The icy Galilean satellites: ultraviolet phase curve analysis

Icarus 173 (2005) 29–49 www.elsevier.com/locate/icarus

The icy Galilean satellites: ultraviolet phase curve analysis Amanda R. Hendrix a,∗ , Deborah L. Domingue b , Kimberly King b,1 a Jet Propulsion Laboratory/California Institute of Technology, 4800 Oak Grove Dr., MS 230-250, Pasadena, CA 91109, USA b Johns Hopkins University/Applied Physics Laboratory, 1100 Johns Hopkins Rd., Laurel, MD 20723, USA

Received 10 June 2003; revised 12 May 2004 Available online 11 September 2004

Abstract Ultraviolet disk-integrated solar phase curves of the icy galilean satellites Europa, Ganymede, and Callisto are presented, using combined data sets from the International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and the Galileo Ultraviolet Spectrometer. Global, disk-integrated solar phase curves for all three satellites, in addition to disk-integrated solar phase curves for Europa’s leading, trailing, jovian, and anti-jovian hemispheres, are modeled using Hapke’s equations for 7 broadband UV wavelengths between 260 and 320 nm. The sparse coverage in solar phase angle, particularly for Ganymede and Callisto, and the noise in the data sets poorly constrain some of the photometric parameter values in the model. However, the results are sufficient for forming a preliminary relationship between the effects of particle bombardment on icy surfaces and photometric scattering properties at ultraviolet wavelengths. Callisto exhibits a large UV opposition surge and a surface comprised of relatively low-backward scattering particles. Europa’s surface displays a dichotomy between the jovian and antijovian hemispheres (the anti-jovian hemisphere is more backward scattering), while a less pronounced hemispherical variation was detected between the leading and trailing hemispheres. Europa’s surface, with the exception of the trailing hemisphere region, appears to have become less backscattering between the late-1970s–early-1980s and the mid-1990s. These results are commensurate with the bombardment history of these surfaces by magnetospheric charged particles.  2004 Elsevier Inc. All rights reserved. Keywords: Ultraviolet; Spectroscopy; Icy satellites; Photometry

1. Introduction 1.1. Background The galilean satellites are each phase-locked with Jupiter, so that one hemisphere (the jovian hemisphere, centered on 0◦ longitude) is always facing Jupiter. The hemisphere leading in the direction of orbital motion (leading hemisphere) is centered on 90◦ W longitude, while the trailing hemisphere is centered on 270◦ W. Because Jupiter’s magnetosphere corotates at a rate faster than the orbital speed of these moons, the satellites’ trailing hemispheres are preferentially affected by magnetospheric particle bombardment. Some ef* Corresponding author. Fax: (818)-393-4669.

E-mail address: [email protected] (A.R. Hendrix). 1 Mentor student from Eleanor Roosevelt High School, Greenbelt MD.

Currently with University of Maryland, College Park, MD. 0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2004.06.017

fects are implantation of magnetospheric ions, sputtering, erosion and grain size alteration. The leading hemispheres of these moons are preferentially affected by micrometeorite bombardment, while the jovian and anti-jovian hemispheres may be affected by dust and/or neutral wind particles traveling radially within the jovian system. Europa’s surface displays a distinctive longitudinal albedo variation from leading to trailing hemisphere at visible wavelengths (Stebbins, 1927; Stebbins and Jacobsen, 1928; Johnson, 1971; Morrison et al., 1974; Millis and Thompson, 1975; Nelson and Hapke, 1978; McEwen, 1986). The surface albedo varies as a cosine function and is lowest at the apex of the trailing hemisphere and is greatest at the apex of the leading hemisphere (Johnson et al., 1983; McEwen, 1986). This has been attributed to the preferential ion bombardment of the trailing hemisphere (Johnson et al., 1988; Pospieszalska and Johnson, 1989) from ions originating in Io’s torus and picked up by Jupiter’s magnetosphere.

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Europa’s UV spectrum also varies with longitude (e.g., Nelson et al., 1987). Lane et al. (1981) first noted, using data from the International Ultraviolet Explorer (IUE), an absorption feature centered at 280 nm that was strongest on Europa’s trailing hemisphere. Subsequent observations with the Hubble Space Telescope (HST) have also detected this trailing hemisphere absorption feature in ratioed spectra of the leading and trailing hemispheres (Noll et al., 1995). Lane et al. (1981) proposed that this absorption was due to an S–O bond lattice, which produces an absorption feature resembling SO2 gas. Noll et al. (1995) offered an alternative explanation, proposing that the 280 nm absorption feature is due to SO2 frost layered over water frost based on the laboratory data of Sack et al. (1992). This implies an endogenic origin for the SO2 with the magnetospheric bombardment creating erosion to expose the underlying SO2 on the trailing hemisphere. Disk-resolved observations of Europa’s trailing hemisphere by the Galileo Ultraviolet Spectrometer (UVS) indicate that Europa’s surface is darkest near 270◦ W and increases in brightness away from the trailing hemisphere apex. The 280 nm absorber, however, is much more confined to the trailing hemisphere apex region than the darkening; the two effects may thus be caused by slightly different processes (Hendrix et al., 1998). Hendrix et al. (1998) proposed that corotating ions are implanted into Europa’s surface more deeply close to the trailing hemisphere apex, causing both the darkening and the 280 nm absorption; away from 270◦ W, the ions are implanted less deeply, causing some darkening but creating lower amounts of the 280 nm absorber. More recent results using higher spatial resolution Galileo UVS observations suggest a stronger correlation between the UV absorber and the infrared-measured hydrate (associated primarily with dark terrain) (McCord et al., 1999; Carlson et al., 1999), than between the UV absorber and longitude (Hendrix et al. 2003, in preparation). This may indicate a relationship between the UV absorber and the dark terrains on Europa’s trailing hemisphere. Hydrogen peroxide (H2 O2 ) has also been detected on Europa’s surface using measurements from the Galileo NIMS and UVS (Carlson et al., 1999; Hendrix et al., 1999b), likely due to radiolysis and/or photolysis of the water ice-rich surface, particularly on the leading hemisphere. The H2 O2 feature appears most obviously away from the trailing hemisphere, where fewer contaminants are mixed with the water ice. Ganymede displays hemispherical albedo variations at visible wavelengths similar to Europa (Stebbins, 1927; Stebbins and Jacobsen, 1928; Morrison et al., 1974; Millis and Thompson, 1975; Nelson et al., 1987). The longitudinal albedo variation on Ganymede, as on Europa, is correlated to the preferential bombardment of the trailing hemisphere by magnetospheric ions (Lane et al., 1981; Nelson et al., 1987; Johnson et al., 1988; Pospieszalska and Johnson, 1989). This effect is complicated by the presence of an internal magnetic field at Ganymede (Kivelson et al., 1996). Ganymede, orbiting at 15RJ , also experiences a lower charged particle flux

than Europa (9RJ ). Ganymede’s UV spectral properties indicate different UV absorption properties than observed on Europa. Ganymede’s UV spectral characteristics were first observed by Nelson et al. (1987) with IUE, who proposed is the presence of ozone in addition to sulfur to explain the differences in Europa’s and Ganymede’s UV spectral properties. More recent HST observations by Noll et al. (1996) confirmed Nelson et al.’s (1987) preliminary evidence for the presence of ozone in the surface of Ganymede’s trailing hemisphere with the detection of a broad absorption band centered near 260 nm. Disk-resolved observations by the Galileo UVS indicate an increase in the 260 nm absorption feature near Ganymede’s north and south poles (Hendrix et al., 1999a). An explanation for Ganymede’s 260 nm feature is that the “ozone-like” absorber is concentrated in regions of high water ice amounts (such as the polar caps). Alternately, the absorption may be stronger in the polar regions due to additional bombardment there by particles traveling along Ganymede’s magnetic field lines. Ultraviolet spectra of Ganymede from Galileo also display the H2 O2 feature initially seen on Europa in both the IR and UV (Carlson et al., 1999; Hendrix et al., 1999b). Callisto displays the opposite hemispherical dichotomy of the other two icy galilean satellites; its trailing hemisphere has a slightly higher albedo than its leading hemisphere. This has been ascribed to preferential bombardment of the leading hemisphere by micrometeorites and/or debris from the outer small satellites of Jupiter (Buratti, 1991). Magnetospheric ion bombardment (and its associated sputtering, implantation and grain size alteration) is a weaker alteration process at Callisto’s distant orbit compare to at the orbits of Europa and Ganymede, where the ion fluxes are higher. An interesting absorption feature has been detected on Callisto (Lane and Domingue, 1997; Noll et al., 1997) that resembles the 280 nm absorption feature seen on Europa. This feature on Callisto has been detected on the leading jovianfacing hemisphere, in contrast to Europa, where it is seen on the trailing hemisphere. The correlation with the jovianfacing hemisphere suggests a possible formation by neutral sulfur implantation from the jovian neutral wind (Lane and Domingue, 1997). 1.2. Goals of this work The optical and mechanical properties of the icy galilean satellite surfaces based on photometric modeling using Hapke’s theory are well studied in the visible wavelength region (Buratti, 1985, 1991; Helfenstein, 1986; Domingue et al., 1991, 1995; Domingue and Hapke, 1992; Domingue and Verbiscer, 1997). Similar modeling at far- to near-ultraviolet wavelengths has not yet been examined because reliable modeling requires observational data over a wide range of phase angles. The Galileo UVS instrument provided the first high-phase angle observations of the galilean satellites at these wavelengths. Used in combination with the low phase angle IUE and HST observations, the UVS observations pro-

UV solar phase curves

vide an excellent opportunity to use Hapke’s photometric equations to model the photometric characteristics of the icy galilean satellite surfaces for the first time at ultraviolet wavelengths. This study investigates endogenic versus exogenic processes affecting the surfaces of the satellites by characterizing the photometric properties of the leading and the trailing hemispheres, as well as the jovian and anti-jovian hemispheres, where the data coverage allows. Our goal is to understand how the environment of the jovian system alters the optical and mechanical properties of the regoliths of the icy satellites and how these alterations affect the photometric properties of icy surfaces.

