Icarus 191 (2007) 193–202 www.elsevier.com/locate/icarus

The origin of ’s polar caps

Krishan K. Khurana a,∗, Robert T. Pappalardo b, Nate Murphy c, Tilmann Denk d

a Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095, USA b Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA c Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0392, USA d Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstraße 74-100, 12249 Berlin, Germany Received 7 August 2006; revised 13 April 2007 Available online 5 May 2007

Abstract Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of at was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite’s equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter’s magnetosphere, and our neglect of higher-order terms in modeling Ganymede’s internal field. We postulate that leading-trailing brightness differences in Ganymede’s low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite’s surface. © 2007 Elsevier Inc. All rights reserved.

Keywords: Ganymede; Satellites, surfaces; Jupiter; Jupiter, magnetosphere; Europa

1. Introduction suggest that Ganymede’s polar caps are the polar remnants of a once-global water ice layer that formed in the satellite’s early Images of Ganymede acquired by revealed that history when it was geologically active and that, subsequently, Ganymede’s high latitude regions are noticeably brighter than the ice layer was removed from the equatorial region by subli- its equatorial regions (Smith et al., 1979). Initially, it was be- mation. Shaya and Pilcher hypothesized that the initial ice layer lieved that these polar caps formed through thermal migration must have been at least a few meters thick, because micromete- of water from the equatorial region to higher latitudes by subli- orite induced impact-gardening would obliterate a thinner frost mation and subsequent redeposition. However, detailed studies layer even from the poles over the history of the satellite. showed that because of extremely low temperatures in the polar A competing model put forward by Sieveka and Johnson regions, thermal migration is incapable of transporting appre- (1982) attributed the polar cap formation to a competition be- ◦ ciable water beyond a latitude of ∼50 (Purves and Pilcher, tween ion-induced sputtering and thermal sublimation. They 1980; Shaya and Pilcher, 1984). This led Purves and Pilcher to suggested that sputtering of Ganymede’s surface by ions from Jupiter’s magnetosphere led to frost generation, whereas subli- * Corresponding author. Fax: +1 (310) 206 8042. mation destroyed it. Thus, in the polar regions where thermal E-mail address: [email protected] (K.K. Khurana). sublimation is minimal, plasma sputtering might maintain a

0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.04.022 194 K.K. Khurana et al. / Icarus 191 (2007) 193–202 bright frost layer, whereas in regions of darker surface (mainly by redistributing water frost into optically thick patches of at low latitudes), frost formation is inhibited by sublimation. ice-rich material. First, in Section 2.1 we discuss the vary- Johnson (1985) later shelved the idea of direct frost formation ing external field and plasma environment in which Ganymede from sputtered molecules, noting that in laboratory experiments is embedded. In Section 2.2 we describe the contributions of even a rapid deposition of water molecules on an extremely Ganymede’s internal and induction (from a putative ocean) cold surface leads to a relatively clear (non-scattering) surface. fields to the local field environment. In the next section, we He instead proposed that changes in reflectance properties were calculate the boundaries of those polar regions on Ganymede’s caused by an increase in defects and voids and growth of den- surface that are connected directly with Jupiter’s magnetic field dritic crystals in the bombarded surficial ice layer. He also sug- lines and are therefore open to plasma bombardment from gested that the higher surface temperatures in the equatorial belt Jupiter’s magnetosphere. Finally, in Section 3, we compare the caused thermal reprocessing and annealing of the icy surface, regions of open field lines with the polar caps identified from which consequently remains non-scattering and dark. the imaging data and show that there is a close match between Hillier et al. (1996) performed a photometric analysis of them, thus confirming that the polar caps are created by the in- Voyager images in terms of a two layer model, assuming that ability of Ganymede’s magnetosphere to shield its polar regions the upper layer consists of a fine grained clean water frost and from Jupiter’s plasma. the lower layer is the same as material found in Ganymede’s Galileo made four close flybys of Ganymede during its equatorial region. They found that the optical depth of the as- primary mission (G1, G2, G7, and G8), and two additional sumed upper layer is at most a few mm near the poles. Hillier et close passes during its extended mission (G28 and G29). The al. argued that such a thin frost layer must be continually regen- field and particle measurements from these flybys revealed erated by plasma bombardment, because micro-meteor impact that Ganymede possesses an internal magnetic field strong gardening would have otherwise obliterated the thin veneer. enough to carve out a miniature magnetosphere of its own in- From the close flybys of Ganymede by Galileo, it became side Jupiter’s magnetosphere (Gurnett et al., 1996; Kivelson clear that Ganymede possesses an internal field strong enough et al., 1996, 1997, 1998; Williams et al., 1997). Because the to create a secondary magnetosphere of its own within the jovian magnetospheric plasma is essentially co-rotational with magnetosphere of Jupiter (Kivelson et al., 1996, 1997). The Jupiter near Ganymede’s orbit (Frank et al., 1997), it over- Ganymede magnetosphere strongly inhibits the flow of jovian takes Ganymede in its orbit. Consequently, reconnection in plasma on field lines that map onto mid-to-equatorial latitudes the trailing hemisphere of Ganymede’s magnetosphere opens on Ganymede. Prompted by this result, Johnson (1997) rein- Ganymede’s field lines and convects them over the satellite’s forced the surficial-defects hypothesis for brightening the polar polar regions (Fig. 1). Analogy with the Earth’s magnetosphere regions, and he further suggested that annealing effects be- suggests that the open flux would ultimately reconnect on the comes pronounced in the equatorial region in the absence of downstream (leading) side and convect back to the upstream surficial defects induced by plasma bombardment. side (trailing hemisphere) at low latitudes. The interconnection In this work we combine Galileo SSI (Solid State Imager) of Ganymede and Jupiter’s field lines results in three types of data and Galileo fields and particles data to demonstrate that field lines (Fig. 1). Fully open field lines are connected at both charged particle bombardment creates Ganymede’s polar caps ends to Jupiter (dashed blue lines), whereas fully closed field

