\
Pergamon
Planet[ Space Sci[\ Vol[ 35\ No[ 8:09\ pp[ 0110Ð0124\ 0887
Þ 0887 Elsevier Science Ltd
All rights reserved
9921Ð9522:87:,*see front matter
PII ] S9921Ð9522"87#99965Ð1
Cassini UVIS observations of Saturn|s rings$
Larry W[ Esposito\ Joshua E[ Colwell and William E[ McClintock
Laboratory for Atmospheric and Space Physics\ University of Colorado\ Boulder\ Colorado 79298!9281\ U[S[A[
Received 8 February 0887 ^ revised 02 May 0887 ^ accepted 07 May 0887
Abstract[ The Cassini Ultra!violet Imaging Spec!
trograph "UVIS# is part of the remote sensing payload
of the Cassini Orbiter spacecraft[ Its science objectives
include investigation of the chemistry\ clouds\ and
energy balance of the Titan and Saturn atmospheres ^
neutrals in the magnetosphere ^ D:H ratio for Titan
and Saturn ^ and structure and evolution of Saturn|s
rings[ The UVIS has two spectrographic channels
which provide images and spectra covering the ranges
from 45Ð007 nm and 009Ð089 nm[ A third optical path
with a solar blind CsI photocathode is used for high
signal to noise ratio stellar occultations by rings and
atmospheres[ A separate hydrogen!deuterium absorp!
tion cell "HDAC# measures the relative abundance D:H
from their Lyman!alpha emission[
The rings of Saturn are the best!studied of planetary
rings and contain the majority of the ring material in
the solar system[ The four!year Cassini tour provides
multiple observation opportunities and long time
coverage[ The UVIS observations include photometry\
imaging\ spectroscopy\ and stellar occultations[
Numerous di}raction!limited star occultations by the
rings are a prime objective for the UVIS[ The 1 ms
integration period in this mode will give a ring radial
resolution of better than 19 m[ The counting rate is
49× greater than the Voyager star occultations in a
resolution element 4× smaller[ Multiple opportunities
on the same Saturn passage will de_ne temporal and
azimuthal variation[ We expect to observe waves\
wakes and ring edges*all characteristics of ring
dynamics and history[ The imaging resolution is 0
mrad\ or 0999 km from a viewing range of 095 km[ The
UVIS is sensitive to the shortest wavelengths of all
the remote sensing experiments\ and thus the scattered
light from the smallest ring particles[ In combination
with images from ISS and VIMS\ CIRS spectra\ and
Correspondence to ] L[ W[ Esposito[ Tel[ ] 990 292 381 6566 ^ fax ]
990 292 381 5835
$ Presented at the International Cassini Conference\ Bologna\
Italy\ November 0885[
radio occultations\ we will de_ne the size distribution
of ring particles larger than r 9[0 mm[ Observing the
di}erential opacity of the rings from 009Ð089 nm and
the possible {{aureole|| created by ring particle forward
scattering will further constrain the ring particle size
distribution[ Water ice absorption is strong in this por!
tion of the UV ] some compositional implications for
the ring particles are possible from the ring spectral
re~ectance[ The UVIS is sensitive to H "0105 A
ý # and O
"0293\ 0245 A
ý # emissions near the rings which will
better de_ne the ring atmosphere[ The atmosphere
observations constrain ring!magnetosphere inter!
actions\ ring evaporation and particle lifetime\ as well
as interchange of material between rings\ moons and
magnetosphere in the Saturn system[ The high spectral\
spatial\ azimuthal and time coverage from Cassini will
provide data to re_ne and test our best current models
of the short!term and long!term ring dynamics[ Þ 0887
Elsevier Science Ltd[ All rights reserved
Introduction
Planetary rings\ which just two decades ago were thought
unique to the planet Saturn\ have now been observed
around all the giant planets "for a recent review\ see Espo!