2. Data sets and data reduction The photometric properties of a scattering surface are characterized through the values of the parameters in a photometric model. In order to derive meaningful photometric parameter values using any photometric model a data set with a wide phase angle range is needed. Combining the IUE and HST observations (mostly smaller phase angles, < 12◦ ) with the Galileo UVS data (mostly larger phase angles, but with some phase angle overlap with IUE and HST) provides the most robust data set currently available for the icy galilean satellites. The data sets used in this study are listed in Tables 1, 2, and 3 for Europa, Ganymede, and Callisto, respectively. The tables list the observation name and date, the solar phase angle and the sub-observer longitude. The IUE data used in this study were obtained from the INES (IUE Newly-Extracted Spectra) website (http://ines. stsci.edu/ines/). The NEWSIPS versions of these spectra have been published elsewhere (Nelson et al., 1987; Lane and Domingue, 1997). The INES files benefit from an improved geometric correction, a more accurate photometric correction and an increased signal-to-noise ratio of the extracted IUE data through the use of new processing algorithms (Wamsteker and Gonzelez-Riestra, 1998). The IUE observations of the icy galilean satellites span the 1978– 1996 time period. Spectra were taken with the Long Wavelength Prime and Redundant (LWP and LWR) spectrographs that cover the 185–335 nm wavelength range with a spectral spacing of 0.27 nm. The HST data were obtained from the MAST (Multimission Archive at Space Telescope) archive and are spectra that have previously been published elsewhere (Noll et al., 1995, 1996, 1997). The Faint Object Spectrograph (FOS) observations cover the 222–330 nm wavelength range with a spectral spacing of 0.05 nm. To determine the reflectance measured in the IUE and HST observations, we subtracted off the small amount of background measurement (first several spectral elements averaged), then converted the spectra to photons/(cm2 s Å) for division by the solar spectrum. For all observations, we used a solar spectrum from UARS SOLSTICE (Rottman et al., 1993), corrected to a Earth–Sun distance of 1.0 AU. The

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Table 1 Europa disk-integrated observations Instrument/observation

Phase angle

Central longitude

IUE year

IUE/LWR02339 IUE/LWR02349 IUE/LWR02350 IUE/LWR02351 IUE/LWR02352 IUE/LWR02371 IUE/LWP02955 IUE/LWP02957 IUE/LWP02967 IUE/LWP02969 IUE/LWP03673 IUE/LWP03683 IUE/LWP 04277 IUE/LWP05803 IUE/LWP05805 IUE/LWP05808 IUE/LWR06404 IUE/LWR06407 IUE/LWR06437 IUE/LWR06440 IUE/LWR06449 IUE/LWP06567 IUE/LWP06578 IUE/LWR06605 IUE/LWP06925 IUE/LWR10587 IUE/LWR10588 IUE/LWR10591 IUE/LWR10592 IUE/LWR10596 IUE/LWR10883 IUE/LWR10884 IUE/LWR12752 IUE/LWR12756 IUE/LWR13672 IUE/LWR15272 IUE/LWR15356 IUE/LWR16349 IUE/LWR16350 IUE/LWR16353 IUE/LWR16367 IUE/LWR16368 IUE/LWR16369 IUE/LWR16383 IUE/LWR16384 IUE/LWR16387 IUE/LWR16388 IUE/LWR16389 IUE/LWR16801 IUE/LWR16805 IUE/LWP30014 IUE/LWP30015 IUE/LWP30058 IUE/LWP32190 IUE/LWP32229 UVS/G8PH07 UVS/C3PH68 UVS/G8PH65 UVS/C9PH03 UVS/C9PH05 UVS/G1ECL01 UVS/G1ECL04

8.3 8.4 8.4 8.4 8.4 8.6 10.5 10.5 10.6 10.6 0.143 0.1 11.1 11.2 11.2 11.2 10.2 10.2 10.0 10.0 9.9 0.42 0.1 8.1 11.0 8.3 8.3 8.3 8.3 8.4 10.7 10.7 8.1 8.1 10.7 10.2 10.6 8.7 8.7 8.7 8.9 8.9 8.9 9.1 9.1 9.1 9.1 9.1 10.3 10.3 10.3 10.3 10.4 10.6 10.5 7.16 68.7 64.8 1.53 4.3 49.0 44.5

111.4 234.0 236.8 241.3 245.7 88.9 288.0 307.0 139.0 148.0 283.4 77.9 24.1 263.9 275.7 286.2 270.8 282.2 223.4 233.3 332.9 135.0 242.0 200.7 50.9 16.7 21.1 35.4 37.9 91.9 59.1 62.2 248.0 278.0 60.0 154.3 150.4 316.2 319.8 330.2 100.0 103.0 106.0 283.6 286.5 300.3 303.5 307.4 25.4 41.1 117.0 121.0 62.0 165.0 124.0 20.0 25.0 30.0 40.0 40.0 40.0 50.0

1978 1978 1978 1978 1978 1978 1984 1984 1984 1984 1984 1984 1984 1985 1985 1985 1979 1979 1979 1979 1979 1985 1985 1980 1985 1981 1981 1981 1981 1981 1981 1981 1982 1982 1982 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1995 1995 1995 1996 1996

(continued on next page)

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Table 1 (continued) Instrument/observation UVS/G2PH60 UVS/C3PH57 UVS/C3PH57 UVS/C3PH40 UVS/G2PH43 UVS/C3PH41 UVS/G2PH58 UVS/G2PH79 UVS/C10LON250 UVS/C10LON270 UVS/C10LON290 UVS/C10PH93 UVS/C10LON100 UVS/C10LON300 HST/y2p60603t HST/y2p60503t

Phase angle 60.8 57.3 56.8 39.4 42.2 41 58 79 103.3 96.4 81.8 93.6 91.8 64.1 10.2 4.3

Central longitude

IUE year

45.0 50.0 50.0 90.0 120.0 200.0 225.0 330.0 270.0 280.0 290.0 100.0 125.0 300.0 294.0 81.0

observations were corrected for satellite–Sun and satellite– Earth distances at the time of each measurement. Thus, the equation that was used to determine spectral reflectance for each IUE and HST measurement was
 ref(λ) = D(λ) − bk ∗ λ ∗ K ∗ ∆2 /F (λ) ∗ Ω, where D(λ)—raw data, in ergs/(cm2 s Å); bk—spectral background, the average of spectral pixels 100:150 (shortward of 200 nm), where contribution from reflected solar light is negligible; K—conversion factor from ergs to photons, 5.028e7; Ω—satellite radius (km)2 /δ (km)2; F (λ)— solar spectrum, in photons/(cm2 s Å); ∆—distance between satellite and Sun, in AU; δ—distance between satellite and Earth. Error bars on the IUE reflectance data were computed by the optimal extraction algorithm in the INES processing pipeline. The noise in the IUE data is determined after a complex procedure has been applied to the raw data (Wamsteker and Gonzelez-Riestra, 1998). The Galileo observations used in this study (primarily of Europa, as very few disk-integrated observations of Ganymede and Callisto exist) were taken using the UVS Fchannel (162–323 nm, with a 0.32 nm spectral element spacing). Some of these spectra have been previously published (Hendrix et al., 1998, 1999a, 1999b). For each observation, the background signal was subtracted, then the calibration curve applied; the resultant measured spectral intensity was then divided by the solar spectrum. Generally the background signal is due to radiation background and is approximated by averaging the first ∼ 100 spectral elements, or the signal shortward of ∼ 200 nm, where no reflected sunlight is measured. In determining the reflectance the filling factor was accounted for, since these disk-integrated observations were performed at distances such that the 0.1◦ × 0.4◦ Fchannel FOV was not filled. Error bars on the UVS reflectance data were computed based on calibration error, statistical error, and integration period (Hendrix et al., 1998). In this analysis, the data were binned into 7 wavelength

Table 2 Ganymede disk-integrated observations Instrument/observation

Phase angle

Central longitude

IUE year

IUE/LWP02958 IUE/LWP02959 IUE/LWP02979 IUE/LWR03140 IUE/LWP03679 IUE/LWP04269 IUE/LWP04278 IUE/LWP04279 IUE/LWP05270 IUE/LWP05796 IUE/LWP05797 IUE/LWP05806 IUE/LWP05817 IUE/LWP06331 IUE/LWP06332 IUE/LWR06599 IUE/LWP06725 IUE/LWP09793 IUE/LWR10614 IUE/LWR10887 IUE/LWR13670 IUE/LWR15275 IUE/LWR15276 IUE/LWR16201 IUE/LWR16351 IUE/LWR16352 IUE/LWR16390 IUE/LWR16392 IUE/LWR16800 IUE/LWR16804 IUE/LWP30044 IUE/LWP30099 IUE/LWP30100 IUE/LWP32171 IUE/LWP32178 IUE/LWP32182 IUE/LWP32197 IUE/LWP32198 IUE/LWP32202 IUE/LWP32209 IUE/LWP32249 IUE/LWP32253 IUE/LWP32281 IUE/LWP32283 IUE/LWP32285 UVS/C10PH77 HST/y2p60703t HST/y2p60803t

10.5 10.5 10.7 8.3 0.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.2 6.4 6.4 8.2 7.5 10.8 8.5 10.7 10.6 10.2 10.2 4.9 8.7 8.7 9.1 9.1 10.3 10.3 10.4 10.5 10.5 10.7 10.7 10.7 10.6 10.6 10.6 10.5 10.3 10.3 10.1 10.1 10.2 77.2 4.8 6.7

102.0 103.0 292.0 54.9 79.0 54.0 73.0 75.0 56.0 229.0 230.0 280.0 337.0 231.0 233.0 123.8 188.0 97.0 312.5 224.7 53.3 11.5 11.5 14.8 218.2 219.6 31.0 34.4 43.2 51.3 265.0 100.0 101.0 125.0 146.0 152.0 227.0 229.0 238.0 272.0 72.0 81.0 234.0 238.0 242.0 30.0 71.0 255.0

1984 1984 1984 1978 1984 1984 1984 1984 1984 1985 1985 1985 1985 1985 1985 1980 1985 1986 1981 1981 1982 1983 1983 1983 1983 1983 1983 1983 1983 1983 1995 1995 1995 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996

groups to study spectral variations in Hapke parameters. The reflectance in each wavelength bin was determined by averaging over 21 spectral samples for IUE and Galileo data, and 114 samples for HST data. The seven wavelength bins are 260, 270, 280, 290, 300, 310, and 320 nm, each with a 6 nm bandpass.