Fig. 1. The configuration of Ganymede’s magnetosphere in the X–Z (left) and Y –Z (right) planes. The x-axis is parallel to the corotation direction (effectively parallel to the Ganymede orbital direction), y is positive inward toward Jupiter, and z is parallel to the spin axis of Jupiter (effectively parallel to the Ganymede spin axis). Red lines show the last fully open field lines that are connected at both ends to Jupiter. Open arrows mark the path of newly opened field lines, and the dark arrow marks the direction of jovian magnetospheric flow. Ganymede’s polar caps 195 lines are connected at both ends to Ganymede (black lines). Fi- nally, there are partially open field lines (green lines), which have one end connected to Jupiter and the other end connected to the high latitude regions of Ganymede. The plasma on the fully open field lines does not have ready access to Ganymede, but the plasma on the partially open field lines can precipitate onto the polar regions of Ganymede. Cooper et al. (2001) have used Galileo orbiter measure- ments of energetic ions (20 keV to 100 MeV) and electrons (20–700 keV) and electron flux estimates from the JPL elec- tron model (<40 MeV) of energetic electron fluxes in Jupiter’s magnetosphere (Divine and Garrett, 1983) to compute irradia- tion effect on the surfaces of the icy Galilean satellites. Their detailed calculations using actual particle trajectories in the Ganymede magnetosphere show that the average energy flux of the particles reaching the surface is 5.4 × 109 keV cm−2 s−1 in the polar cap region and only 2.6 × 108 keV cm−2 s−1 in the equatorial region. In the polar cap region, fluxes are dominated by electrons, while electrons above the energy range of tens of keV are completely excluded from the equatorial regions. In- Fig. 2. The xy component of the magnetic field of Jupiter’s magnetosphere in terestingly, their calculations show that some heavy, multiply the frame of Ganymede. For information on coordinate system, see Fig. 1.Open charged ions (like S3+) can impinge upon Ganymede’s surface circles mark the times corresponding to Galileo’s flybys. even in the equatorial region, though the energy flux is small (1.4 × 108 keV cm−2 s−1). (E<6keV) (McNutt et al., 1981) and energetic plasma In Ganymede’s equatorial plane, the sharp magnetopause (Krimigis et al., 1981) are lower by a factor of ∼3–5 in regions boundary separates the fully open field lines from the fully outside of Jupiter’s plasma sheet compared to values inside the closed field lines and provides an estimate of the scale size plasma sheet. Therefore, Ganymede can be expected to face of Ganymede’s magnetosphere at ∼4RG. The interconnection the most intense plasma bombardment when it is located near ◦ ◦ geometry is defined by the internal field of Ganymede and its the west System III longitudes of 112 and 292 . However, magnetosphere and the external field of jovian magnetospheric Ganymede spends only a short time in the plasma sheet (less origin. Next we will see that in Ganymede’s frame, the exter- than two hours) during each synodic rotation period of Jupiter. nal field is variable; thus, the interconnection geometry evolves continually. 2.2. Ganymede’s internal magnetic field

