First far-ultraviolet disk-integrated phase curve analysis of Mimas, Tethys and
Dione from the Cassini-UVIS data sets
Emilie M. Royera,, Amanda R. Hendrixb,
a University
of Colorado, LASP, 3665 Discovery Dr., Boulder, CO 80303
b PSI, Tucson, AZ
Abstract
We perform an analysis of the photometric properties of the icy Saturnian satellites at 180 nm, based on the first
far-UV disk-integrated phase curves of Mimas, Tethys and Dione. Their interactions with the environment (the E-ring
and the magnetosphere) are investigated, leading to a better understanding of the effects of exogenic processes on the
system of Saturn. We find that Tethys and Dione have a leading hemisphere brighter than their trailing hemisphere
at far-UV wavelengths, while Mimas exhibits a quite uniform reflectance on its surface. No asymmetry is observed
between the Saturnian and anti-Saturnian hemispheres of those satellites, indicating that exogenic processes are important primarily on the leading and trailing hemispheres. Tethys shows a narrower opposition effect, suggesting a
more porous regolith on its surface than on Dione and Mimas. This could be the consequence of more significant
bombardment by the E-ring grains at the orbit of Tethys. Dione’s photometric properties reveal a more absorbing surface, which could be explained by a lower amount of E-ring grain bombardment and/or by the deposit of a darkening
agent mainly on its trailing side.
Keywords: Saturn, satellites, Satellites, surfaces, Ultraviolet observations, ices, UV spectroscopy, spectrophotometry
1. Introduction
The midsize satellites of Saturn have been observed
since 1980 by two space missions: Voyager and Cassini.
While the first close-up images were obtained in the
early 1980s, the planetary community waited 24 more
years for the arrival of the Cassini spacecraft around
Saturn on July, 1 st 2004 , to investigate the Saturnian
system in detail. For the first time, a significant amount
of information on those satellites have been acquired at
far-ultraviolet (FUV) wavelengths.
Buratti and Veverka (1984) were the first to analyze
Tethys and Dione disk-integrated phase curves, using
Voyager clear filter data. No information was available
on the opposition surge, due to a lack of data at solar
phase angles smaller than 8◦ . They observed a leading/trailing asymmetry (where the leading hemisphere
is centered on 90◦ W and the trailing hemisphere is centered on 270◦ W) with a leading hemisphere brighter on
Tethys and Dione; they found Mimas to have a slightly
brighter trailing hemisphere, despite the poor quality of
Email address: Emilie.Royer@lasp.colorado.edu (Emilie
M. Royer)
Preprint submitted to Icarus
the data. Verbiscer and Veverka (1992) also reported a
slightly brighter trailing hemisphere (TH) on Mimas at
0.48 µm using Voyager data. This leading versus trailing hemisphere brightness has also been predicted by
the model of Hamilton and Burns (1994): as written in
note 16 of that paper, the E-ring grains overtaking Mimas in its orbit should primarily impact the trailing side.
Buratti et al. confirmed this result again in 1998, using
ground-based data at 0.9 µm.
Moreover, Buratti et al. (1998) showed that at 0.9
µm Mimas, Enceladus, Tethys, Dione and Rhea are far
brighter than any other class of object in the Solar system, each of them having a leading hemisphere (LH)
geometric albedo greater than 0.70. This is also true in
near-UV and visible wavelengths (Buratti and Veverka,
1984; Nelson et al., 1987). The high albedos suggest
that these water-ice surfaces contain few low albedo
contaminants and also support the idea that the optical properties of the mid-size Saturnian satellites are
strongly affected by deposits of bright ice grains from
the E-ring (Buratti et al., 1998). Previously, no correlations between the geological units and the color and
albedo patterns were found for the five midsize satellites
of Saturn, suggesting that the photometric properties are
July 21, 2014
due largely to exogenic alterations (Buratti et al., 1990).
More recently, in 2007, Verbiscer et al. used Hubble
Space Telescope (HST) data to show a positive correlation between the E-ring grain flux and the geometric
albedo of satellites.
Detailed studies of the photometric and spectral properties of the Saturnian satellites started with the analysis of the Cassini data sets. Among them, we can
note in 2010, the papers of Pitman et al. and Filacchione et al., which also reported a leading hemisphere
brighter than the trailing one on Tethys and Dione, both
using the Cassini-VIMS (Visual and Infrared Mapping
Spectrometer) instrument. Pitman et al. (2010) analyzed
disk-integrated spectrophotometric properties of the five
midsize satellites, in the visible and infrared between
0.35 and 5.01 µm, and also derived the bolometric Bond
albedos of Mimas (0.67±0.10), Tethys (0.61±0.09) and
Dione (0.52±0.08). They gave new refined values of the
major photometric quantities for both leading and trailing hemispheres.
An extended study comes from Filacchione et al.
(2010) , who provided comparative study of the spectral characteristics of the mid-size and minor Saturnian
satellites. They investigated the visible spectral slopes
(0.3µm - 0.55 µm) and found a strong forward scattering component on Tethys. They investigated the diskintegrated composition of the satellites and the amount
of contaminant present in the water ice, as well as the
grain size distribution. Carbon dioxide was suggested
as a possible contaminant of the water ice on the most
exterior satellites (Hyperion, Iapetus and Phoebe). That
study found no particular hemispheric brightness asymmetry at Mimas; the largest hemispheric albedo dichotomy was found on Dione. A large amount of contaminant covering Dione’s trailing side could explain it,
as suggested by Clark et al. (2008).
Schenk et al. (2011) created global color maps on
the midsize icy satellites in three colors (UV-Green-IR).
Color patterns were highlighted, especially on the leading hemispheres of Tethys and Mimas, where a blueish lens shape at the equator, extending ±20◦ in latitude
on Tethys and ±40◦ on Mimas, was clearly observed.
While Voyager images already showed this equatorial
band across the leading hemisphere of Tethys (Smith
et al., 1981; Stooke, 1989, 2002; Buratti et al., 1990),
it was the first time that it was observed on Mimas.
Howett et al. (2011, 2012) with Cassini-CIRS near-IR
data produced results correlated with the observations
of Schenk et al. (2011). A thermal inertia anomaly was
discovered, matching the location of the blue lenses on
Mimas and Tethys. Energetic electron bombardment is
the main hypothesis proposed for both the thermal iner-
tia anomalies and the observed color patterns (Paranicas
et al., 2012).
The 1980’s and 1990’s hosted the development of
several photometric models (Hapke, 1981, 1984, 1986;
Shkuratov et al., 1999; Lumme and Bowell, 1981; Buratti, 1985). Cassini scientists have thus benefited from
this advancement in theory to retrieve more information
from phase curves. The greater amount of data of a better quality and resolution brought by Cassini, as well
as the better coverage in solar phase angles and these
model improvements have led to numerous discoveries
about these midsize satellites of Saturn. Cassini provides the first opportunity to observe this region in detail
in the FUV domain, over a long period of time and with
a wide variety of solar phase angles not accessible from
Earth. While acquiring unexpected observations in visible and infrared, the UV represents a third piece of the
puzzle to obtain a global understanding of the system of
Saturn.
The ultraviolet wavelengths are particularly sensitive to relatively small amounts of surface weathering
(Hapke, 2001; Hendrix et al., 2003). By probing the
uppermost layers of the icy satellite surfaces, they allow the study of the exogenic processes that alter them,
such as bombardment by E-ring grains, charged particles and plasma. Ion bombardment of ices is known
to produce defects in the ice. These processes altering the surfaces, create voids and bubbles that affect the
light scattering properties of the surface. They can also
change the chemistry by trapping gases, which can produce spectral absorption features (Johnson, 1997; Johnson and Quickenden, 1997; Kouchi and Kuroda, 1990;
Sack et al., 1992). Heavy (less-penetrating) damaging
ions have been seen to brighten surfaces in the visible
(Sack et al., 1992). Bombardment of charged particle
can implant new chemical species, can drive chemical
reactions and species creation, alter grain size and other
microstructure and even sputter away the surface.
We present here the first disk-integrated far-UV phase
curves of three midsize icy satellites of Saturn: Mimas,
Tethys and Dione. The layout of the paper is as follows; Section 2 presents the instrument and datasets.