UV solar phase curves

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Table 3 (continued)

Table 3 Callisto disk-integrated observations Instrument/observation

Phase angle

Central longitude

IUE year

IUE/LWP02970 IUE/LWP02971 IUE/LWP03684 IUE/LWP04274 IUE/LWP04273 IUE/LWP04274 IUE/LWR06601 IUE/LWR06602 IUE/LWP06727 IUE/LWR07134 IUE/LWR07137 IUE/LWR07138 IUE/LWR07141 IUE/LWP09714 IUE/LWP09792 IUE/LWR09933 IUE/LWR09935 IUE/LWR09938 IUE/LWR09948 IUE/LWR10581 IUE/LWR10589 IUE/LWR10597 IUE/LWR10615 IUE/LWR10869 IUE/LWR10888 IUE/LWR12386 IUE/LWR12731 IUE/LWR12732 IUE/LWR12753 IUE/LWR12768 IUE/LWR13671 IUE/LWR13749 IUE/LWR13890 IUE/LWR15273 IUE/LWR15274 IUE/LWR15358 IUE/LWR16798 IUE/LWR16802 IUE/LWP29976 IUE/LWP30079 IUE/LWP30083 IUE/LWP30084 IUE/LWP30103 IUE/LWP32184 IUE/LWP32185 IUE/LWP32187 IUE/LWP32188 IUE/LWP32189 IUE/LWP32193 IUE/LWP32195 IUE/LWP32196 IUE/LWP32199 IUE/LWP32207 IUE/LWP32212 IUE/LWP32215 IUE/LWP32227 IUE/LWP32230 IUE/LWP32267 IUE/LWP32268 IUE/LWP32273 IUE/LWP32276

10.6 10.6 0.38 11.1 11.1 11.1 8.1 8.1 7.7 2.9 2.9 2.9 2.9 11.3 10.8 6.7 6.7 6.7 6.6 8.3 8.3 8.4 8.5 10.7 10.7 10.7 8.4 8.4 8.2 7.9 10.6 10.8 10.5 10.2 10.2 10.6 10.4 10.3 10.2 10.5 10.5 10.5 10.5 10.6 10.6 10.6 10.6 10.6 10.6 10.6 10.6 10.6 10.6 10.5 10.5 10.5 10.5 10.2 10.2 10.2 10.2

286.8 286.0 49.6 74.0 75.8 76.5 209.3 209.8 136.0 61.9 64.9 65.3 67.1 259.0 157.0 270.6 271.7 273.5 295.7 280.0 288.1 303.4 331.7 303.5 346.7 335.5 218.5 219.0 260.2 302.1 104.2 299.1 315.6 12.2 12.7 242.5 333.3 342.5 297.0 107.0 111.0 112.0 135.0 246.0 247.0 250.0 251.0 252.0 259.0 267.0 268.0 270.0 279.0 291.0 293.0 319.0 321.0 27.0 27.0 43.0 47.0

1984 1984 1984 1984 1984 1984 1980 1980 1985 1980 1980 1980 1980 1986 1986 1981 1981 1981 1981 1981 1981 1981 1981 1981 1981 1982 1982 1982 1982 1982 1982 1982 1982 1983 1983 1983 1983 1983 1995 1995 1995 1995 1995 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 (continued)

Instrument/observation

Phase angle

Central longitude

IUE year

IUE/LWP32277 IUE/LWP32278 IUE/LWP32282 UVS/G8PH78 HST/y32z0308t HST/y3er0103t

10.2 10.1 10.1 77.8 9.3 10.1

48.0 63.0 66.0 140.0 91.8 279.4

1996 1996 1996

3. Analysis 3.1. Rotational phase curves Disk-integrated brightness variations in planetary observations are governed by variations due to rotational phase (which longitude region is being observed) and to solar phase (the viewing geometry of the observation). In order to study and model solar phase brightness variations, the changes in brightness due to rotational phase must first be removed. In this study, the IUE data set was used to establish the UV rotational phase curves of each satellite at each wavelength. The rationale in using only the IUE data is that it removes variations due to different instrumentation and the IUE data set spans such a short range of solar phase values that we can assume the solar phase brightness contribution is constant within this data set. The IUE data set used to determine the rotational phase curve covered 8◦ to 11.4◦ in solar phase angle for each of the satellites. Nelson et al. (1987) first measured the UV rotational phase curves of all four galilean satellites using IUE observations. The rotational phase curve analysis presented here also includes IUE data taken since the Nelson et al. (1987) work and does not account for temporal variations in the rotational phase curves (Domingue and Hendrix, 2004). A rotational phase curve of the form R = mθ + b + A[cos((2πθ/P ) + ρ)] was initially fit to each data set (as a function of satellite and wavelength), where R is the calculated brightness (or reflectance), θ is the rotational phase angle, and the parameters m, b, A, P , and ρ (hereafter referred to as the rotational variables) are constants. The rotational variables were found by minimizing the root mean square (RMS) values between the measured IUE reflectance and the calculated brightness. Once a solution to the rotational phase curve was found for all wavelengths, the goodness of fit between the 300 nm observations and the calculated rotational phase curve was examined. All data within the 300 nm data set that varied by more than 15% of the predicted value from the fitted rotational phase curve were tagged as suspicious. (We chose 15% as the conservative limit in the culling process, at just over twice the value of the largest standard deviation in all data sets between the measured reflectance and the model rotational phase curve.) The spectral data corresponding to these data points were removed from the IUE data set and not used in this analysis. The rotational phase curve equation was then refit to the remaining data using the same algorithm. Once again, using the 300 nm data set,

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(a) Europa

(b) Ganymede

(c) Callisto

(d)

Fig. 1. Rotational lightcurves of the icy galilean satellites at 300 nm. The observations (asterisks) are from the refined IUE data set. The solid line represents the best fit rotational phase curve. (a) Europa; (b) Ganymede; (c) Callisto; (d) all three satellites. The RMS values between the curves and the data are 1.2%, 6%, and 3% for Europa, Ganymede, and Callisto, respectively. Error bars represent measurement errors and do not include error due to small amount (< 10%) of solar variability across solar cycle.

those observations that varied by more than 15% of the new solution were removed from the spectral data set and the rotational phase curve equation was refit to the remaining data. This solution to the rotational phase curve equation was then used to define the rotational phase curve behavior for that particular data set. The remaining IUE data points (at other phase angles) plus the HST and Galileo observations (hereafter referred to as the refined data set) were then used for the solar phase curve analysis. Figure 1 shows the rotational phase curve fit to the 300 nm observations for each of the satellites. This figure demonstrates that the rotational equation used to model the data provides a good measure (less than 2% RMS variability) of the reflectance trends with rotational phase. The RMS values for the 300 nm fit are 0.012, 0.006, and 0.003 for Europa, Ganymede, and Callisto, respectively, with the respective range of RMS values over all the wavelengths being 0.01–0.017, 0.005–0.009, and 0.003–0.004. These rotational phase curves are consistent with earlier IUE-measured UV rotational phase curves (Nelson et al., 1987). As in the visible, Europa’s leading hemisphere at UV wavelengths is

brighter than the trailing hemisphere; this is also the case for Ganymede. And, as in the visible, Callisto’s trailing hemisphere in the UV is brighter than the leading hemisphere. The calculated rotational phase curves as a function of wavelength for each satellite are shown in Fig. 2. The rotational phase curves become systematically brighter as wavelength increases from 260 to 310 nm for all the satellites. The rotational phase curves are similar at 310 and 320 nm for each satellite. The amplitude of Ganymede’s rotational phase curve increases with decreasing wavelength, so that the leading-trailing difference at 260 nm is more drastic than at 310 nm. This is consistent with increased absorption centered near 260 nm by ozone in the ice on Ganymede’s trailing hemisphere (Noll et al., 1996; Hendrix et al., 1999a). On Europa and Callisto, the amplitude of the rotational phase curve at 280 nm is greater than at the other UV wavelengths. This is commensurate with increased absorption centered near 280 nm by the SO2 -like material on Europa’s trailing hemisphere (Lane et al., 1981; Noll et al., 1995; Hendrix et al., 1998) and on Callisto’s leading hemisphere (Lane and Domingue, 1997; Noll et al., 1997).

UV solar phase curves

(a) Europa

(b) Ganymede

(c) Callisto Fig. 2. The rotational phase curves for each of the icy galilean satellites as a function of wavelength. (a) Europa; (b) Ganymede; (c) Callisto.

35

tion of longitude: (1) leading hemisphere data, between 0◦ and 180◦ W longitude, and corrected to 90◦ W longitude; (2) trailing hemisphere data, between 180◦ and 360◦ W longitude, and corrected to 270◦ W longitude; (3) jovian hemisphere data, between 270◦ and 90◦ W longitude, and corrected to 0◦ W longitude; and (4) anti-jovian hemisphere data, between 90◦ and 270◦ W longitude, and corrected to 180◦ W longitude. For Ganymede and Callisto, since the data sets have sparser coverage in longitude than Europa, only a globally-averaged solar phase curve was determined, correcting to 90◦ W longitude. This was also done for Europa, for purposes of comparison. The solar phase curves include data from IUE, HST, and Galileo. Most of the IUE reflectance values are in one of several small phase angle ranges. The IUE reflectance values in these ranges were corrected for rotational phase variations and then averaged. The resulting average angles and brightnesses were used in the solar phase curve data sets along with the HST and Galileo observations. For the Europa leading and jovian hemisphere data sets, as well as for the global Europa data set, a distinct difference was noted within the IUE data set. Reflectance values from the “Voyager era” of IUE, 1978–1984, were found to be higher than those measured at similar phase angles in the “Galileo era,” the 1990’s. The “Galileo era” IUE data (all from ∼ 10◦ solar phase angle) are in better agreement with the HST and Galileo data at similar phase angles, when present, than with the “Voyager era” IUE data. For these two hemispheres, therefore, we have separated the overall solar phase curves into a Voyager era phase curve and a Galileo era phase curve. The trailing hemisphere data set does not show this dichotomy: the HST observation agrees with the Voyager era IUE data, as do the IUE data (at 10◦ phase) from the Galileo era. The anti-jovian hemisphere IUE observations from the Galileo era (at ∼ 10◦ phase) indicate a lower reflectance than for the Voyager era data; however, since we do not have HST or Galileo UVS data for this hemisphere to fill out the Galileo era phase curve, we model only the Voyager era anti-jovian hemisphere phase curve. The Voyager era solar phase curves for the Europa global, leading hemisphere and jovian hemisphere data sets do not include any large solar phase angle observations. For this analysis, the Galileo era large phase angle observations were used with the Voyager-era small phase angle data; this may affect interpretation of results. No temporal variations were noted in the Ganymede and Callisto data sets, which could be due to the paucity of observations of these satellites. In particular, no Galileo UVS observations at small phase angles exist for comparison with IUE and HST.

