JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,
Ocean Worlds in the Outer Solar System F. Nimmo1 and R.T. Pappalardo2
Abstract. Many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells. This article first reviews how such oceans are detected. We then discuss how they are maintained, when they formed, and what the oceans’ likely char- acteristics are. We focus in particular on Europa, Ganymede, Callisto, Titan and Ence- ladus, bodies for which there is direct evidence of subsurface oceans. We also consider candidate ocean worlds such as Pluto and Triton.
1. Introduction consequences of these oceans; and the extent to which they might be habitable. We begin with a general discussion of The outer solar system contains a stunningly diverse ar- each of these topics, and then discuss applications to differ- ray of planetary bodies (Figs 1,2): Europa, with its bizarre ent ocean worlds. We focus in particular on the five bodies array of surface features; tiny, geologically-active Enceladus; for which oceans are currently best established - Europa, Titan, the only moon with a substantial atmosphere; Pluto, Ganymede, Callisto, Enceladus and Titan - but where ap- with its nitrogen glaciers; and many others. Over the last propriate we mention other potential ocean worlds, includ- twenty-five years spacecraft measurements have revealed ing Triton and Pluto. that many of these bodies are “ocean worlds”, possessing large volumes of liquid water beneath insulating ice shells. In this article we will review the exploration history of these 2. History of Exploration and Ideas ocean worlds, what we currently know about them, and what revelations the next twenty-five years may bring. Twenty-five years ago, essentially all our knowledge of the Ocean worlds are important for several reasons, but the outer solar system satellites was thanks to two spacecraft. most compelling is also the simplest: they could be abodes Although the Pioneer 10 and 11 spacecraft acquired a few of life. Life as we know it requires liquid water, in addition images of Jovian and Saturnian moons, little was known un- to energy and nutrients, and all three requirements can po- til the Voyager flybys. Voyager 1 encountered the Jupiter tentially be satisfied within some of these bodies (Section 9). system in 1979 and the Saturn system in 1980, while Voy- Future spacecraft investigations are likely to be increasingly ager 2 had similar encounters in 1979 and 1981, respectively, focused on determining the extent to which particular bod- and then went on to encounter the Uranian and Neptunian ies are potentially habitable. systems in 1986 and 1989. The Jovian, Saturnian and initial There are at least two other reasons that ocean worlds Uranian results are documented in books edited by Morri- are important to study. First, they represent systems that son [1982], Burns and Matthews [1986] and Bergstralh et al. are both more complex and less well understood than the [1991]. terrestrial planets. For example, on many ocean worlds the In 1991 it was unclear whether any of the outer solar sys- main source of heat is energy extracted from their orbits tem satellites were truly ocean worlds. Europa was consid- (Section 5.2). There is thus a strong coupling between ther- ered to be heavily tectonically deformed and to have a sur- mal and orbital evolution that is almost entirely absent from face that was young and/or warm [Lucchitta and Soderblom, the terrestrial planets. Similarly, the dynamics of global 1982; Malin and Pieri, 1986]. A subsurface ocean was con- oceans beneath tidally flexing ice shells represents a rich set sidered plausible [Ojakangas and Stevenson, 1989], but there of problems which have barely begun to be explored (Section was no direct evidence. Enceladus was also known to be 8). Second, the characteristics of these worlds provide clues deformed and to have young, resurfaced regions and re- to their history, and the evolution of the solar system as a laxed craters [Kargel and Pozio, 1996]. Although the then- whole. For instance, the fact that tiny Enceladus has man- available images did not cover the now-famous active south aged to retain a global subsurface ocean may be telling us polar region, it was recognized that the existence of the ten- something profound about both its orbital history (Section uous E-ring centred on Enceladus could be the result of ge- 5.2) and the evolution of the Saturnian system. ological activity [Pang et al., 1984]. Ganymede’s extended The main aim of this paper is to summarize how our history of deformation was known [Shoemaker et al., 1982], understanding of ocean worlds has developed over the last and the possibility of at least a transient internal ocean rec- twenty-five years. The subject is large enough that we can- ognized [Kirk and Stevenson, 1987]. Here cryovolcanism not provide a comprehensive review. Instead, we have de- was thought to have been an important resurfacing process, cided to focus on a series of questions, and to point the though more recent observations have revealed that tecton- reader to other reviews where individual topics are treated ics has a dominant role in resurfacing [Head et al., 2002]. in more detail. We discuss in turn the current understand- The surface and interior of Titan, owing to its opaque atmo- ing of where oceans are located; how they are detected; how sphere, were essentially