First far-ultraviolet disk-integrated phase curve analysis of Mimas, Tethys and Dione from the Cassini-UVIS data sets

Icarus 242 (2014) 158–171

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First far-ultraviolet disk-integrated phase curve analysis of Mimas, Tethys and Dione from the Cassini-UVIS data sets Emilie M. Royer a,⇑, Amanda R. Hendrix b a b

University of Colorado, LASP, 3665 Discovery Dr., Boulder, CO 80303, United States PSI, Tucson, AZ 85719-2395, United States

a r t i c l e

i n f o

Article history: Received 19 May 2014 Revised 17 July 2014 Accepted 19 July 2014 Available online 14 August 2014 Keywords: Saturn, satellites Satellites, surfaces Ultraviolet observations Ices, UV spectroscopy Spectrophotometry

a b s t r a c t 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. Ó 2014 Elsevier Inc. All rights reserved.

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, 1st 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 the data. Verbiscer and Veverka (1992) also reported a slightly brighter trailing hemisphere (TH) on Mimas at 0.48 lm using Voyager data. This leading versus trailing

⇑ Corresponding author. E-mail address: [email protected] (E.M. Royer). http://dx.doi.org/10.1016/j.icarus.2014.07.026 0019-1035/Ó 2014 Elsevier Inc. All rights reserved.

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. (1998) confirmed this result again in 1998, using ground-based data at 0.9 lm. Moreover, Buratti et al. (1998) showed that at 0.9 lm 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 due largely to exogenic alterations (Buratti et al., 1990). More recently, in 2007, Verbiscer et al. (2007) 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.

E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171

(2010) and Filacchione et al. (2010), 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 lm, 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 (0.3–0.55 lm) and found a strong forward scattering component on Tethys. They investigated the disk-integrated 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 inertia anomalies and the observed color patterns (Paranicas et al., 2012). The 1980s and 1990s 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

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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. Observations and datasets 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 one-dimensional 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.1, A.2 and A.3 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 Fig. 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 s, thus an observation of few minutes contains multiple measurements that are averaged. Background signals, from the Radioisotope Thermoelectric Generators (RTG) and the sky background, are removed prior to

Fig. 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.

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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 ¼ I=F, where I is the measured signal from the satellite and pF 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). 2.2. Data sets Fig. 2 displays the distribution in longitudes and solar phase angles (a, 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.1, A.2 and A.3. 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. 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 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). 3. Phase curves We choose to focus on the 180 nm wavelength (±5 nm), given the spectral shape of icy satellites as displayed in Fig. 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 180 nm. Figs. 4 and 5 present the first far-ultraviolet disk-integrated phase curves of Mimas, Tethys and Dione at 180 ± 5 nm. In Fig. 4, we divide the satellite surfaces into two hemispheres: the leading and the trailing hemispheres, as defined in Fig. 2. A

Fig. 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 165 nm and displays the water ice absorption edge at 165 nm.

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 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. Fig. 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 135–225° 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 Fig. 4. Conversely, no differences can be observed between the saturnian and antisaturnian 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. Fig. 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 does not 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. 4. Hapke modeling

Fig. 2. Distribution in longitude and solar phase angle (in degrees) for UVIS diskintegrated 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).

Among the few analytical photometric models cited in the introduction, Hapke’s equations have seen the widest application

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(a) Mimas

(b) Tethys

(c) Dione Fig. 4. Leading/trailing hemispheres of Mimas, Tethys and Dione. The leading side spans from 0° to 180°, while the trailing side spans from 180° to 360°.

to icy surfaces. Thus, in order to make comparisons with previous studies, we propose 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 Levenberg–Marquardt 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 x, the scattering lobe parameters b and c, the geometric albedo Ap, the roughness H, 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. 4.1. Parameters Because our data sets are somewhat sparse at very small phase angles and we are observing quite UV-dark 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 do not show any evidence for CBOE on these icy satellites at 180 nm.

Because our datasets cover a wide range of phase angles, we choose to use a two parameters Henyey–Greenstein function (2PHG) (Domingue et al., 1991): 2

PðaÞ ¼

2

ð1  cÞ  ð1  b Þ 2

ð1 þ 2b cosðaÞ þ b Þ

3 2

þ

c  ð1  b Þ 2

3

ð1  2b cosðaÞ þ b Þ2

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 2P-HG 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; a is the solar phase angle. The values of x, b, and c require information on both the forward (a > 100 ) and backward scattering (a < 30 ) directions, to be constrained. The roughness parameter, H, has been fixed in our analysis. It requires observations at phase angles greater than 90° to be well constrained. In addition, H is not expected to be wavelengthdependent. 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

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(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

Fig. 5. Detailed view of the Saturn, anti-saturn, leading and trailing hemispheres of Mimas, Tethys and Dione. Each hemisphere spans 90° of longitude.

(1992) in order to retrieve the x, 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:

h¼

3 lnðPÞ Y; 8

where Y ¼

pffiffiffi 3 lnðrrsl Þ

ð2Þ

This is given assuming an uniform grain size lunar-like distribution. r l =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 x, ±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. 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

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and 8 show the models and corresponding Hapke model solutions at 180 nm. Fig. 8 shows the same modeled solar phase curves as in Fig. 7, but they are over-plotted, allowing an easier comparison of the three satellites. 5.1. Single particle scattering terms

(a) Mimas

The single-scatter albedo x 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 x than their trailing hemispheres, while both Mimas hemispheres are equivalent in x, 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 x values signal that singlescattering 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. Fig. 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. 5.2. Opposition effect parameters

(b) Tethys

(c) Dione Fig. 6. Rotational phase curves of Mimas, Tethys and Dione at about 15° phase angle.

