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Europa's icy shell: Past and present state, and future exploration
Icarus 177 (2005) 293–296 www.elsevier.com/locate/icarus
Editorial
Europa’s icy shell: Past and present state, and future exploration Europa, the Moon-sized ice-covered satellite of Jupiter, ranks with Mars and Titan in its astrobiological significance. Beneath its icy surface lies a putative water ocean up to ∼100 km deep (Zimmer et al., 2000), providing a potential habitat for life (Lipps and Rieboldt, 2005, this issue) and a compelling target for future space missions. The Galileo mission to the jovian system is now history, but the startling results have prompted renewed calls for a return to take a much closer look at Europa (Fig. 1) and her siblings Io, Ganymede, and Callisto sometime in the next decade. Despite a crippling antenna failure, full analysis of Galileo’s observations is ongoing and continues to provide surprises. For instance, spectacular examples of europan topography (see cover), generated by stereo and shape-from-shading techniques reveal that, contrary to pre-Galileo expectations, Europa is in fact a locally rugged satellite, exhibiting local relief of up to 1 km and a global dynamic range of 2 km (Schenk, 2001). The collection of reports presented here is a direct result of a 2.5 day workshop entitled “Europa’s Icy Shell: Past, Present and Future” hosted by the Lunar and Planetary Institute in Houston on February 6–8, 2004 (Schenk et al., 2004). A total of 78 scientists from the terrestrial and planetary science communities in the U.S. and Europe gathered to discuss the importance and limitations of available data on the state of the icy shell with a post-Galileo perspective. 1. The icy shell The outer icy shell of Europa covers and conceals the putative ocean, and contains a record of the satellite’s geological history. However, despite the fundamental importance of the shell, many of its properties are presently poorly understood, in particular its thickness, composition, mechanical behavior and evolution through time. The reports presented here cover all these issues, and reflect many of the major themes raised at the Workshop. An overarching theme of both the Workshop and this Special Issue is that of shell heterogeneity. Early models of Europa tended to assume a shell that was compositionally simple and homogeneous, and of a uniform thickness that did not vary with time. As we outline below, the reports presented here (and recent results published elsewhere) suggest that none of these assumptions is correct: the shell is very likely a complex, het0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.08.002
erogeneous layer with properties varying in both space and time. While this complexity makes analysis of the shell more challenging, our ability to resolve even a few of these details is a testament to the resounding success of the Galileo mission. 2. Shell thickness The thickness of the ice shell is a quantity of fundamental importance, both because it influences the astrobiological potential of the underlying ocean, and because it provides information on the balance between heat production and heat loss in the satellite. A thin (few km) shell implies direct surface-ocean communication, with the (slim) possibility of sustaining photosynthetic organisms. A thicker shell (>10 km) requires indirect ocean-surface communication via impacts, cryovolcanism and diapirism and probably restricts organisms to chemosynthetic types similar to those that populated early Earth and today’s mid-ocean ridges (Chyba and Phillips, 2002). The shell thickness is controlled by the amount of tidal dissipation within the ice shell (e.g., Ruiz, 2005, this issue); thin shell scenarios also require a partially molten silicate interior, similar to that of Io. Unfortunately, estimates of shell thickness span a range exceeding an order of magnitude (see Billings and Kattenhorn, 2005, this issue), and have led to considerable disagreements in the past. Although many of these disagreements may in fact arise from temporal or spatial variability in shell properties, no consensus has yet been reached. Analysis of surface features provides conflicting evidence. Cycloid formation apparently requires cracks to penetrate the entire ice shell, and has been interpreted as requiring a ∼1 km thick ice shell (Greenberg et al., 1998). The mechanics and formation of cycloids and other fractures are examined by Marshall and Kattenhorn (2005, this issue) and Lee et al. (2005, this issue). The large amplitude of surface topography now seen with analysis of Galileo stereo images (e.g., Prockter and Schenk, 2005, this issue, also the cover) is probably indirect evidence for a relatively thick shell at the present day: it is difficult to envision how a 2–3 km thick shell could sustain topographic variations of similar magnitude. Topography of this magnitude is also difficult to produce with purely convective models for deformation of the interior of the icy shell; complications such as compositional variations (Nimmo and Giese, 2005, this issue) or yielding (Showman and Han, 2005, this is-
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F. Nimmo et al. / Icarus 177 (2005) 293–296
Fig. 1. Galileo image of Conamara Chaos on Europa, showing ridged plains and chaos units. The matrix material containing the broken plates is associated with reddish material. Brighter regions may also indicate compositional differences. Scene width is ∼100 km.