sito\ 0882#[ These rings are composed of many particles
with a broad range in size[ The observed rings systems are
quite diverse "Fig[ 0#[ Jupiter|s ring is optically thin and
composed of dust!like small particles "Showalter et al[\
0876#[ Saturn|s rings are broad\ bright\ and opaque "Cuzzi
et al[\ 0873\ Esposito et al[\ 0873#[ Uranus has narrow\
dark rings among broad lanes of dust which are invisible
from Earth "French et al[\ 0880\ Esposito et al[\ 0880#[
Neptune|s rings include incomplete arcs restricted to a
small range of the circumference "Porco et al[\ 0884#[ All
rings lie predominantly within their planet|s Roche limit\
where tidal forces would destroy a self!gravitating body
"Nicholson and Dones\ 0880#[ They are also within the
planet|s magnetosphere\ and in the case of Uranus\ they
are within the upper reaches of the planetary atmosphere[
0111
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 0[ Rings and moons of the outer planets\ scaled to the size of the primary[ Dashed line ] synchronous orbit[ Dot!dashed ] Roche
limit[ From Nicholson and Dones "0880#
The structure of the rings and their composition "as
shown by their colors# vary between the planets\ and like!
wise within each ring system[ The broadest set of rings
and the most identi_ed processes are found in Saturn|s
system[ Saturn|s rings display many diverse structural fea!
tures ] vertical thickness considerably greater than the
average particle size ^ dark lanes\ gaps\ and other opacity
variations ^ eccentric shapes and inclined orientations ^
sharp edges ^ azimuthal brightness variations\ arcs\ and
clumps ^ waves and wakes ^ and incomplete\ kinked\ and
apparently {{braided|| con_gurations[ Some of these fea!
tures have been explained as a result of a range of gravi!
tational interactions with nearby moons "e[g[ Borderies et
al[\ 0873#[
Beyond the interactions with moons\ the ring particles
interact with the planet|s magnetosphere via charging\
plasma drag\ and forces applied by the ambient magnetic
and electric _elds[ Electrostatic forces may lift small par!
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
ticles o} the surface of the larger ring particles to create
the dark\ radial lanes seen in the Voya`er Saturn pictures
which were termed {{spokes||[ The particles in the lowest
orbits may experience drag from Saturn|s exosphere[
The size distribution of ring particles spans many
decades\ extending from sub!micron dust\ through meter!
sized particles\ to small imbedded moons\ including the
recently discovered {{Pan||\ about 09 km in radius
"Showalter\ 0880#[ Perhaps 099Ð0999 moons bigger than
0 km orbit each of the giant planets\ but are too small to
be detected by Voyager|s cameras "Colwell and Esposito\
0882#[ Theoretical expectations and some data support
the idea that the particles in a ring will segregate in size\
both radially and vertically[
Radio occultations at two wavelengths have provided
size information for Saturn|s rings at a number of
locations "unfortunately excluding Saturn|s B ring
because of its opacity# in the range of roughly 0Ð09 m[
Information on smaller particles is from photometry and
di}erential opacity in stellar occultations[ The derived
size distribution can be characterized by broad power!
law distributions\ with power!law exponents of 1[4Ð2[4
"Zebker et al[\ 0874#[
The composition of ring particles is well known only
for Saturn[ Spectroscopic\ occultation\ and neutron
measurements all imply that Saturn|s particles resemble
the nearby small moons and probably contain at least
some silicate material "Cuzzi et al[\ 0873\ Entrada and
Cuzzi\ 0885#[ Color variations may indicate compositional
di}erences in di}erent parts of the rings[
We have a _rst order understanding of the dynamics
and key processes in Saturn|s rings\ much of it based on
previous work in galactic and stellar dynamics[ The rings
are a kinetic system\ where the deviations from perfect
circular\ equatorial motion can be considered as random
velocities in viscous ~uid[ Unfortunately\ the present
theoretical models are often idealized "e[g[\ treating all
particles as hard spheres of the same size# and cannot
yet predict many phenomena in the detail observed by
spacecraft observations "e[g[\ sharp edges#[
The rings show many youthful features ^ Saturn|s ice
is bright\ Uranus| rings are narrow\ Neptune|s arcs are
constrained to a small range of longitude\ and Jupiter|s
particles are so small that they are transported away in a
thousand years or less[ The momentum transferred
between rings and the nearby moons should have caused
them to spread much further apart than they are now[
Further\ Saturn|s small moons discovered by Voya`er
also could not have survived the ~ux of interplanetary
meteoroids for the age of the solar system "Colwell and
Esposito\ 0881#[ In much less time\ these small moons
would be shattered by an impacting object[ These impacts
may not only destroy the moon\ but they can also re!