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 Table 1 gives the best-fit Hapke parameters determined in this analysis for both hemispheres of Mimas, Tethys and Dione. Figs. 7

A comparison of the shape of the phase curves and IMSA 2012 Hapke models (Figs. 4, 5, 7 and 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 h. However, it is necessary to be cautious with a possible bias because we do not 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 ¼ 1:00. Assuming a lunar-like uniform grain size distribution as explained in Eq. (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 Fig. 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

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Table 1 Best-fit IMSA Hapke parameters. Satellite

Hemisphere

x

b

c

B0

h

rms

Mimas

Leading

0.516 ±0.02 0.495 ±0.03

0.46 ±0.02 0.48 ±0.03

1.00 ±0.02 1.00 ±0.03

0.85 ±0.10 0.84 ±0.15

0.641 ±0.05 1.00 ±0.50

0.197

0.618 ±0.03 0.387 ±0.02

0.48 ±0.02 0.49 ±0.02

1.00 ±0.02 1.00 ±0.02

0.80 ±0.02 0.96 ±0.02

0.134 ±0.05 0.064 ±0.07

0.039

0.484 ±0.02 0.211 ±0.02

0.35 ±0.02 0.44 ±0.02

1.00 ±0.02 1.00 ±0.02

0.95 ±0.15 0.95 ±0.15

0.457 ±0.07 0.505 ±0.10

0.031

Trailing Tethys

Leading Trailing

Dione

Leading Trailing

0.123

0.022

0.012

Fig. 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.

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. 5.3. Amount of bombardment The xPð0Þ 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 ¼ Sð0Þ=xPð0Þ. The term Sð0Þ characterizes the contribution of light scattered from near the front surface of the particles as part of the opposition surge, while xPð0Þ describes the total light scattered from the particles, both from internal and surface scattering. Hendrix et al. (2005) linked the xPð0Þ 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. For both Tethys and Dione, we observe a higher value of xPð0Þ on the LH than on the TH (Fig. 11), when we expect more bombard-

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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 Ering grain bombardment on its trailing side (Hamilton and Burns, 1994). 6. Discussion

Fig. 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.

Fig. 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.

In this paper we have presented the first FUV disk-integrated 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% 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 x (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

Fig. 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 has not 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.

ment 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

Fig. 11. Amount of bombardment. The quantity xPð0Þ could be related to the amount of bombardment received at the surface, Tethys and Dione LH having higher values than their TH.

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(suggested by Clark et al. (2008)), is another possible hypothesis. A combination of both processes (lower flux of E-ring grains on LH, darkening 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 x 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 antisaturnian 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 density. Our results can also be linked with the amount of E-ring 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 xPð0Þ values, while the Mimas LH and TH and Tethys LH have lower B0 and higher xPð0Þ values. As discussed by Hendrix et al. (2005), the xPð0Þ 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.

7. Conclusion 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.

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 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.

Appendix A. Tables of observations Appendix A contains tables of observations used for Mimas, Tethys and Dione. a represents the solar phase angle, U 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.

Table A.1 Mimas disk-integrated observations. a is the averaged solar phase angle of the observation in degree, U is the central longitude. Name

a

U

Altitude

Filing factor

FUV2005_016_13_03_13_UVIS_00CMI_ICYLON008_VIMS FUV2005_016_13_25_58_UVIS_00CMI_ICYLON008_VIMS FUV2005_016_15_04_29_UVIS_00CMI_ICYLON009_VIMS FUV2005_017_13_06_20_UVIS_00CMI_ICYLON012_ISS FUV2005_017_17_28_51_UVIS_00CMI_ICYLON014_ISS

133.60 136.20 144.56 128.31 131.09

256.50 261.03 277.30 197.15 265.92

285879.82 304400.81 391507.94 775945.04 1016915.20

0.96 0.83 0.51 0.13 0.08

167

E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171 Table A.1 (continued) Name