sue) are probably required. Whether some topographic features can be explained by cryovolcanism Miyamoto et al., 2005, this issue) remains an open question. Topographic profiles may be used to estimate the elastic rigidity of the surface, and thus the shell thickness Billings and Kattenhorn, 2005, this issue; Hurford et al., 2005, this issue); such estimates vary widely depending on the type of geological feature investigated. The best evidence for the present-day ice shell thickness may come from studies of impact crater morphology. The ability of larger craters to support central peaks, and the unusual morphologic transitions compared to craters on Ganymede and Callisto both suggest that the shell is greater than 5 km and could be as thick as 20 km (e.g., Turtle and Pierazzo, 2001; Schenk, 2002). But the lack of a global crater inventory and limited compositional, topographic and geophysical data hampers our ability to use these features as true probes of the deeper ice layers on Europa. 3. Shell composition The composition of the ice shell is important because it may well reflect the chemistry of the underlying ocean, assuming there has been communication between the ocean and the shell. Furthermore, lateral variations in composition can significantly affect surface topography. Although the outer shell of Europa is dominantly composed of water ice, other dark reddish materials are present on the surface, and appear to be correlated with geological features such as chaos regions (Fig. 1). The nature of these non-ice contaminants is unclear, although most workers have now concluded from Galileo’s spectroscopic observations that they include some kind of hydrated sulfates, such as hydrated magnesium sulfate, sulfuric acids, or possibly hydronium (McCord et al., 1999; Dalton et al., 2005, this issue; Carlson and Anderson, 2005, this issue). The relative abundances of these different species remain unknown, but it is increasingly clear that no single species can completely satisfy the currently available spectroscopic constraints. Nor are the sources of the non-ice components unambiguously known:
sulfur from Io is one possibility, but the association of contaminants with surface cracks and chaos features suggests an internal origin for at least some of the contaminants. We are also hampered by our inability to resolve small concentrated deposits of non-ice materials in the very low-resolution Galileo infrared data. The composition of the ocean is even less well known than that of the ice shell. The composition has important implications for the style of fluid flow within the ocean Vance and Brown, 2005, this issue) and whether or not clathrate hydrates may form Prieto-Ballesteros et al., 2005, this issue). One of the more exciting possibilities of a future Europa orbiter is its potential ability to determine the electrical conductivity of the ocean (and thus its composition) as well as its thickness (Khurana et al., 1998). 4. Shell evolution in time A growing community consensus, reinforced at the Workshop, is that Europa’s icy shell may be laterally heterogeneous and also subject to significant temporal variations in thickness, mechanical properties, and composition, over periods comparable to the inferred mean surface age of ∼60 Myr (Zahnle et al., 2003). Models of coupled orbital–thermal evolution and ice shell stability (e.g., Hussmann and Spohn, 2004) demonstrate that it is possible for the ice shell to undergo periodic thickening and thinning. As geologic processes in a floating shell are likely to be dependent on shell thickness, surface observations may be able to constrain the evolution of the shell. For instance, changes in shell thickness are expected to have significant tectonic consequences (Nimmo, 2004; Mitri and Showman, 2005, this issue). Similarly, convective features such as diapirs may form only in a thicker shell (McKinnon, 1999). Despite the extreme youth of the surface and the lack of global mapping, there does appear to be some evidence for changing styles of geological behavior with time (e.g., Figueredo and Greeley, 2004). Europa is dominated by apparent tectonic features of both exogenic and endogenic origins. Likely stress mechanisms have
Editorial