2. Modeling Ganymede’s polar cap boundary Data from Galileo’s first two encounters with Ganymede (G1 and G2) suggested that Ganymede has a southward-oriented ◦ 2.1. Varying external field and plasma environment dipole tilted by ∼10 with respect to the south geographic pole and pointing toward 340◦ W longitude. The surface field The external field at Ganymede’s location results from the strength was estimated to be ∼750 nT at the equator (Kivel- combination of two fields: (1) the mainly dipolar field from son et al., 1996, 1997). A new modeling study (Kivelson et Jupiter, and (2) the field from a thin current sheet (half thick- al., 2002) using observations from the four close flybys of ness ∼2.5RJ ) located near Jupiter’s dipole magnetic equator Ganymede suggests that in addition to a permanent internal (Khurana, 1997). Because the magnetic dipole axis of Jupiter field, a conductor located within Ganymede (presumably a is tilted by ∼10◦ with respect to the planet’s rotation axis, the global subsurface ocean) generates a dipolar induction response nearly corotational current sheet bobs up and down with respect to the changing (in its frame) magnetic field of Jupiter’s mag- to Ganymede in each synodic rotation period of Jupiter. The netosphere. The dipole moment of the revised model is 719 nT ◦ relative motion of Ganymede with respect to Jupiter’s dipole and the dipole axis is tilted only by ∼4 with respect to the geo- ◦ equator and current sheet causes a varying field and plasma en- graphic south pole and points toward 336 W longitude. The in- vironment in Ganymede’s rest frame. In Fig. 2 we show the duction response is ∼84% that of a perfectly conducting sphere. expected background field experienced by Ganymede during a As discussed below, because of the changing background field, synodic rotational period of Jupiter. The coordinate system used the induced field changes its strength and direction over time. is defined in Fig. 1. As Ganymede orbits Jupiter, the z compo- nent of the magnetic field remains essentially constant, but the 2.3. The open/closed field line (OCFL) boundary traced from x and y components vary with an elliptical polarization at the the best-fit model synodic period of Jupiter’s rotation. Observations from the two Voyager spacecraft showed that Here we have used the permanent and induced dipole model near Ganymede’s radial distance, the fluxes of warm plasma of Kivelson et al. (2002) along with the model of Jupiter’s mag- 196 K.K. Khurana et al. / Icarus 191 (2007) 193–202 netospheric field (Khurana, 1997) to trace field lines around the violet/green ratio serves as a good proxy for the location of Ganymede. Field lines were traced outward from Ganymede’s small-grain-size frost. Relatively fresh impact craters do ap- surface starting at low latitudes for all longitudes to deter- pear, as they probably contain fine-grained frost, but these can mine whether the field lines returned to Ganymede’s other be visually excluded. As noted above, the applied photometric hemisphere (and therefore are closed) or diverge away from corrections are imperfect (especially at high emission angles), Ganymede (and therefore are open). The latitude was then resulting in bright artifacts near the edges of the individual ◦ advanced by 1 and the process repeated. Once the approxi- global images. We find that a violet/green ratio of 0.80–0.84 mate location of the open/close field line (OCFL) boundary had serves well to delineate the polar cap material, based on com- been found on Ganymede’s surface, its location was refined by parison to clear-filter image data (Fig. 3a). repeating the process with a 0.1◦ longitude/latitude grid. As discussed above, the contributions from the external field of 3.2. Comparison to the OCFL boundary Jupiter’s magnetosphere and from Ganymede’s induction re- sponse are each time variable. Therefore, we have calculated Also shown in Fig. 3b are the calculated OCFL boundaries, the location of the OCFL boundary for three situations: when for cases in which Ganymede is located above Jupiter’s plasma Ganymede is farthest above the plasma sheet (which occurs ◦ sheet (red curves), in the middle of the plasma sheet (green), when Ganymede is located near 202 west system III jovian ◦ ◦ and below the plasma sheet (blue). The ∼4 tilt of Ganymede’s longitude), when it is farthest below the plasma sheet (22 intrinsic field means that the OCFL boundaries are biased to- west system III longitude), and when it is located in the mid- plane of the plasma sheet (292◦ west system III longitude). ward more southern latitudes in the anti-jovian hemisphere, We note that at any Ganymede longitude, the surface region and toward more northern latitudes in the trailing hemisphere. poleward of the most poleward OCFL boundary always experi- The first-order latitudinal correspondence of the OCFL bound- ences plasma bombardment. Similarly, the region equatorward aries to the polar cap boundaries is quite apparent. This sug- of the most equatorward OCFL boundary never experiences gests that Ganymede’s polar caps owe their existence in some low-energy plasma bombardment. Between the poleward and manner to charged particle bombardment of the surface. As equatorward OCFL boundary extremes, the plasma flux varies already discussed above, Ganymede experiences its most in- with time. tense plasma bombardment when located near the mid-plane of Jupiter’s plasma sheet; however, Ganymede spends most 3. Ganymede’s polar caps: Observations and comparison of its time above or below the plasma sheet. If relatively in- to OCFL boundary tense plasma bombardment is necessary to create and maintain the polar caps, the effective cap boundaries might be expected 3.1. Global color image ratios to be located near the OCFLs that describe Ganymede in the plasma mid-plane (green curves of Fig. 3b). If the cap ma- Ganymede’s polar caps appear as bright (high albedo) re- terials can be formed and maintained by less intense plasma gions at high latitudes (greater than ∼40◦) in broadband visible bombardment, the boundary might be located near envelope images. Fig. 3a is a composite mosaic constructed from Galileo describing the greatest equatorial extents of the three OCFL images at a variety of resolutions and observation geometries, curves. where appropriate photometric corrections (McEwen, 1991) To provide a better understanding of the relationship be- have been applied to the constituent images in a first-order tween the polar caps and the OCFL boundaries, we have pro- attempt to derive absolute albedo of surface materials. How- jected the ratio image into northern and southern polar stereo- ever, the photometric behavior of Ganymede’s surface has not graphic projections (Figs. 4a and 4b). Artifacts of image geom- ◦ yet been fully refined based on Galileo images, so our photo- etry (especially 190–250 longitude), and bright material away metric correction of these images is imperfect and results in from the caps as related to bright fresh craters, should be ig- notable mismatches along image boundaries. Moreover, local nored. Again there is a good overall match between the OCFLs reflectance is also affected by local morphology and geological and the polar cap boundaries. However, it is apparent that the fit ◦ processes (bright or dark terrain, and craters and their ejecta), so is not ideal, with latitudinal mismatches of ∼10 , notably near ◦ brightness in these images is an unreliable tool in distinguishing 150 W longitude. In modeling the OCFL boundary, we have between polar cap frost and other materials present elsewhere. so far neglected the potentially important effect of Ganymede’s Because the fine grained frosts of the polar caps scatter pref- complex current sheet/magnetopause field. It is well known erentially at shorter wavelengths (Johnson et al., 1983), a bet- from studies of the Earth’s ionospheric “polar cap” that the pri- ter discrimination is provided by the intensity ratio of Galileo mary effect of the current sheet/magnetopause field is to shift violet (413 nm effective wavelength) to green (560 nm) im- the center of the Earth’s OCFL boundary towards local mid- ◦ ages. night by ∼5 .InFigs. 4c and 4d, we have applied a shift of sim- ◦ Fig. 3b shows the violet/green ration of global-scale color ilar 5 magnitude in Ganymede’s magnetosphere and moved the images from the Galileo Solid State Imaging (SSI) system, OCFL boundaries towards lower latitudes in the leading hemi- obtained during orbits G1, E14, and C10. In this image, sphere and toward higher latitudes in the trailing hemisphere. Ganymede’s surface features—specifically, bright and dark ter- This shift appears to improve the overall match to the imaging- rains and other surface relief—are not apparent, confirming that derived cap boundary. Ganymede’s polar caps 197