3.2. Solar phase curves

3.3. Hapke analysis

To generate solar phase curves for each satellite, rotational phase corrections were applied based on the solutions to the rotational phase curve equation described above. For Europa, the data set was divided into four groups as a func-

The resulting solar phase curves for each wavelength bin were modeled using Hapke’s model (Hapke, 1984, 1986, 1993, Eq. (8.89)). Because our data sets are sparse in very small phase angle data, we use a simple form of the Hapke

36

A.R. Hendrix et al. / Icarus 173 (2005) 29–49

model that assumes that the opposition surge is created by the shadow-hiding process and does not account for the coherent-backscatter process. This will affect the interpretation of the results. However, this modeling effort is similar to that published for the visible solar phase curves of the icy galilean satellites (Domingue and Verbiscer, 1997) and will allow for interpretations based on comparisons with past modeling efforts of these and other objects within the Solar System (such as the Moon). The disk-resolved reflectance is integrated over the disk of the body to result in a disk-integrated brightness Φ(α), as given by Hapke (1984, 1986)  
  r0 ω Φ(α) = P (α) 1 + B(α) − 1 + (1 − r0 ) 8 2       α α α × 1 − sin tan ln cot 2 2 4 

2 2 sin(α) + (π − α) cos(α) + r0 , 3 π which is multiplied by the geometric albedo Ap and the k term (for surface roughness) in this model (after Domingue et al., 1998), where   ω r0 r0 Ap = (1 + B0 )p(0) − 1 + C 1+ , 8 2 3   α tan Θ¯ k = exp − 0.32Θ¯ tan 2  α − 0.52Θ¯ tan Θ¯ tan , 2 
C = 1 − 0.048Θ¯ + 0.0041Θ¯ 2 r0 
− 0.33Θ¯ − 0.0049Θ¯ 2 r02 , √ 1− 1−ω r0 = √ . 1+ 1−ω The Hapke model parameters include ω (single scattering albedo), B0 (shadow-hiding opposition amplitude), h (shadow-hiding opposition half-width), Θ¯ (surface roughness), and the single particle scattering function parameters b and c. The terms µ and µ0 are the cosines of the emission and incidence angles, respectively. The single particle scattering function, P (α), used in this analysis is the double Henyey–Greenstein function of the form P (α) =

(1 − c)(1 − b2 ) c(1 − b2 ) + . [1 − 2b cos α + b2 ]3/2 [1 + 2b cos α + b2 ]3/2

The Hapke model was fit to each wavelength data set using a root mean square grid search. This same algorithm and software was used to fit the Hapke model to the visible solar phase curve observations of these satellites (Domingue and Verbiscer, 1997). The best-fit values of each parameter in the Hapke model were found by minimizing the RMS values between the

modeled reflectance (Rm ) and the observed reflectance (Ro ):  (Rm − Ro )2 RMS = . N Because none of the data sets has observations at phase angles
105◦ , and observations at phase angles > 90◦ (preferably > 120◦ ) are needed to constrain the Θ¯ parameter, we chose to hold Θ¯ constant. For Europa and Ganymede, we used Θ¯ values from the visible wavelength analysis (using values from Domingue and Verbiscer (1997) of Θ¯ = 10◦ for Europa and 28.5◦ for Ganymede). For Callisto, we initially used the visible Θ¯ value of 42◦ , but found that a smaller value produced better fits to the UV solar phase curves (see below). For each data set, four test cases were run initially to understand how well each parameter was constrained: (1) Θ¯ was held constant at its visible value, while all other parameters were allowed to vary; (2) Θ¯ and B0 were held constant at their visible values, while all other parameters were allowed to vary; (3) Θ¯ and h were held constant at their visible values, while all other parameters were al¯ B0 , and h were held constant at their lowed to vary; (4) Θ, visible values, while all other parameters were allowed to vary. For each case, we initially varied each parameter on a coarse grid; the case was run again on a finer grid over ranges of each parameter covering 1% of the original RMS value. The initial range covered for each parameter was as follows: ω was 0.1–0.80, with an initial step size of 0.05; b and c were 0–1.0, with an initial step size of 0.1; B0 was 0–1.0, with an initial step size of 0.1; h was 0–0.05, with an initial step size of 0.01. The results of these four test cases showed that, for each of the data sets, the ω, b, and c terms did not vary significantly from case to case. Final values of ω, b, and c were determined by averaging the results from the four cases. The resultant values of ω, b, and c were used in a final test case where B0 and h were varied on a fine grid (h step size of 0.0001 and B0 step size of 0.01) to get final values for those parameters. For Ganymede and Callisto, the data sets were much sparser in phase angle coverage than the Europa data sets, particularly at high and very low phase angles, so additional test cases were run. With just a single data point at α < 5◦ , it is impossible to constrain the h term; we therefore chose to hold the h term constant for Ganymede and Callisto, at the average h value obtained for Europa of 0.005. For Ganymede, we ran two additional test cases, where h was held constant and all other parameters were allowed to vary. The resultant Θ¯ values were consistent with the average visible value of 28.5◦, so in the final Ganymede case, we held Θ¯ constant at 28.5◦ , h constant at 0.005 and varied the other parameters to get the best fit values. Similarly, for Callisto we ran two additional test cases, where h was held constant at the average Europa value of 0.005 and all other parameters were allowed to vary. At all wavelengths, the best fit value of Θ¯ was 0◦ , indicating that this data set does not allow us to place constraints on the

UV solar phase curves

37

Table 4 Single-particle scattering terms: Europa global data set 260 nm Voyager

270 nm Galileo

Voyager

280 nm Galileo

Voyager

290 nm Galileo

Voyager

300 nm Galileo

Voyager

310 nm Galileo

Voyager

320 nm Galileo

Voyager

Galileo

ω

0.53 0.68 0.53 0.48 0.63 0.52 0.56 0.56 0.64 0.60 0.66 0.64 0.68 0.68 (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) (0.02) b 0.40 0.70 0.42 0.30 0.48 0.30 0.33 0.25 0.40 0.30 0.40 0.25 0.38 0.20 (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) c 0.63 0.95 0.45 0.15 0.60 0.15 0.08 0.0 0.28 0.10 0.20 0.0 0.15 0.0 (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) rms 0.01099 0.01128 0.01149 0.01252 0.01116 0.00967 0.01622 0.01410 0.01625 0.01479 0.02042 0.01714 0.02339 0.01743

Table 5 Single-particle scattering terms: Ganymede and Callisto global data sets

Ganymede

Callisto

ω b c rms ω b c rms

260 nm

270 nm

280 nm

290 nm

300 nm

310 nm

320 nm

0.21 (0.03) 0.37 (0.05) 0.0 (0.02) 0.0161893 0.21 (0.03) 0.0 (0.05) 0.0 (0.10) 0.00657927

0.28 (0.03) 0.34 (0.05) 0.0 (0.03) 0.0130031 0.24 (0.03) 0.0 (0.05) 0.0 (0.10) 0.00422833

0.25 (0.03) 0.40 (0.05) 0.0 (0.03) 0.0148896 0.22 (0.03) 0.08 (0.05) 0.06 (0.10) 0.00497429

0.34 (0.03) 0.33 (0.05) 0.0 (0.07) 0.0133461 0.25 (0.03) 0.06 (0.05) 0.08 (0.05) 0.00383564

0.33 (0.03) 0.36 (0.05) 0.0 (0.07) 0.0141358 0.24 (0.03) 0.10 (0.05) 0.0 (0.05) 0.00380669

0.37 (0.03) 0.35 (0.05) 0.0 (0.07) 0.0142390 0.24 (0.03) 0.13 (0.05) 0.0 (0.02) 0.00376817

0.36 (0.03) 0.36 (0.05) 0.01 (0.07) 0.0127491 0.26 (0.03) 0.11 (0.05) 0.0 (0.07) 0.00384310

macroscopic roughness term. In other test cases (described above), when we held Θ¯ constant at the visible value of 42◦ , the single-scatter albedos were unrealistically high. We chose to hold Θ¯ constant at 0◦ and h constant at 0.005 for our final test case for Callisto; the other parameters were varied to obtain the best fit values. 3.4. Error bars on Hapke photometric parameters The accuracy to which any photometric model parameters can be determined is highly dependent on the quality of the data set being modeled. The initial error bars for all data sets were as follows: ±0.01 for ω, ±0.02 for b and c, ±0.10 for B0 , and ±0.01 for h. These initial error bars correspond to the fine grid step sizes that were used in these analyses. After completing the least-squares fit analyses, the error bars were reviewed to consider scatter in the data sets and phase angle coverage in the solar phase curves. Final error bars on each parameter (listed in parentheses in Tables 4–9) were estimated 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 in the data. We also judged the constraints on the variation in any single parameter based on the phase angle coverage in each data set. In order to constrain the values of the single particle scattering function terms (ω, b, and c), information is needed in both the forward scattering (α > 100◦ ) and the backward scattering (α < 30◦ ) directions. The opposition term B0 requires information between 0◦ and 30◦ , while the opposition term h requires several data points at α < 5◦ in order to be well constrained.