a

U

Altitude

Filing factor

FUV2005_017_17_36_51_UVIS_00CMI_ICYLON014_ISS FUV2005_017_20_08_31_UVIS_00CMI_ICYLON015_ISS FUV2005_018_20_09_39_UVIS_00CMI_ICYLON018_ISS FUV2005_019_13_08_41_UVIS_00CMI_ICYLON019_ISS FUV2005_049_10_28_54_UVIS_003MI_ICYLON006_ISS FUV2005_049_14_41_44_UVIS_003MI_ICYLON007_ISS FUV2005_049_18_55_04_UVIS_003MI_ICYLON008_ISS FUV2005_049_23_21_54_UVIS_003MI_ICYLON009_ISS FUV2005_050_13_46_34_UVIS_003MI_ICYLON010_ISS FUV2005_065_15_11_01_UVIS_004MI_ICYLON002_ISS FUV2005_066_14_08_51_UVIS_004MI_ICYLON005_ISS FUV2005_069_08_42_24_UVIS_004MI_ICYLON007_ISS FUV2005_071_07_40_13_UVIS_004MI_ICYLON008_ISS FUV2005_071_11_47_23_UVIS_004MI_ICYLON009_ISS FUV2005_072_15_11_23_UVIS_004MI_ICYLON010_ISS FUV2005_213_02_22_00_UVIS_012MI_ICYLON002_ISS FUV2005_263_21_11_30_UVIS_015MI_ICYLON007_ISS FUV2005_265_00_26_50_UVIS_015MI_ICYLON017_ISS FUV2005_265_06_11_00_UVIS_015MI_ICYLON023_ISS FUV2005_265_19_18_49_UVIS_015MI_ICYLON025_ISS FUV2005_285_23_29_28_UVIS_016MI_ICYLON002_ISS FUV2005_286_03_37_08_UVIS_016MI_ICYLON003_ISS FUV2005_286_06_58_08_UVIS_016MI_ICYLON004_ISS FUV2005_287_02_56_49_UVIS_016MI_ICYLON005_ISS FUV2005_287_07_22_59_UVIS_016MI_ICYLON006_ISS FUV2005_287_21_52_58_UVIS_016MI_ICYLON007_ISS FUV2005_288_02_01_28_UVIS_016MI_ICYLON008_ISS FUV2005_288_06_32_38_UVIS_016MI_ICYLON009_ISS FUV2005_288_20_42_39_UVIS_016MI_ICYLON010_ISS FUV2005_289_00_54_19_UVIS_016MI_ICYLON011_ISS FUV2005_304_18_52_37_UVIS_017MI_ICYLON001_ISS FUV2005_304_23_01_48_UVIS_017MI_ICYLON002_ISS FUV2005_305_03_27_59_UVIS_017MI_ICYLON003_ISS FUV2005_306_02_42_28_UVIS_017MI_ICYLON004_ISS FUV2005_334_18_49_57_UVIS_018MI_ICYLON001_ISS FUV2006_018_16_02_08_UVIS_020MI_ICYLON001_ISS FUV2006_018_20_56_36_UVIS_020MI_ICYLON006_ISS FUV2006_019_01_12_56_UVIS_020MI_ICYLON008_ISS FUV2006_019_15_31_36_UVIS_020MI_ICYLON011_ISS FUV2006_019_20_18_07_UVIS_020MI_ICYLON013_ISS FUV2006_020_00_31_27_UVIS_020MI_ICYLON016_ISS FUV2006_020_14_41_16_UVIS_020MI_ICYLON017_ISS FUV2006_022_07_12_07_UVIS_020MI_ICYLON019_ISS FUV2006_052_06_21_18_UVIS_021MI_ICYLON001_ISS FUV2006_052_10_57_18_UVIS_021MI_ICYLON002_ISS FUV2006_052_15_07_18_UVIS_021MI_ICYLON003_ISS FUV2006_052_20_31_28_UVIS_021MI_ICYLON004_ISS FUV2006_053_14_16_21_UVIS_021MI_ICYLON005_ISS FUV2006_054_13_18_08_UVIS_021MI_ICYLON006_ISS FUV2006_054_19_07_07_UVIS_021MI_ICYLON007_ISS FUV2006_054_23_39_08_UVIS_021MI_ICYLON008_ISS FUV2006_081_09_27_47_UVIS_022MI_ICYLON003_ISS FUV2006_083_11_17_28_UVIS_022MI_ICYLON004_ISS FUV2006_208_10_05_53_UVIS_026MI_ICYLON001_ISS FUV2006_208_10_34_52_UVIS_026MI_ICYLON001_ISS FUV2006_209_11_00_53_UVIS_026MI_ICYLON002_ISS FUV2006_209_13_02_53_UVIS_026MI_ICYLON002_ISS FUV2006_209_13_18_53_UVIS_026MI_ICYLON002_ISS FUV2006_254_02_35_16_UVIS_028MI_PHOTOM035_ISS FUV2006_340_16_24_02_UVIS_034MI_STARE001_ISS FUV2006_340_17_02_01_UVIS_034MI_STARE001_ISS FUV2006_340_17_14_01_UVIS_034MI_STARE001_ISS FUV2006_340_18_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_18_14_02_UVIS_034MI_STARE001_ISS FUV2006_340_19_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_19_14_02_UVIS_034MI_STARE001_ISS FUV2006_340_20_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_20_14_02_UVIS_034MI_STARE001_ISS FUV2006_340_21_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_21_14_02_UVIS_034MI_STARE001_ISS FUV2006_340_22_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_22_14_02_UVIS_034MI_STARE001_ISS FUV2006_340_23_02_02_UVIS_034MI_STARE001_ISS FUV2006_340_23_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_00_02_02_UVIS_034MI_STARE001_ISS

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 88.44 122.64 126.38 132.61 123.63 115.38 157.01 150.75 153.95 146.87 142.03 145.02 140.17 137.61 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

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 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

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 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

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 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 (continued on next page)

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E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171