create the ring systems that are gradually spreading and
being ground to dust[ Thus\ the moons not only sculpt
the rings| structure [ [ [ they may be the reservoirs for past
and future ring systems[ Consequently\ most rings are
likely much younger than the solar system and new rings
episodically created by the destruction of small moons
near the planets[ This idea is one example of how the
recent spacecraft observations have indicated a larger
role for catastrophic events in the history of the solar
system[
0112
Dones "0886# presents an alternative possibility that the
rings of Saturn could be primordial\ by re!interpreting the
evidence for ring youth and criticizing the present models
for torque transferred between moons and rings and the
proposed darkening of rings by meteoroid bombardment[
We note that no viable model exists for creating Saturn|s
extensive ring system in the recent past\ unless it was a very
unlikely event "Dones\ 0880#[ However\ the destruction of
small moons provides a ready source for lesser rings[
The range of phenomena seen in Saturn|s rings was
unexpected and gives insight into the processes in other
~attened astrophysical systems[ An interesting parallel
can be drawn between the processes observed now in
planetary ring systems and those that occurred at the time
of the origin of the planets[ Thus\ Saturn|s rings provide
tests for theories of origin and evolution of the solar
system[
This paper gives a brief description of the UVIS experi!
ment\ and planned Cassini observations[
Instrument description
In this section\ we describe the constraints\ optical con!
_guration and performance of the UVIS which enable the
scienti_c investigation "McClintock et al[\ 0881\ 0882#[
The UVIS design represents a balance between the science
objectives and the constraints of mass\ power\ volume and
operability for a spacecraft instrument[ Figure 1 and Table
0 summarize the optical and mechanical characteristics of
the instrument[ It consists of two moderate resolution
telescope!spectrographs covering the wavelength ranges
45Ð007 "EUV# and 009Ð089 nm "FUV#\ a sensitive high!
speed photometer "HSP#\ and a hydrogen!deuterium
absorption cell "HDAC#[ The separate channels are alig!
ned for simultaneous observation[
Because of the breadth of science objectives addressed
by the UVIS "Esposito et al[\ in preparation#\ the UVIS
uses a variety of observation techniques[ These include
producing maps and images by moving the slit to a
sequence of locations through rastering\ slewing and drif!
ting the spacecraft optical axis[ Limb drifts provide high
resolution at the target|s limb[ Occultations of the sun are
observed with the EUV channel ^ the HSP can observe
stellar occultations in concert with the EUV and FUV[
The UVIS will encounter a wide range of signal streng!
ths[ Saturn system atmospheric and magnetospheric emis!
sions at wavelengths shorter than 199 nm are very faint\
with radiances of the order of 9[0Ð09 Rayleighs in indi!
vidual emission lines[ In contrast\ sunlight scattered from
the disk of Saturn and its icy satellites produces radiance
in the range of 0Ð09 kR:nm[
Spectro`raphic channels
The basic instrument design adapts proven design
concepts\ using a grating spectrometer followed by multi!
element detector[ We chose imaging pulse!counting mic!
rochannel plate detectors because of more than a decade
of experience using this kind of detector equipped with a
CODACON readout anode[
0113
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 1[ UVIS opticalÐmechanical con_guration
Table 0[ Summary*ultraviolet imaging spectrograph and HSP
FUV "009Ð089 nm#
EUV "44Ð004 nm#
HSP
Telescope
Focal length Size "mm#
Entrance pupil size "mm#
Mirror size "mm#
Re~ecting surface
) re~ectivity
19×19
11×29
11×29
Al¦MgF1
79
099
19×19
11×29
SIC
39
199
Toroidal gratings
Size "mm#
Grating radii "mm#
Grating surface
) Grating absolute
Grooves:mm
Input angle a "degrees#
Out angles b "degrees#
59×59
299\ 185[0
A0¦MgF1
39
0957
8[11
21[8
59×59
299\ 185[7
SIC
09
0399
7:92
−3[97\ ¦0[61
2!Position slits
Slit widths "microns#
Dl "nm# atmosphere
Field of View "mrad#
Filters "Acton Research#
64\ 049\ 799
9[13\ 9[37\ 1[4
"[64\ 0[4\ 7#×53
None
099\ 199\ 799
9[13\ 9[37\ 0[8
"0\ 1\ 7#×53
None
CsI
7
MgF1
14[5×5[3
0913×53
14×099
KBr
14
None
14[5×5[3
0913×53
14×099
Detectors
Photocathode
) Maximum QE
Detector window
Detector size "mm#
Pixel format "l×#
Pixel size "m#
Window
Pulse resolution
024×29
Al¦MgF1
5×5
CsR
Hamamatsu
MgF1
49 ns
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
The CODACON acts as a photon locator[ For 1M×1N
pixels on the anode array\ the output electronics generate
a logic compatible M¦N bit address for each detected
photon[ These photon locations are accumulated in an
external memory to build a picture that is periodically
read out and telemetered[ Detectors for the Cassini UVIS
have a count rate limit 094:s\ set by the speed of the
detection electronics[ Details of the CODACON oper!