been identified, including diurnal tides of up to 30 m, slow non-synchronous rotation, shell thickening, and internal dynamics such as solid-state overturn (and possibly localized melt-through) within the shell, but the history, magnitude and variability of these stresses over time remains only partly understood. The rate of tectonic deformation, in particular, is almost entirely unconstrained by observations, though the paper by Stempel et al. (2005, this issue) provides one such constraint. The tectonic record of Europa consists of innumerable lineations and displacements formed by different stress mechanisms that have apparently shifted location over time. This tectonic history can be thought of as like a ball of twine, with each tectonic strand cross-cutting or overprinting the strands preceding it. The added complication is that the tectonic strands on Europa are not continuous but occur in short lengths from 10s to a few 1000 km. Structural and diapiric features overlap and cross-cut in a bewildering tangle. Our job is further complicated by the Galileo high-gain antenna malfunction, limiting data rates to a few 10s of bits per second. At mission’s end in 2003, mappable imaging of Europa was chiefly limited to two narrow north–south swaths at ∼200 m resolution, plus a few additional smaller areas. It is as if we divided Europa’s ball of twine into 10 segments like an orange, but only have two of these segments to map. Unraveling this stratigraphy with such large gaps remains a daunting challenge (e.g., Prockter and Schenk, 2005, this issue), but will be key to deciphering Europa’s history. Laborious detailed geologic mapping (e.g., Figueredo and Greeley, 2004) has revealed numerous episodes of deformation from both exogenic and endogenic processes. These maps, although limited in extent, further hint at possible changes in deformation mechanisms over time. Another line of evidence that the rate and style of geological activity on Europa may have changed significantly over time lies in the apparent fact that the 150 or so known impact craters larger than 1 km across (Moore et al., 2001) show almost no signs of tectonic degradation. This suggests there may have been a rapid decline in geologic activity shortly after the formation of the presently observed ice shell. A similar scenario has been proposed to explain the observed crater population on Venus, but the lack of a global crater survey renders such hypotheses difficult to test. This is the subject of ongoing research. A final peculiarity of Europa’s tectonics is the preponderance of extensional over compressional features identified. There appear to be only two solutions to this problem: either Europa really is expanding, perhaps because of shell thickening (Nimmo, 2004); or there are compressional features which have not yet been identified, such as the small folds detected by Prockter and Pappalardo (2000), or compressional bands (e.g., Sarid et al., 2002). Again, careful mapping and rigorous analysis of high resolution images may help to resolve this question. 5. Summary After nearly a decade of investigation, it is becoming increasingly clear that the geologic history of Europa’s surface and its floating ice shell are considerably more complex than originally thought when Galileo returned its first images in
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1996. The shell is compositionally and perhaps mechanically heterogeneous, and has a thickness that has probably varied laterally and over time. The evolving shell thickness depends on the thermal and thus orbital evolution of the satellite, and exerts a major influence on the style of ice shell deformation. Thus, it may be ultimately possible to use geological observations to constrain the orbital history of Europa—a good demonstration of the cross-disciplinary nature of planetary sciences. As the reports presented here make clear, the Galileo data have provided important clues to Europa’s evolution, and will continue to do so for the foreseeable future. At the same time, many of the lessons learned from the Galileo mission will be directly applicable to the saturnian satellites, currently being investigated by the Cassini spacecraft. Nonetheless, it is clear that many questions of fundamental importance to Europa will remain unresolved with the available data. Mapping coverage of Europa’s surface is uneven (in the extreme), ranging from 10 m in a few locations to ∼1–4 km over 75% of the surface. Topographic data is even more limited (and based solely on stereo and shape-from-shading). Infrared mapping spectroscopy is limited and resolved