Tyre and Pwyll: Galileo orbital remote sensing of mineralogy versus morphology at two selected sites on Europa

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. E9, PAGES 22,647-22,655, SEPTEMBER 25, 2000 Tyre and Pwyll: Galileo orbital remote sensing of mineralogy versus morphology at two selected sites on Europa Fraser P. Fanale,,2 James C. Granahan, 2,3 Ronald Greeley, 4 Robert Pappalardo, s James Head III, s James Shirley, 6 Robert Carlson, 6 Amanda Hendrix,? Jeffrey Moore, s Thomas B. McCord, Michael Belton, 9 and the Galileo NIMS and SSI Instrument Teams Abstract. Observational data from the Galileo Orbiter's remote-sensing instruments, including solid-state imaging (SSI), the Near-Infrared Mapping Spectrometer (NIMS), and the ultraviolet spectrometer (UVS), are analyzed and interpreted in terms of the history of the Tyre and Pwyll impact sites and the nature of Europa's crustal zonal structure. Material characterized by asymmetric 1.4 and 2.0/xm bands and visible coloration characterizes each site where morphologic evidence suggests disruption of the topmost crust. The material is not H20 ice, is endogenic, and is common to the linea, impact basins, and dark trailing side terrain. Differences between Tyre and 11 are interpreted in terms of Europa's peculiar energy history, while other evidence may suggest flooding of the endogenic material as a liquid. The pure ice appearance of most of Europa's crust is probably superficial, while beneath a thin patina of sputtered H20 molecules the crust is everywhere laced with numerous generations of intrusions and extrusions of an aqueous phase. Our results, together with numerous laboratory experiments and theoretical analysis, suggesthat the aqueous phase is dominated by the SO2 anion. Nothing is rich but the inexhaustible wealth of Nature. She shows us only surfaces, but she is a million fathoms deep. R. W. Emerson 1. Introduction Data from several remote-sensing instruments aboard the Galileo Orbiter are analyzed. Imaging and spectral data are coregistered in an attempt to understand the relationship between morphology and mineralogy and its implications for Europa's geologic history and present crustal zonal structure. Although all previous relevant global data are incorporated in the analysis, this study focuses on high- to moderate-resolution data obtained at two selected sites: Tyre and Pwyll. These sites contain major impact craters, exhibit clear-cut superposition of features (allowing for relative geochronology), and were the subject of advanced planning to allow effective coregistration of data from instruments of intrinsically very different spatial resolution in order to implement this study. 2. Composition Map Procedures Both the Tyre and Pwyll ice and "salt" maps (Plates 1, 3, and 4) were constructed on the basis of a linear mixing model of endmembers found in the Near-Infrared Mapping Spectrometer (NIMS) Tyre and Pwyll observations. The end-members were selected by a manual interrogation of the NIMS cubes and principal component analysis. In each case (on the basis of the experience of McCord et al. [1998, 1999]), only two endmembers were selected. These two are an ice and a "salt" end-member, where the ice end-member had the most symmetric water ice bands and the salt end-member had the most asymmetric water ice bands. The extreme spectra as they occurred in each site are shown in Figures 1 and 2. The Tyre and Pwyll maps were then generated by using these end-members in a linear mixing model to see how these spectra were distributed. The resulting maps were then projected over correspond- Hawaii Institute of Geophysics and Planetology, School of Ocean ing solid-state imaging (SSI) green filter images of the approand Earth Science and Technology, University of Hawaii, Honolulu. priately projected region to show the relationship between 2Science and Technology International, Honolulu. 3Marconi Integrated Systems San Diego, California. composition and morphology. Residuals for the Pwyll case 4Department of Geology, Arizona State University, Tempe. were low (---5% maximum), suggesting that these two compo- 5Department of Geological Sciences, Brown University, Providence. nents are the primary spectral components in that region. Re- 6jet Propulsion Laboratory, Pasadena, California. siduals for some of Tyre were as high as 10% (forming a ring 7Laboratory of Atmospheric and Space Physics, University of Colorado, Boulder. in the Tyre region and suggesting an additional component). SSpace Sciences Division, NASA Ames Research Center, Moffett The spectral end-members in each site were evaluated using a Field, California. linear spectral mixing model of water ice spectra [Clark et al., 9National Organization of Astronomical Observatories, Tucson, Ar- 1981a, b] and the NIMS observations of Rhadamanthys Linea. izona. These represent naturally occurring examples of the extremes Copyright 2000 by the American Geophysical Union. of spectral properties observed on Europa. The false color SSI Paper number 1999JE images (Plate 2 and Figure 2) are red, green, blue composites /00/1999JE of various SSI filter images (red is 0.989, green is 0.560, and 22,647