Fig. 3. Global image mosaics of Ganymede and predicted locations of the open/closed field line (OCFL) boundary. (a) Mosaic of Voyager and Galileo clear filter (broadband) images at a variety of resolutions. (b) Green/violet ratio composite image of Galileo color data from orbits E14 (west, 10.4 km/pixel), G1 (central, 13.1 km/pixel), and C10 (east, 7.3 km/pixel green and 14.7 km/pixel violet). Superimposed are the OCFL boundaries for three different configurations: red, when Ganymede is farthest above Jupiter’s plasma sheet, green when Ganymede is located in the middle of the plasma sheet, and blue when Ganymede is located farthest below the plasma sheet. Image mosaics are in simple cylindrical projection and were produced by the U.S. Geological Survey, Flagstaff, Arizona.

3.3. Polar cap characteristics based on imaging ent, with albedo effects dominating instead. These images show that the surface is segregated into bright and dark patches on a Observations of both dark and bright terrains in Ganymede’s small spatial scale, similar to the equatorial regions but with mid-latitude and near-equatorial regions indicate that bright bright patches covering a greater surface fraction within the po- regions are segregated from darker regions on 1 km scales lar caps. Visual inspection of crater walls in the northern high- (Pappalardo et al., 1998; Prockter et al., 1998; Oberst et al., latitude images indicates that bright material is preferentially 1999). To examine the small-scale characteristics of Gany- located on colder (north- and east-facing) slopes. The bright mede’s high latitude terrain, a sequence of high-resolution SSI deposits appear to be locally thick within some craters (e.g., images was obtained in a transect from (56◦ N, 174◦ W) to at black arrow in Fig. 5), plausibly on the orders of meters, as (65◦ N, 165◦ W) during Galileo’s G2 encounter (Fig. 5). The they appear to mantle underlying topography (Pappalardo et al., position of these images ≈30◦ from the terminator (incidence 1997). angle ≈58–66◦) implies that few if any actual shadows are vis- Voyager-based derivations of the single scattering albedos of ible, since small-scale slopes on Ganymede are generally less darker polar materials were in the range 0.5–0.7, and not easily than 20◦ (Oberst et al., 1999). The brightest slopes (oriented distinguished from some equatorial dark materials (Helfenstein, NE-SW) betray the direction of incoming sunlight (white arrow 1985), consistent with an interpretation as ice-poor materials. of Fig. 5), but overall shading effects are only subtly appar- Helfenstein et al. (1997) analyzed small patches of dark ma- 198 K.K. Khurana et al. / Icarus 191 (2007) 193–202

Fig. 4. Northern (a) and southern (b) polar projections of the Ganymede green/violet ratio image shown in Fig. 3b, and the corresponding OCFL boundaries obtained ◦ from field line mapping. The corresponding figures (c) and (d) are the same images but with the OCFL boundaries shifted by 5 in the direction of plasma flow; i.e., ◦ toward the leading point of the satellite’s orbital motion (toward 90 W longitude). terials in Galileo images of Uruk Sulcus and inferred that they surroundings (Spencer, 1990), and local concentration of low have normal albedos (i.e., albedos extrapolated to incidence and albedo material will reinforce warmer temperatures; thus, low- emission angles normal to the surface) averaging ∼0.25 and as lying regions are expected to be relatively ice free. If any ice low as 0.12, similar to dark materials on Callisto and Europa, does persist in darkened topographic lows, the added warmth and suggesting that dark polar materials are depleted in ice rel- might promote grain growth; however, in the very cold high lat- ative to bright materials. The average single-scattering albedos itude regions it is unlikely that grain growth can be significant inferred for bright polar patches in Voyager images are in the (Clark et al., 1983). Galileo NIMS data are consistent with rela- range 0.7–0.9, consistent with relatively clean ice (Helfenstein, tively small (50 µm) grain sizes in the polar regions (Hibbitts 1985). The normal albedos of small patches of polar bright et al., 2003), while concentrations of amorphous versus crys- materials have not been accurately determined from Galileo talline ice on Ganymede are quite variable across Ganymede images, but they are several times higher than dark materials, (Hansen and McCord, 2004). consistent with an interpretation as deposits of relatively clean These observations are consistent with thermally driven seg- ice. Dark patches might represent larger grain size and greater regation (Spencer, 1987) of icy and ice-poor materials into path length through the ice (Clark, 1981), and/or compositional colder and warmer patches, respectively, as an active process differences. In lower latitude regions, stereo data demonstrates even at high latitudes on Ganymede. The close correspondence the tendency for dark material to reside preferentially in lo- of the polar cap boundaries with the OCFL boundaries sug- cal depressions, suggesting that compositionally distinct dark gests that water molecules are more readily mobilized by sput- lag material has sloughed into topographic lows (Oberst et al., tering than across the rest of Ganymede’s surface, and that 1999). This interpretation is supported by the presence of sharp sputter-redistributed frost in Ganymede’s polar regions is sub- boundaries dividing some material units (Prockter et al., 1998). sequently thermally segregated into bright (icy) and dark (ice- Local depressions are expected to be slightly warmer than their poor) patches. The relatively bright polar cap regions may have Ganymede’s polar caps 199