The Europa global data set (Fig. 3a) from the Voyager era includes 3 observations at phase angles < 1◦ (0.1◦ , 0.14◦ , and 0.42◦ ) and 6 observations in the 8◦ –11◦ range. The Galileo era data set includes one observation at 1.5◦ phase and 4 observations in the 4◦ –11◦ range. Both the Voyager era and the Galileo era global Europa data sets used 14 Galileo observations in the 40◦ –80◦ range, and 4 observations in the 90◦ –105◦ range. The scatter in the Galileo observations in the 40◦ –100◦ range constrained the error on the singlescatter terms ω, b, and c for these two data sets. For both the Voyager era and the Galileo era data sets, the final error bars on ω were set at ±0.02, while the error bars on the b and c terms were set at ±0.03. We used the observations at α < 30◦ to determine the error bars on the B0 term. In the Voyager era data set, the error was as low as ±0.04 (at 300 nm) to ±0.15 (at 260 nm, where there is more scatter in the data). In the Galileo era data set, the B0 error bars are between ±0.04 and ±0.10. For both the Voyager era and the Galileo era data sets, the error bars on the h term were set at double the best-fit value for each wavelength, as there were few small phase angle observations to constrain this parameter. Both the Ganymede and Callisto data sets (Figs. 3b and 3c, respectively) include one observation at solar phase angle < 1◦ (0.1◦ for Ganymede, 0.37◦ for Callisto), several observations (10 for Ganymede, 8 for Callisto) in the 3◦ –11◦ phase angle range, and one Galileo observation at a larger phase angle (77◦ for both Ganymede and Callisto). At all wavelengths, the Ganymede and Callisto data in the 3◦ –11◦ phase angle range include a significant amount of scatter. In the Callisto data set, the IUE data in the 3◦ –11◦ range appear to smoothly increase with decreasing wavelength, with the

38

Table 6 Single-particle scattering terms: Europa hemispheric data sets 260 nm Leading

ω b

Trailing

rms ω b c

Jovian

rms ω b c

Antijovian

rms ω b c rms

280 nm

290 nm

300 nm

310 nm

320 nm

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

0.42 (0.02) 0.25 (0.03) 0.10 (0.03) 0.00909 0.25 (0.02) 0.40 (0.03) 0.45 (0.03) 0.005008 0.40 (0.02) 0.30 (0.03) 0.60 (0.03) 0.00566 0.31 (0.02) 0.40 (0.03) 0.33 (0.03) 0.00466

0.42 (0.02) 0.25 (0.03) 0.13 (0.03) 0.01274

0.49 (0.02) 0.34 (0.03) 0.24 (0.03) 0.01035 0.28 (0.02) 0.38 (0.03) 0.45 (0.03) 0.004073 0.37 (0.02) 0.36 (0.03) 0.38 (0.03) 0.00676 0.32 (0.02) 0.38 (0.03) 0.02 (0.03) 0.005105

0.48 (0.02) 0.30 (0.02) 0.15 (0.03) 0.03052

0.52 (0.02) 0.35 (0.03) 0.23 (0.03) 0.00886 0.360 (0.02) 0.45 (0.03) 0.65 (0.03) 0.003473 0.45 (0.02) 0.42 (0.03) 0.58 (0.03) 0.00604 0.33 (0.02) 0.41 (0.03) 0.05 (0.03) 0.00574

0.51 (0.02) 0.29 (0.03) 0.08 (0.03) 0.01054

0.57 (0.02) 0.32 (0.03) 0.02 (0.03) 0.01497 0.33 (0.02) 0.38 (0.03) 0.40 (0.03) 0.005001 0.44 (0.02) 0.32 (0.03) 0.13 (0.03) 0.00827 0.40 (0.02) 0.38 (0.03) 0.03 (0.03) 0.008995

0.59 (0.02) 0.26 (0.03) 0.04 (0.03) 0.01516

0.60 (0.02) 0.34 (0.03) 0.05 (0.03) 0.01656 0.38 (0.02) 0.43 (0.03) 0.47 (0.03) 0.005761 0.48 (0.02) 0.34 (0.03) 0.19 (0.03) 0.00928 0.44 (0.02) 0.41 (0.03) 0.04 (0.03) 0.008015

0.63 (0.02) 0.26 (0.03) 0.03 (0.03) 0.01525

0.67 (0.02) 0.37 (0.03) 0.17 (0.03) 0.01802 0.42 (0.02) 0.425 (0.03) 0.43 (0.03) 0.009201 0.52 (0.02) 0.34 (0.03) 0.15 (0.03) 0.01438 0.48 (0.02) 0.40 (0.03) 0.01 (0.03) 0.007547

0.67 (0.02) 0.24 (0.03) 0.04 (0.03) 0.01746

0.68 (0.02) 0.32 (0.03) 0.04 (0.03) 0.01848 0.44 (0.02) 0.38 (0.03) 0.35 (0.03) 0.01032 0.60 (0.02) 0.34 (0.03) 0.20 (0.03) 0.01639 0.52 (0.02) 0.40 (0.03) 0.15 (0.03) 0.008883

0.69 (0.02) 0.20 (0.03) 0.0 (0.03) 0.01738

0.50 (0.02) 0.47 (0.03) 0.82 (0.03) 0.00676

0.36 (0.02) 0.31 (0.03) 0.32 (0.03) 0.00709

0.40 (0.02) 0.30 (0.03) 0.35 (0.03) 0.00484

0.44 (0.02) 0.23 (0.03) 0.08 (0.03) 0.00822

0.47 (0.02) 0.24 (0.03) 0.08 (0.03) 0.02257

0.52 (0.02) 0.21 (0.03) 0.10 (0.10) 0.01050

0.59 (0.02) 0.17 (0.03) 0.08 (0.03) 0.01192

A.R. Hendrix et al. / Icarus 173 (2005) 29–49

c

270 nm

UV solar phase curves

39

Table 7 Opposition effect terms: Europa global data set 260 nm Voyager

270 nm Galileo

Voyager

280 nm Galileo

Voyager

290 nm Galileo

Voyager

300 nm Galileo

Voyager

310 nm Galileo

320 nm

Voyager

Galileo

Voyager

Galileo

B0

0.48 0.14 0.62 0.15 0.47 0.16 0.71 0.44 0.56 0.20 0.61 0.05 0.66 0.22 (0.15) (0.10) (0.06) (0.08) (0.05) (0.04) (0.09) (0.08) (0.04) (0.40) (0.05) (0.06) (0.09) (0.05) S(0) 0.42 0.10 0.80 0.17 0.70 0.19 1.11 0.55 1.04 0.29 1.28 0.07 1.36 0.28 h 0.012 0.050 0.004 0.045 0.004 0.044 0.008 0.014 0.005 0.01 0.003 0.049 0.003 0.011 (0.012) (0.050) (0.004) (0.045) (0.004) (0.044) (0.008) (0.014) (0.005) (0.01) (0.003) (0.049) (0.003) (0.011) rms 0.01099 0.01128 0.01149 0.01252 0.01116 0.00967 0.01622 0.01410 0.01625 0.01479 0.02042 0.01714 0.02339 0.01743

Table 8 Opposition effect terms: Ganymede and Callisto global data sets

Ganymede

C17 allisto

B0 S(0) rms B0 S(0) rms

260 m

270 nm

280 nm

290 nm

300 nm

310 nm

320 nm

0.35 (0.15) 0.25 0.0161893 0.74 (0.20) 0.16 0.00657927

0.60 (0.17) 0.52 0.0130031 1.0 (0.12) 0.24 0.00422833

0.45 (0.17) 0.44 0.0148896 1.0 (0.17) 0.28 0.00497429

0.55 (0.12) 0.55 0.0133461 1.0 (0.12) 0.29 0.00383564

0.55 (0.12) 0.60 0.0141358 1.0 (0.12) 0.33 0.00380669

0.55 (0.12) 0.65 0.01423(0.4790) 1.0 (0.12) 0.36 0.00376817

0.75 (0.12) 0.89 0.0127491 1.0 (0.12) 0.36 0.00384310

The h term was set to 0.005 at all wavelengths for both Ganymede and Callisto.

two HST observations somewhat offset; in the Ganymede data set, there is scatter even within the IUE observations. The scatter in these data sets translates to a lower amount of certainty in the single-particle scattering terms derived in the Hapke analysis. The entire range of phase angles is affected by variations in ω. The error bar on that term was set to ±0.03 for Ganymede and Callisto. Because no observations exist for Ganymede and Callisto in the ∼ 15–70◦ range, it is difficult to place constraints on the b term, and the error bars on that term were set at ±0.05. The final error bars on the c term were between ±0.02 and ±0.07 for Ganymede, and between ±0.02 and ±0.10 for Callisto, dependent upon wavelength. The scatter in the observations at α < 30◦ was used to constrain the B0 term. For Ganymede, the error on the B0 term was as low as ±0.12 for the longer wavelengths, and up to ±0.20 at 260 nm where there was increased scatter in the data. For Callisto, the error bar on the B0 term was ±0.12, except at 260 nm where it was ±0.20 due to a greater amount of scatter in the data. The Europa leading hemisphere data set (Fig. 4a) from the Voyager era includes two observations at phase angles < 1◦ (at 0.1◦ and 0.42◦ ) and 2 observations in the 8◦ –11◦ range. The Galileo era data set includes one observation at 1.5◦ phase, 5 observations in the 4◦ –11◦ phase angle range, 9 observations in the 40◦ –70◦ phase angle range, and 2 observations in the 90◦ –95◦ phase angle range. The Europa trailing hemisphere data set (Fig. 4b) includes two observations at phase angles < 1◦ (0.1◦ and 0.14◦), 3 observations in the 8◦ –11◦ range, 5 observations in the 40◦ –80◦ range, and 2 observations in the 95◦ –105◦ range. The Europa jovian hemisphere data set (Fig. 4c) from the Voyager era includes 2 observations at phase angles < 1◦ (0.1◦ and 0.14◦ ) and 2 observations in the 8◦ –11◦ phase angle range. The Galileo era

data set includes one observation at 1.5◦ phase, 4 observations in the 4◦ –11◦ phase angle range, 11 observations in the 40◦ –80◦ phase angle range, and 2 observations in the 95◦ – 105◦ phase angle range. The Europa anti-jovian hemisphere data set (Fig. 4d) includes two observations at phase angles < 1◦ (at 0.1◦ and 0.42◦ ), 2 observations in the 8◦ –11◦ range, 4 observations in the 40◦ –60◦ range, and 3 observations in the 90◦ –105◦ range. This amount of phase angle coverage, and the scatter in the hemispheric data sets, allowed us to set the error bars on the hemispheric single-scattering terms at ±0.02 for ω and ±0.03 for b and c, as with the global Europa data set. The error bars on the hemispheric B0 terms varied between ±0.05 and ±0.12, depending on scatter in the data set at any wavelength, while the error bars on the h terms were again set at double the best-fit value.

4. Results The best-fit Hapke parameters determined in this analysis are given in Tables 4–9. Examples of the solar phase curves and corresponding Hapke model solutions are shown in Figs. 3 and 4. Figure 3 shows the global solar phase curves at 300 nm for Europa, Ganymede and Callisto. The global solar phase curves were corrected to the central longitude of the leading hemisphere, 90◦ W, using the rotational phase curve corrections. For Europa, Fig. 3a shows the solar phase curve and model fits for both the Voyager era and the Galileo era data sets. Figure 4 shows the solar phase curves at 300 nm for the Europa hemispheric data sets. For Europa’s leading and jovian hemispheres, data were acquired in both the Voyager and Galileo eras; those solar phase curves and model fits are plotted (Figs. 4a, 4c).