Table A.1 (continued) Name

a

U

Altitude

Filing factor

FUV2006_341_00_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_01_02_02_UVIS_034MI_STARE001_ISS FUV2006_341_01_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_02_02_02_UVIS_034MI_STARE001_ISS FUV2006_341_02_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_03_02_02_UVIS_034MI_STARE001_ISS FUV2006_341_03_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_04_02_02_UVIS_034MI_STARE001_ISS FUV2006_341_04_14_02_UVIS_034MI_STARE001_ISS FUV2006_341_05_02_02_UVIS_034MI_STARE001_ISS FUV2006_341_05_15_02_UVIS_034MI_STARE001_ISS FUV2007_163_16_40_40_UVIS_046MI_ICYLON001_ISS FUV2007_298_20_45_41_UVIS_051MI_GLOCOLB001_ISS FUV2007_298_23_25_52_UVIS_051MI_094W010PH001_ISS FUV2007_299_12_06_51_UVIS_051MI_310W014PH001_ISS FUV2008_196_14_22_59_UVIS_076MI_ICYLON001_CIRS FUV2008_235_10_06_11_UVIS_081MI_ICYLON002_ISS FUV2008_235_10_38_11_UVIS_081MI_ICYLON002_ISS FUV2008_272_06_32_22_UVIS_086MI_ICYLON001_ISS FUV2008_278_21_33_07_UVIS_087MI_ICYLON001_ISS FUV2008_293_22_46_00_UVIS_089MI_ICYLON001_ISS FUV2009_023_14_26_21_UVIS_101MI_ICYLON001_ISS FUV2009_023_14_52_01_UVIS_101MI_ICYLON001_ISS FUV2009_176_18_15_02_UVIS_113MI_ICYLON001_ISS FUV2009_176_18_15_02_UVIS_113MI_iCYLON001_ISS FUV2009_176_18_15_02_UVIS_113MI_iCYLON001_ISS FUV2009_176_18_15_02_UVIS_113MI_iCYLON001_ISS FUV2009_176_18_15_02_UVIS_113MI_iCYLON001_ISS FUV2009_287_13_48_36_UVIS_119MI_ICYLON001_ISS FUV2009_287_13_48_36_UVIS_119MI_ICYLON001_ISS FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_226_19_38_00_UVIS_170MI_LOPHASE001_PIE FUV2012_267_17_18_40_UVIS_172MI_ICYLON001_ISS FUV2012_267_17_43_22_UVIS_172MI_ICYLON001_ISS FUV2012_267_18_07_21_UVIS_172MI_ICYLON001_ISS FUV2012_267_18_27_00_UVIS_172MI_ICYLON001_ISS FUV2013_092_10_02_20_UVIS_185MI_ICYLON001_ISS FUV2013_092_10_11_00_UVIS_185MI_ICYLON001_ISS FUV2013_092_10_26_40_UVIS_185MI_ICYLON001_ISS FUV2013_092_10_41_59_UVIS_185MI_ICYLON001_ISS FUV2013_092_10_56_45_UVIS_185MI_ICYLON001_ISS FUV2013_092_11_11_39_UVIS_185MI_ICYLON001_ISS FUV2013_092_11_26_29_UVIS_185MI_ICYLON001_ISS FUV2013_127_03_01_00_UVIS_189MI_ICYLON001_PRIME FUV2013_127_17_24_30_UVIS_189MI_ICYLON002_PRIME FUV2013_174_20_08_30_UVIS_193MI_ICYLON001_ISS FUV2013_174_20_24_10_UVIS_193MI_ICYLON001_ISS FUV2013_174_20_39_30_UVIS_193MI_ICYLON001_ISS FUV2013_174_20_54_15_UVIS_193MI_ICYLON001_ISS FUV2013_174_21_09_09_UVIS_193MI_ICYLON001_ISS FUV2013_178_09_47_20_UVIS_193MI_LOPHASE001_PIE FUV2013_178_10_37_20_UVIS_193MI_LOPHASE001_PIE

161.23 160.83 160.38 159.91 159.42 158.93 158.46 158.01 157.61 157.24 156.93 15.32 10.33 9.80 13.71 62.09 15.25 14.33 14.23 14.90 14.21 149.78 151.74 11.39 12.49 13.50 14.48 15.16 1.98 2.37 2.47 1.50 0.61 0.79 1.50 2.53 158.37 160.36 162.14 163.57 159.64 159.09 158.40 157.79 157.26 156.78 156.45 58.60 68.51 151.29 152.13 152.92 153.82 154.69 1.42 1.29

31.11 37.86 44.94 52.29 60.04 68.08 76.49 85.15 93.75 103.06 112.10 276.88 50.15 95.29 307.52 17.62 41.60 55.63 88.10 103.48 91.88 76.14 109.36 64.77 71.83 76.62 80.82 83.60 261.74 262.67 18.13 20.09 22.42 24.44 26.16 28.60 4.08 8.03 11.55 14.39 75.19 78.02 81.91 85.80 89.64 93.59 96.79 204.02 56.60 289.30 293.12 296.54 300.21 303.60 7.24 14.60

1739927.40 1724712.50 1720103.10 1698881.90 1692963.30 1667383.90 1660611.40 1632645.10 1625546.30 1597413.00 1589997.00 610275.64 1215459.70 1144121.20 1480275.50 294910.58 1336495.50 1318405.20 1238290.10 1106735.70 1197051.20 576011.46 556850.30 593830.99 593830.99 593830.99 593830.99 593830.99 334207.08 334207.08 804360.65 804360.65 804360.65 804360.65 804360.65 804360.65 891984.11 882061.85 871160.42 861321.34 605838.58 598036.42 583815.49 569799.27 556219.78 542486.15 528824.91 1183584.90 1406615.00 841024.45 848247.47 855254.37 861898.08 868481.29 1164865.90 1167213.60

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.22 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

Table A.2 Tethys disk-integrated observations. a is the averaged solar phase angle of the observation in degree, U is the central longitude. Name

a

U

FUV2005_017_15_10_10_UVIS_00CTE_ICYLON012_ISS FUV2005_017_16_40_11_UVIS_00CTE_ICYLON014_ISS FUV2005_017_22_19_11_UVIS_00CTE_ICYLON016_ISS FUV2005_018_13_59_21_UVIS_00CTE_ICYLON017_ISS FUV2005_019_00_30_00_UVIS_00CTE_ICYLON018_ISS FUV2005_049_12_29_34_UVIS_003TE_ICYLON006_ISS FUV2005_049_22_58_34_UVIS_003TE_ICYLON008_ISS FUV2005_050_15_28_34_UVIS_003TE_ICYLON010_ISS FUV2005_051_11_46_54_UVIS_003TE_ICYLON011_ISS FUV2005_065_08_58_49_UVIS_004TE_ICYLON001_ISS FUV2005_065_17_22_01_UVIS_004TE_ICYLON002_ISS

109.92 108.67 109.51 122.23 111.31 97.87 85.15 100.00 82.95 34.82 38.43

78.95 90.20 136.19 273.79 345.09 22.28 91.79 238.63 22.62 96.86 165.56

Altitude 1036869.7 1014101.4 935656.5 1419638.5 1895205.6 1348686.2 1242793.1 1357412.6 2084398.7 1674436.4 1311345.9