◦ ◦ Fig. 5. A mosaic of high resolution (92 m/pixel) Galileo images within the northern polar cap (centered near 56 N, 173 W), with location indicated by dashed region of the inset. The existence of small-scale bright and dark patches suggests that thermal effects have segregated water-frosts into locally colder locations. White arrow indicates direction of insolation, and black arrow indicates one of many areas where relatively thick frost deposits appear to mantle a crater wall. The background image is the best Voyager image of the area, at a resolution of about 1.4 km/pixel. a relatively large fraction of cold-trapped icy material compared trapped. Sputter-redistribution offers a more likely means of to other regions on Ganymede. mobilizing ice to build patchy frost deposits in Ganymede’s po- If brightening were an exclusive result of radiation damage lar regions than radiation damage alone. of Ganymede’s polar terrains (Johnson, 1997), it is expected that the entire polar cap surface should be uniformly bright- 4. Discussion ened, presumably in proportion to the ice content of the original surface. Instead, segregation into ice-rich and ice-poor patches The agreement between the polar cap and OCFL boundaries suggests that ice is mobilized and redistributed through charged is imperfect, and there are several possible reasons for this dis- particle bombardment and sputtering, then subsequently cold crepancy. The most obvious source of inaccuracy in modeling 200 K.K. Khurana et al. / Icarus 191 (2007) 193–202 the plasma bombardment boundary is the neglect of higher or- tant mechanism for brightening of Europa’s surface. This exo- der terms in the determination of the internal magnetic field of genic means of brightening is in contrast to suggestions that Eu- Ganymede. The short durations of the Galileo flybys, the long ropa’s surface is brightened by active deposition of water frost rotation period of Ganymede (so that only a limited longitude vented from a global subsurface ocean (Squyres et al., 1983; region is sampled in any flyby), the highly variable background Eviatar et al., 1985), but is instead a natural consequence of field of Jupiter, and the strong varying fields from Ganymede’s its radiation environment. It is plausible that charged particle putative ocean and magnetopause make it virtually impossible bombardment, sputter redistribution, and thermal segregation to determine terms higher than the dipole in the internal field of operate in tandem to brighten originally dark features and create Ganymede. However, a quadrupole field that has a magnitude the small-scale albedo segregation observed on Europa’s sur- of 10–20% of the dipole field could easily shift the OCFL by face, with Europa’s bright material being only a thin patina that several degrees in latitude. coats an intrinsically darker substrate (Pappalardo et al., 1999; Another source of discrepancy in the modeling arises from Fanale et al., 2000). the assumption that charged particles remain tied to the field In addition to the strong latitudinal albedo and color ra- lines during their travel from Jupiter’s magnetosphere to Gany- tio asymmetries linked to the OCFL geometry on Ganymede, mede’s. However, there are several physical effects that would Fig. 3b also suggests a longitudinal asymmetry in the green/ tend to change the trajectories of charged particles as they violet color ratio. The leading hemisphere has a measurably travel along the field lines and precipitate in Ganymede’s polar higher ratio (∼0.79) than the trailing hemisphere (∼0.73), regions. For energetic particles, the dominant effects modify- implying a greater abundance of fine-grained frost in the ing particle trajectories are the gradient and curvature drifts in leading hemisphere. Leading/trailing hemisphere asymmetries a non-uniform magnetic field. For low energy plasma (a few have been previously reported for Ganymede (Clark et al., keV to 10 keV), the dominant effect is the modification of 1986; Calvin et al., 1995) and more significantly for Europa the electric field environment around Ganymede from its in- (McEwen, 1986; Sack et al., 1992). McEwen (1986) ana- teraction with the jovian plasma. Detailed trajectory analyses lyzed Voyager color image data and suggested that the hemi- of the charged particles in the complex magnetic and electric spheric asymmetries on Europa have arisen from two exogenic field environment of Ganymede are beyond the scope of this processes, namely, impact gardening and magnetospheric in- paper. teraction, the latter including both sulfur-ion implantation and Comparison between the OCFL boundary and the bound- sputtering effects. Sack et al. (1992) using UV spectra from the ary of the polar cap determined from color ratio images could IUE spacecraft and Calvin et al. (1995) using composite spectra be significantly improved through additional image process- compiled from numerous sources further demonstrated Eu- ing that includes refinement of the photometric function for ropa’s distinct leading/trailing hemispheric asymmetry. More Ganymede’s surface. Future work should also consider addi- recently, Paranicas et al. (2001) have used a radiance derived tional Galileo C30 global color data that closes the gap along from Galileo NIMS data over the surface of Europa to in- ∼230◦ W longitude. Here we have examined the cap bound- fer the hydrate content of surface materials (Carlson et al., ary subjectively based on violet/green color ratios, while pho- 1999). They show that the hydrate map of Europa has both tometric refinement would allow for quantitative assessment leading/trailing and latitudinal asymmetries, consistent with of the polar cap location and its fit to the OCFL bound- McEwen’s Voyager-based analysis. Further, by modeling the aries. Such assessment should include consideration of bright- energetic (10 keV–10 MeV) electrons in Europa’s environment, ness changes as measured in longitudinal profiles of the vio- Paranicas et al. (2001) show that the distribution of energetic let/green ratio across the surface. Additionally, there is promise electrons is strongly correlated with the hydrate content of sur- for correlation of the OCFL boundaries to Galileo 1-µm/green face materials, suggesting that energetic electrons rather than color image ratios, where preliminary analysis suggests higher ions are the principal source of energy for driving radiolytic ratios (“redder” color) in the unshielded polar cap regions chemistry on Europa’s surface. (Denk et al., 1999). Finally, during the final G29 Galileo en- The observations of leading/trailing hemispheric asymme- counter with Ganymede, additional higher resolution color data tries from Ganymede are more ambiguous. Domingue and (∼700 m/pixel in the sample direction) were obtained across Verbiscer (1997) used Voyager images to analyze the shape the northern cap boundary in Perine Regio, and these data have of the solar phase curves to understand the properties of yet to be analyzed. surface materials. They concluded that inferred differences These results for Ganymede’s surface have important im- in surface albedo, texture, grain transparency and particle plications for Europa as well. We infer that Ganymede’s po- compaction rates contribute to a leading/trailing asymmetry. lar regions are brightened in response to being open to jov- Burratti (1991), on the other hand, analyzing a similar dataset, ian plasma. Europa’s entire surface is bright, and that satel- determined that intrinsic albedo differences could explain the lite lacks a substantial intrinsic magnetic field (Khurana et al., photometric differences in the leading and trailing hemispheres. 1998), implying that most co-rotational plasma reaches Eu- Spectral studies of Clark et al. (1986) which utilize ice band ropa’s surface. Hillier et al. (1996) have shown that the large- depths to understand crystallinity of ice suggest that the trailing scale photometric properties of Ganymede’s polar caps are sim- hemisphere of Ganymede has larger ice grain size, and Calvin ilar to those of the whole of Europa. Thus, it seems likely that et al. (1995) suggest that the presence of O2 may also contribute sputter-induced redistribution of water molecules is an impor- to some of the observed band depths on the trailing hemisphere. Ganymede’s polar caps 201