Section 3 deals with the disk-integrated phase curves
retrieved from the observations. Section 4 details the
Hapke model we use, followed by its results and analysis in Section 5. Section 6 deals with the interpretation
of such results.
2
2. Observations and datasets
Background signals, from the Radioisotope Thermoelectric Generators (RTG) and the sky background, are
removed prior to apply calibration corrections. We determine an average background per row by averaging
the signal from several rows far from the satellite. We
analyze data in terms of reflectance: r = I/F, where
I is the measured signal from the satellite and πF is
the incident solar flux. The solar spectra come from
data measured by SOLSTICE on the SORCE spacecraft on the day of the observation adjusted for the
solar longitude (McClintock et al., 2000). The solar
spectrum is corrected to the heliocentric distance, using heliocentric distances from the HORIZONS website
(http://ssd.jpl.nasa.gov/horizons.cgi).
2.1. Observations
The UVIS (Ultraviolet Imaging Spectrograph subsystem) instrument, described in detail by Esposito et al.
(2004), is composed of two-dimensional CODACON
detectors that provide simultaneous spectral and onedimensional spatial images. The far-UV channel covers
wavelengths from 111.5 nm to 191.2 nm. The detector
format is 1024 spectral pixels by 64 spatial pixels. Each
spectral pixel is 0.25 mrad and each spatial pixel is 1.0
mrad projected on the sky. Our study focuses on observations using the low-resolution slit, giving a spectral
resolution of 0.48 nm and a spatial FOV of 1.5 mrad in
the spectral dimension.
We use disk-integrated observations spanning the entire Cassini mission from 2004 until the most recent
ones in 2013, as listed in Tables A.2, A.3 and A.4 in
appendix. A filling factor, which is the ratio of the area
of the satellite on the area of a pixel projected to the sky,
normalizes each observation to a common distance. As
shown in Figure 1, the satellite usually appears on one
or two spatial pixels of the detector. Signals from each
pixel containing the satellite are summed. The integration time is usually 120 seconds, thus an observation
of few minutes contains multiple measurements that are
averaged.
2.2. Data sets
Figure 2 displays the distribution in longitudes and
solar phase angles (α, hereafter referred to in the text
as phase angles) of our data sets for Mimas, Tethys
and Dione. Additional information is given in Tables A.2, A.3 and A.4. The leading hemisphere, between 0 and 180◦ W longitude contains more observations for each satellite, especially at phase angles lower
than 10◦ . The three satellites exhibit a regular longitude
visibility pattern, due to the orbit of Cassini in the Saturn system. Data over a wide range of phase angles,
from 0.3◦ to 168.2◦ , have been obtained.
Figure 2: Distribution in longitude and solar phase angle (in degrees)
for UVIS disk-integrated observations of Mimas, Tethys and Dione.
The dashed line divides the leading hemisphere (from 0 to 180◦ W)
from the trailing one (from 180 to 360◦ W).
Figure 1: A sample observation showing the satellite Dione in the
UVIS slit. Each rectangle, limited by white lines, represents a spatial
pixel. The notations indicate that Dione is partially on pixel 32 and
pixel 31. This is a disk-integrated configuration, Dione being smaller
than a pixel. On the bottom left, some details are given about the date,
time, observational geometry and the position of the spacecraft.
The Mimas data set has 137 observations with phase
angles spanning from 0.61◦ to 163.57◦ . The Tethys data
set has 84 observations and phase angles spanning from
3
0.32◦ to 163.75◦ , while Dione’s has 90 observations
with phase angles spanning from 0.49◦ to 168.22◦ .
Changes in brightness with longitude, known as the
rotational phase, are partially taken into account by
dividing our data set into leading and trailing hemispheres. More complete longitude coverage will be necessary to apply a full rotational correction. This could
be important in particular for Dione, which displays
bright fractures on its trailing hemisphere near 240◦ W
(Stephan et al., 2010).
greater. Dione and Tethys are of similar size, with diameters of 531.1 km and 561.4 km respectively. In the
FUV, our results for Tethys and Dione show a leading
hemisphere brighter than the trailing by a factor about
1.5. Mimas shows similar leading and trailing hemispheres in FUV wavelengths.
Derived from the UVIS observations, the geometric albedos of Tethys’ leading hemisphere and Mimas
are equivalent with value about 0.48 at 180 ± 5 nm.
The trailing hemisphere of Tethys and the leading hemisphere of Dione have a geometric albedo of about 0.32
at the same wavelength. Dione’s trailing hemisphere is
the darkest of all with a geometric albedo of about 0.18.
These values correlate with the distance to Saturn, especially for the trailing hemispheres where the brightest
satellite is Mimas, following by Tethys then Dione. The
Tethys leading and trailing sides also seem to exhibit
a more intense opposition surge than Mimas or Dione;
the slope of the solar phase curve appears to be steeper
below 10◦ of phase angle.
Figure 5 displays a more detailed representation of
the phase curves for each hemisphere, where we subdivide further: the Saturnian hemisphere is between
315 and 45◦ ; the leading hemisphere is between 45 and
135◦ , the anti-Saturnian hemisphere covers 135 to 225◦
and the trailing hemisphere is between 225 and 315◦ .
The leading/trailing phase curves formed from these
narrower hemispheres lead to the same conclusion as
in Figure 4. Conversely, no differences can be observed
between the Saturnian and anti-Saturnian hemispheres
of each satellites. These results suggest that processes
that alter the surfaces act primarily on the leading and
trailing hemispheres. We note that the reduced number
of data points does not always allow a definitive conclusion about hemispherical asymmetries, especially on
Mimas where we do not have anti-Saturnian observations at low phase angles.
Figure 6 represents the rotational phase curves of
Mimas, Tethys and Dione at about 15◦ phase angle.
The leading/trailing asymmetry still appears on the
Tethys and Dione phase curves. Nevertheless, the small
amount of points doesn’t allow to properly fit these
curves. We need more complete coverage in longitude
in order to be able to perform a rotational correction on
our data sets.
3. Phase curves
We choose to focus on the 180 nm wavelength (±5
nm), given the spectral shape of icy satellites as displayed in Figure 3. At wavelengths shorter than 165 nm
the spectrum is very dark, a consequence of the edge
of a water-ice absorption band (Hendrix and Hansen,
2008). The spectrum is the brightest and provide
the best signal over noise ratio (SNR) around 180nm.
Figures 4 and 5 present the first far-ultraviolet diskintegrated phase curves of Mimas, Tethys and Dione at
180±5 nm .
Figure 3:
Tethys reflectance spectrum from observation
FUV2013 166 21 53 00 UVIS 192TE LOPHASE001 PIE around
1◦ solar phase angle. The spectrum is very dark below 165nm and
displays the water ice absorption edge at 165 nm.
In Figure 4, we divide the satellite surfaces into two
hemispheres: the leading and the trailing hemispheres,
as defined in Figure 2. A zoom on the phase angles from
0◦ to 15◦ is included for each satellite. The background
subtraction accounts for a big part in the value of the
error bars. Because Mimas is the smallest of the three
satellites with a diameter of 198.2 km, its error bars are
4. Hapke modeling
Among the few analytical photometric models cited
in the introduction, Hapke’s equations have seen the
widest application to icy surfaces. Thus, in order to
make comparisons with previous studies, we propose
4
(a) Mimas
(b) Tethys
(c) Dione
Figure 4: Leading/Trailing hemispheres of Mimas, Tethys and Dione. The leading side spans from 0 to 180 degrees, while the trailing side spans
from 180 to 360 degrees.
here an analysis of our data sets using the Isotropic Multiple Scattering Approximation (IMSA) of the Hapke
model (Hapke, 2012). Our methodology follows the one
described by Hendrix et al. (2005); we use a LevenbergMarquardt algorithm to determine the best fits.