Filing factor 0.55 0.58 0.67 0.29 0.16 0.32 0.38 0.31 0.13 0.22 0.39

169

E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171 Table A.2 (continued) Name

a

U

Altitude

Filing factor

FUV2005_066_10_17_01_UVIS_004TE_ICYLON003_ISS FUV2005_071_09_58_24_UVIS_004TE_ICYLON006_ISS FUV2005_072_16_12_03_UVIS_004TE_ICYLON010_ISS FUV2005_215_01_08_39_UVIS_012TE_ICYLON015_ISS FUV2005_263_21_45_59_UVIS_015TE_ICYLON008_ISS FUV2005_264_05_47_10_UVIS_015TE_ICYLON012_ISS FUV2005_264_14_57_10_UVIS_015TE_ICYLON013_ISS FUV2005_286_22_58_58_UVIS_016TE_ICYLON004_ISS FUV2005_287_20_48_58_UVIS_016TE_ICYLON005_ISS FUV2005_288_05_06_48_UVIS_016TE_ICYLON006_ISS FUV2005_288_22_28_09_UVIS_016TE_ICYLON007_ISS FUV2005_305_00_32_07_UVIS_017TE_ICYLON001_ISS FUV2005_305_18_03_27_UVIS_017TE_ICYLON002_ISS FUV2005_305_18_11_27_UVIS_017TE_ICYLON002_ISS FUV2005_306_05_11_48_UVIS_017TE_ICYLON003_ISS FUV2005_307_07_57_58_UVIS_017TE_ICYLON004_ISS FUV2005_332_20_51_58_UVIS_018TE_ICYLON001_VIMS FUV2005_333_01_58_58_UVIS_018TE_ICYLON002_VIMS FUV2005_333_04_38_58_UVIS_018TE_ICYLON003_VIMS FUV2006_018_19_34_58_UVIS_020TE_ICYLON005_ISS FUV2006_019_12_26_28_UVIS_020TE_ICYLON009_ISS FUV2006_019_21_38_36_UVIS_020TE_ICYLON015_ISS FUV2006_022_07_31_28_UVIS_020TE_ICYLON020_ISS FUV2006_052_10_07_58_UVIS_021TE_ICYLON001_ISS FUV2006_052_21_01_48_UVIS_021TE_ICYLON002_ISS FUV2006_083_15_08_08_UVIS_022TE_ICYLON002_ISS FUV2006_123_06_27_58_UVIS_023TE_ICYLON069_ISS FUV2006_223_00_51_34_UVIS_027TE_ICYLON001_ISS FUV2006_254_01_38_02_UVIS_028TE_PHOTOM035_ISS FUV2006_255_09_07_31_UVIS_028TE_ICYATM001_PRIME FUV2006_255_09_25_16_UVIS_028TE_ICYATM001_PRIME FUV2006_255_09_42_58_UVIS_028TE_ICYATM001_PRIME FUV2006_255_10_00_43_UVIS_028TE_ICYATM001_PRIME FUV2006_255_10_18_28_UVIS_028TE_ICYATM001_PRIME FUV2006_255_10_36_13_UVIS_028TE_ICYATM001_PRIME FUV2006_255_10_53_58_UVIS_028TE_ICYATM001_PRIME FUV2006_255_11_11_43_UVIS_028TE_ICYATM001_PRIME FUV2006_278_23_44_02_UVIS_030TE_STARE001_PRIME FUV2006_279_01_34_02_UVIS_030TE_STARE001_PRIME FUV2006_279_07_08_01_UVIS_030TE_STARE001_PRIME FUV2007_205_05_06_40_UVIS_048TE_ICYLON002_ISS FUV2007_298_19_45_11_UVIS_051TE_166W012PH001_ISS FUV2007_299_03_04_32_UVIS_051TE_238W021PH001_ISS FUV2007_299_12_54_51_UVIS_051TE_310W016PH001_ISS FUV2007_300_00_24_21_UVIS_051TE_022W012PH001_ISS FUV2007_301_02_59_42_UVIS_051TE_238W011PH001_ISS FUV2007_302_17_06_52_UVIS_051TE_166W021PH001_ISS FUV2007_302_17_16_52_UVIS_051TE_166W021PH001_ISS FUV2007_303_20_56_42_UVIS_051TE_022W024PH001_ISS FUV2007_358_15_19_50_UVIS_054TE_ICYLON001_PRIME FUV2008_029_19_09_11_UVIS_057TE_ICYLON001_ISS FUV2008_029_19_09_11_UVIS_057TE_ICYLON001_ISS FUV2008_063_19_39_49_UVIS_060TE_ICYLON001_ISS FUV2008_063_19_39_49_UVIS_060TE_ICYLON001_ISS FUV2008_105_20_40_10_UVIS_064TE_ICYLON001_ISS FUV2008_173_16_00_00_UVIS_073TE_ICYLON001_CIRS FUV2008_220_08_20_47_UVIS_079TE_ICYLON001_ISS FUV2008_220_08_20_47_UVIS_079TE_ICYLON001_ISS FUV2008_228_01_07_17_UVIS_080TE_ICYLON001_ISS FUV2008_228_01_07_17_UVIS_080TE_ICYLON001_ISS FUV2008_228_01_07_17_UVIS_080TE_ICYLON001_ISS FUV2008_235_08_23_37_UVIS_081TE_ICYLON001_ISS FUV2008_235_08_23_37_UVIS_081TE_ICYLON001_ISS FUV2008_294_01_02_47_UVIS_089TE_ICYLON001_ISS FUV2008_294_01_02_47_UVIS_089TE_ICYLON001_ISS FUV2008_301_13_25_47_UVIS_090TE_ICYLON001_ISS FUV2008_301_13_25_47_UVIS_090TE_ICYLON001_ISS FUV2008_301_13_25_47_UVIS_090TE_ICYLON001_ISS FUV2013_166_21_53_00_UVIS_192TE_LOPHASE001_PIE FUV2013_166_21_53_00_UVIS_192TE_LOPHASE001_PIE FUV2013_166_21_53_00_UVIS_192TE_LOPHASE001_PIE FUV2013_166_21_53_00_UVIS_192TE_LOPHASE001_PIE FUV2013_166_21_53_00_UVIS_192TE_LOPHASE001_PIE