40

Table 9 Opposition Effect terms: Europa hemispheric data sets 260 nm Leading

B0

Trailing

rms B0 S(0) h

Jovian

rms B0 S(0) h

Antijovian

rms B0 S(0) h rms

280 nm

290 nm

300 nm

310 nm

320 nm

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

Voyager

Galileo

0.45 (0.05) 0.39 0.042 (0.042) 0.00909 0.51 (0.10) 0.29 0.002 (0.002) 0.005008 0.64 (0.05) 0.34 0.038 (0.038) 0.00566 0.56 (0.07) 0.47 0.003 (0.003) 0.00466

0.52 (0.12) 0.44 0.05 (0.05) 0.01274

0.51 (0.05) 0.61 0.02 (0.02) 0.01035 0.64 (0.05) 0.37 0.022 (0.022) 0.004073 0.51 (0.06) 0.41 0.007 (0.007) 0.00676 0.54 (0.05) 0.59 0.004 (0.004) 0.005105

0.69 (0.15) 0.77 0.005 (0.005) 0.03052

0.56 (0.05) 0.75 0.04 (0.04) 0.00886 0.42 (0.05) 0.28 0.011 (0.011) 0.003473 0.44 (0.05) 0.39 0.011 (0.011) 0.00604 0.42 (0.05) 0.53 0.005 (0.005) 0.00574

0.34 (0.07) 0.42 0.006 (0.006) 0.01054

0.64 (0.05) 1.03 0.009 (0.009) 0.01497 0.62 (0.05) 0.47 0.021 (0.021) 0.005001 0.60 (0.05) 0.67 0.005 (0.005) 0.00827 0.45 (0.05) 0.61 0.012 (0.012) 0.008995

0.55 (0.07) 0.71 0.005 (0.005) 0.01516

0.54 (0.05) 0.94 0.023 (0.023) 0.01656 0.54 (0.05) 0.51 0.018 (0.018) 0.005761 0.54 (0.05) 0.67 0.011 (0.011) 0.00928 0.40 (0.05) 0.67 0.005 (0.005) 0.008015

0.38 (0.07) 0.53 0.005 (0.005) 0.01525

0.46 (0.05) 0.90 0.008 (0.008) 0.01802 0.54 (0.05) 0.59 0.013 (0.013) 0.009201 0.58 (0.05) 0.80 0.014 (0.014) 0.01438 0.42 (0.05) 0.75 0.005 (0.005) 0.007547

0.40 (0.07) 0.56 0.002 (0.002) 0.01746

0.55 (0.05) 1.01 0.015 (0.015) 0.01848 0.66 (0.07) 0.70 0.020 (0.020) 0.01032 0.61 (0.05) 0.91 0.007 (0.007) 0.01639 0.52 (0.05) 0.91 0.004 (0.004) 0.008883

0.60 (0.07) 0.76 0.009 (0.009) 0.01738

0.40 (0.15) 0.23 0.005 (0.005) 0.00676

0.40 (0.12) 0.29 0.005 (0.005) 0.00709

0.40 (0.10) 0.30 0.005 (0.005) 0.00484

0.40 (0.10) 0.34 0.005 (0.005) 0.00822

0.40 (0.07) 0.37 0.005 (0.005) 0.02257

0.45 (0.10) 0.42 0.005 (0.005) 0.01050

0.50 (0.10) 0.47 0.005 (0.005) 0.01192

A.R. Hendrix et al. / Icarus 173 (2005) 29–49

S(0) h

270 nm

UV solar phase curves

(a) Europa

41

(b) Ganymede

(c) Callisto Fig. 3. Global solar phase curves for Europa, Ganymede and Callisto at 300 nm, corrected to 90◦ W, with Hapke fits overplotted. Error bars represent error in measurements, with rotational corrections applied. The error bars do not include error due to the small amount (< 10%) of solar variability across solar cycle. (a) Europa solar phase curve at 300 nm. Voyager era IUE data: The 0.1◦ point is the average of two observations. The point at 8.47◦ is the average of 20 observations. The point at 9.2◦ is the average of 6 observations. The point at 10.4◦ is the average of 15 observations. The point at 11.1◦ is the average of 5 observations. Galileo era IUE data: The 10.4◦ point is the average of 5 observations; (b) Ganymede solar phase curve at 300 nm: The 6.4◦ point is the average of 2 observations. The 8.4◦ point is the average of 5 observations. The 9.1◦ point is the average of 2 observations. The 10.47◦ point is the average of 25 observations. The 11.1◦ point is the average of 8 observations. (c) Callisto solar phase curve at 300 nm: The 2.9◦ point is the average of 4 observations. The 6.7◦ point is the average of 4 observations. The 7.8◦ point is the average of 2 observations. The 8.3◦ point is the average of 9 observations. The 10.47◦ point is the average of 39 observations. The 11.1◦ point is the average of 4 observations.

The best-fit Hapke parameters were used to compute the UV spectral geometric albedo for the global Europa, Ganymede, and Callisto data sets (Fig. 5a) and for the Europa hemispheric data sets (Fig. 5b). These geometric albedos are consistent with measurements from IUE (Nelson et al., 1987) and HST (Noll et al., 1996, 1997). The global geometric albedo of Europa is significantly higher and steeper (spectrally red) than the global geometric albedo of both Ganymede and Callisto. Ganymede’s global geometric albedo is higher and slightly redder in shape than Callisto’s. The geometric albedo of Europa’s leading hemisphere is significantly higher than that of the trailing hemisphere (Fig. 5b), while the geometric albedo of the anti-jovian hemisphere is only slightly higher than that of the jovian hemisphere. The geometric albedo of the trailing hemisphere displays a hint of the absorption

feature at 280 nm (Lane et al., 1981; Noll et al., 1995; Hendrix et al., 1998), while the albedo of the leading hemisphere flattens out spectrally toward the longer wavelengths, likely due to greater amounts of hydrogen peroxide on that hemisphere (Carlson et al., 1999). The longitudinal variations in geometric albedo at each wavelength (Fig. 5b) mimic the derived rotational phase curves (Fig. 2a). 4.1. Single-particle scattering terms The best-fit single-scatter albedos and single particle scattering function terms b and c are given in Tables 4–6 for the global and Europa hemispheric data sets, respectively. Europa’s single-scatter albedo is the highest of the three satellites, and increases with wavelength, consistent with this satellite’s geometric albedo (Fig. 5a), although the spectral

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(a) Europa leading hemisphere

(b) Europa trailing hemisphere

(c) Europa jovian hemisphere

(d) Europa anti-jovian hemisphere

Fig. 4. Hemispheric solar phase curves for Europa at 300 nm, with Hapke models overplotted. Error bars represent error in measurements, with rotational corrections applied. The error bars do not include error due to the small amount (< 10%) of solar variability across solar cycle. (a) Europa leading hemisphere. The 10.4◦ point is an average of 5 IUE observations (from the Galileo era). The 10.6◦ point is an average of 11 IUE observations (from the Voyager era). The 8.5◦ point is the average of 10 observations (Voyager era). (b) Europa trailing hemisphere. The 8.64◦ point is the average of 15 IUE observations. The 10.48◦ point is the average of 13 IUE observations. (c) Europa jovian hemisphere. The 8.6◦ point is the average of 14 IUE observations (Voyager era). The 10.6◦ point is the average of 15 IUE observations (Voyager era). The 10.6◦ point is the average of 2 observations (Galileo era). (d) Europa anti-jovian hemisphere. The 8.47◦ point is the average of 11 IUE observations. The 10.38◦ point is the average of 12 IUE observations. The HST observations are from 294◦ W and 81◦ W, so no HST data point appears in the anti-jovian hemisphere phase curve.

shape of the single-scatter albedo is not as red as the geometric albedo. Ganymede’s single-scatter albedo is ∼ 40% higher than Callisto’s at 290–320 nm. At visible wavelengths (0.47 µ), the single-scatter albedos (averaged from the leading and trailing hemispheres) of Europa, Ganymede and Callisto are 0.92, 0.85, and 0.61, respectively (Domingue and Verbiscer, 1997). Ganymede thus has the steepest-sloped (reddest) single-scatter albedo in the UV–visible wavelength range. Europa’s leading hemisphere has a higher single-scatter albedo than the trailing hemisphere, while the jovian and anti-jovian hemispheres have intermediate single-scatter albedos, consistent with the geometric albedos of these hemispheres (Fig. 5b). The jovian hemisphere has a slightly higher single-scatter albedo (ω ∼ 0.47 at 300 nm) than the anti-jovian hemisphere (ω ∼ 0.44 at 300 nm). Compared to the visible results of Domingue and Verbiscer (1997),

who obtained ω values for Europa’s leading and trailing hemispheres of 0.94 and 0.9 at 0.47 µ, respectively, the UV trailing hemisphere has a much lower single-scatter albedo relative to the visible, than does the leading hemisphere. The UV–visible spectral slope of the trailing hemisphere is therefore steeper than the leading hemisphere. Single-particle scattering functions are shown plotted in Fig. 6 (global data sets at 300 nm) and Fig. 7a (Europa hemispheric data sets at 300 nm). Ganymede is more backscattering, and less forward scattering, than Europa. Callisto’s single-particle scattering function indicates that it is dramatically less backscattering than Europa and Ganymede. At visible wavelengths, Domingue and Verbiscer (1997) found that Europa’s leading and trailing hemispheres are more backscattering than the leading and trailing hemispheres of the other satellites, while Callisto’s leading hemisphere is the least backscattering. Thus, we find a difference between

UV solar phase curves

43

(a) Fig. 6. Global single-particle scattering functions for Europa (Voyager era and Galileo era), Ganymede, Callisto at 300 nm.