Photometric properties can lead to enhanced understanding of the structure of a regolith. Parameters such
as the single scattering albedo ω, the scattering lobe parameters b and c, the geometric albedo Ap, the roughness Θ, and the shadow hiding parameters B0 and h can
be determined by spectrophotometry, allowing for the
characterization of a surface. Photometric properties are
also useful to fully investigate the composition and grain
sizes of a surface.
surfaces, we use a simple form of the IMSA Hapke
model, which assumes that the opposition surge is created only by the shadow-hiding process. We do not account for the coherent-backscatter process, as it requires
phase angles smaller than 2◦ to be constrained and it is
more commonly observed on brighter surfaces (Nelson
et al., 1998), although some studies show that it could be
important on low-albedo surfaces also (Shkuratov and
Helfenstein, 2001; Verbiscer et al., 2005; Buratti et al.,
2010). In addition, we have tested the possibility of
a Coherent Backscattering Opposition Effect (CBOE)
with the 2012 IMSA Hapke model and preliminary results don’t show any evidence for CBOE on these icy
satellites at 180 nm.
4.1. Parameters
Because our datasets cover a wide range of phase
angles, we choose to use a two parameters HenyeyGreenstein function (2P-HG) (Domingue et al., 1991):
Because our data sets are somewhat sparse at very
small phase angles and we are observing quite UV-dark
5
(a) Mimas Saturn/anti-Saturn hemispheres
(b) Mimas Leading/Trailing hemispheres
(c) Tethys Saturn/anti-Saturn hemispheres
(d) Tethys Leading/Trailing hemispheres
(e) Dione Saturn/anti-Saturn hemispheres
(f) Dione Leading/Trailing hemispheres
Figure 5: Detailled view of the Saturn, anti-Saturn, Leading and Trailing hemispheres of Mimas, Tethys and Dione. Each hemisphere spans 90
degrees of longitude.
6
P(α) =
(1 − c) ∗ (1 − b2 )
(1 + 2b cos(α) + b2 )
3
2
+
c ∗ (1 − b2 )
3
(1 − 2b cos(α) + b2 ) 2
Domingue and Verbiscer (1997) demonstrated that
using the 2P-HG or a 3P-HG is equivalent, as long as
the coverage in large phase angles is adequate. The 2PHG is the most commonly used, where b describes the
amplitude of the scattering lobe, and the c parameters
describes the direction of scattering, forward or backward. Both parameters vary between 0 and 1. A value
of c = 0 implies pure forward scattering; α is the solar
phase angle. The values of ω, b, and c require information on both the forward (α >100◦ ) and backward
scattering (α <30◦ ) directions, to be constrained.
The roughness parameter, Θ, has been fixed in our
analysis. It requires observations at phase angles greater
than 90◦ to be well constrained. In addition, Θ is not
expected to be wavelength-dependent. Based on previous literature and on the work of Verbiscer and Veverka
(1992) we decided to fix it to the value of 20◦ .
The Shadow-Hiding Opposition Effect (SHOE) parameters are B0 , the amplitude, which requires phase
angles less than 30◦ to be constrained and h, the angular width of the opposition surge, linked to the porosity,
which requires phase angles smaller than 5◦ (Domingue
et al., 1998). B0 was allowed to vary between 0 and 1, 1
being characteristic of an opaque particle. Both parameters were initially fixed to the values from Verbiscer
and Veverka (1992) in order to retrieve the ω, b and c
parameters. Then the B0 and h values were refined.
The h parameter can be related to the porosity P by
the equation:
√
3
3
(2)
h = − ln(P) Y , where Y =
8
ln( rrsl )
(a) Mimas
(b) Tethys
This is given assuming an uniform grain size lunarlike distribution. rl /r s is the ratio of the effective radius
of the largest grain to the effective radius of the smallest
grain (Hapke, 1986; Helfenstein and Veverka, 1987).
4.2. Error bars
The initial error bars on the Hapke photometric parameters are taken to be equal to the fine grid step sizes
used in our analysis. They are as follows: ±0.01 for ω,
±0.02 for b and c, ±0.01 for B0 and ±0.10 for h. The
accuracy to which these parameters can be determined
is highly dependent on the quality of the data sets being
modeled. The revised error bars, after completing of
the Levenberg-Marquardt fit analysis, take into account
scatter in the data sets and phase angle coverage.
(c) Dione
Figure 6: Rotational phase curves of Mimas, Tethys and Dione at
about 15◦ phase angle.
7
The estimation of empirical errors bars on each parameter was made by comparing the measured solar
phase curve of each data set with the model generated
using the best-fit parameters, with one parameter being
varied to check the effect of the fit on the phase curve,
including the scatter of data. The constraints on the variation in any single parameter based on the phase angle
coverage in each data set was also judged. We report
these error in each of the final figures as the best a posteriori estimates of the parameter errors. The final error
bars, derived from the quality of fit, are shown in Table 1.
5. Results
Figure 8: IMSA Hapke model (Hapke, 2012) at 180 nm. The Leading hemisphere (LH) of Tethys and Dione are clearly brighter than
their trailing hemisphere (TH). Tethys LH and TH are also brighter
than Dione. This is consistent with the fact that Tethys is closer to
Saturn and Enceladus, so it receives more E-ring grains and energetic
electrons. Both hemispheres of Mimas are similar. At phase angles
smaller than 20◦ , Tethys exhibits steeper curves on its leading and
trailing sides. This demonstrates a different opposition effect.
Table 1 gives the best-fit Hapke parameters determined in this analysis for both hemispheres of Mimas,
Tethys and Dione. Figures 7 and 8 show the models and
corresponding Hapke model solutions at 180 nm. Figure 8 shows the same modelled solar phase curves as in
Figure 7, but they are over-plotted, allowing an easier
comparison of the three satellites.
5.1. Single particle scattering terms
The single-scatter albedo ω and the b and c terms
are the parameters most robustly characterized by the
Hapke model. The leading hemispheres of Tethys and
Dione have higher values of ω than their trailing hemispheres, while both Mimas hemispheres are equivalent
in ω, within the error bars. The single scattering albedo
is driven by composition. These results show that the
Mimas TH and LH likely have roughly similar composition, while the trailing hemispheres of Tethys and Dione
include additional absorbing species. The relatively low
ω values signal that single-scattering likely dominates
over multiple scattering.
The b parameter is the amplitude of the scattering
lobes. The 2P-HG assumes that the half-width of the
forward and backward scattering lobes are equivalent
and that the relative amplitude varies. The c parameter relates to the direction of scatter. No forwardscattering components have been found in this analysis, only backward-scattering. Figure 9 illustrates the
2P-HG functions. The main differences appear at phase
angles lower than 40◦ . We can consider that the c values are equivalent for each hemisphere of the satellites,
and that the b parameter values are equivalent for both
hemispheres of Mimas, Tethys and the trailing hemisphere of Dione. The exception is the Dione LH, which
has the lowest b value.
Figure 9: Phase functions: 2P-HG functions at 180 nm. Within the
error bars, it is delicate to make differences between these phase functions, except that Dione’s leading hemisphere is less backscattering.
All the other curves are the similar within error bars.
5.2. Opposition effect parameters
A comparison of the shape of the phase curves and
IMSA 2012 Hapke models (Figs. 4, 5, 7, 8) indicates
that Tethys has a different behavior than Mimas and
Dione. At phase angles smaller than 20◦ , Tethys displays a narrower curve, suggesting a strong opposition
effect on this satellite in both hemispheres. This could
be the result of a different microstructure of the regolith.
The Hapke model results reflect this difference, with
both the Tethys LH and TH having smaller values of
8
Figure 7: Hapke best fit of the leading and trailing hemispheres of Mimas, Tethys and Dione at 180 nm. The model is very sensitive, strongly
dependent, to the data set phase angle coverage, that can introduce a bias in the interpretation.
Satellite
hemisphere
Leading
ω
0.516
±0.02
b
0.46
±0.02
c
1.00
±0.02
B0
0.85
±0.10
h
0.641
±0.05
rms
0.197
Trailing
0.495
±0.03
0.618
±0.03
0.48
±0.03
0.48
±0.02
1.00
±0.03
1.00
±0.02
0.84
±0.15
0.80
±0.02
1.00
±0.50
0.134
±0.05
0.123
0.387
±0.02
0.484
±0.02
0.49
±0.02
0.35
±0.02
1.00
±0.02
1.00
±0.02
0.96
±0.02
0.95
±0.15
0.064
±0.07
0.457
±0.07
0.022
0.211
±0.02
0.44
±0.02
1.00
±0.02
0.95
±0.15
0.505
±0.10
0.012
Mimas
Leading
0.039
Tethys
Trailing
Leading
0.031
Dione
Trailing
Table 1: best-fit IMSA Hapke parameters.