2 22,648 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA blue is 0.410/ m). These are used to establish the relationship between colored material, material with asymmetric bands, and morphology. There are three points to bear in mind when viewing our false color products. First, it must be remembered that most of Europa's optical surface is covered with relatively pure H20 ice. We consider the relationship between the spectral presence of other materials and morphology to be of importance but wish to emphasize that the regions where the presence of other materials dominates the spectra are important but limited in area. Second, even in these areas, it is unlikely that any field of view of NIMS is truly viewing an "ice-free" area, considering the ongoing random redistribution of H20 molecules by sputtering (see below). Third, the "percentages" we list do not refer to actual weight percent of end-members but simply to the weight of end-members in affecting the spectral signatures. It is well known that very small amounts of nonicy material can dominate the spectrum of H20 ice, at least in the visible. We consider the presence of significant amounts of non-h20 ice materials and their distribution relative to morphological features to be critical to our analysis, but their abundance in terms of weight percent is indeterminate at this time, and our analysis does not depend on determining those values. 3. Tyre Macula Tyre Macula was chosen as an especially suitable site for this interdisciplinary study for several reasons: The site contains a huge 160 x 120 km feature of clear impact origin [Lucchitta and Soderblom, 1982; Malin and Pieri, 1986] as well as several prominent lineae. The morphology of this area has been studied and interpreted in detail [Greeley et al., 1998]. These studies ½++ _ ß Ice + Tyre Asymmetric End-member, 0.1 " - + e + +_ Wavelength (microns) Figure 1. Here we depict the o spectra] end-members utilized to create Plate 1. The Tyre asymmetric band end-member can be replicated by ab6ut 1/3 Rhadamanthys Linea material and 2/3 water ice. The water ice end-member is consistent with a water ice spectrum. We expect the high water ice component in the Tyre asymmetric end-member to be due to the high proportions of surface water ice that are present in the NIMS pkels (6.26 km/pkel) along with regions of Tyre that contain pure asymmetric band material. This results in an areal spectral mkture which dilutes the asymmetric band end-member with water ice components. ß e 0,25 ' '... + Pwyll ß Pwyll "Asymmetric" Ice Endmember I :, c: :... : ß i-i, g +: ß,,, ½-... ;... :-e... ;... :... m- ß ß, ee e 11, i Wavelength (microns) Figure 2. This plot depicts the two spectral end-members selected to create the Pwyll asymmetric abundance map. The illumination levels for this NIMS data set are lower owing to the 85 ø phase angle of the observation. The asymmetric band end-member used is expected to have shadow components, which is probably why the overall albedo is low for this spectrum. This has also appeared to increase noise and decrease the albedo values in the water ice end-member spectrum as well. show that the crater Tyre was emplaced while lineae were forming. Some linea transect Tyre, while Tyre is superposed on others. This sequence of superposition allows assignment of relative age among the lineae. This is especially significant since there appears to be a definite aging effect; that is, the color in the visible fades with age [Geissler et al., 1998; Clark et al., 1998] and the non-h20 ice material becomes less abundant. This observation is evident in our false color NIMS/SSI maps of the site (Plates 1, 2, and 3). Yet another reason for selecting this site is that the NIMS and SSI spatial resolutions are more similar than elsewhere. Indeed, if taken under identical conditions, NIMS spatial resolution would be 50 times less than that of SSI. However, the NIMS and SSI were carefully coordinated in the case of this site. In part, these observations were made to satisfy different lighting conditions for the two sets of observations (SSI often prefers low lighting angle for definition of shadowed regions to highlight the topography, while NIMS requires high lighting angles for spectral observations). In the case of Tyre Macula the two teams planned observations at different times for an additional reason: the two sets of observations were coordi- nated to decrease the discrepancy in spatial resolution from x50 to x8 with the specific intent of allowing more effective coregistration of the two data sets and implementing this interdisciplinary study. The resulting NIMS-based compositional map [Granahan et al., 1997b, 1998], when superimposed upon the morphological features, shows the following: the non-h20 ice or salt component is very clearly and closely associated with the lineae (Plates 1 and 3). In the case of the crater itself is, the ring structure is associated with apparent deposits of the "non- H20 ice" component. The spectral dominance is not as great as in some of the lineae themselves but is similar to that in the area which envelops and parallels the lineae. Note that the central portion of Tyre is relatively free of non-h20 material. Plate 2 shows a SSI spectral map of the same region. A1-

3 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA 22, km.t OOo 50øo 100Oo "Salt" End- member Plate 1. This map is a composite of Galileo Near-Infrared Mapping Spectrometer (NIMS) and solid-state imaging (SSI) data. It is a map of relative "salt" or asymmetric water band concentrations in the Tyre region. The Tyre impact scar is approximately centered on 34 ø latitude and ø longitude. The NIMS data have a spatial resolution of 6.26 km/pixel and are represented here by the color-coded relative abundances. These abundances were determined using a linear spectral mixing model of water ice and asymmetric band endmembers (see text). The water ice end-members' abundances are approximately the inverse abundances of the asymmetric band member material in this plate. The SSI data have a resolution of 159 m/pixel and provide the spatial context for this map.

4 22,650 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA E

5 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA

6 22,652 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA 10 km Plate 5. This is a false color image of the Pwyll crater region collected by the Galileo SSI camera. It is a red, green, blue composite where red is 0.989, green is 0.560, and blue is 0.410/zm. The resolution of these data is 106 m/pixel. Only this strip had full coverage in these spectral bands owing to data gaps from data lost during playback. though the SSI photometric studies are constrained with broad spectral resolution of seven filters (as opposed to hundreds of units: (1) a spectrally red darker unit and (2) a much less red "lighter" unit. Once again, as with Tyre, we note the key point spectral bands in the case of NIMS), they are independently that the material which is colored in the visible correlates well important. Important for this study is the fact that the SSI photometric data extend to shorter wavelengths than the NIMS spectra. The SSI photometric map of the site shows a clear anticorrelation between 1.04/zm reflectance and depth of color. This is to be expected, since the filter which includes the H20 ice absorption band at 1.04/ m indicates the dominance of relatively pure H20 ice, whereas we attribute the coloration or "redness" of the visible spectrum to non-h20 ice materials. The key point here is that by comparing Plates 1, 2, and 3, it with the material exhibiting asymmetric bands. This repeated observation is critical because, although the cause of the band asymmetry can conceivably be debated (see section 5), it cannot be contended that pure H20 ice produces the visible coloration. It is clear by comparison of the apparent topography in Plate 4 with the degree of coloration in Plate 5 that there is a tendency for the coloration to avoid the higher areas. This prompts speculation that the non-h20 ice material may have been emplaced as a liquid. can be seen that the coloring agent in the visible correlates very well with the distribution of band-asymmetric material (see section 5). 5. Discussion This discussion deals with the implications for Europa's his- 4. P 11 tory of the relationships among (1) SSI multifilter data, (2) SSI morphological observations, (3) NIMS data in the near infrared, and (4) ultraviolet spectrometer (UVS) global data. So far, the most intense discussion in the literature has focused almost exclusively on the interpretation of the 1.4 and 2.0/zm bands defined in detail by NIMS. Therefore we will deal with this discussion in isolation from all the other remote-sensing data provided by Galileo and then examine the implications of the latter data for the interpretation of the NIMS spectra and the history of Europa. The presence of materials other than H20 ice in some regions of Europa was deduced from the very asymmetric nature of the 1.4 and 2.0/zm bands and was interpreted as suggesting the presence of hydrated minerals [Carlson eta!., 1996; Grana- We now consider the area containing the well-studied crater Pwyll. Pwyll is a much smaller crater (26 km diameter) than Tyre. However, it is of great interest because of its unusual morphology. It was recognized as a crater from Voyager images [McEwen, 1986]. It has recently been studied in detail, on the basis of the Galileo imagery, by Greeley et al. [1998] and Moore et al. [1998]. It is a relatively flat-bottomed crater with what appear to be several peaks at various places within. It is one of the youngest features on Europa, and its far-flung rays cover all preexisting features over a large (>1000 km wide region) of the surface. As with Tyre, darker material has been apparently excavated from depth (<1 km) by impact [Greeley et al., 1998] or at least exposed by the impact. Given the preceding morphological context, the NIMS results are of particular interest. These results are shown in Plate 4 where the crater appears to be flooded (however, see Moore et al. [1998]) with material that we have designated as "dark" or "salt-rich" material characterized by asymmetric bands. It is bordered sharply by the surrounding material (beige or green in Plate 4, blue in Figure 2), which is more ice rich. In Plate 4, detail can be seen which shows that there are some small areas which clearly show exceptionally high concentrations of nonicy material. There are also detectable areas when the concentra- tion of ice is higher than the ambient concentration within the crater itself. Next, we compare the NIMS map with a false color map of the same area based on SSI multifilter data (Plate 5). It can be seen that the Pwyll region consists of two primary hah et al., 1997a, 1997]. McCord et al. [1998] suggested that hydrated salt minerals were the most likely candidates [McCord eta!., 1997]. However, Dalton and Clark [1998] suggested that coarse ice, with long optical path lengths, could also exhibit the same asymmetric bands. However, McCord et al. [1998, 1999] point out that published spectra of t t20 ice do not exhibit 1.4 and 2.0 tzm bands as asymmetric as those seen in NIMS spectra of certain regions of Europa. In addition, Mc- Cord et al. [1997] present arguments against the substantial presence of alternative (to salts) hydrated minerals, such as clays, especially the absence of features in the tzm region which would characterize clay spectra. Recently, Carl- son et al. [ 1999] suggested that the infrared bands could be well fit with those of sulfuric acid. Thus the composition of what we refer to as "non-h20 ice" or "sulf" remains in doubt. Before

7 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA 22,653 including in this discussion the wide variety of other data pro- terrain, and the dark trailing side very well and were very vided by the Galileo spectra, an additional argument specific to different from those parameters as exhibited by H20 ice specthese two infrared absorptions should be noted. While Clark tra. Moreover, the visible spectrum of the leach evaporite [1981a, b] have shown that the 1.4 m band in H20 ice spectra matched the visible spectrum of the lineae and those of the exhibits a great range of band minimum positions, no pub- mottled terrain and dark side quite well. The chemical analysis lished spectra of H20 ice of any grain size exhibits a 2.0 m and infrared spectra precluded any major components other band minimum with band centers shifted to wavelengths as than a mixture of sulfates and sulfur (except for possible trace short as those (almost identically) exhibited by (1) lineae, (2) of chlorides, which could not produce coloration and which the mottled terrain, or (3) the dark trailing side terrain [Fanale have no infrared signature). et al., 1998, 1999]. An ambiguity has arisen owing to the fact that S ions are a We now consider all other relevant data and arguments. The primary constituent of the magnetosphere, and, at the same McCord et al. [1998, 1999] suggestion of hydrated salts and the time, almost the entire surface of Io, whenever it has been expressed preference for sulfates as the leading candidates are disrupted, is covered with compounds containing S. Lane et al. in harmony with many laboratory and theoretical studies. [1981] showed that the occurrence of trailing side S on Europa These studies, published over a period of 20 years, have re- was caused by magnetospheric bombardment since the S conpeatedly suggested on the basis of experimental investigations centration correlated with the area of greatest bombardment. and theoretical modeling that hydrated salts and, specifically, This was based on global-resolution UV data. As moderate sulfates would be expected to be a major constituent of the resolution UVS data on Europa have become available, a