(b)

more backscattering than the trailing hemisphere (Fig. 7a), which is the same trend as seen at visible wavelengths (Domingue and Verbiscer, 1997), while the anti-jovian hemisphere is significantly more backscattering than the jovian hemisphere. Figures 6 and 7b indicate that globally, Europa’s surface was less backscattering and less forward scattering during the Galileo era than in the Voyager era. The Voyager era leading hemisphere is more backscattering than the Galileo era leading hemisphere; similarly, we find that the Voyager era jovian hemisphere is more backscattering than the Galileo era jovian hemisphere (Fig. 7b). The discrepancy in reflectance on Europa’s leading and jovian hemispheres, between the Voyager era IUE data and the Galileo era IUE data (Figs. 4a, 4c), which agree with the HST and Galileo data, is not measured in the single-scatter albedo. The single-scatter albedos for these data sets are very close (within 5% for the leading hemisphere, and within 12% for the jovian hemisphere, except at 260 nm). The difference in the reflectances is due to variations in the single-particle scattering functions in the two epochs. 4.2. Opposition effect terms

(c) Fig. 5. Geometric albedos determined using the best-fit Hapke parameters (from the Voyager era for Europa). (a) Global geometric albedos for Europa, Ganymede and Callisto. Geometric albedos of Europa’s leading, trailing, jovian and anti-jovian hemispheres (b) vs. wavelength; (c) vs. longitude. Representative error bars for each data set are shown, which correspond to the error in the derived Hapke parameters.

the backscattering properties of Ganymede and Europa at UV and visible wavelengths, where Ganymede becomes more backscattering and Europa becomes less backscattering at UV wavelengths compared to visible wavelengths. In both wavelength ranges, Callisto is the least backscattering of the icy satellites. The leading hemisphere of Europa is

The best-fit shadow-hiding opposition effect terms B0 and h are listed in Tables 7–9 for the global data sets and the Europa hemispheric data sets, respectively. The opposition effect amplitude B0 values for Europa (Voyager era) and Ganymede are in the 0.4–0.7 range. The Callisto B0 values are higher than for Europa and Ganymede, at ∼ 1.0. The B0 values for the Europa hemispheric data sets are all in the 0.4–0.7 range as well, with the jovian hemisphere B0 values higher than the anti-jovian hemisphere B0 values at each wavelength. These results are consistent with the visible results of Domingue and Verbiscer (1997), who found higher B0 values for Callisto, similar values for Europa and Ganymede, and similar values for Europa’s leading and trailing hemispheres.

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(a) (a)

(b)

(b) Fig. 7. (a) Europa hemispheric single-particle scattering functions at 300 nm (Voyager era); (b) Europa leading and jovian hemisphere single-particle scattering functions: Voyager era vs. Galileo era.

To understand these results, we consider the B0 parameter in terms of its components, 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. The B0 term is thus a measure of the opacity of particles, with more opaque particles exhibiting lower amounts of surface scattering. Figure 8a shows the ωp(0) values vs. B0 values for the global data sets in this study. At each wavelength, Europa has higher ωp(0) values than Ganymede, while Callisto’s ωp(0) values are the lowest of the three satellites. This indicates that Europa’s particles reflect the most total (surface plus internal) light. Callisto, with the highest B0 values, has the most opaque particles. Because Europa’s particles are slightly more opaque (higher B0 values at wavelengths except 320 nm) than Ganymede’s, this suggests that Europa’s particles scatter more light from their surfaces than do Ganymede’s particles. This is also indicated in the S(0) values listed in Tables 7 and 8.

(c) Fig. 8. The total scattered light (internal plus surface scattering) from particles, ωp(0), plotted vs. the B0 term (amplitude of shadow-hiding opposition effect). (a) Global data sets; (b) Europa leading and trailing hemispheres; (c) Europa jovian and anti-jovian hemispheres. As discussed in the text, the relative ωp(0) value corresponds to the relative importance of charged particle bombardment vs. annealing on the surfaces.

Figure 8b displays ωp(0) vs. B0 for Europa’s leading and trailing hemispheres at each UV wavelength. At each wavelength, Europa’s trailing hemisphere has lower ωp(0) values than the leading hemisphere, along with similar B0 values. This suggests that the leading and trailing hemisphere parti-

UV solar phase curves

cles have similar opacities, but that the leading hemisphere particles scatter more light from their surfaces. This is also indicated in the S(0) values listed in Table 9. Figure 8c displays ωp(0) vs. B0 for Europa’s jovian and anti-jovian hemispheres. Europa’s jovian hemisphere has lower ωp(0) values than the anti-jovian hemisphere at each wavelength, with higher B0 values (except at 280 nm). This suggests that the particles on the anti-jovian hemisphere scatter more light both internally and externally. But because the S(0) values are similar (for wavelengths of 290 nm and longer), this suggests that the particles on the anti-jovian hemisphere scatter more light internally than those on the jovian hemisphere. Europa’s opposition surge width (characterized by the h term) is narrow, with an average value of 0.005. The Europa jovian hemisphere has a slightly wider angular opposition surge (average h value of 0.008) than the anti-jovian hemisphere (average h values of 0.005). The h values for Europa’s leading and trailing hemispheres are higher (with average values of 0.02 and 0.015, respectively) than for the jovian and anti-jovian hemispheres. Domingue and Verbiscer (1997) found h values for Europa of ∼ 0.0016, consistent with the UV results obtained here. Qualitatively, because the h term is related to surface porosity, we find that on a global scale, Europa’s anti-jovian hemisphere is more porous than the jovian hemisphere, while the trailing hemisphere is more porous than the leading hemisphere. To determine quantitative porosities from the h results, we use the relationship h = −(3/8) ln(P )Y and assume a uniform grain size distribution (Domingue and Verbiscer, 1997). A lunar-like grain size distribution is √ Y = 3/ ln(rl /rs ), where rl /rs is the ratio of the effective radius of the largest grain to the effective radius of the smallest grain (Helfenstein and Veverka, 1987). Using this relationship and assuming rl /rs = 1000 with average h values, we find that Europa’s global porosity is ∼ 95%. Europa’s leading and trailing hemispheres have similar porosities at ∼ 81% and 82%, respectively. The anti-jovian hemisphere may have a somewhat greater porosity, at ∼ 95%, than the jovian hemisphere, at 92%. These porosity values are similar to previously obtained results at visible wavelengths (Domingue and Verbiscer, 1997), but we must note that the values are extremely high. These results suggest that a different grain size distribution should be used for icy satellites than that derived √ for the Moon. In particular, a relationship closer to Y = 0.04/ ln(rl/rs) would yield porosities in the ∼ 60% range. Another alternative is that the opposition surges of the icy satellites are caused not only by shadow hiding (as modeled in this study) but also by coherent backscatter. Recent work (Hapke et al., 1993, 1998; Nelson et al., 1998; Hapke, 2002) has shown that coherent backscatter is a very important contributor to the opposition effect, even in dark materials, so it is quite likely that this process contributes to the opposition effect at UV wavelengths. The contribution to the opposition effect by coherent backscatter may mean that the derived porosities are inaccurate. The quali-

45

tative porosities may remain unchanged, but the quantitative values may be unrealistically high. Helfenstein et al. (1998) used Galileo SSI camera data of Europa at very small phase angles ( 5◦ ) to measure both the shadow hiding and the coherent backscatter opposition surges on Europa in the visible. They state that a narrow coherent backscatter opposition surge is consistent with particles that are more transparent (less opaque) and does not require large porosities. For coherent backscatter, the angular width of the opposition surge should increase with decreasing particle albedo (Helfenstein et al., 1998). We do not detect this type of trend in our analysis, but cannot do a satisfactory job of measuring the coherent backscatter opposition effect with these data sets, since the small phase angle coverage is sparse. Clearly we are detecting the shadow-hiding opposition effect: Because we expect that multiply scattered photons illuminate particle shadows, the strength of the shadow hiding opposition surge should decrease with increasing albedo, which is what our results indicate. Coherent backscatter is also likely contributing to the opposition surges measured in this study, but without better small phase angle coverage, we cannot determine the contributions from each process individually.

5. Discussion As previously discussed, exogenic processes, particularly charged particle bombardment, significantly affect the surface chemistry of the icy galilean satellites, leading to products that absorb efficiently in the UV but not in the visible (e.g., SO2 , O3 , H2 O2 ). Exogenic processes also affect the structure of the icy surfaces, in turn affecting the scattering properties. In this section, we summarize the processes that could potentially affect the scattering properties of the icy galilean satellites; we then discuss the processes in relation to the results from our analysis. We expect to find variations in scattering properties of the icy satellites that can be related to the different experiences of each surface. The primary processes occurring in the Jupiter system that are expected to potentially affect the scattering properties of the satellites are: charged-particle bombardment, micrometeorite bombardment, annealing and ionian wind particle bombardment. We discuss these processes in light of the results found in this study to understand which processes are dominant. We expect to be able to understand how these processes affect the scattering properties of the icy surfaces by looking at the results in terms of satellite-to-satellite comparisons, longitudinal variations (on Europa), temporal variations (on Europa), and wavelength variations in Hapke parameters. 5.1. The effects of exogenic processes on scattering properties Charged particle bombardment in the jovian magnetosphere, as previously discussed, has been widely accepted