9
h. However, it is necessary to be cautious with a possible bias because we don’t have much data at very small
phase angles on the TH.
The h parameter is related to the porosity and needs
phase angles lower than 5◦ to be well constrained. Our
results should therefore be considered as a test of the
Hapke model for this parameter, with moderately low
phase angle coverage. We conclude h cannot be determined for Mimas TH; there are not enough data points,
which gives a value of h = 1.00.
Assuming a lunar-like uniform grain size distribution
as explained in equation (2), we derive porosities of
25% on the LH of Tethys and 49 % on its TH, while
Mimas and Dione have very low porosities (<5%). If
the CBOE were contributing significantly, the derived
porosity can be inaccurate, providing values that are
high (Helfenstein et al., 1998). However, our preliminary tests show that CBOE is not significant.
The B0 parameter is a representation of the transparency of the particles, a value of 1 meaning an opaque
particle. The three satellites have equivalent values,
suggesting quite opaque particles. But, here again, the
error bars are large due to less than complete coverage
at small phase angles.
As shown on Figure 10, and in reference to the work
of Verbiscer et al. (2007), we have plotted the opposition effect amplitude as a function of its angular width.
The results form two distinct groups, with Tethys as a
member of the first group and, Mimas and Dione as
members of the second one, since Tethys exhibits much
lower values of the h parameter. This result is in agreement with the work of Verbiscer et al. (2007), who also
formed the same groups. However, the opposition surge
behavior varies with wavelength. Verbiscer et al. (2007)
found Mimas and Dione to have higher values of B0
than Tethys in the visible (along with larger h values),
whereas in the FUV we find all B0 values to be similar.
Figure 10: The opposition effect amplitude vs. its angular width for
Mimas, Tethys and Dione leading hemisphere (LH) and trailing hemisphere (TH). The Mimas TH h value hasn’t been able to be fitted due
to a lack of data at very low phase angle in this hemisphere. Exception
of the Mimas TH, we can distinguish 2 groups: the first one composed
of Dione with the Mimas’ LH and the second one composed of Tethys.
For both Tethys and Dione, we observe a higher value
of ωP0 on the LH than on the TH (Fig. 11), when we expect more bombardment by E-ring grains on their leading side. Dione also has lower values than Tethys; this is
consistent with the fact that Dione is further from Enceladus, the source of the E-ring. We expect Dione to experience less bombardment by E-ring grains than Tethys
and Mimas. Mimas displays a slightly higher value on
the TH, but with large error bars. We expect that Mimas
should experience more E-ring grain bombardment on
its trailing side (Hamilton and Burns, 1994).
5.3. Amount of bombardment
The ωP(0) value describes the total light scattered
from the particles, both from internal and surface scattering (Hendrix et al., 2005). The B0 term being a measure of the opacity of particles, it indicates the level
of surface scattering, where B0 = S(0) / ωP(0). The
term S(0) characterizes the contribution of light scattered from near the front surface of the particles as part
of the opposition surge, while ωP(0) describes the total light scattered from the particles, both from internal
and surface scattering. Hendrix et al. (2005) linked the
ωP(0) value to the amount of bombardment experienced
by the satellite surfaces, based on a study of the Galilean
satellites. Our results also support this interpretation.
Figure 11: Amount of bombardment. The quantity ωP(0) could be
related to the amount of bombardment received at the surface, Tethys
and Dione LH having higher values than their TH.
10
6. Discussion
agent on TH) acting simultaneously is conceivable.
We were expecting a slight difference between both
hemispheres of Mimas, however the quality of the UVIS
datasets on Mimas do not allow us to detect such an
asymmetry. The leading and trailing hemispheres have
different coverage in phase angle, and in particular we
are lacking observations at very low phase angles on
the trailing side. Thus, the Hapke model cannot retrieve realistic opposition surge parameters B0 and h
on this side. The Hapke parameters being quite correlated with each other, this can also influence the ω
and 2P-HG function parameters. On Mimas, E-ring
grains are expected to impact primarily the trailing side
(Hamilton and Burns, 1994), while the energetic electrons impact the leading side (Paranicas et al., 2012).
More equivalent phase angle coverage on both hemispheres would give some indication of the photometric behavior induced by one or the other exogenic processes. Hendrix et al. (2012) showed for Mimas that the
FUV wavelengths are largely not sensitive to effects of
energetic charged particle bombardment due to the shallower sensing depths; however they used an observation
centered on the anti-Saturnian hemisphere, so a clear
comparison between central LH and central TH has yet
to be accomplished.
Our results demonstrate a different photometric behavior of Tethys surface at low phase angles compared
to Mimas and Dione, when looking at the opposition
surge parameters. While Mimas and Dione have similar
shadow-hiding parameters, Tethys has a narrower width
of its opposition surge peak in the FUV. The smaller
values of h of Tethys indicate a possibly different microstructure of the regolith, compared with Mimas and
Dione. This can be linked to porosity. Tethys has derived porosity values of 25% for the LH and 49%
for the TH, while porosity values for Dione and Mimas are less than 1%. We need to remember that these
porosity estimates are made under the assumption of a
lunar-like grain distribution, with significant error bars
on the h term. The CBOE was not taken into account,
based on preliminary results showing that it is negligible
on these satellites. However, our results are consistent
with the visible-regime results of Verbiscer et al. (2007),
suggesting some amount of consistency in opposition
surge width of a wide wavelength range. Nevertheless,
at FUV wavelengths, the amplitude of the opposition
surge, related to the particles opacity, seems to remain
the same for all satellites, contrary to the visible wavelength results. These opposition surge results suggest
that the regolith microstructure and granularity as well
as the composition, driving the opacity can be related to
orbital position of the satellites relative to E-ring grain
In this paper we have presented the first FUV diskintegrated phase curves of Mimas, Tethys and Dione.
Observations show a leading hemisphere brighter than
the trailing on Tethys and Dione, while Mimas has a
quite uniform brightness across its surface. While the
Mimas and Tethys leading hemisphere reflectance peaks
at about 0.5, Tethys’ trailing side and Dione have much
lower values of reflectance. Dione’s trailing side has
a value of I/F equal to about 0.18 at zero phase angle. Dione’s hemispheric albedo asymmetry is also
more pronounced than on Tethys: the leading/trailing
brightness ratio is about 1.50 for Tethys and about 1.78
for Dione. Nevertheless, none of the three satellites
seems to display an asymmetry between their Saturnian
and anti-Saturnian hemispheres. This observation suggests significant interactions between those moons with
the E-ring particles and Saturnian magnetosphere, two
agents expected to act mainly on the leading and trailing
faces of these satellites.
Model results must be interpreted with caution. The
UVIS data points are quite scattered at low phase angles and longitudes, especially on Dione. The absence
of rotational correction for the three satellites, due to
this lack of data points at different longitudes is an additional source of error. An overall uncertainty about
10 to 20 percent on the value of the data points should
thus be considered. We also need more observations at
very low phase angles to draw some conclusions with
more conviction. Model parameters show a strong dependance to the coverage in data points at small phase
angle. Great caution must be applied when it comes to
the opposition surge absolute values given by the Hapke
model.
The scattering properties display some differences
between the three satellites, especially on Dione, showing a more absorbent surface. Combining the phase
function parameters, b and c, with the single scattering
albedo ω (Fig. 11) shows that Dione exhibits a slightly
different behavior where absorption dominates clearly
over scattering on both hemispheres. This is not so obvious on Mimas and Tethys. Dione’s greater distance
from Saturn and/or Enceladus could explain this effect.
Dione is expected to receive fewer amount of E-ring
grains from Enceladus and thus should exhibit less fresh
bright water-ice on its surface. In addition, an exogenic
process acting only on the trailing side, such as a darkening agent coming from outside the system of Saturn
for example (suggested by Clark et al. (2008)), is another possible hypothesis. A combination of both processes (lower flux of E-ring grains on LH, darkening
11
density.