unknown, although the possibility of they are maintained; when they formed; characteristics and liquid ethane at the surface had been recognized [Sagan and Dermott, 1982; Lunine et al., 1983]. Important theoretical work was carried out during, and 1Department of Earth and Planetary Sciences, University even prior, to the Pioneer and Voyager missions. In par- of California, Santa Cruz, California, USA ticular, Lewis [1971] showed that radioactive decay alone 2Jet Propulsion Laboratory, Pasadena, California, USA was likely sufficient to produce subsurface oceans in large moons, particularly if ammonia was present, while Goldre- ich and Soter [1966] explained the role of tides in driving Copyright 2016 by the American Geophysical Union. outwards satellite migration, damping eccentricities and de- 0148-0227/16/$9.00 termining spin rates. Peale et al. [1979] pointed out the 1 X - 2 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM potentially important role of tidal heating, famously pre- Titan’s two degree-two gravity coefficients have been in- dicting Io’s extreme volcanic activity just prior to the Voy- dependently determined, and indicate that it is very close ager encounter. Much subsequent work investigated the role to hydrostatic equilibrium, so its MoI can be deduced [Iess of tidal heating on satellite evolution and structure. For et al., 2010]. The fact that Titan’s shape is not hydrostatic instance, Cassen et al. [1979] suggested that tidal heating can be explained by variations in the thickness of a com- could potentially sustain a Europa ocean, while Ojakangas pensated ice shell [Nimmo and Bills, 2010]. Although its and Stevenson [1989] argued that in this case a floating nominal MoI suggests that Titan is not completely differ- entiated, more recent analyses have suggested that a fully- ice shell would experience thickness variations. A sugges- differentiated body could also be consistent with the ob- tion that satellites in resonance could experience oscillatory servations [Gao and Stevenson, 2013; Baland et al., 2014]. tidal heating [Ojakangas and Stevenson, 1986] turns out to The shapes of Europa, Ganymede and Callisto do not pro- be potentially important for both the Jovian and Saturnian vide tight constraints on whether hydrostatic equilibrium is systems (Section 5.2). maintained, while in none of these cases has it been possi- Since 1991, a wealth of new information has been gener- ble to obtain independent measures of J2 and C22 [Schubert ated. In particular, the Galileo mission entered the Jupiter et al., 2004]. We thus do not know whether any of these bod- system in 1995 and spent the next 8 years returning data, ies is in hydrostatic equilibrium. The oft-repeated assertion despite the failure of its main antenna. Cassini arrived in that Callisto’s MoI indicates only partial differentiation [An- the Saturn system in 2004 and is still operating at the time derson et al., 2001] is thus based on an assumption which of this writing. And most recently the New Horizons space- may not be correct [McKinnon, 1997]. For Europa, the as- craft encountered Pluto and Charon in July 2015. sumption of hydrostatic equilibrium is reasonable, given its Earth-based observations have also contributed to the presumed thin ice shell, but again we do not know for sure. growing body of knowledge. Highlights include a possible Although the data are not as good as for Titan, the grav- ity moments of Enceladus have been measured [Iess et al., detection by the Hubble Space Telescope (HST) of eruptive 2014]. Enceladus is non-hydrostatic at the ∼5% level, but activity at Europa [Roth et al., 2014], astrometric observa- by using a combination of shape and gravity data the hydro- tions of satellite orbital evolution [Lainey et al., 2009, 2012] static contributions have been isolated and its MoI deduced and the ongoing detection and characterization of Kuiper [Iess et al., 2014; McKinnon, 2015]. An important result of Belt Objects [Brown, 2012, e.g.]. this study was the conclusion that Enceladus’s rocky core A pre-New Horizons summary of Pluto and Charon is in has a low density, due to either high porosity or hydrother- the book edited by Stern and Tholen [1997], while Triton mal alteration. is described in articles in the book edited by Cruikshank Even in the absence of observational constraints, it is [1996]. Post-Galileo understanding of the Jovian satellites common to assume that ocean-bearing bodies are differen- is summarized in books edited by Bagenal et al. [2004] and tiated. This is because melting of water ice will lead to Pappalardo et al. [2009] while books documenting our cur- efficient downwards segregation of denser silicates. Mod- rent understanding of the Saturnian satellites include Brown els allowing ocean formation while not requiring complete et al. [2010], Dougherty et al. [2009] and M¨uller-Wodarg differentiation have been proposed [Nagel et al., 2004] but, et al. [2014]. A useful review of satellite orbital and thermal as noted above, the evidence for incompletely differentiated ocean-bearing bodies is weak. evolution is Peale [1999]. A generic icy satellite will thus consist of an H2O layer sit- ting atop a silicate core. For a small satellite like Enceladus, 3. Where are oceans located? any ocean will sit directly above the silicates and below a solid icy shell. For a larger ice-rich body like Ganymede, To understand