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to affect the trailing hemispheres of the satellites, and to have the strongest effect on Europa, which being the closest to Jupiter experiences the highest flux of particles, and the least effect on Callisto, with the most distant orbit and the lowest particle flux. Ganymede receives the second-highest dose, but is shielded to a certain extent by its magnetosphere. (Ganymede may receive a particularly large amount of charged particles along the open field lines in the polar regions.) The cold ions and energetic electrons of the magnetosphere preferentially bombard the trailing hemisphere, due to the higher rate of motion of the particles associated with the magnetosphere, compared with the orbital speeds of the satellites. However, flow vectors indicate that hot ions bombard the anti-jovian hemisphere preferentially (Paranicas et al., 2002). Furthermore, the tilt of the Jupiter’s magnetic field lines relative to the rotational poles of the satellites means that an asymmetry in introduced in the jovian/anti-jovian direction, which becomes more pronounced as the distance from Jupiter increases (Paranicas et al., 1999). Callisto, due to its distance from Jupiter, is not always fully immersed in the magnetosphere, but travels in and out due to the magnetospheric tilt (Cooper et al., 2001). One of the primary effects of ion bombardment of ices is the production of defects in the ice (Johnson, 1997; Johnson and Quickenden, 1997). The extended defects, in the form of voids and bubbles, affect the light scattering properties of the surfaces, in addition to trapping gases which can produce spectral absorption features. Incident ions deposit energy over a small region about their path through the ice, causing local damage and amortization; or, depending on the excitation density and cooling rate, they can cause local crystallization and grain growth. Crystalline ice exposed to UV radiation at cold temperatures (70 K) has been shown to convert to amorphous ice, which has then converted back to crystalline form upon warming to ∼ 150 K (Kouchi and Kuroda, 1990). Heavy (less-penetrating) damaging ions have been seen to brighten the surface in the visible by producing voids and bubbles (Sack et al., 1992). Depending on the size and density of these features, a clear surface can become brighter due to enhanced scattering in the visible. Electrons and the lower energy ions have smaller penetration depths, and the quality of the change produced in the surface can differ from that for the very fast ions (Johnson, 1997). The radiation damage produced by plasma bombardment brightens the low-temperature water ice in the polar regions of Ganymede, creating the polar caps (Johnson, 1997). This occurs on Ganymede due to the high flux of particles traveling along the magnetic field lines. On Io, the same process darkens the SO2 in the polar regions, due to chemical damage that results in products such as S2 that absorb efficiently in the UV and visible (Johnson, 1997). The plasma-induced brightening competes with annealing, which proceeds efficiently in regions where the temperature exceeds ∼ 100 K, making equatorial regions less light-scattering despite heavy plasma bombardment. Sack

et al. (1991) showed that annealing induced by ion bombardment can reduce the density of internal scatterers. Since the surface at low latitudes on Ganymede is partially protected from the plasma, the ion flux is low and annealing dominates, resulting in larger grains and hence, a darker surface. The icy satellites experience additional bombardment from the neutral species streaming radially out from Io. This bombardment preferentially impacts the jovian hemispheres. Finally, micrometeorites preferentially bombard the leading hemispheres of the satellites, introducing new material and also acting to garden the surfaces and produce finer particles. 5.2. Implications for phase curve analysis The processes discussed above can be related to the UV phase curve results presented here, to understand the photometric parameter variations in terms of longitudinal, temporal and satellite-to-satellite variations in scattering properties. These differences may be related to variations in exogenic processes that affect grain size, compaction and chemical makeup of the surface. 5.2.1. Variations in scattering properties with satellite and longitude We find that the measured ωp(0) values, representing the total (internal plus surface) scattered light from particles on each of the satellites, correspond directly to the relative amount of charged particle bombardment experienced by the satellites (Fig. 8a). Europa receives the highest flux of particles and has the highest ωp(0) values at each wavelength, while Callisto receives the lowest flux and correspondingly has the lowest ωp(0) values. This suggests that the amount of total scattering by the particles on the icy satellites is directly related to the amount of charged particle bombardment seen by the surface and is consistent with the results of Sack et al. (1992). As shown in Fig. 8b, we find that Europa’s leading hemisphere has consistently higher ωp(0) values than the trailing hemisphere. From the Fig. 8a discussion above, this suggests that the leading hemisphere experiences more charged particle bombardment than the trailing hemisphere, which is not expected to be the situation. We must therefore consider the amount of annealing occurring on the surface, which would lower the amount of total scattering, and counteract the effects of the charged particle bombardment. We propose that although the trailing hemisphere may actually experience higher amounts of bombardment than the leading hemisphere, its ωp(0) values are lower due to greater amounts of annealing occurring on the trailing hemisphere. This annealing could be the result of higher surface temperatures on the trailing hemisphere, due to the lower albedo of the dark material covering that hemisphere, or due to the high amounts of bombardment itself. The effects of micrometeorite bombardment on Europa’s leading hemisphere may also contribute to that hemisphere’s more backscattering nature. However, we do not see significant effects of mi-

UV solar phase curves

crometeorite bombardment in the scattering properties of the surface. It may be that the energy deposition from micrometeoroids is insignificant relative to the effects of charged particle bombardment (Cooper et al., 2001). Using the same reasoning on Europa’s jovian and antijovian hemispheres as for the leading and trailing hemispheres (above), we find that the anti-jovian hemisphere has higher ωp(0) values, likely due to greater amounts of bombardment experienced by that hemisphere (Fig. 8c). Although jovian hemisphere likely experiences a higher amount of bombardment from iogenic particles than the anti-jovian hemisphere, the anti-jovian hemisphere receives a higher flux of hot ions (Paranicas et al., 2002) than the jovian hemisphere. The greater amount of bombardment on the anti-jovian hemisphere contributes to the higher amounts of internal scattering demonstrated in the particles on that hemisphere. This is consistent with results from McGuire and Hapke (1995) and Hartman and Domingue (1998), who have found that particles with moderate to high densities of internal scatterers are more backscattering than those with little to no quantities of internal scatterers. This suggests that the increased amount of iogenic particles experienced by the jovian hemisphere does not significantly affect the scattering properties of that surface. Furthermore, the opposition surge width (characterized by the h term) of Europa’s jovian hemisphere is wider than for the anti-jovian hemisphere, suggesting a less porous jovian-facing surface, consistent with lower amounts of charged particle bombardment on that region. 5.2.2. Temporal variations In this study, data were available from both the Galileo and Voyager eras for Europa’s jovian and leading hemispheres, allowing the analysis of temporal variations in scattering properties in those regions. The trailing hemisphere does not display temporal variations in brightness; the Galileo era IUE and HST observations are in agreement with the Voyager era IUE data. We were not able to look for temporal variations on the anti-jovian hemisphere, due to a lack of HST and Galileo UVS data in that region. (In a related study, Domingue and Hendrix (2004) performed an analysis of temporal variations of the rotational phase curves of the icy galilean satellites, using only IUE data at ∼ 10◦ phase angle from the Voyager and Galileo eras. That study found temporal variations on Europa’s leading hemisphere, but did not include enough coverage of the jovian hemisphere to detect a difference. In this study, we also use HST and Galileo UVS data on the jovian hemisphere and can detect a temporal variation.) We find that Europa’s jovian and leading hemispheres had higher geometric albedos (Fig. 4) during the Voyager era than during the Galileo era, because the surfaces were more backscattering (by a multiplicative factor of 1.3 at 300 nm) in the Voyager era than in the Galileo era. Based on the above comparisons between the satellites and Europa’s hemispheric results, this implies lower amounts of bom-

47

bardment of the leading and jovian hemispheres by charged particles during the Galileo timeframe than during the Voyager era. Such a scenario would be consistent with a decrease in particle population in the Jupiter system between the late-1970s–early-1980s and the mid-1990s timeframes. Although no published data exist to support the idea that there was a decrease in the population of charged particles at Europa’s orbit between the Voyager and Galileo epochs, analysis has shown a depletion in hot ions near Io’s orbit, as far out as 7.6RJ (Mauk et al., 1998). The Voyager era measured values were between 2.2 and 5.5 times higher than those measured during the Galileo era, for quantities such as hot ion particle intensity and energy intensity. While we cannot say with certainty that this trend continued out to Europa’s orbit (at 9RJ ), this result does suggest that such temporal changes are possible. We expect that although Europa’s leading and jovian hemispheres became less backscattering between the Voyager and Galileo epochs due to a decrease in particle bombardment, no change was measured on the trailing hemisphere because of the dominance of annealing on that relatively dark hemisphere. It is not clear that such temporal changes in scattering properties have been detected at visible wavelengths. A study of Europa’s visible opposition effect as measured by Galileo (Helfenstein et al., 1998) discusses only the opposition effect and does not directly compare surface brightness measured by Galileo with that measured by Voyager. An indepth study of the complete visible solar phase curves during both eras has not been published. However, it is possible that such temporal changes in scattering processes are detectable primarily at UV wavelengths and may not be seen in the visible. Ultraviolet wavelengths are particularly sensitive to relatively small amounts of weathering (e.g., Hapke, 2001; Hendrix et al., 2003) and it is thus possible that the UV brightness of the galilean satellites is variable with time, dependent upon the amount of charged particle bombardment occurring during any particular era. Surfaces may brighten at UV wavelengths due to an increase in bombardment and may darken at UV wavelengths when charged particle bombardment slows down. Effects of such variations in charged particle population could be seen on timescales of a few years (Cooper et al., 2001). We expect that variations in UV brightness due to fluctuations in charged particle population are most readily detected on surfaces that are relatively bright, where annealing is not dominant.

6. Conclusions The analysis of the ultraviolet solar phase curves of Europa, Ganymede, and Callisto yields information regarding the scattering properties of these icy surfaces and the effects of their environments. Though the data sets are somewhat sparse and scattered, useful results are obtained, including the measurements of Callisto’s large opposition surge and low-backscattering nature, and Europa’s highly backscatter-

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ing anti-jovian hemisphere; there is also a notable decrease in the amount of backscattering from Europa’s leading and jovian hemispheres between the late-1970s–early-1980s and the mid-1990s. These key results can be tied to the amount of bombardment the surfaces experience. On Callisto, the surface is likely dominated by annealing due to the dark surface and a relative lack of charged particle bombardment. As a result, the grains become large, dark, and opaque, leading the surface to have a large opposition surge and relatively low amount of backscattering. Europa’s anti-jovian hemisphere, in contrast, experiences large amounts of bombardment from hot ions, increasing the number of internal scatterers in the grains, and making the region very backscattering. Europa’s trailing hemisphere is less backscattering than the leading hemisphere because although it experiences more charged particle bombardment, the surface is darker and annealing dominates the scattering properties. Europa’s surface became less backscattering between the Voyager and Galileo eras, which could be due to a decrease in the charged particle population in Jupiter’s magnetosphere during this period. In conclusion, we find that two competing processes affect the amount of light scattered by particles on surfaces in the jovian system: charged particle bombardment and annealing. Surfaces that experience a high amount of charged particle bombardment and little annealing tend to have particles that scatter high amounts of light (internal and surface scattering). Conversely, surfaces that experience relatively high amounts of annealing and/or lower amounts of bombardment have particles that scatter lower amounts of light. This is likely due to the creation of defects in the particles by bombardment, causing the particles to be more backscattering and may make particles brighter as well. Annealing will tend to mask such defects, making particles less backscattering and may also may the particles darker.

Acknowledgments This work was supported by NASA Grant #NAG5-8178. Kimberly King was supported through the APL Summer High School Student Program. We are grateful for the detailed reviews from Alan Stern and another reviewer, and for helpful comments from Bob Nelson.

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Further reading Hapke, B., 1981. Bidirectional reflectance spectroscopy. 1. Theory. J. Geophys. Res. 86, 3039–3054.