gravity data are non-existent. Chief among the unresolved questions are the lateral and temporal variability of the (floating) icy shell, and its current state (including thickness). The mechanical and thermal properties of ice are also relevant to understanding the observed geologic features but are relatively poorly understood and require dedicated investigation. Our understanding of these properties will also influence the ability of radar sounding to penetrate an ice shell of uncertain composition (e.g., Chyba et al., 1998). These questions, coupled with Europa’s potential habitability, are likely to ensure that a return to Jupiter and Europa in particular remain at the top of NASA’s exploration priorities. References Billings, S.E., Kattenhorn, S.A., 2005. The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges. Icarus 177, 397–412. Carlson, R.W., Anderson, M.S., 2005. Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate. Icarus 177, 461–471. Chyba, C.F., Phillips, C.B., 2002. Europa as an abode of life. Origins Life Evol. Biosphere 32, 47–68. Chyba, C.F., Ostro, S.J., Edwards, B.C., 1998. Radar detectability of a subsurface ocean on Europa. Icarus 134, 292–302. Dalton, J.B., Prieto-Ballesteros, O., Kargel, J., Jamieson, C.S., Jolivet, J., Quinn, R., 2005. Spectral comparison of highly hydrated sulfate salts to disrupted terrains on Europa. Icarus 177, 472–490. Figueredo, P.H., Greeley, R., 2004. Resurfacing history of Europa from poleto-pole geological mapping. Icarus 167, 287–312. Greenberg, R., et al., 1998. Tectonic processes on Europa: Tidal stresses, mechanical response and visible features. Icarus 135, 64–78. Hurford, T., Beyer, R., Schmidt, B., Preblich, B., Sarid, A., Greenberg, R., 2005. Flexure of Europa’s lithosphere due to ridge-loading. Icarus 177, 380–396. Hussmann, H., Spohn, T., 2004. Thermal-orbital evolution of Io and Europa. Icarus 171, 391–410. Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., Walker, R.J., Polanskey, C., 1998. Induced magnetic fields as evidence for subsurface oceans on Europa and Callisto. Nature 395, 777–780. Lee, S., Pappalardo, R.T., Makris, N.C., 2005. Mechanics of tidally-driven fractures in Europa’s ice shell. Icarus 177, 367–379.
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Lipps, J.H., Rieboldt, S., 2005. Habitats and taphonomy of Europa. Icarus 177, 515–527. Marshall, S.T., Kattenhorn, S.A., 2005. A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries. Icarus 177, 341–366. McCord, T.B., et al., 1999. Hydrated salt minerals on Europa’s surface from the Galileo near-infrared mapping spectrometer (NIMS) investigation. J. Geophys. Res. 104, 11827–11851. McKinnon, W.B., 1999. Convective instability in Europa’s floating ice shell. Geophys. Res. Lett. 26, 951–954. Mitri, G., Showman, A., 2005. Convective-conductive transitions and sensitivity of a convecting ice shell to perturbations in heat flux and tidal heating rate: Implications for Europa. Icarus 177, 447–460. Miyamoto, H., Mitri, G., Showman, A.P., Dohm, J.M., 2005. Putative ice flows on Europa: Geometric patterns and relation to topography collectively constrain material properties and effusion rates. Icarus 177, 413–424. Moore, J.M., et al., 2001. Impact features on Europa: Results of the Galileo Europa Mission (GEM). Icarus 151, 93–111. Nimmo, F., 2004. Stresses generated in cooling viscoelastic ice shells: Application to Europa. J. Geophys. Res. 109, E12001. Nimmo, F., Giese, B., 2005. Thermal and topographic tests of Europa chaos formation models from Galileo E15 observations. Icarus 177, 327–340. Prieto-Ballesteros, O., Kargel, J.S., Fernandex-Sampedro, M., Selsis, F., Sebastian-Martinez, E., 2005. Evaluation of the possible presence of clathrate hydrates in Europa’s icy shell or seafloor. Icarus 177, 491–505. Prockter, L.M., Pappalardo, R.T., 2000. Folds on Europa: Implications for crustal cycling and accommodation of extension. Science 289, 941–943. Prockter, L.M., Schenk, P., 2005. Origin and evolution of Castalia Macula, an anomalous young depression on Europa. Icarus 177, 305–326. Ruiz, J., 2005. The heat flow of Europa. Icarus 177, 438–446. Sarid, A., Greenberg, R., Hoppa, G., Hurford, T., Tufts, B., Geissler, P., 2002. Polar wander and surface convergence of Europa’s icy shell. Icarus 158, 24–41. Showman, A.P., Han, L., 2005. Effects of plasticity on convection in an ice shell: Implications for Europa. Icarus 177, 425–437.