Our results can also be linked with the amount of Ering grain bombardment experienced by each satellite
hemisphere. As shown in Fig 11, the Dione LH and TH
and Tethys TH have higher B0 values, and lower ωP(0)
values, while the Mimas LH and TH and Tethys LH
have lower B0 and higher ωP(0) values. As discussed
by Hendrix et al. (2005), the ωP(0) term relates to total
(internal + surface) scattering, while the B0 term relates
to the amount of surface scattering (and the opaqueness
of the particles). Thus, the surface with lower overall
amounts of E-ring grain bombardment (Dione TH, LH
and Tethys TH) have more contributions to scattering
from surface scattering from the grains, while the grains
on the Mimas LH and TH and Tethys LH, with more
E-ring grain bombardment, demonstrate more overall
scattering, with a greater relative contribution from internal scattering due to the relatively transparent nature
of these grains.
funding. The authors would like to thanks the reviewers, as well as Larry W. Esposito for rereading and discussions. We are grateful to Linda Spilker for helpful
comments and conversations and to Todd Bradley and
Josh Colwell for their help with the observations and
the geometry software.
9. References
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Though the data sets are still somewhat limited in this
study, useful results are obtained. We provide the first
FUV values for the geometric albedo of Mimas, Tethys
and Dione, as well as the first estimates of porosity from
FUV data sets. More data at very low phase angles are
required to characterize the opposition surge more accurately.
We observe a LH/TH asymmetry on Tethys and
Dione consistent with the visible and IR observations,
while the reflectance is quite uniform across Mimas’
surface. No difference in reflectance between the Saturnian and anti-Saturnian hemispheres have been observed on those satellites.
Our analysis shows that Dione has a more absorbing surface than the other satellites. We explain it by
a less intense bombardment by E-ring grains and/or a
darkening agent acting on its trailing side. Tethys exhibits a different opposition effect in the FUV domain
than Mimas and Dione, reflecting a more porous surface. Intense bombardment by E-ring grains could be
responsible for gardening the surface and changing the
microstructure on Tethys.
8. Acknowledgments
This research was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under
contract with NASA. We thanks the Cassini Project for
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13
Appendix A. Tables of observations
Appendix A contains tables of observations used for Mimas, Tethys and Dione. α represents the solar phase angle,
Φ is the central longitude. The altitude (in km) from the Cassini spacecraft to the satellite, as well as the filling factor
(the portion of a pixel, which contain the satellite, 1 meaning that the satellite fills the entire pixel) are also given. The
sequence number of the name of the observation is defined as follow: the first 3 letters indicates that the observation
is made in the FUV wavelength. It is then followed by the year, day and time of observation. UVIS is the name of the
instrument. The following three numbers stand for the reference number of revolution around Saturn, followed by the
initial of the observed satellite (MI for Mimas, TE for Tethys and DI for Dione). The final letter sequence indicates
the type of observation and the instrument, which was prime for it.
Name
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
016
016
016
017
017
017
017
018
019
049
049
049
049
050
065
066
069
071
071
072
213
263
265
265
265
285
286
286
287
287
287
288
288
288
289
304
304
305
306
334
13
13
15
13
17
17
20
20
13
10
14
18
23
13
15
14
08
07
11
15
02
21
00
06
19
23
03
06
02
07
21
02
06
20
00
18
23
03
02
18
03
25
04
06
28
36
08
09
08
28
41
55
21
46
11
08
42
40
47
11
22
11
26
11
18
29
37
58
56
22
52
01
32
42
54
52
01
27
42
49
13
58
29
20
51
51
31
39
41
54
44
04
54
34
01
51
24
13
23
23
00
30
50
00
49
28
08
08
49
59
58
28
38
39
19
37
48
59
28
57
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
00CMI ICYLON008 VIMS
00CMI ICYLON008 VIMS
00CMI ICYLON009 VIMS
00CMI ICYLON012 ISS
00CMI ICYLON014 ISS
00CMI ICYLON014 ISS
00CMI ICYLON015 ISS
00CMI ICYLON018 ISS
00CMI ICYLON019 ISS
003MI ICYLON006 ISS
003MI ICYLON007 ISS
003MI ICYLON008 ISS
003MI ICYLON009 ISS
003MI ICYLON010 ISS
004MI ICYLON002 ISS
004MI ICYLON005 ISS
004MI ICYLON007 ISS
004MI ICYLON008 ISS
004MI ICYLON009 ISS
004MI ICYLON010 ISS
012MI ICYLON002 ISS
015MI ICYLON007 ISS
015MI ICYLON017 ISS
015MI ICYLON023 ISS
015MI ICYLON025 ISS
016MI ICYLON002 ISS
016MI ICYLON003 ISS
016MI ICYLON004 ISS
016MI ICYLON005 ISS
016MI ICYLON006 ISS
016MI ICYLON007 ISS
016MI ICYLON008 ISS
016MI ICYLON009 ISS
016MI ICYLON010 ISS
016MI ICYLON011 ISS
017MI ICYLON001 ISS
017MI ICYLON002 ISS
017MI ICYLON003 ISS
017MI ICYLON004 ISS
018MI ICYLON001 ISS
14
α
133.60
136.20
144.56
128.31
131.09
130.96
126.01
113.88
107.26
94.97
94.97
106.44
103.36
90.13
36.04
28.55
114.04
88.87
94.15
87.23
30.90
57.88
50.42
38.84
43.07
106.23
111.78
120.28
99.69
106.56
89.73
93.15
98.97
85.48
88.44
122.64
126.38
132.61
123.63
115.38
Φ
256.50
261.03
277.30
197.15
265.92
269.62
320.96
319.09
211.58
95.27
95.27
238.09
311.54
166.18
100.24
92.14
171.21
167.88
237.37
310.29
260.01
239.96
310.30
22.15
236.02
100.99
165.92
230.46
165.58
238.18
94.84
165.85
238.71
92.53
165.06
95.01
166.74
238.80
239.07
22.32
Altitude
285879.82
304400.81
391507.94
775945.04
1016915.20
1026174.50
1240933.10
1657658.90
1646898.50
1005021.00
1005021.00
1089668.50
1379393.90
1335561.90
1606313.20
1261312.60
569577.96
1437102.90
1557006.20
2088974.90
839228.39
1576129.20
1367500.90
1334603.60
779480.13
766263.92
709589.01
820995.09
1163516.50
1306242.20
1604984.30
1494910.30
1625383.30
1875073.60
1748906.20
1213967.60
1133877.50
1285592.10
1655618.10
1967090.80
Filing factor
0.96
0.83
0.51
0.13
0.08
0.08
0.05
0.03
0.03
0.08
0.08
0.07
0.04
0.05
0.03
0.05
0.25
0.04