ation are described by McClintock et al[ "0881#\ and Lawr!
ence and McClintock "0885#[
A two!dimensional format for the CODACON detec!
tors allows simultaneous spectral and spatial coverage[
This allows two!dimensional spatial mapping as described
above[ The detector format is 0913×53 with a pixel size
of 9[914×9[09 mm[ Two separate detectors cover the
wavelength range[ A windowless detector is required for
the extreme ultraviolet "EUV# wavelength range 45Ð007
nm[ We chose KBr as the photocathode material for this
detector because of its ability to withstand a moderate
amount of exposure to atmospheric water vapor[ CsI was
also considered as the EUV photocathode\ but it requires
a door with a hermetic seal to protect it before launch[
CsI is the photocathode material for the far!ultraviolet
"FUV# wavelength range 009Ð089 nm[ This detector is
enclosed in a separate vacuum housing with a MgF1
window[ Although other materials such as CsTe are more
e.cient photocathodes than CsI for wavelengths longer
than 029 nm\ we use CsI for the FUV because it is much
less sensitive to longer wavelength scattered light within
the spectrograph[ This scattered light usually dominates
and can often obliterate weak planetary FUV emiss!
ions when a CsTe or a more red!sensitive photocathode
is used[
The optical requirements for the EUV channel are
unique because the low re~ectivity of optical materials
at these wavelengths require that the number of optical
elements in the system be as small as possible[ Our design
is a single!mirror telescope followed by an entrance slit
and a concave grating used in a Rowland circle mount[
Fig[ 2[ FUV optical con_guration
0114
We found that the EUV channel con_guration also
provides satisfactory spatial and spectral resolution in the
FUV channel[ The two channels are identical except for
the optical coatings\ di}raction grating rulings\ and detec!
tor photocathodes[ To achieve the desired spectral resol!
ution\ the optics must provide image quality in the spec!
trograph dispersion direction that is nearly equal to
CODACON spatial resolution limit of two pixels "9[94
mm#[ Toroidal gratings used with the angle of di}raction
equal to zero in the center of the detector have the best
imaging properties because\ for a given angle of incidence\
they produce stigmatic images at two wavelengths in the
focal plane[
The FUV channel is shown in Fig[ 2[ It is our best
compromise for spectral resolution\ image quality\ and
size[ The EUV channel is identical to the FUV except that
the detector window and ion pump are not present[ Each
telescope consists of an o}!axis section of a parabolic
mirror with a 099 mm focal length[ A 19×19 mm aperture
which is 022 mm in front of the telescope mirror acts as
a Lyot stop conjugate to the grating[ We equipped the
telescope with a sunshade and ba/e system to minimize
scattered light background during limb scan measure!
ments[ We achieve a point source rejection ratio of 095 for
sources located 0> or more away from the _eld of view
"Fig[ 3#[
Each spectrograph consists of a set of three inter!
changeable entrance slits "9[964Ð9[5 mm wide×5[3 mm
tall ^ see Table 0# and a toroidal grating with a 299!mm
horizontal radius of curvature used in a Rowland circle
mount[ The spectrum is thus formed on a cylinder of
radius 299 mm[ We placed the detector on a chord of
the Rowland circle to minimize average defocus over its
planar 14[5×5[3 mm sensitive area[ The grating ruling
spacing is set by the spectral coverage requirement for
each channel[
Mechanical constraints require the spectrograph hous!
ings to have the angle aÐb 8[1> identical for both chan!
nels "a and b are the angles of incidence and di}raction\
0115
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 3[ HSP o} axis response "contours ] log09 of sensitivity relative to maximum signal#
respectfully#[ This con_guration results in b 9> at the
center of the detector for the FUV channel[ The detector
subtends angles Db 21[34> along the Rowland circle[
The grating radii yields the stigmatic wavelengths at
bs 20[12>[ The detector plane also intersects the Row!
land circle at these wavelengths[ This con_guration thus
gives the best average imaging over the entire detector[
Stellar and solar occultation modes
In addition to observing planetary emissions\ the UVIS
spectroscopic channels will view occultations of stars by
the atmospheres of both Saturn and Titan\ and by Sat!