Schenk, P., 2001. Topographic variability on Europa from Galileo stereo images. Lunar Planet. Sci. XXXII. Abstract 2078. Schenk, P.M., 2002. Thickness constraints on the icy shells of the Galilean satellites from a comparison of crater shapes. Nature 417, 419–421. Schenk, P., Nimmo, F., Prockter, L., 2004. Europa’s ice shell: A bridge between its surface and ocean. EOS Trans. AGU 85 (33), 311. Stempel, M.M., Barr, A.C., Pappalardo, R.T., 2005. Model constraints on the opening rates of bands on Europa. Icarus 177, 297–304. Turtle, E.P., Pierazzo, E., 2001. Thickness of a Europan ice shell from impact crater simulations. Science 294, 1326–1328. Vance, S., Brown, J.M., 2005. Layering and double-diffusion style convection in Europa’s ocean. Icarus 177, 506–514. Zahnle, K., Schenk, P., Levison, H., Dones, L., 2003. Cratering rates in the outer solar system. Icarus 163, 263–289. Zimmer, C., Khurana, K.K., Kivelson, M.G., 2000. Subsurface oceans on Europa and Callisto: Constraints from Galileo magnetometer observations. Icarus 147, 329–347.
Francis Nimmo University of California, Santa Cruz, CA, USA Louise Prockter ∗ Applied Physics Laboratory, Johns Hopkins University, Baltimore, MD, USA E-mail address: [email protected] Paul Schenk Lunar and Planetary Institute, Houston, TX, USA * Corresponding author.
Available online 12 September 2005
Editorial
Europa’s icy shell: Past and present state, and future exploration Europa, the Moon-sized ice-covered satellite of Jupiter, ranks with Mars and Titan in its astrobiological significance. Beneath its icy surface lies a putative water ocean up to ∼100 km deep (Zimmer et al., 2000), providing a potential habitat for life (Lipps and Rieboldt, 2005, this issue) and a compelling target for future space missions. The Galileo mission to the jovian system is now history, but the startling results have prompted renewed calls for a return to take a much closer look at Europa (Fig. 1) and her siblings Io, Ganymede, and Callisto sometime in the next decade. Despite a crippling antenna failure, full analysis of Galileo’s observations is ongoing and continues to provide surprises. For instance, spectacular examples of europan topography (see cover), generated by stereo and shape-from-shading techniques reveal that, contrary to pre-Galileo expectations, Europa is in fact a locally rugged satellite, exhibiting local relief of up to 1 km and a global dynamic range of 2 km (Schenk, 2001). The collection of reports presented here is a direct result of a 2.5 day workshop entitled “Europa’s Icy Shell: Past, Present and Future” hosted by the Lunar and Planetary Institute in Houston on February 6–8, 2004 (Schenk et al., 2004). A total of 78 scientists from the terrestrial and planetary science communities in the U.S. and Europe gathered to discuss the importance and limitations of available data on the state of the icy shell with a post-Galileo perspective. 1. The icy shell The outer icy shell of Europa covers and conceals the putative ocean, and contains a record of the satellite’s geological history. However, despite the fundamental importance of the shell, many of its properties are presently poorly understood, in particular its thickness, composition, mechanical behavior and evolution through time. The reports presented here cover all these issues, and reflect many of the major themes raised at the Workshop. An overarching theme of both the Workshop and this Special Issue is that of shell heterogeneity. Early models of Europa tended to assume a shell that was compositionally simple and homogeneous, and of a uniform thickness that did not vary with time. As we outline below, the reports presented here (and recent results published elsewhere) suggest that none of these assumptions is correct: the shell is very likely a complex, het0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.08.002