0.03
0.02
0.12
0.03
0.04
0.05
0.13
0.14
0.16
0.11
0.06
0.05
0.03
0.04
0.03
0.02
0.03
0.06
0.06
0.05
0.03
0.02
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2007
018
018
019
019
019
020
020
022
052
052
052
052
053
054
054
054
081
083
208
208
209
209
209
254
340
340
340
340
340
340
340
340
340
340
340
340
340
340
340
341
341
341
341
341
341
341
341
341
341
341
341
163
16
20
01
15
20
00
14
07
06
10
15
20
14
13
19
23
09
11
10
10
11
13
13
02
16
17
17
18
18
19
19
20
20
21
21
22
22
23
23
00
00
01
01
02
02
03
03
04
04
05
05
16
02
56
12
31
18
31
41
12
21
57
07
31
16
18
07
39
27
17
05
34
00
02
18
35
24
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
14
02
15
40
08
36
56
36
07
27
16
07
18
18
18
28
21
08
07
08
47
28
53
52
53
53
53
16
02
01
01
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
02
40
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
020MI
020MI
020MI
020MI
020MI
020MI
020MI
020MI
021MI
021MI
021MI
021MI
021MI
021MI
021MI
021MI
022MI
022MI
026MI
026MI
026MI
026MI
026MI
028MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
034MI
046MI
ICYLON001 ISS
ICYLON006 ISS
ICYLON008 ISS
ICYLON011 ISS
ICYLON013 ISS
ICYLON016 ISS
ICYLON017 ISS
ICYLON019 ISS
ICYLON001 ISS
ICYLON002 ISS
ICYLON003 ISS
ICYLON004 ISS
ICYLON005 ISS
ICYLON006 ISS
ICYLON007 ISS
ICYLON008 ISS
ICYLON003 ISS
ICYLON004 ISS
ICYLON001 ISS
ICYLON001 ISS
ICYLON002 ISS
ICYLON002 ISS
ICYLON002 ISS
PHOTOM035 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
STARE001 ISS
ICYLON001 ISS
15
157.01
150.75
153.95
146.87
142.03
145.02
140.17
137.61
74.72
79.27
78.15
70.60
72.27
63.71
50.29
41.12
149.07
161.98
148.78
146.53
150.74
150.12
150.05
150.38
155.67
156.31
157.08
157.85
158.59
159.30
159.96
160.55
161.05
161.47
161.76
161.95
162.01
161.97
161.81
161.56
161.23
160.83
160.38
159.91
159.42
158.93
158.46
158.01
157.61
157.24
156.93
15.32
22.27
94.20
167.19
21.99
93.20
166.98
22.52
308.93
166.40
237.94
311.12
23.00
313.66
310.50
21.25
86.13
260.81
311.23
186.09
213.47
230.64
250.40
256.97
193.33
285.93
292.82
300.73
308.36
315.60
322.57
329.23
335.69
341.93
348.06
354.08
180.07
5.87
12.11
18.28
24.58
31.11
37.86
44.94
52.29
60.04
68.08
76.49
85.15
93.75
103.06
112.10
276.88
1169997.00
1082838.50
1011445.50
1625641.40
1518850.5
1431573.0
2000482.90
2465572.90
1950185.40
1961484.00
2107489.80
2099582.00
1767571.80
1355127.80
1312629.40
1053960.60
1050820.70
2045935.30
1664875.70
1673643.20
1980807.10
2083264.00
2098846.80
1195110.60
1576381.60
1605278.40
1614226.70
1648570.20
1656684.50
1686676.50
1693472.60
1717392.90
1722492.80
1739062.20
1742208.50
1750658.20
1751725.40
1751823.30
1750820.00
1742862.7
1739927.40
1724712.50
1720103.10
1698881.90
1692963.30
1667383.90
1660611.40
1632645.10
1625546.30
1597413.00
1589997.00
610275.64
0.06
0.07
0.08
0.03
0.03
0.04
0.02
0.01
0.02
0.02
0.02
0.02
0.03
0.04
0.05
0.07
0.07
0.02
0.03
0.03
0.02
0.02
0.02
0.06
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.22
FUV2007
FUV2007
FUV2007
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2009
FUV2009
FUV2009
FUV2009
FUV2009
FUV2009
FUV2009
FUV2009
FUV2009
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2012
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
298
298
299
196
235
235
272
278
293
023
023
176
176
176
176
176
287
287
226
226
226
226
226
226
267
267
267
267
092
092
092
092
092
092
092
127
127
174
174
174
174
174
178
178
20
23
12
14
10
10
06
21
22
14
14
18
18
18
18
18
13
13
19
19
19
19
19
19
17
17
18
18
10
10
10
10
10
11
11
03
17
20
20
20
20
21
09
10
45 41 UVIS 051MI GLOCOLB001 ISS
10.33
50.15 1215459.70
25 52 UVIS 051MI 094W010PH001 ISS
9.80
95.29 1144121.20
06 51 UVIS 051MI 310W014PH001 ISS
13.71 307.52 1480275.50
22 59 UVIS 076MI ICYLON001 CIRS
62.09
17.62
294910.58
06 11 UVIS 081MI ICYLON002 ISS
15.25
41.60 1336495.50
38 11 UVIS 081MI ICYLON002 ISS
14.33
55.63 1318405.20
32 22 UVIS 086MI ICYLON001 ISS
14.23
88.10 1238290.10
33 07 UVIS 087MI ICYLON001 ISS
14.90 103.48 1106735.70
46 00 UVIS 089MI ICYLON001 ISS
14.21
91.88 1197051.20
26 21 UVIS 101MI ICYLON001 ISS
149.78 76.14
576011.46
52 01 UVIS 101MI ICYLON001 ISS
151.74 109.36 556850.30
15 02 UVIS 113MI ICYLON001 ISS
11.39
64.77
593830.99
15 02 UVIS 113MI iCYLON001 ISS
12.49
71.83
593830.99
15 02 UVIS 113MI iCYLON001 ISS
13.50
76.62
593830.99
15 02 UVIS 113MI iCYLON001 ISS
14.48
80.82
593830.99
15 02 UVIS 113MI iCYLON001 ISS
15.16
83.60
593830.99
48 36 UVIS 119MI ICYLON001 ISS
1.98
261.74 334207.08
48 36 UVIS 119MI ICYLON001 ISS
2.37
262.67 334207.08
38 00 UVIS 170MI LOPHASE001 PIE
2.47
18.13
804360.65
38 00 UVIS 170MI LOPHASE001 PIE
1.50
20.09
804360.65
38 00 UVIS 170MI LOPHASE001 PIE
0.61
22.42
804360.65
38 00 UVIS 170MI LOPHASE001 PIE
0.79
24.44
804360.65
38 00 UVIS 170MI LOPHASE001 PIE
1.50
26.16
804360.65
38 00 UVIS 170MI LOPHASE001 PIE
2.53
28.60
804360.65
18 40 UVIS 172MI ICYLON001 ISS
158.37
4.08
891984.11
43 22 UVIS 172MI ICYLON001 ISS
160.36
8.03
882061.85
07 21 UVIS 172MI ICYLON001 ISS
162.14 11.55
871160.42
27 00 UVIS 172MI ICYLON001 ISS
163.57 14.39
861321.34
02 20 UVIS 185MI ICYLON001 ISS
159.64 75.19
605838.58
11 00 UVIS 185MI ICYLON001 ISS
159.09 78.02
598036.42
26 40 UVIS 185MI ICYLON001 ISS
158.40 81.91
583815.49
41 59 UVIS 185MI ICYLON001 ISS
157.79 85.80
569799.27
56 45 UVIS 185MI ICYLON001 ISS
157.26 89.64
556219.78
11 39 UVIS 185MI ICYLON001 ISS
156.78 93.59
542486.15
26 29 UVIS 185MI ICYLON001 ISS
156.45 96.79
528824.91
01 00 UVIS 189MI ICYLON001 PRIME 58.60 204.02 1183584.90
24 30 UVIS 189MI ICYLON002 PRIME 68.51
56.60 1406615.00
08 30 UVIS 193MI ICYLON001 ISS
151.29 289.30 841024.45
24 10 UVIS 193MI ICYLON001 ISS
152.13 293.12 848247.47
39 30 UVIS 193MI ICYLON001 ISS
152.92 296.54 855254.37
54 15 UVIS 193MI ICYLON001 ISS
153.82 300.21 861898.08
09 09 UVIS 193MI ICYLON001 ISS
154.69 303.60 868481.29
47 20 UVIS 193MI LOPHASE001 PIE
1.42
7.24
1164865.90
37 20 UVIS 193MI LOPHASE001 PIE
1.29
14.60 1167213.60
Table A.2: Mimas disk-integrated observations. α is the averaged solar
phase angle of the observation in degree, Φ is the central longitude.