urn|s rings[ The _eld of view for a 9[964Ðmm!wide
entrance slit is 9[64 mrad[ Spacecraft pointing accuracy is
limited to21 mrad ^ therefore\ we set each telescope _eld
of view to be 7 mrad wide for stellar occultation experi!
ments[ To change the _eld of view\ we included a three!
position slit mechanism in each channel "Fig[ 2#[ We need
at least two slits ^ one 9[964 mm wide "FUV# or 9[09
Fig[ 4[ High!speed photometer layout
mm wide "EUV# to meet the highest spectral resolution
requirements for extended sources and one 9[79 mm wide
for point sources\ to accommodate the spacecraft pointing
capability[
We also added a solar occultation capability to the
EUV channel ] stars are not useful because interstellar
atomic hydrogen blocks their radiation at wavelengths
shorter than 80[1 nm[ In this observing mode "see Fig[ 1#\
light enters the EUV channel through a small aperture
located 19> away from the normal viewing direction and
is directed toward the telescope mirror by a small grazing
incidence mirror[ A two!position mechanism is used to
block the light path when the solar viewing mode is not
used[
Hi`h!speed photometer
In addition to the spectroscopic channels\ the UVIS con!
tains a high!speed photometer with an integration time of
1[9 ms to observe occultation of stars by the rings of
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Saturn[ Figure 4 shows the con_guration for the HSP[ It
consists of a telescope mirror that is approximately 09
times as large as those used in the spectroscopic channels\
followed by an aperture to limit the _eld of view to
5 mrad\ a MgF1 lens\ and a Hamamatsu model R0970
photomultiplier tube with a CsI photocathode used in
pulse!counting mode[ The lens images the telescope mirror
onto the photocathode of the photomultiplier[ Without
this lens\ small changes in spacecraft pointing would cause
the image of the star to move around on the nonuniform
detector photocathode\ resulting in unwanted signal vari!
ations[ In Fig[ 4\ we also show the detector electronics\
which include a high!voltage power supply "HVPS# and a
pulse!ampli_er discriminator "PAD#[
The spectral response of the HSP is limited at short
wavelengths to about 004 nm by the MgF1 detector win!
dow and at long wavelengths to about 089 nm by the
work function of the CsI photocathode[ We chose CsI for
its low sensitivity to solar light because the spacecraft
orbit geometry requires that most of the occultation obser!
vations be made while looking through the sunlit rings[
The photometer _eld of view must be at least 5 mrad to
account for pointing errors of 21 mrad[ If we used a
photomultiplier with either a CsTe or a Bi!alkali pho!
tocathode with this large a _eld of view\ sunlight re~ected
from the rings would produce a background at least 099
times larger than the signal from the brightest stars[ Using
a CsI photocathode we are limited to stars with spectral
class O and B "e}ective temperatures in the range 39\999Ð
09\999 K#\ but the background from re~ected sunlight is
equal to the signal from a B4 star with a visual magnitude
MV of 3[4[ During the mission\ we expect over 249 stellar
occultation opportunities for the rings with stars that are
at least 09 times brighter than MV of 3[4[
Sensitivity
We use the following equations to relate the signal and
noise output of the UVIS to its design parameters[ For
viewing extended sources\
Fig[ 5[ UVIS sensitivity\ in counts:s:KR
0116
S"l# I"l#(AT"l#:F1T(Apix(t"l#(QE"l#T
"0#
N1"l# S"l#¦BT
"1#
where
S signal in counts per pixel
l wavelength
I source radiance in photons per cm1 per ster!
adian per s
AT:FT1 square of the optical system focal ratio
Apix area of a single detector pixel in cm1
t optical system transmission
QE detector quantum e.ciency
T observation time in seconds
N noise in counts per pixel
B detector background or {{dark count|| in
counts per pixel per s[
Figure 5 shows the system sensitivity of the UVIS for
each channel and for two of the slit widths[ Curves for the
occultation slits are not shown[ They are a factor of 09[6
higher than the curves for a 64!mm slit[ Each curve is
labeled by the slit width in micrometers and nominal
resolution Dl in nanometers[ In limb scan mode\ we orient
the entrance slit parallel to the planet limb and slew the
spacecraft to obtain vertical pro_les[ At each altitude\ we
sum all 53 spatial pixels into a 0913×0 spectrum[ Map!