new surface of any differentiated satellite in the outer solar system interpretation has arisen. While it is obviously impossible to [Fanale et al., 1977, 1982, 1998; Kargel, 1991]. We believe that deny a role for magnetospheric implantation, it appears that the close correspondence between the spectral interpretations of the 1.4 and 2.0 m bands and independent prediction based on earlier experimental studies and theoretical predictions cannot be ignored. Recently, however, Carlson et al. [1999] have pointed out that sulfuric acid also can produce 1.4 and 1.9 m bands with a shape very nearly identical to those exhibited by the non-hno ice materials on Europa. Thus the identity of the material we refer to as salt remains in doubt, owing to the charged particle erosion, rather than simple implantation, may dominate. This erosion has apparently removed the cover of nearly pure H20 ice which covers almost all of Europa and revealed endogenic material with spectral properties similar to those found by NIMS in the immediate vicinity of the lineae, impacts, and mottled terrain [Hendrix et al., 1998; Hendrix, 1998]. In the latter cases there is less ambiguity to begin with. Magnetospheric implantation could hardly be focused on parnumerous processes which could affect and compositionally ticular targets such as the lineae. alter the optical surface; the relationship to the composition of any putative aqueous phase below also remains in doubt: We now consider the importance of the close and immediate spatial correlation among three types of terrain: (1) terrain exhibiting the strongly asymmetric bands, (2) terrain which exhibits strong coloration in the visible, and (3) terrain where there are clear morphological indicators that the icy crust has been disrupted. By disrupted terrain we include large impact craters, lineae, and the dark trailing side terrain. In comparing the SSI false color images of the Tyre and Pwyll regions (Plates Thus it now appears that some form of S of endogenic origin accompanies or is produced from sulfate salts, also of endogenic origin, and that these components are released at each particular site where tectonics, impact, or magnetospheric erosion disrupts the nearly ubiquitous surface of nearly pure H20 ice. With this skeletal hypothesis in mind, we will consider several seemingly anomalous observations and show that they are also consistent with this hypothesis. Why was the smaller (and younger) Pwyll impact apparently more effective in disrupting the H20 ice crust and exposing 2, 4, and 5) with our maps of the spatial distribution of spectra non-hno ice material than was the larger impact at Tyre (comexhibiting asymmetric 1.4 and 2.0 m bands (Plates 1 and 4), this correspondence is evident as is the association with the pare Plate 1 and Figure 1 with Plate 4 and Figure 2)? We believe that, in the particular case of Europa, this does not fractures associated with the lineae and concentric features in constitute an anomaly. First of all, tidal stresses are almost the the basins. Despite the continuing discussion of the infrared bands, it has never been suggested that pure HnO ice could be strongly colored in the visible. Therefore there exists firm evidence that indisputably non-hno ice material is always assoexclusive source of crustal disruption and heating. These are not only asymmetrically applied to the globe of Europa, but the places where the maximum effects are felt may vary with time owing to nonsynchronous rotation [Greenberg et al., 1998; ciated spatially with NIMS spectra exhibiting the asymmetric Geissler et al., 1998]. bands. Further, both are associated with local morphological evidence for disruption of the crust. Salts are typically highly reflective and neutral in the visible, but sulfur has been known to be ubiquitous in the system, both in the magnetosphere and on the surface of the satellites [e.g., Fanale et al., 1974]. Sulfur allotropes have been identified on Europa [Spencer et al., 1995; Lane et al., 1981]. Fanale et al. [1998] reported results of an experiment in which material from the Murchison meteorite, which was selected on the basis of cosmochemical reasoning alone, was As indicated above, the correlation of what appear to be high peaks with a paucity of non-hno ice material may suggest that the latter may have been introduced as a liquid. Since this would have dramatic implications, and since there may be alternative explanations, we should perhaps regard this conclusion as speculative. Also, note that there are several examples of geyser-like and cryovolcanic flows elsewhere on the Galilean satellites [Greeley et al., 1998]. Another issue is the cause of the aging of features. Tyre provides clear data on superposition and relative age of lineae leached in boiling distilled H20 to produce a "best guess" versus each other and versus the impact event. Geissler et al. version of what European ocean material should be. An evaporite deposit was produced from the leach solution. The 1.4 and 2.0 m band asymmetry and the band minima of the infrared spectrum matched those of the lineae, the mottled [1998] also noted that the lineae appeared to brighten in the visible and fade with age on the basis of the SSI multifilter data. Clark et al. [1998] pointed out that they do not brighten at 1 m and may even darken in the infrared. We believe the