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 15.82 11.73 11.36 21.08 20.95 24.40 28.63 8.99 9.13 0.32 0.38 26.59 49.74 6.75 6.70 9.31 9.75 10.17 9.88 9.63 8.65 8.86 9.77 9.84 9.96 1.49 1.09 0.90 1.15 1.55

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 92.17 310.00 169.10 237.49 310.76 22.33 238.14 164.96 166.40 21.79 32.10 96.95 99.96 106.78 107.79 219.73 177.77 80.18 86.13 106.13 110.72 114.65 60.10 65.44 93.92 100.24 85.17 89.37 92.85 190.22 192.35 195.12 198.05 200.15

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 1560629.6 1861939.0 1764316.3 2055504.2 2054977.2 2828910.8 2312366.0 1135196.4 1135196.4 913484.6 913484.6 1148263.2 774580.7 1219627.5 1219627.5 1107990.5 1107990.5 1107990.5 1335446.0 1335446.0 1147685.2 1147685.2 1203826.6 1203826.6 1203826.6 809317.6 809317.6 809317.6 809317.6 809317.6

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 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

170

E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171

Table A.3 Dione disk-integrated observations. a is the averaged solar phase angle of the observation in degree, U is the central longitude. Name

a

U

Altitude

Filing factor

FUV2005_017_14_11_30_UVIS_00CDI_ICYLON012_ISS FUV2005_050_17_24_33_UVIS_003DI_ICYLON010_ISS FUV2005_051_16_14_04_UVIS_003DI_ICYLON011_ISS FUV2005_065_17_43_01_UVIS_004DI_ICYLON001_ISS FUV2005_066_07_53_00_UVIS_004DI_ICYLON002_ISS FUV2005_070_17_38_12_UVIS_004DI_ICYLON004_ISS FUV2005_071_08_18_04_UVIS_004DI_ICYLON006_ISS FUV2005_264_15_17_30_UVIS_015DI_ICYLON014_ISS FUV2005_265_03_17_00_UVIS_015DI_ICYLON020_ISS FUV2005_286_22_19_57_UVIS_016DI_ICYLON012_ISS FUV2005_288_06_01_58_UVIS_016DI_ICYLON013_ISS FUV2005_288_21_13_59_UVIS_016DI_ICYLON014_ISS FUV2005_305_01_04_57_UVIS_017DI_ICYLON001_ISS FUV2005_305_23_58_58_UVIS_017DI_ICYLON002_ISS FUV2005_307_06_16_58_UVIS_017DI_ICYLON003_ISS FUV2005_325_02_58_57_UVIS_018DI_ICYLON001_ISS FUV2006_018_11_54_23_UVIS_020DI_ICYLON001_ISS FUV2006_019_16_47_26_UVIS_020DI_ICYLON012_ISS FUV2006_052_02_36_31_UVIS_021DI_ICYLON001_ISS FUV2006_052_16_29_28_UVIS_021DI_ICYLON002_ISS FUV2006_053_16_19_29_UVIS_021DI_ICYLON003_ISS FUV2006_062_12_51_00_UVIS_021DI_ICYLON004_ISS FUV2006_082_19_36_26_UVIS_022DI_ICYLON001_ISS FUV2006_083_12_47_26_UVIS_022DI_ICYLON002_ISS FUV2006_123_14_07_25_UVIS_023DI_ICYLON070_ISS FUV2006_124_15_07_26_UVIS_023DI_ICYLON073_ISS FUV2006_125_21_02_26_UVIS_023DI_ICYLON075_ISS FUV2006_126_09_27_25_UVIS_023DI_ICYLON076_ISS FUV2006_126_21_37_25_UVIS_023DI_ICYLON077_ISS FUV2006_223_05_01_55_UVIS_027DI_ICYLON001_ISS FUV2006_255_21_15_43_UVIS_028DI_STARE001_PRIME FUV2006_256_00_13_47_UVIS_028DI_STARE001_PRIME FUV2006_256_00_35_47_UVIS_028DI_STARE001_PRIME FUV2006_295_19_19_50_UVIS_031DI_STARE002_PRIME FUV2006_295_20_42_15_UVIS_031DI_STARE002_PRIME FUV2006_334_22_08_58_UVIS_034DI_ICYATM005_PRIME FUV2006_334_22_26_09_UVIS_034DI_ICYATM005_PRIME FUV2006_334_22_43_19_UVIS_034DI_ICYATM005_PRIME FUV2006_334_23_00_29_UVIS_034DI_ICYATM005_PRIME FUV2006_334_23_17_39_UVIS_034DI_ICYATM005_PRIME FUV2006_334_23_34_49_UVIS_034DI_ICYATM005_PRIME FUV2006_334_23_51_59_UVIS_034DI_ICYATM005_PRIME FUV2006_335_00_09_09_UVIS_034DI_ICYATM005_PRIME FUV2006_335_00_26_18_UVIS_034DI_ICYATM005_PRIME FUV2006_335_00_43_28_UVIS_034DI_ICYATM005_PRIME FUV2006_335_01_00_39_UVIS_034DI_ICYATM005_PRIME FUV2006_335_01_17_49_UVIS_034DI_ICYATM005_PRIME FUV2006_335_01_34_59_UVIS_034DI_ICYATM005_PRIME FUV2006_341_17_32_07_UVIS_034DI_STARE001_PRIME FUV2006_341_18_34_07_UVIS_034DI_STARE001_PRIME FUV2006_341_19_28_22_UVIS_034DI_STARE001_PRIME FUV2007_034_07_30_00_UVIS_038DI_ICYLON001_ISS FUV2007_034_10_20_00_UVIS_038DI_ICYLON002_ISS FUV2007_037_01_44_29_UVIS_038DI_ICYLON001_CIRS FUV2007_037_02_05_19_UVIS_038DI_ICYLON001_CIRS FUV2007_204_10_28_30_UVIS_048DI_ICYLON002_ISS FUV2007_205_03_07_30_UVIS_048DI_ICYLON003_ISS FUV2007_299_04_10_02_UVIS_051DI_166W010PH001_ISS FUV2007_299_15_07_01_UVIS_051DI_238W018PH001_ISS FUV2007_302_01_07_01_UVIS_051DI_166W019PH001_ISS FUV2007_302_12_44_32_UVIS_051DI_238W014PH001_ISS FUV2007_303_18_18_02_UVIS_051DI_022W025PH001_ISS FUV2007_303_18_40_02_UVIS_051DI_022W025PH001_ISS FUV2008_177_03_31_30_UVIS_073DI_ICYLON001_ISS FUV2008_177_07_59_00_UVIS_073DI_ICYLON002_ISS FUV2008_177_15_03_00_UVIS_073DI_ICYLON003_ISS FUV2008_181_03_56_00_UVIS_074DI_ICYLON001_ISS FUV2008_190_13_49_00_UVIS_075DI_ICYLON001_PRIME FUV2008_190_14_40_00_UVIS_075DI_ICYLON001_PRIME FUV2008_221_01_14_48_UVIS_079DI_ICYLON008_ISS FUV2008_221_01_14_48_UVIS_079DI_ICYLON008_ISS FUV2008_234_11_34_48_UVIS_081DI_ICYLON001_ISS FUV2008_234_11_34_48_UVIS_081DI_ICYLON001_ISS FUV2008_242_16_26_30_UVIS_082DI_ICYLON001_ISS