ping mode uses the full 1!dimensional capability of the
detector\ and we generate a spectral by spatial map of
0913×53 pixels[
We have plotted the detection limit "i[e[ the signal
required to produce a signal!to!noise ratio of 2[9# for a
2599 s observation in limb scan and mapping modes[ Solid
lines are for typical observed CODACON backgrounds
of 1 counts per detector "54\425 pixels# per second[ The
dashed lines in Fig[ 6 show detection limits for the total
expected background[ Three radioisotope thermoelectric
generators "RTGs# provide the spacecraft with electrical
power[ We estimate that the gamma rays emitted by the
RTGs will increase the background by an additional 19
0117
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 6[ UVIS sensitivity limits for integration period of one h[ Solid ] for the low resolution and high resolution slits[ Dashed ] summing
all spatial information along the slit
counts per detector per second[ The dashed lines in Fig[ 6
show SNR curves that include both contributions to the
background[
During Titan encounters\ its disk will have an angular
diameter of 1[5> on the sky at 4[1 h before closest approach
at a distance of about 0[1×094 km[ At that time\ a single
detector pixel subtends 1[4> in latitude at the subspacecraft
point[ The satellite just _lls the length of the spectrograph
entrance slit at T−2[6 h[ Thus T−4[1 h to T−2[6 h is an
ideal time to make maps to provide data for pho!
tochemical models and aerosol studies[ We will have
about 4399 s to record 13 longitude positions across the
disk[ Limb scans and atmospheric occultation measure!
ments will occur nearer to closest approach[
To make thermosphere measurements at an altitude of
0999 km with the disk 4> away from the optic axis\ we
must be within 00×092 km[ This occurs at about T−9[4
h[ At this distance\ a 64!mm!wide entrance slit subtends
about 8 km at the limb and the spacecraft drifts about 0
scale height "49 km# every 7 s[ Occultation measurements
of the atmosphere can be made at larger distances\
especially at night when there is no scattered light from
the sunlit disk of Titan to interfere with the atmospheric
observations[
system "Fig[ 7#[ Conversely\ the azimuthal coverage is
more sketchy[ As to temporal coverage\ the data
encompass only a few weeks around each encounter\ with
the best resolution data for each from a period of less than
a day[ The images are all from the Voyager RETICON
cameras ] the spectral sensitivity of their silicon detectors
explains why we have no pictures in the UV or IR[ This
information is now supplemented by data from the
ground\ HST and IUE\ which all lack the radial\ azi!
muthal and temporal resolution provided brie~y by the
spacecraft encounters[ A new data set from the spectacular
0884 ring plane crossing "see Nicholson et al[\ 0885\
Showalter 0886# provides information on the more trans!
parent rings[ The thickest parts of Saturn|s rings were not
penetrated by either the radio or stellar occultations[
The Cassini mission provides the opportunity to mea!
sure the rings at high resolution in the radial\ azimuthal
and vertical dimensions over a period of four years[ Obser!
vations of the ring environment\ magnetosphere and small
nearby satellites will determine the interactions of the
various parts of the Saturn system with its rings[ The
UVIS experiment has the capability for imaging\ spec!
troscopy and high speed photometry that can contribute
to these scienti_c objectives[
UVIS ring observations
Rin` structure
Voya`er results and Cassini capabilities
Figure 8 shows observations of density waves in Saturn|s
rings from the Voyager PPS occultation "Esposito et al[\
0872a\b#[ These features are but one of a number of
dynamic phenomena evident in the ring radial pro_le[ The
UVIS High Speed Photometer "HSP# channel has been
optimized to follow up on the Voyager investigations[ The
HSP _eld of view is 5×5 mrad\ large enough that no
brightness modulations are expected from pointing vari!
ations[ The CsI photocathode is essentially solar!blind\
Voyager I and II made the closest investigations of Sat!
urn|s rings in 0879 and 0870[ These missions provided
images\ spectra\ radio and stellar occultations as well as
information on the ring environment[ Reviews are pro!
vided by Cuzzi et al[\ 0873 and Esposito et al[\ 0873[ The
radio occultation\ stellar occultation\ and sequences of
images each provided complete radial coverage of the ring
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
0118
Fig[ 7[ Saturn|s rings from the Voyager 1 PPS d Scorpii stellar occultation\ showing ring radial structure "Esposito et al[\ 0872a#
allowing high signal!to!noise occultations of both sunlit
and shadowed rings[ The HSP is bore!sighted with the
spectroscopic channels\ which allows concurrent spec!