8 22,654 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA explanation of both trends is simple and obvious. Magnetospheric sputtering of the surface could probably redistribute enough H20 molecules fast enough to account for both changes. While the precise rate of H20 removal and deposition is not known, it was apparently enough to denude the endogenic underlying material on the dark side in the vicinity of the most intense bombardment. There is an anomalous ocean [Greeley et al., 1998]. However, we reemphasize that our results on Tyre and Pwyll, together with the theoretical geophysical considerations we have discussed, allow for a greatly dichotomous Europa. In this dichotomous Europa we envision places where the ice (although laced with salts from previous activity) may constitute a very thick (> 10 kin) crust which has remained intact during vast portions of geologic time. aspect of this process: Geissler et al. [1998] report that not only do the lineae brighten to match the albedo of the plains material, but they continue to brighten until their albedo actually appears to exceed that of the surrounding plains. We can think 6. Conclusions 1. There is a very close and exclusive association among of no explanation for this reported "excess" brightening. A final issue may be termed "commonality." Fanale et al. [1998] and McCord et al. [1999] have noted the great similarity among spectra of various lineae, mottled terrain spectra, and dark side spectra. This could be quickly interpreted as evidence for communication of solutes in a global ocean. Our data may be too sparse to justify embracing the simple explanation without considering alternatives. To widen the discussion, we will suggest a speculative alternative which does include a global ocean or regional oceans close to the surface at many places. If we consider what we could refer to as the "implicit" model, just described, we find that Europa has an ice crust of nearly pure water ice. Below that, at various depths there is an ocean with three characteristics of parts of Europa's surface: (1) sites where the morphology suggests that disruption of the largely pure H O ice crust has occurred (by tectonics, impact, or magnetospheric effects); (2) sites where reflectance spectra exhibit asymmetric 1.4 and 2.0/ m infrared H O bands and a 2.0/ m band minimum position not found for even coarse H20 ice; and (3) colored visible spectra which definitely cannot be attributed to H O ice. Taken together, with the fact that the spectra of all these sites are remarkably similar in both the near infrared and the visible, and the unlikelihood of magnetospheric implantation focusing on tectonic and impact features, this suggests that all the non-h O material is endogenic and has a generally common origin. abundant solutes, mainly sulfates, which is exposed on the 2. Tyre is a larger and older impact than 11. Both have optical surface every time the ice crust is disrupted by any of concentrations of non-h20 ice material, but Tyre's are in the the alternative mechanisms we have discussed. Our somewhat form of concentric rings, whereas 11 appears to be more different speculative alternative is suggested by the following observations: (1) The new moderate-resolution UVS data suggest that particle erosion may have denuded the vast area called the dark side terrain. (2) It appears that lineae are disrupted and then erased by H O redistribution on a timescale short relative to the age of Europa's visible surface and much shorter than geologic time. (3) Sputtering is capable of redistributing H O molecules on a global scale and can do so in a highly discriminating manner relative to nonicy constituents at the target site [Johnson, 1990]. (4) It is unlikely that the dark terrain is unique in its internal history, as opposed to its bombardment history, and unlikely to be unique in frequency completely filled. We do not consider this to be anomalous, only a reflection of two special properties of Europa: The heating that may determine the depth to any aqueous phase is not largely due to radioactive decay, so despite the 1.3 billion year composite half-lives of 238U, 235U, 232Th, and 40K, there