigma detectable level of 10 counts above
background, instrument sensitivity at 130.4 nm of 3.4 cts/kR-sec, integration time of ~ 10 hours, and solar wind values for
the electron density and temperature at Saturn.
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SUMMARY
System scans are continuing, to monitor the oxygen in the Saturn system and
better understand its source.
Rings observations are revealing the structure of the rings; any more stellar
& solar ring occultations are coming up.
Titan observations and investigations are ongoing, to probe the atmospheric
structure.
Icy satellites studies are focusing on the search for tenuous atmospheres,
compositional modeling and investigating possible presence of escaping
hydrogen.
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