troscopy of the transmitted starlight from 80Ð089 nm[
Simultaneously\ we can obtain a long slit image of the
occultation point up to a distance 229 mrad\ allowing
the observation of di}racted light in the stellar aureole[
The time resolution of 1 ms can give spatial resolution of
19 m or better on the rings\ comparable to the di}raction
limit[
The large mirror and ampli_ers in the HSP allow very
high counting rates[ We expect 1999 counts in 1 ms for
the occultation star d Sco[ This compares to the Voyager
PPS observation of d Sco of 28 counts every 09 ms "Espo!
sito et al[\ 0872a#[ Thus the Cassini UVIS HSP can probe
structures _ve times narrower than Voyager\ with 49 times
the signal in each integration period[ This high sensitivity
and resolution will be used to probe wakes\ waves and
ring edges[
Occultation tracks at multiple ring longitudes will likely
reveal azimuthal asymmetries in the rings as was seen by
the Voyager PPS at Uranus "Colwell et al[\ 0889# and
ground!based stellar occultation observations of Nep!
tune|s rings "Elliot et al[\ 0874\ Covault et al[\ 0875\ French
et al[\ 0882#[ These asymmetries may provide clues to the
origin of the ring features and imply the presence of nearby
perturbing satellites[ These data will be used to constrain
physical models of their origin and evolution[
Radial optical depth pro_les will be generated from
stellar occultation measurements "cf\ Colwell\ 0878\ Col!
well et al[\ 0889#[ Accurate absolute determinations of the
locations of edges\ gaps\ and wave features in the data
will allow the identi_cation of new ring!satellite reson!
ances\ and quite possibly the indirect detection of new
small satellites within the ring system "e[g[\ Horn et al[\
0889#[ Such satellites are linked to the evolution and the
origin of the rings[ Their number and distribution within
the rings will help constrain theories of ring origin "Col!
well\ 0883\ Sicardy and Lissauer 0881#[
During HSP observations of stellar occultations\ the
UVIS far ultraviolet channel "FUV# will measure ring
background brightness[ These measurements will also
provide information on the size distribution of small
"micron!sized# dust particles in the rings during occul!
tations by the shadowed rings[ Dust in Saturn|s rings
will produce a di}raction aureole detectable by the FUV
channel of the UVIS[ The _rst dark di}raction ring will
show up in FUV spectral!spatial images as a decrease in
the ring brightness away from the stellar image\ that is
seen along a sloping locus of pixels in the image "Fig[ 09#[
The location of this di}raction signature is sensitive to the
characteristic dust particle size[ Size distributions based
on di}erent physical models of the creation\ transport\
and destruction of dust will be tested against the obser!
vations[ The abundance\ size\ and distribution of dust in
the rings is a useful diagnostic for the size distribution\
velocity distribution\ and surface properties of the larger
ring particles which act as sources and sinks of dust in
the rings "Colwell and Esposito\ 0889a\b#[ Data on these
larger particles will be obtained from stellar occultations
and combining UVIS re~ectance data with images from
the Cassini imager "ISS# and near infrared camera
"VIMS#[ The FUV data will also be used to determine the
magnitude of the ring background signal to be removed
0129
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 8[ Density waves seen in the PPS occultation "Esposito et al[\ 0872b#[ Compare the resolution to Fig[ 7
Fig[ 09[ Aureole observations during stellar occultation\ shown as a spectral!spatial UVIS image[ Location of _rst dark di}raction
ring is the central sloping line[ This would be the center of a dark band in the image[ The calculation is for spherical particles with
4 mm radius[ The two outer lines indicate the dark band|s full width at half!maximum
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
0120
Fig[ 00[ Numerous ring stellar occultation opportunities are shown with CASPER "Cassini Sequence Planner# software in this near!
polar view of Saturn and its ring system[ Plus symbols indicate stars ^ larger symbols are brighter stars[ The stippled region indicates
Saturn|s shadow[ Stellar occultations by the shadowed rings are best for ring aureole observations[ The sub!spacecraft point is
indicated by a cross on the planet
from the HSP occultation data[ Figures 00\ 01\ and 02
show the multitude of available opportunities and the
progress of a stellar occultation from Cassini tour 81!90[
UVIS ima`es
During remote sensing ring observations\ the UVIS will
slew its slit to make images of the rings[ At a range of 095
km\ the UVIS resolution of 0 mrad gives a picture element
of 0999 km width[ Typical images will be 53 elements tall
"along the slit# with the image width determined by the
total slew duration[ Because UVIS has the shortest wave!