Name
FUV2005 017 15 10 10 UVIS 00CTE ICYLON012 ISS
FUV2005 017 16 40 11 UVIS 00CTE ICYLON014 ISS
16
α
109.92
108.67
Φ
78.95
90.20
Altitude
1036869.7
1014101.4
0.05
0.06
0.04
0.82
0.05
0.05
0.06
0.07
0.06
0.25
0.35
0.24
0.25
0.25
0.25
0.25
0.73
0.73
0.13
0.13
0.13
0.13
0.13
0.13
0.10
0.11
0.11
0.11
0.23
0.23
0.24
0.26
0.27
0.28
0.30
0.06
0.04
0.11
0.11
0.11
0.11
0.11
0.06
0.06
Filing factor
0.55
0.58
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2007
FUV2007
FUV2007
017
018
019
049
049
050
051
065
065
066
071
072
215
263
264
264
286
287
288
288
305
305
305
306
307
332
333
333
018
019
019
022
052
052
083
123
223
254
255
255
255
255
255
255
255
255
278
279
279
205
298
299
22
13
00
12
22
15
11
08
17
10
09
16
01
21
05
14
22
20
05
22
00
18
18
05
07
20
01
04
19
12
21
07
10
21
15
06
00
01
09
09
09
10
10
10
10
11
23
01
07
05
19
03
19
59
30
29
58
28
46
58
22
17
58
12
08
45
47
57
58
48
06
28
32
03
11
11
57
51
58
38
34
26
38
31
07
01
08
27
51
38
07
25
42
00
18
36
53
11
44
34
08
06
45
04
11
21
00
34
34
34
54
49
01
01
24
03
39
59
10
10
58
58
48
09
07
27
27
48
58
58
58
58
58
28
36
28
58
48
08
58
34
02
31
16
58
43
28
13
58
43
02
02
01
40
11
32
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
00CTE ICYLON016 ISS
00CTE ICYLON017 ISS
00CTE ICYLON018 ISS
003TE ICYLON006 ISS
003TE ICYLON008 ISS
003TE ICYLON010 ISS
003TE ICYLON011 ISS
004TE ICYLON001 ISS
004TE ICYLON002 ISS
004TE ICYLON003 ISS
004TE ICYLON006 ISS
004TE ICYLON010 ISS
012TE ICYLON015 ISS
015TE ICYLON008 ISS
015TE ICYLON012 ISS
015TE ICYLON013 ISS
016TE ICYLON004 ISS
016TE ICYLON005 ISS
016TE ICYLON006 ISS
016TE ICYLON007 ISS
017TE ICYLON001 ISS
017TE ICYLON002 ISS
017TE ICYLON002 ISS
017TE ICYLON003 ISS
017TE ICYLON004 ISS
018TE ICYLON001 VIMS
018TE ICYLON002 VIMS
018TE ICYLON003 VIMS
020TE ICYLON005 ISS
020TE ICYLON009 ISS
020TE ICYLON015 ISS
020TE ICYLON020 ISS
021TE ICYLON001 ISS
021TE ICYLON002 ISS
022TE ICYLON002 ISS
023TE ICYLON069 ISS
027TE ICYLON001 ISS
028TE PHOTOM035 ISS
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
028TE ICYATM001 PRIME
030TE STARE001 PRIME
030TE STARE001 PRIME
030TE STARE001 PRIME
048TE ICYLON002 ISS
051TE 166W012PH001 ISS
051TE 238W021PH001 ISS
17
109.51
122.23
111.31
97.87
85.15
100.00
82.95
34.82
38.43
43.96
97.64
75.29
144.21
50.02
58.72
56.6
113.50
86.29
91.58
97.17
124.72
128.91
128.77
114.91
118.52
123.30
121.32
123.31
146.64
157.80
153.82
131.93
81.42
69.70
162.40
162.85
163.75
151.12
152.95
152.95
152.97
153.01
153.06
153.13
153.22
153.32
163.15
159.53
158.24
12.18
12.03
20.99
136.19
273.79
345.09
22.28
91.79
238.63
22.62
96.86
165.56
309.15
237.49
93.61
310.30
166.12
238.17
310.18
310.06
93.82
165.84
309.86
165.96
309.58
310.49
22.34
238.37
72.38
117.46
139.58
93.85
241.26
310.26
22.70
310.53
23.19
238.57
236.93
165.13
7.27
252.97
255.39
257.81
260.20
262.57
264.92
267.26
269.58
34.61
70.85
92.17
310.00
169.10
237.49
935656.5
1419638.5
1895205.6
1348686.2
1242793.1
1357412.6
2084398.7
1674436.4
1311345.9
1477132.7
1460006.4
1983476.7
835580.7
1381004.1
1375994.6
1579871.9
1422165.8
1567849.8
1422107.4
2055226.4
10521315.2
1784960.0
1791150.3
2064689.3
1953265.5
1078844.3
994807.95
960096.9
1036143.5
1232063.4
1722851.5
2650114.8
2241331.2
2197578.7
1823381.6
1877859.1
2126953.9
1277133.2
1401984.3
1415988.9
1430128.1
1444381.1
1458731.1
1473160.4
1487651.3
1502186.4
2335653.0
2288487.4
2080257.9
2019682.8
797282.12
1035424.6
0.67
0.29
0.16
0.32
0.38
0.31
0.13
0.22
0.39
0.27
0.27
0.15
0.83
0.31
0.31
0.24
0.29
0.24
0.29
0.14
0.07
0.18
0.18
0.14
0.15
0.51
0.61
0.65
0.55
0.39
0.20
0.08
0.12
0.12
0.18
0.17
0.13
0.36
0.30
0.29
0.29
0.28
0.27
0.27
0.26
0.26
0.11
0.13
0.14
0.14
0.93
0.54
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
299
300
301
302
302
303
358
029
029
063
063
105
173
220
220
228
228
228
235
235
294
294
301
301
301
166
166
166
166
166
12
00
02
17
17
20
15
19
19
19
19
20
16
08
08
01
01
01
08
08
01
01
13
13
13
21
21
21
21
21
54 51 UVIS 051TE 310W016PH001 ISS
15.82 310.76 1560629.6
24 21 UVIS 051TE 022W012PH001 ISS
11.73
22.33
1861939.0
59 42 UVIS 051TE 238W011PH001 ISS
11.36 238.14 1764316.3
06 52 UVIS 051TE 166W021PH001 ISS
21.08 164.96 2055504.2
16 52 UVIS 051TE 166W021PH001 ISS
20.95 166.40 2054977.2
56 42 UVIS 051TE 022W024PH001 ISS
24.40
21.79
2828910.8
19 50 UVIS 054TE ICYLON001 PRIME 28.63
32.10
2312366.0
09 11 UVIS 057TE ICYLON001 ISS
8.99
96.95
1135196.4
09 11 UVIS 057TE ICYLON001 ISS
9.13
99.96
1135196.4
39 49 UVIS 060TE ICYLON001 ISS
0.32
106.78
913484.6
39 49 UVIS 060TE ICYLON001 ISS
0.38
107.79
913484.6
40 10 UVIS 064TE ICYLON001 ISS
26.59 219.73 1148263.2
00 00 UVIS 073TE ICYLON001 CIRS
49.74 177.77
774580.7
20 47 UVIS 079TE ICYLON001 ISS
6.75
80.18
1219627.5
20 47 UVIS 079TE ICYLON001 ISS
6.70
86.13
1219627.5
07 17 UVIS 080TE ICYLON001 ISS
9.31
106.13 1107990.5
07 17 UVIS 080TE ICYLON001 ISS
9.75
110.72 1107990.5
07 17 UVIS 080TE ICYLON001 ISS
10.17 114.65 1107990.5
23 37 UVIS 081TE ICYLON001 ISS
9.88
60.10
1335446.0
23 37 UVIS 081TE ICYLON001 ISS
9.63
65.44
1335446.0
02 47 UVIS 089TE ICYLON001 ISS
8.65
93.92
1147685.2
02 47 UVIS 089TE ICYLON001 ISS
8.86
100.24 1147685.2
25 47 UVIS 090TE ICYLON001 ISS
9.77
85.17
1203826.6
25 47 UVIS 090TE ICYLON001 ISS
9.84
89.37
1203826.6
25 47 UVIS 090TE ICYLON001 ISS
9.96
92.85
1203826.6
53 00 UVIS 192TE LOPHASE001 PIE
1.49
190.22
809317.6
53 00 UVIS 192TE LOPHASE001 PIE
1.09
192.35
809317.6
53 00 UVIS 192TE LOPHASE001 PIE
0.90
195.12
809317.6
53 00 UVIS 192TE LOPHASE001 PIE
1.15
198.05
809317.6
53 00 UVIS 192TE LOPHASE001 PIE
1.55
200.15
809317.6
Table A.3: Tethys disk-integrated observations. α is the averaged solar
phase angle of the observation in degree, Φ is the central longitude.