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 69.50 156.96 146.05 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 10.38 12.84 13.18 6.51 6.01 7.49

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 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 51.43 88.28 90.67 60.56 63.35 59.82

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 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 1033680.5 1182535.2 1182535.2 1222943.7 1222943.7 1358785.2

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 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 0.63 0.48 0.48 0.44 0.45 0.37

171

E.M. Royer, A.R. Hendrix / Icarus 242 (2014) 158–171 Table A.3 (continued) Name FUV2008_242_16_26_30_UVIS_082DI_ICYLON001_ISS FUV2008_286_15_34_30_UVIS_088DI_ICYLON001_ISS FUV2008_286_15_34_30_UVIS_088DI_ICYLON001_ISS FUV2009_019_03_12_50_UVIS_100DI_ICYLON001_ISS FUV2009_019_03_12_50_UVIS_100DI_ICYLON001_ISS FUV2009_307_03_06_04_UVIS_120DI_ICYSTARE001_PRIME FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE FUV2013_178_07_35_00_UVIS_193DI_LOPHASE001_PIE

a 7.48 4.78 4.78 8.87 8.87 85.89 1.30 0.94 0.49 0.53 0.89 1.24 1.63 2.03 2.36 2.66

References Buratti, B.J., 1985. Applications of a radiative transfer model to bright icy satellite. Icarus 61, 208–216. Buratti, B.J., Veverka, J., 1984. Voyager photometry of Rhea, Dione, Tethys, Enceladus and Mimas. Icarus 58, 254–264. Buratti, B., Mosher, J., Torrence, V., 1990. Albedo and color maps of the saturnian satellites. Icarus 87, 339–357. Buratti, B., Mosher, J., Nicholson, P., McGhee, C., French, R., 1998. Near-infrared photometry of the saturnian satellites during ring plane crossing. Icarus 136, 223–231. Buratti, B., Bauer, J., Hicks, M., Mosher, J., Filacchione, G., Momary, T., Baines, K., Brown, R., Clark, R., Nicholson, P., 2010. Cassini spectra and photometry 0.25– 5.1 micrometers of the small inner satellites of Saturn. Icarus 206, 524–536. Clark, R. et al., 2008. Compositional mapping of Saturn’s satellite Dione with Cassini VIMS and implications of dark material in the Saturn system. Icarus 193, 372– 386. Domingue, D.L., Verbiscer, A., 1997. Re-analysis of the solar phase curves of the icy Galilean satellites. Icarus 128, 49–74. Domingue, D., Hapke, B., Lockwood, G., Thompson, D., 1991. Europa’s phase curve: Implications for surface structure. Icarus 90, 30–42. Domingue, D.L., Lockwood, G.W., Kubala, A.E., 1998. Supplementary analysis of Io’s disk-integrated solar phase curve. Icarus 134, 113–136. Esposito, L.W. et al., 2004. The Cassini ultraviolet imaging spectrograph investigation. Space Sci. Rev. 115, 299–361. Filacchione, G., Capaccioni, F., Clark, R., Cuzzi, J., Cruikshank, D., Coradini, A., Cerroni, P., Nicholson, P., McCord, T., Brown, R., Buratti, B., Tosi, F., Nelson, R., Jaumann, R., Stephan, K., 2010. Saturn’s icy satellites investigated by Cassini-VIMS. II. Results at the end of nominal mission. Icarus 206, 507–523. Hamilton, D.P., Burns, J.A., 1994. Origin of Saturn’s E ring: Self-sustained, naturally. Science 264, 550–553. Hapke, B., 1981. Bidirectional reflectance spectroscopy: 1. Theory. J. Geophys. Res. 86, 3039–3054. Hapke, B., 1984. Bidirectional reflectance spectroscopy: 3. Correction for macroscopic roughness. Icarus 59, 41–59. Hapke, B., 1986. Bidirectional reflectance spectroscopy: 4. The extinction coefficient and the opposition effect. Icarus 67, 264–280. Hapke, B., 2001. Space weathering from Mercury to the asteroid belt. J. Geophys. Res. 106, 10,039–10,073. Hapke, B., 2012. Theory of reflectance and emittance spectroscopy, second ed. Cambridge University Press. Helfenstein, P., Veverka, J., 1987. Photometric properties of lunar terrains derived from Hapke’s equations. Icarus 72, 342–357. Helfenstein, P., Currier, N., Clark, B., Veverka, J., Bell, M., Sullivan, R., Klemaszewski, J., Greeley, R., Pappalardo, R., Head III, J., Jones, T., Klaasen, K., Magee, K., Geissler, P., Greenberg, R., McEwen, A., Phillips, C., Colvin, T., Davies, M., Denk, T., Neukum, G., Belton, M., 1998. Galileo observations of Europa’s opposition effect. Icarus 135, 41–63. Hendrix, A.R., Hansen, C.J., 2008. The albedo dichotomy of Iapetus measured at UV wavelengths. Icarus 193, 344–351. Hendrix, A., Vilas, F., Festou, M., 2003. Vesta’s UV lightcurve: Hemispheric variation in brightness and spectral reversal. Icarus 162, 1–9. Hendrix, A.R., Domingue, D.L., Kimberly, K., 2005. The icy Galilean satellites: Ultraviolet phase curve analysis. Icarus 173, 29–49. Hendrix, A.R., Cassidy, T.A., Buratti, B.J., Paranias, C., Hansen, C.J., Teolis, B., Roussos, E., Bradley, E.T., Kollmann, P., Johnson, R.E., 2012. Mimas’ far-UV albedo: Spatial variations. Icarus 220 (2), 922–931.