length of any of the remote sensing instruments\ it will be
more sensitive to the smallest particles in the rings\ with
sizes as small as 9[90Ð9[0 m[ The images at di}erent UV
wavelengths can be compared to camera images to deter!
mine the dust contribution and extend the size distri!
bution[ The rings| {{spokes|| are one target[ By combining
the UV observations with CIRS spectra\ ISS\ VIMS
images and radio occultations\ the size distribution of the
ring particles can be measured from sub!micron to meter
sizes[
Rin` composition
Figure 03 shows the re~ectance spectrum of Saturn|s rings
from IUE and Voyager[ The water absorption edge at
0549 _ is clearly seen[ We can map the depth and wave!
length of this feature across the rings to determine com!
positional and age variations[ At wavelengths 0799Ð1999
_\ water ice is transparent\ and the non!icy ring com!
ponent may be studied[
During systematic spectroscopy of the Saturn system\
the UVIS will map the emissions from neutral hydrogen
"0105 _# and oxygen "0293\ 0245 _#[ Emissions near the
rings will de_ne the ring atmosphere "Broadfoot et al[\
0870\ Hall et al[\ 0885# and the interactions of the ring
with its local environment[ This will constrain the long!
term evolution of the ring particles[ The UVIS will also
measure remotely a number of magnetospheric con!
stituents "particularly H\ O\ N\ and D# that will de_ne the
broader ring!magnetosphere interactions[
Rin` history
The UVIS and the other remote sensing and in situ Cassini
experiments will provide a rich data set on the current
state of the rings and their present interactions with other
parts of the Saturn system[
A comprehensive model of the origin and large!scale
evolution of Saturn|s rings will be developed based on
the ensemble of Cassini observations at Saturn[ Crater
statistics on the larger satellites and the anticipated dis!
covery of smaller {{ring moons|| will allow development
0121
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 01[ The star o Canis Major is occulted by the shadowed rings of Saturn[ The FUV occultation slit and the HSP aperture are
shown around the star[ The stippling indicates shadowed regions of the planet and rings[ The distance to Saturn is 381\999 km
of an accurate model of the time!dependent interplanetary
impacting ~ux at intermediate and large sizes[ This ~ux\
when combined with theoretical and experimental results
on catastrophic fragmentation\ will drive the global evol!
ution of the ring system[ Disruption of satellites or {{rubble
piles|| creates new ring particles[ These then undergo
orbital evolution\ in~uenced by gravitational per!
turbations from other satellites\ and partial accretion
"Canup and Esposito\ 0884#[ Dust is produced within the
ring due to interparticle collisions and micrometeoroid
bombardment[ These processes can be combined into a
single global Markov chain simulation of the ring system
"Colwell and Esposito\ 0882# for time scales of the age of
the solar system[ Monte Carlo simulations can be con!
ducted using the same transition probabilities in order to
create simulated histories of the rings system for direct
comparison with Cassini observations[
Summary
Scienti_c objectives
Variations of the abundance of dust between di}erent
ring features will provide additional information on ring
particle characteristics\ including collision velocities and
regolith properties[ Dynamically active regions may exhi!
bit di}erent dust abundances than quiescent regions in
the rings[ Features such as arcs\ kinks\ braids\ and other
Fig[ 02[ An animated sequence of a stellar occultation across the shadowed rings in a geometry similar to that in Fig[ 01[ The UVIS FUV occultation slit and HSP aperture are shown[
The duration of the occultation is approximately 1[4 h
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
0122
0123
L[ W[ Esposito et al[ ] Cassini UVIS observations of Saturn|s rings
Fig[ 03[ Re~ectance of Saturn|s rings from Voyager and IUE[ Source ] Jay Holberg "personal communication#
azimuthal asymmetries may indicate a mode of origin
from satellite disruption or complex gravitational inter!
actions with nearby satellites[ The distribution of small
moons\ their resonances\ and density waves and gaps
within the rings will be used to characterize the large!scale
transport of angular momentum through the system for
use in global evolution models of the rings[
The UVIS investigation will provide "0# a com!
prehensive study of features in Saturn|s rings\ including
their dynamical links to embedded satellites ^ "1# a deter!
mination of the abundance and size distribution of dust
within the rings as a function of location within the rings ^
"2# modeling of ring processes and system interactions at
short time scales ^ "3# models of the global evolution of
the rings over the age of the solar system[
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