Our ratio map of Fig. 3b suppresses intrinsic albedo differ- Clark, R.N., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Im- ences but is supportive of the conclusion (Clark et al., 1986) plications for remote sensing of planetary surfaces. Icarus 56, 233–245. that the material of the trailing hemisphere has larger ice grain Clark, R.N., Fanale, F.P., Gaffey, M.J., 1986. Surface composition of satellites. In: Burns, J., Matthews, M.S. (Eds.), Satellites. University of Arizona Press, size than the leading hemisphere. Unlike the Europa case, the Tucson, pp. 437–491. leading-trailing differences cannot be explained by radiolytic Cooper, J.F., Johnson, R.E., Mauk, B.H., Garrett, H.B., Gehrels, N., 2001. En- chemistry driven by energetic electrons, because the equatorial ergetic ion and electron irradiation of the icy Galilean satellites. Icarus 149, region is shielded from electrons over all longitudes. However, 133–159. reconnection of field lines in the leading sector (see Fig. 1 and Denk, T., Neukum, G., Head, J.W., Pappalardo, R.T., Galileo SSI Team, 1999. the discussion in Section 1 above) could lead to some sputter- Global color contrasts on Ganymede. Bull. Am. Astron. Soc. 31. Ab- stract 70.03. ing of surface material at low latitudes from recently trapped Divine, N., Garrett, H.B., 1983. Charged particle distributions in Jupiter’s mag- particles, though at much lower rates than in the polar caps. Im- netosphere. J. Geophys. Res. 88, 6889–6903. plantation of energetic ions is unlikely to cause darkening of Domingue, D., Verbiscer, A., 1997. Re-analysis of the solar phase cures of the the trailing hemisphere, since the region is shielded from in- icy Galilean satellites. Icarus 128, 49–74. coming plasma by Ganymede’s intrinsic dipole field (Cooper Eviatar, A., Bar-Nun, A., Podolak, M., 1985. Europan surface phenomena. Icarus 61, 185–191. et al., 2001). Therefore, we favor sputter-induced redistribution Fanale, F.P., Granahan, J.C., Greeley, R., Pappalardo, R., Head, J., Shirley, J., of frosts as the cause for the longitudinal asymmetry at lower Carlson, R., Hendrix, A., Moore, J., McCord, T.B., Belton, M., The Galileo latitudes, with reconnected field lines serving as the source of NIMS and SSI Instrument Teams, 2000. Tyre and Pwyll: Galileo orbital leading hemisphere plasma. remote sensing of mineralogy versus morphology at two selected sites on Finally, we would like to comment on the effect of induction Europa. J. Geophys. Res. 105, 22647–22655. Frank, L.A., Paterson, W.R., Ackerson, K.L., Bolton, S.J., 1997. Outflow of field on the open/close boundary that has been included in our hydrogen ions from Ganymede. Geophys. Res. Lett. 24, 2151–2154. model. The induction response from a putative ocean generates Gurnett, D.A., Kurth, W.S., Roux, A., Bolton, S.J., Kennel, C.F., 1996. Evi- a time varying magnetic dipole whose pole lies in the equa- dence for a magnetosphere at Ganymede from plasma wave observations torial plane of Ganymede. When this equatorial dipole (with by the Galileo spacecraft. Nature 384, 535–537. a peak surface field strength of ∼80 nT) is added to the z-axis Hansen, G.B., McCord, T.B., 2004. Amorphous and crystalline ice on the Galilean satellites: A balance between thermal and radiolytic processes. J. aligned internal dipole field (surface strength of ∼1500 nT), the ◦ Geophys. Res. 109, doi:10.1029/2003JE002149. E01012. resulting dipole is tilted additionally by as much as 3 . This ad- Helfenstein, P., 1985. Derivation and analysis of geological constraints on the ditional tilt results in a shift of the open/close boundary at a emplacement and evolution of terrains on Ganymede from applied differen- level of < 6◦, which is close to the uncertainties associated with tial photometry. Ph.D. thesis, Brown University, Providence, RI. the present day modeling of the boundary. Thus, the current