Name
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
FUV2005
017
050
051
065
066
070
071
264
265
286
288
288
305
305
307
325
14
17
16
17
07
17
08
15
03
22
06
21
01
23
06
02
11
24
14
43
53
38
18
17
17
19
01
13
04
58
16
58
30
33
04
01
00
12
04
30
00
57
58
59
57
58
58
57
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
00CDI ICYLON012 ISS
003DI ICYLON010 ISS
003DI ICYLON011 ISS
004DI ICYLON001 ISS
004DI ICYLON002 ISS
004DI ICYLON004 ISS
004DI ICYLON006 ISS
015DI ICYLON014 ISS
015DI ICYLON020 ISS
016DI ICYLON012 ISS
016DI ICYLON013 ISS
016DI ICYLON014 ISS
017DI ICYLON001 ISS
017DI ICYLON002 ISS
017DI ICYLON003 ISS
018DI ICYLON001 ISS
18
α
105.37
78.11
94.29
50.56
46.38
106.84
99.69
34.61
36.70
117.46
89.68
79.95
112.76
129.30
109.05
69.50
Φ
90.67
94.46
237.34
238.63
311.40
237.00
309.80
94.93
167.00
237.40
21.59
93.98
94.12
237.71
21.95
168.15
Altitude
916127.4
1496263.7
1628098.6
1356107.2
1588315.6
1165571.8
1840019.7
1336159.8
846023.7
1011514.7
2064213.1
1822131.3
1270235.3
1481941.1
2460187.3
2224134.8
0.24
0.17
0.19
0.14
0.14
0.07
0.11
0.46
0.46
0.71
0.71
0.44
0.99
0.41
0.41
0.50
0.50
0.50
0.34
0.34
0.47
0.47
0.43
0.43
0.43
0.88
0.88
0.88
0.88
0.88
Filing factor
0.79
0.29
0.25
0.36
0.26
0.48
0.19
0.37
0.92
0.64
0.15
0.20
0.41
0.30
0.11
0.13
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2006
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2007
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
018
019
052
052
053
062
082
083
123
124
125
126
126
223
255
256
256
295
295
334
334
334
334
334
334
334
335
335
335
335
335
335
341
341
341
034
034
037
037
204
205
299
299
302
302
303
303
177
177
177
181
190
11
16
02
16
16
12
19
12
14
15
21
09
21
05
21
00
00
19
20
22
22
22
23
23
23
23
00
00
00
01
01
01
17
18
19
07
10
01
02
10
03
04
15
01
12
18
18
03
07
15
03
13
54
47
36
29
19
51
36
47
07
07
02
27
37
01
15
13
35
19
42
08
26
43
00
17
34
51
09
26
43
00
17
34
32
34
28
30
20
44
05
28
07
10
07
07
44
18
40
31
59
03
56
49
23
26
31
28
29
00
26
26
25
26
26
25
25
55
43
47
47
50
15
58
09
19
29
39
49
59
09
18
28
39
49
59
07
07
22
00
00
29
19
30
30
02
01
01
32
02
02
30
00
00
00
00
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
UVIS
020DI
020DI
021DI
021DI
021DI
021DI
022DI
022DI
023DI
023DI
023DI
023DI
023DI
027DI
028DI
028DI
028DI
031DI
031DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
034DI
038DI
038DI
038DI
038DI
048DI
048DI
051DI
051DI
051DI
051DI
051DI
051DI
073DI
073DI
073DI
074DI
075DI
ICYLON001 ISS
ICYLON012 ISS
ICYLON001 ISS
ICYLON002 ISS
ICYLON003 ISS
ICYLON004 ISS
ICYLON001 ISS
ICYLON002 ISS
ICYLON070 ISS
ICYLON073 ISS
ICYLON075 ISS
ICYLON076 ISS
ICYLON077 ISS
ICYLON001 ISS
STARE001 PRIME
STARE001 PRIME
STARE001 PRIME
STARE002 PRIME
STARE002 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
ICYATM005 PRIME
STARE001 PRIME
STARE001 PRIME
STARE001 PRIME
ICYLON001 ISS
ICYLON002 ISS
ICYLON001 CIRS
ICYLON001 CIRS
ICYLON002 ISS
ICYLON003 ISS
166W010PH001 ISS
238W018PH001 ISS
166W019PH001 ISS
238W014PH001 ISS
022W025PH001 ISS
022W025PH001 ISS
ICYLON001 ISS
ICYLON002 ISS
ICYLON003 ISS
ICYLON001 ISS
ICYLON001 PRIME
19
156.96
146.05
72.58
63.69
77.99
131.97
161.09
162.08
160.40
162.17
151.08
154.93
159.57
168.22
159.55
161.33
163.10
155.33
157.09
150.79
150.95
151.10
151.23
151.35
151.45
151.54
151.62
151.68
151.73
151.77
151.80
151.82
159.69
159.09
155.47
5.45
0.80
67.23
67.08
12.76
18.54
0.95
17.68
19.17
13.55
25.13
25.40
2.84
1.52
7.60
59.33
11.10
22.88
165.62
28.00
94.44
240.23
22.29
221.65
310.34
166.81
310.00
94.56
167.21
240.36
310.45
321.35
328.43
335.16
116.82
137.96
225.21
227.19
229.16
231.10
233.03
234.94
236.84
238.71
240.57
242.41
244.23
246.03
247.98
46.88
50.07
78.23
256.16
272.75
215.18
216.73
312.03
22.04
165.22
237.64
165.99
237.71
24.41
26.30
74.36
100.01
145.16
282.49
45.83
1254364.3
1114807.9
2505889.4
1929718.6
1406476.2
2795999.4
1407983.3
2190630.6
1775146.2
2583134.7
2438440.8
2285692.3
2502285.6
2594757.4
1900589.7
1994509.5
2004826.5
1728809.1
1681815.0
838641.8
841669.4
844908.9
848352.4
851988.9
855807.4
859796.7
863945.8
868243.3
872678.9
877241.8
881920.1
886703.8
1901883.6
1882658.6
1864336.1
925313.9
1022042.2
1188271.3
1195576.4
1779976.1
2178018.8
881576.2
1199874.7
1821668.5
2070996.4
2875793.3
2873018.8
1080377.8
969638.1
817122.1
817411.3
1039584.2
0.42
0.53
0.10
0.18
0.33
0.08
0.33
0.14
0.21
0.10
0.10
0.13
0.10
0.10
0.17
0.16
0.16
0.22
0.25
0.94
0.93
0.92
0.91
0.91
0.90
0.89
0.88
0.87
0.86
0.85
0.84
0.83
0.18
0.19
0.22
0.76
0.60
0.46
0.46
0.20
0.14
0.85
0.45
0.20
0.15
0.08
0.08
0.57
0.72
0.99
0.98
0.61
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2008
FUV2009
FUV2009
FUV2009
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
FUV2013
190
221
221
234
234
242
242
286
286
019
019
307
178
178
178
178
178
178
178
178
178
178
14
01
01
11
11
16
16
15
15
03
03
03
07
07
07
07
07
07
07
07
07
07
40 00 UVIS 075DI ICYLON001 PRIME
10.38
51.43 1033680.5
14 48 UVIS 079DI ICYLON008 ISS
12.84
88.28 1182535.2
14 48 UVIS 079DI ICYLON008 ISS
13.18
90.67 1182535.2
34 48 UVIS 081DI ICYLON001 ISS
6.51
60.56 1222943.7
34 48 UVIS 081DI ICYLON001 ISS
6.01
63.35 1222943.7
26 30 UVIS 082DI ICYLON001 ISS
7.49
59.82 1358785.2
26 30 UVIS 082DI ICYLON001 ISS
7.48
63.30 1358785.2
34 30 UVIS 088DI ICYLON001 ISS
4.78
80.25 1189997.4
34 30 UVIS 088DI ICYLON001 ISS
4.78
84.20 1189997.4
12 50 UVIS 100DI ICYLON001 ISS
8.87
75.56 1235935.6
12 50 UVIS 100DI ICYLON001 ISS
8.87
79.28 1235935.6
06 04 UVIS 120DI ICYSTARE001 PRIME 85.89
65.88
945777.3
35 00 UVIS 193DI LOPHASE001 PIE
1.30
8.08
1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
0.94
8.70
1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
0.49
9.65
1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
0.53
10.67 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
0.89
11.40 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
1.24
12.03 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
1.63
12.71 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
2.03
13.39 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
2.36
13.97 1322038.2
35 00 UVIS 193DI LOPHASE001 PIE
2.66
14.49 1322038.2
Table A.4: Dione disk-integrated observations. α is the averaged solar
phase angle of the observation in degree, Φ is the central longitude.
20
0.63
0.48
0.48
0.44
0.45
0.37
0.37
0.48
0.48
0.45
0.45
0.75
0.38
0.38
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0.37