erogeneous layer with properties varying in both space and time. While this complexity makes analysis of the shell more challenging, our ability to resolve even a few of these details is a testament to the resounding success of the Galileo mission. 2. Shell thickness The thickness of the ice shell is a quantity of fundamental importance, both because it influences the astrobiological potential of the underlying ocean, and because it provides information on the balance between heat production and heat loss in the satellite. A thin (few km) shell implies direct surface-ocean communication, with the (slim) possibility of sustaining photosynthetic organisms. A thicker shell (>10 km) requires indirect ocean-surface communication via impacts, cryovolcanism and diapirism and probably restricts organisms to chemosynthetic types similar to those that populated early Earth and today’s mid-ocean ridges (Chyba and Phillips, 2002). The shell thickness is controlled by the amount of tidal dissipation within the ice shell (e.g., Ruiz, 2005, this issue); thin shell scenarios also require a partially molten silicate interior, similar to that of Io. Unfortunately, estimates of shell thickness span a range exceeding an order of magnitude (see Billings and Kattenhorn, 2005, this issue), and have led to considerable disagreements in the past. Although many of these disagreements may in fact arise from temporal or spatial variability in shell properties, no consensus has yet been reached. Analysis of surface features provides conflicting evidence. Cycloid formation apparently requires cracks to penetrate the entire ice shell, and has been interpreted as requiring a ∼1 km thick ice shell (Greenberg et al., 1998). The mechanics and formation of cycloids and other fractures are examined by Marshall and Kattenhorn (2005, this issue) and Lee et al. (2005, this issue). The large amplitude of surface topography now seen with analysis of Galileo stereo images (e.g., Prockter and Schenk, 2005, this issue, also the cover) is probably indirect evidence for a relatively thick shell at the present day: it is difficult to envision how a 2–3 km thick shell could sustain topographic variations of similar magnitude. Topography of this magnitude is also difficult to produce with purely convective models for deformation of the interior of the icy shell; complications such as compositional variations (Nimmo and Giese, 2005, this issue) or yielding (Showman and Han, 2005, this is-
294
F. Nimmo et al. / Icarus 177 (2005) 293–296
Fig. 1. Galileo image of Conamara Chaos on Europa, showing ridged plains and chaos units. The matrix material containing the broken plates is associated with reddish material. Brighter regions may also indicate compositional differences. Scene width is ∼100 km.
sue) are probably required. Whether some topographic features can be explained by cryovolcanism Miyamoto et al., 2005, this issue) remains an open question. Topographic profiles may be used to estimate the elastic rigidity of the surface, and thus the shell thickness Billings and Kattenhorn, 2005, this issue; Hurford et al., 2005, this issue); such estimates vary widely depending on the type of geological feature investigated. The best evidence for the present-day ice shell thickness may come from studies of impact crater morphology. The ability of larger craters to support central peaks, and the unusual morphologic transitions compared to craters on Ganymede and Callisto both suggest that the shell is greater than 5 km and could be as thick as 20 km (e.g., Turtle and Pierazzo, 2001; Schenk, 2002). But the lack of a global crater inventory and limited compositional, topographic and geophysical data hampers our ability to use these features as true probes of the deeper ice layers on Europa. 3. Shell composition The composition of the ice shell is important because it may well reflect the chemistry of the underlying ocean, assuming there has been communication between the ocean and the shell. Furthermore, lateral variations in composition can significantly affect surface topography. Although the outer shell of Europa is dominantly composed of water ice, other dark reddish materials are present on the surface, and appear to be correlated with geological features such as chaos regions (Fig. 1). The nature of these non-ice contaminants is unclear, although most workers have now concluded from Galileo’s spectroscopic observations that they include some kind of hydrated sulfates, such as hydrated magnesium sulfate, sulfuric acids, or possibly hydronium (McCord et al., 1999; Dalton et al., 2005, this issue; Carlson and Anderson, 2005, this issue). The relative abundances of these different species remain unknown, but it is increasingly clear that no single species can completely satisfy the currently available spectroscopic constraints. Nor are the sources of the non-ice components unambiguously known:
sulfur from Io is one possibility, but the association of contaminants with surface cracks and chaos features suggests an internal origin for at least some of the contaminants. We are also hampered by our inability to resolve small concentrated deposits of non-ice materials in the very low-resolution Galileo infrared data. The composition of the ocean is even less well known than that of the ice shell. The composition has important implications for the style of fluid flow within the ocean Vance and Brown, 2005, this issue) and whether or not clathrate hydrates may form Prieto-Ballesteros et al., 2005, this issue). One of the more exciting possibilities of a future Europa orbiter is its potential ability to determine the electrical conductivity of the ocean (and thus its composition) as well as its thickness (Khurana et al., 1998). 4. Shell evolution in time A growing community consensus, reinforced at the Workshop, is that Europa’s icy shell may be laterally heterogeneous and also subject to significant temporal variations in thickness, mechanical properties, and composition, over periods comparable to the inferred mean surface age of ∼60 Myr (Zahnle et al., 2003). Models of coupled orbital–thermal evolution and ice shell stability (e.g., Hussmann and Spohn, 2004) demonstrate that it is possible for the ice shell to undergo periodic thickening and thinning. As geologic processes in a floating shell are likely to be dependent on shell thickness, surface observations may be able to constrain the evolution of the shell. For instance, changes in shell thickness are expected to have significant tectonic consequences (Nimmo, 2004; Mitri and Showman, 2005, this issue). Similarly, convective features such as diapirs may form only in a thicker shell (McKinnon, 1999). Despite the extreme youth of the surface and the lack of global mapping, there does appear to be some evidence for changing styles of geological behavior with time (e.g., Figueredo and Greeley, 2004). Europa is dominated by apparent tectonic features of both exogenic and endogenic origins. Likely stress mechanisms have
Editorial
been identified, including diurnal tides of up to 30 m, slow non-synchronous rotation, shell thickening, and internal dynamics such as solid-state overturn (and possibly localized melt-through) within the shell, but the history, magnitude and variability of these stresses over time remains only partly understood. The rate of tectonic deformation, in particular, is almost entirely unconstrained by observations, though the paper by Stempel et al. (2005, this issue) provides one such constraint. The tectonic record of Europa consists of innumerable lineations and displacements formed by different stress mechanisms that have apparently shifted location over time. This tectonic history can be thought of as like a ball of twine, with each tectonic strand cross-cutting or overprinting the strands preceding it. The added complication is that the tectonic strands on Europa are not continuous but occur in short lengths from 10s to a few 1000 km. Structural and diapiric features overlap and cross-cut in a bewildering tangle. Our job is further complicated by the Galileo high-gain antenna malfunction, limiting data rates to a few 10s of bits per second. At mission’s end in 2003, mappable imaging of Europa was chiefly limited to two narrow north–south swaths at ∼200 m resolution, plus a few additional smaller areas. It is as if we divided Europa’s ball of twine into 10 segments like an orange, but only have two of these segments to map. Unraveling this stratigraphy with such large gaps remains a daunting challenge (e.g., Prockter and Schenk, 2005, this issue), but will be key to deciphering Europa’s history. Laborious detailed geologic mapping (e.g., Figueredo and Greeley, 2004) has revealed numerous episodes of deformation from both exogenic and endogenic processes. These maps, although limited in extent, further hint at possible changes in deformation mechanisms over time. Another line of evidence that the rate and style of geological activity on Europa may have changed significantly over time lies in the apparent fact that the 150 or so known impact craters larger than 1 km across (Moore et al., 2001) show almost no signs of tectonic degradation. This suggests there may have been a rapid decline in geologic activity shortly after the formation of the presently observed ice shell. A similar scenario has been proposed to explain the observed crater population on Venus, but the lack of a global crater survey renders such hypotheses difficult to test. This is the subject of ongoing research. A final peculiarity of Europa’s tectonics is the preponderance of extensional over compressional features identified. There appear to be only two solutions to this problem: either Europa really is expanding, perhaps because of shell thickening (Nimmo, 2004); or there are compressional features which have not yet been identified, such as the small folds detected by Prockter and Pappalardo (2000), or compressional bands (e.g., Sarid et al., 2002). Again, careful mapping and rigorous analysis of high resolution images may help to resolve this question. 5. Summary After nearly a decade of investigation, it is becoming increasingly clear that the geologic history of Europa’s surface and its floating ice shell are considerably more complex than originally thought when Galileo returned its first images in