po- Helfenstein, P., Veverka, J., Denk, T., Neukum, G., Head, J.W., Pappalardo, R., lar cap modeling work cannot confirm or disprove the existence The Galileo Imaging Team, 1997. Dark-floor craters: Galileo constraints on a Ganymede regolith component. Lunar Planet. Sci. 28, 71–72. of an ocean in Ganymede. However, future improvements re- Hibbitts, C., Pappalardo, R.T., Hansen, G., McCord, T.B., 2003. Carbon dioxide sulting from additional analyses of existing magnetic field and on Ganymede. J. Geophys. Res. 108 (E5), doi:10.1029/2002JE001956. imaging data—and some day additional data—will reduce the Hillier, J., Helfenstein, P., Veverka, J., 1996. Latitude variations of the polar uncertainties associated with modeling the polar cap boundary caps on Ganymede. Icarus 124, 308–317. and help constrain the existence of a subsurface ocean. Johnson, R.E., 1985. Polar cap formation on Ganymede. Icarus 62, 344–347. Johnson, R.E., 1997. Polar “caps” on Ganymede and Io revisited. Icarus 128, 469–471. Acknowledgments Johnson, T.V., Soderblom, L.A., Mosher, J.A., Danielson, G.E., Cook, A.F., Kupferman, P., 1983. Global multispectral mosaics of the icy Galilean satel- We thank Steven Joy and Joe Mafi for preparing the mag- lites. J. Geophys. Res. 88, 5789–5805. netic field data sets used in this work, and Tammy Becker, Kivelson, M.G., Khurana, K.K., Russell, C.T., Walker, R.J., Warnecke, J., Coro- Lisa Gaddis, and Tony Rosanova of the U.S. Geological Sur- niti, F.V., Polanskey, C., Southwood, D.J., Schubert, G., 1996. Discovery of Ganymede’s magnetic field by the Galileo spacecraft. Nature 384, 537–541. vey for their work in calibrating and processing of the Galileo Kivelson, M.G., Khurana, K.K., Coroniti, F.V., Joy, S., Russell, C.T., Walker, SSI color data set. Work by K.K. was supported by the National R.J., Warnecke, J., Bennett, L., Polanskey, C., 1997. The magnetic field and Aeronautics and Space Administration through Jet Propulsion magnetosphere of Ganymede. Geophys. Res. Lett. 24, 2155–2158. Laboratory under contract 958694 and by a NASA grant NNG- Kivelson, M.G., Warnecke, J., Bennett, L., Joy, S., Khurana, K.K., Linker, 05103573. R.P. was supported in part by NASA Planetary Geol- J.A., Russell, C.T., Walker, R.J., Polanskey, C., 1998. Ganymede’s mag- netosphere: Magnetometer overview. J. Geophys. Res. 103, 19963–19972. ogy and Geophysics Program. UCLA-IGPP Publication #6297. Kivelson, M.G., Khurana, K.K., Volwerk, M., 2002. The permanent and induc- tive magnetic moments of Ganymede. Icarus 157, 507–522. References Khurana, K.K., 1997. Euler potential models of Jupiter’s magnetospheric field. J. Geophys. Res. 102, 11295–11306. Burratti, B.J., 1991. Ganymede and Callisto: Surface textural dichotomies and Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., photometric analysis. Icarus 92, 312–323. Walker, R.J., Joy, S., Polanskey, C., 1998. Induced magnetic fields as evi- Calvin, W.M., Clark, R.N., Brown, R.H., Spencer, J.R., 1995. Spectra of the icy dence for subsurface oceans in Europa and Callisto. Nature 395, 777–780. Galilean satellites from 0.2 to 5 µm: A compilation, new observations, and Krimigis, S.M., Carbary, J.F., Keath, E.P., Bostrom, C.O., Axford, W.I., Gloeck- a recent summary. J. Geophys. Res. 100, 19041–19048. ler, G., Lanzerotti, L.J., Armstrong, T.P., 1981. Characteristics of hot plasma Carlson, R.W., Johnson, R.E., Anderson, M.S., 1999. Sulfuric acid on Europa in the jovian magnetosphere: Results from the Voyager spacecraft. J. Geo- and the radiolytic sulfur cycle. Science 283, 2062–2064. phys. Res. 86, 8227–8257. Clark, R.N., 1981. The spectral reflectance of water–mineral mixtures at low McEwen, A.S., 1986. Exogenic and endogenic albedo and color patterns on temperatures. J. Geophys. Res. 86, 3074–3086. Europa. J. Geophys. Res. 91, 8077–8097. 202 K.K. Khurana et al. / Icarus 191 (2007) 193–202