U 63.30 80.25 84.20 75.56 79.28 65.88 8.08 8.70 9.65 10.67 11.40 12.03 12.71 13.39 13.97 14.49

Altitude

Filing factor

1358785.2 1189997.4 1189997.4 1235935.6 1235935.6 945777.3 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2 1322038.2

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

Howett, C., Spencer, J., Schenk, P., Johnson, R., Paranicas, C., Hurford, T., Verbiscer, A., Segura, M., 2011. A high-amplitude thermal inertia anomaly of probable magnetospheric origin on Saturn’s moons Mimas. Icarus 216, 221–226. Howett, C., Spencer, J., Hurford, T., Verbiscer, A., Segura, M., 2012. PacMan returns: An electron-generated thermal anomaly on Tethys. Icarus 221, 1084–1088. Johnson, R., 1997. Polar caps on Ganymede and Io revisited. Icarus 128, 469–471. Johnson, R., Quickenden, T., 1997. Photolysis and radiolysis of water ice on outer Solar System bodies. J. Geophys. Res. 102, 10,985–10,996. Kouchi, A., Kuroda, T., 1990. Amorphization of cubic ice by ultraviolet irradiation. Nature 344, 134–135. Lumme, K., Bowell, E., 1981. Radiative transfer in the surfaces of atmosphere less bodies. I. Theory. Astron. J. 86, 1694–1704. McClintock, W.E., Rottman, G.J., Woods, T.N., 2000. Solar stellar irradiance comparison experiment II SOLSTICE II for the NASA Earth observing system’s solar radiation and climate experiment mission. Proc. SPIE Earth Observ. Syst. V 4135, 225–234. Nelson, R.M., Lane, A.L., Matson, D.L., Veeder, G.J., Buratti, B.J., Tedesco, E.F., 1987. Spectral geometric albedos of the Galilean satellites from 0.24 to 0.34 micrometers: Observations with the international ultraviolet explorer. Icarus 72, 358–380. Nelson, R.M., Hapke, B.W., Smythe, W.D., Horn, L.J., 1998. Phase curves of selected particulate materials: The contribution of coherent backscattering to the opposition surge. Icarus 131 (1), 223–230. Paranicas, C. et al., 2012. Energetic charged particle weathering of Saturn’s inner satellites. Planet. Space Sci. 61 (1), 60–65. Pitman, K.M., Buratti, B.J., Mosher, J.A., 2010. Disk-integrated bolometric Bond albedos and rotational light curves of saturnian satellites from Cassini visual and infrared mapping spectrometer. Icarus 206, 537–560. Sack, N., Johnson, R., Boring, J., Baragiola, R., 1992. The effect of magnetospheric ion bombardment on the reflectance of Europa’s surface. Icarus 100, 534–540. Schenk, P., Hamilton, D.P., Johnson, R.E., McKinnon, W.B., Paranicas, C., Schmidt, J., Showalter, M.R., 2011. Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites. Icarus 211, 740– 757. Shkuratov, Y., Helfenstein, P., 2001. The opposition effect and the quasi-fractal structure of regolith: I. Theory. Icarus 152, 96–116. Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G., 1999. A model of spectral albedo of particulate surfaces: Implications for optical properties on the Moon. Icarus 137, 235–246. Smith, B., et al., 1981. Encounter with Saturn: Voyager 1 imaging science results. Science 2012, 163–191. Stephan, K., Jaumann, R., Wagner, R., Clark, R., Cruikshank, D., Hibbitts, C., Roatsch, T., Hoffmann, H., Brown, R., Filiacchione, G., Buratti, B., Hansen, G., McCord, T., Nicholson, P., Baines, K., 2010. Dione’s spectral and geological properties. Icarus 206, 631–652. Stooke, P., 1989. Tethys: Volcanic and structural history. Lunar Planet. Sci. XX, 1071–1072. Stooke, P., 2002. Tethys and Dione: New geological interpretations. Lunar Planet. Sci. XXXIII. Abstract 1553. Verbiscer, A., Veverka, J., 1992. Mimas: Photometry roughness and albedo map. Icarus 99, 63–69. Verbiscer, A., French, R., NcGhee, C., 2005. The opposition surge of Enceladus: HST observations 338–1022 nm. Icarus 173, 66–83. Verbiscer, A., French, R., Showalter, M., Helfenstein, P., 2007. Enceladus: Cosmic graffiti artist caught in the act. Science 315, 815.