is no strong expectation that the depths to any aqueous phase at each site are increasing during the small fraction of geologic time exhibited by Europa's surface. In addition, the evidence for nonsynchronous rotation (on some unknown timescale) together with the asymmetry of energy input and lack of recent time dependence of tidal sources suggesthat at any given place on Europa's surface, the depth to the aqueous of extrusion. Since the maximum bombardment must be fixed phase may be irregularly oscillatory. to the trailing side, it follows that different parts of Europa's crust must be exposed by the nonsynchronous rotation in geologic time. We therefore suggesthat it is conceivable that Europa's 3. In the case of both Tyre and Pwyll there is a notable correlation between elevation and "pureness" of H O ice. The fact that nonicy material is found concentrated in what appear to be concentric low regions and the fact that this correlation "ice" crust does not resemble that seen by terrestrial telescopes holds throughout Pwyll as well suggesthat the effused matemaking hemispheric spectral observations. Instead, it consists rial may possibly have been delivered to the surface in liquid form. of an ice crust with the same salt constituents that are exposed, for example, at the local sites of the linea. That ice crust is a patchwork quilt of numerous past extrusions and, perhaps more important, intrusions. If Europa were denuded of the constantly renewed redeposited H20 from sputtering, it would globally resemble the "dark" terrain. On its surface there always exists a constantly replaced thin patina of fairly pure H20 to which recent effusions stand in contrast. Below this patina which controls the spectral appearance of nearly pure ice 4. As mentioned earlier, the striking spectral commonality among the lineae, impact sites, and mottled terrain in both the visible and infrared suggests a common origin. Naturally, a simple and intriguing explanation is that there exists a global contiguous ocean that is rich in solutes and occasionally breaches what appears to be a very pure H O ice crust. However, if one accepts a major role for particle erosion on the dark trailing side and simultaneously accepts the concept of significant nonsynchronous rotation over geologic time, then a which we obtain from Earth-based hemispheric observations, slightly different view is possible. The exposure of the dark there exists a global layer of ice with the same spectral characteristics (asymmetric bands and shifted band minima) which we associate with the immediate vicinity of the each "breaching" episode. We emphasize that below this second layer of this three-layer alternative model, there surely exists an aqueous layer at some depth which clearly manifests itself in some locations as a crust disrupting near-surface local or regional trailing terrain can hardly be revealing an atypical portion of Europan crust, except by coincidence. Therefore we must assume that the entire Europan crust resembles the dark trailing magnetospheric target zone. In the Jupiter system the position of that zone relative to the shifting crust must vary, always revealing the true nature of the stable crust. It appears that the pure ice crust observed in Earth-based hemispheric spectra

9 FANALE ET AL.: REMOTE SENSING VERSUS MORPHOLOGY ON EUROPA 22,655 may be superficial and that the ice crust is thoroughly laced everywhere with many generations of buried intrusions and extrusions. The remotely observable surface on the leading side and much of the trailing side must be regarded as only a thin patina of H20 molecules that are preferentially removed from the always changing trailing side magnetospheric target and elsewhere and randomly redistributed over the object. This patina, wherever disrupted, is constantly renewed and results in the slow fading of lineae, their brightening in the visible, and their darkening in the infrared. Acknowledgments. We thank W. Calvin and an anonymous reviewer for many useful suggestions for improvement of the manuscript and K. Ogino and M. Miyashiro for assistance in preparation of the manuscript. 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Greeley, Department of Geology, Arizona State University, Box , Tempe, AZ (greeley@asu.edu) J. Head III and R. Pappalardo, Department of Geological Sciences, Brown University, Box 1846, Providence, RI (james_head_iii@ brown.edu; pappalardo@brown.edu) A. Hendrix, Laboratory of Atmospheric and Space Physics, University of Colorado, Campus Box 590, Boulder, CO (he ndrix@ colorado.edu) J. Moore, Space Sciences Division, NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA (jmoore@mail. arc.nasa.gov) (Received June 1, 1999; revised January 12, 2000; accepted February 1, 2000.)