McEwen, A.S., 1991. Photometric functions for photoclinometry and other ap- Prockter, L.M., Head, J.W., Pappalardo, R.T., Senske, D.A., Neukum, G., Wag- plications. Icarus 92, 298–311. ner, R., Wolf, U., Oberst, J., Giese, B., Moore, J.M., Chapman, C.R., McNutt, R.L., Belcher, J.W., Bridge, H.S., 1981. Positive ion observations in Helfenstein, P., Greeley, R., Breneman, H.H., Belton, M.J.S., 1998. Dark the middle magnetosphere of Jupiter. J. Geophys. Res. 86, 8319–8342. terrain on Ganymede: Geological mapping and interpretation of Galileo Re- Oberst, J., Schreiner, B., Giese, B., Neukum, G., Head, J.W., Pappalardo, gio at high resolution. Icarus 135, 317–344. R.T., Helfenstein, P., 1999. The distribution of bright and dark material on Purves, N.G., Pilcher, C.B., 1980. Thermal migration of water on the Galilean Ganymede in relation to surface elevation and slopes. Icarus 140, 283–293. satellites. Icarus 43, 51–55. Pappalardo, R., Head, J.W., Collins, G., Pilcher, C., Helfenstein, P., Veverka, J., Sack, N.J., Johnson, R.E., Boring, J.W., Baragiola, R.A., 1992. The effect of Burns, J., Denk, T., Neukum, G., Belton, M., The Galileo Imaging Team, magnetospheric ion bombardment on the reflectance of Europa’s surface. 1997. Ganymede northern high latitude frosts: Preliminary observations Icarus 100, 534–540. from Galileo SSI data. Lunar Planet. Sci. 28, 1065–1066. Shaya, E.J., Pilcher, C.B., 1984. Polar cap formation on Ganymede. Icarus 58, Pappalardo, R.T., Head, J.W., Collins, G.C., Kirk, R.L., Neukum, G., Oberst, 74–80. J., Giese, B., Greeley, R., Chapman, C.R., Helfenstein, P., Moore, J.M., Sieveka, E.M., Johnson, R.E., 1982. Thermal- and plasma-induced molecular McEwen, A., Tufts, B.R., Senske, D.A., Breneman, H.H., Klaasen, K., redistribution on the icy satellites. Icarus 51, 528–548. 1998. Grooved terrain on Ganymede: First results from Galileo high- Smith, B.A., 19 colleagues, 1979. Galilean satellites and Jupiter—Voyager 2 resolution imaging. Icarus 135, 276–302. imaging science results. Science 206, 927–950. Pappalardo, R.T., Belton, M.J.S., Breneman, H.H., Carr, M.H., Chapman, C.R., Spencer, J.R., 1987. Thermal segregation of water ice on the Galilean satellites. Collins, G.C., Denk, T., Fagents, S., Geissler, P.E., Giese, B., Greeley, R., Icarus 69, 297–313. Greenberg, R., Head, J.W., Helfenstein, P., Hoppa, G., Kadel, S.D., Klaasen, Spencer, J.R., 1990. A rough-surface thermophysical model for airless planets. K.P., Klemaszewski, J.E., Magee, K., McEwen, A.S., Moore, J.M., Moore, Icarus 83, 27–38. W.B., Neukum, G., Phillips, C.B., Prockter, L.M., Schubert, G., Senske, Squyres, S.W., Reynolds, R.T., Cassen, P., Peale, S.J., 1983. Liquid water and D.A., Sullivan, R.J., Tufts, B.R., Turtle, E.P., Wagner, R., Williams, K.K., active resurfacing on Europa. Nature 301, 225–226. 1999. Does Europa have a subsurface ocean? Evaluation of the geological Williams, D.J., Mauk, B.H., McEntire, R.W., Roelof, E.C., Armstrong, T.P., evidence. J. Geophys. Res. 104, 24015–24055. Wilken, B., Roederer, J.G., Krimigis, S.M., Fritz, T.A., Lanzerotti, L.J., Paranicas, C., Carlson, R.W., Johnson, R.E., 2001. Electron bombardment of Murphy, N., 1997. Energetic particle signatures at Ganymede: Implications Europa. Geophys. Res. Lett. 28, 673–676. for Ganymede’s magnetic field. Geophys. Res. Lett. 24, 2163.