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1996. The shell is compositionally and perhaps mechanically heterogeneous, and has a thickness that has probably varied laterally and over time. The evolving shell thickness depends on the thermal and thus orbital evolution of the satellite, and exerts a major influence on the style of ice shell deformation. Thus, it may be ultimately possible to use geological observations to constrain the orbital history of Europa—a good demonstration of the cross-disciplinary nature of planetary sciences. As the reports presented here make clear, the Galileo data have provided important clues to Europa’s evolution, and will continue to do so for the foreseeable future. At the same time, many of the lessons learned from the Galileo mission will be directly applicable to the saturnian satellites, currently being investigated by the Cassini spacecraft. Nonetheless, it is clear that many questions of fundamental importance to Europa will remain unresolved with the available data. Mapping coverage of Europa’s surface is uneven (in the extreme), ranging from 10 m in a few locations to ∼1–4 km over 75% of the surface. Topographic data is even more limited (and based solely on stereo and shape-from-shading). Infrared mapping spectroscopy is limited and resolved gravity data are non-existent. Chief among the unresolved questions are the lateral and temporal variability of the (floating) icy shell, and its current state (including thickness). The mechanical and thermal properties of ice are also relevant to understanding the observed geologic features but are relatively poorly understood and require dedicated investigation. Our understanding of these properties will also influence the ability of radar sounding to penetrate an ice shell of uncertain composition (e.g., Chyba et al., 1998). These questions, coupled with Europa’s potential habitability, are likely to ensure that a return to Jupiter and Europa in particular remain at the top of NASA’s exploration priorities. References Billings, S.E., Kattenhorn, S.A., 2005. The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges. Icarus 177, 397–412. Carlson, R.W., Anderson, M.S., 2005. Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate. Icarus 177, 461–471. Chyba, C.F., Phillips, C.B., 2002. Europa as an abode of life. Origins Life Evol. Biosphere 32, 47–68. Chyba, C.F., Ostro, S.J., Edwards, B.C., 1998. Radar detectability of a subsurface ocean on Europa. Icarus 134, 292–302. Dalton, J.B., Prieto-Ballesteros, O., Kargel, J., Jamieson, C.S., Jolivet, J., Quinn, R., 2005. Spectral comparison of highly hydrated sulfate salts to disrupted terrains on Europa. Icarus 177, 472–490. Figueredo, P.H., Greeley, R., 2004. Resurfacing history of Europa from poleto-pole geological mapping. Icarus 167, 287–312. Greenberg, R., et al., 1998. Tectonic processes on Europa: Tidal stresses, mechanical response and visible features. Icarus 135, 64–78. Hurford, T., Beyer, R., Schmidt, B., Preblich, B., Sarid, A., Greenberg, R., 2005. Flexure of Europa’s lithosphere due to ridge-loading. Icarus 177, 380–396. Hussmann, H., Spohn, T., 2004. Thermal-orbital evolution of Io and Europa. Icarus 171, 391–410. Khurana, K.K., Kivelson, M.G., Stevenson, D.J., Schubert, G., Russell, C.T., Walker, R.J., Polanskey, C., 1998. Induced magnetic fields as evidence for subsurface oceans on Europa and Callisto. Nature 395, 777–780. Lee, S., Pappalardo, R.T., Makris, N.C., 2005. Mechanics of tidally-driven fractures in Europa’s ice shell. Icarus 177, 367–379.
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Francis Nimmo University of California, Santa Cruz, CA, USA Louise Prockter ∗ Applied Physics Laboratory, Johns Hopkins University, Baltimore, MD, USA E-mail address: [email protected] Paul Schenk Lunar and Planetary Institute, Houston, TX, USA * Corresponding author.
Available online 12 September 2005