where subsurface oceans are located, it is pressures are sufficiently high that the ice at depth will first necessary to understand the gross internal structure of transform to higher pressure phases. These phases are icy worlds, and how such structures are deduced. A good denser than Ice I, have melting temperatures that increase with pressure, and have comparable rigidities but higher vis- general introduction is in Hussmann et al. [2015]. cosities than Ice I [Sotin et al., 1998]. If high-pressure ices The internal structure of icy satellites is generally de- are present, there will be a “water sandwich” with the ocean duced from measurements of their bulk density, gravity mo- sitting between the ice-I shell and the higher-pressure phases ments and shape. Bulk density can be used to roughly in- (Figure 4). An important difference between these two cases fer the rock-ice ratio, though this analysis is complicated is that for the small satellite the ocean is in direct contact by the potential role of higher pressure ice phases and/or with the silicates, while the same is not true for the larger porosity, and uncertainties in the correct silicate density to body. assume. Nonetheless a body like Tethys, with a bulk den- Satellite densities are consistent with variable mixes of sity of 970 kgm−3, must be composed almost entirely of ice ice and rock (Figure 3), with more variability at smaller (Fig 3). distances to the primary. Io’s and Europa’s relative deple- Determining the moment of inertia (MoI) of a body can tion in ice are most likely due to either formation conditions help assess whether it has undergone differentiation (separa- in the proto-satellite disk [Lunine and Stevenson, 1982] or tion into rock-dominated and ice-dominated layers) or not. the result of tidal heating causing volatile loss [Dwyer et al., Shape or gravity measurements can in some cases be used to 2013]. Impact disruption of satellites either during accretion [Zahnle et al., 2003, e.g.] or a putative Late Heavy Bom- infer the MoI - if the body is in hydrostatic equilibrium (i.e. dardment [Charnoz et al., 2009; Movshovitz et al., 2015] may behaving like a fluid on long timescales). Unfortunately, have been quite common. In particular, ice-rich bodies like proving that a body is in hydrostatic equilibrium requires ei- Tethys could be fragments of an ancient large body that ther both degree-two gravity coefficients or both degree-two underwent catastrophic disruption [Asphaug and Reufer, shape coefficients to be measured (an object’s gravity can be 2013]. Similarly, Charon’s slightly ice-rich nature relative to decomposed into spherical harmonics; degree-two harmon- Pluto is consistent with it being a fragment of proto-Pluto ics have wavelengths of 180◦). For the gravity coefficients, removed by an ancient impact [Canup, 2011]. this requires at least one roughly equatorial and one roughly polar flyby. Rhea provides a cautionary example of this 4. How are oceans detected? problem: initial analyses based on a single flyby assumed that Rhea was hydrostatic, but a second flyby showed that Although determining the MoI is helpful for inferring bulk the hydrostatic assumption was not correct [Tortora et al., structure, the densities of solid and liquid H2O are too simi- 2016], invalidating the previous MoI determinations. lar to allow the presence of a subsurface ocean to be inferred. NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 3
Instead, other geophysical techniques are used, as described as tides) can potentially detect a subsurface ocean. One below. There are three main geophysical approaches by general drawback of this technique is that it tends to be which subsurface oceans have been (or could be) detected: insensitive to ocean thickness: a thin and a thick ocean pro- magnetic induction, geodetic approaches, and radar sound- vide almost the same degree of decoupling and are thus hard ing. Compositional information can in some cases be used to distinguish (although a thick and thin ice shell produce to argue for the presence of an ocean; observations of surface geological features (Section 4.5) are suggestive, but geophys- notably different results). ical methods are more conclusive. Moreover, compositional 4.2.1. Librations and geological observations cannot necessarily distinguish The most direct technique is to look for forced physical li- between extant and ancient oceans, while the geophysical brations in longitude - modulations of the satellite’s rotation techniques described below are sensitive only to present-day rate over the course of an orbit. For a synchronous satellite oceans. in an eccentric orbit any permanent tidal bulge will rock backwards and forwards across the sub-primary point, re- 4.1. Magnetic Induction sulting in torques which cause the rotation rate to vary (Fig This approach is well described in Khurana et al. [2002]. 5). For a body which is rigid at the present day, the mag- The basic principle is simple: a conductor (such as a salty nitude of the torque depends on the equatorial moment dif- ocean) embedded in a time-varying magnetic field will expe- ference which depends on the degree-two gravity coefficient rience induction currents which generate a secondary mag- C22. The response to the torque depends on the moment netic field. This secondary field can be detected, and will of inertia of the body [Tiscareno et al., 2009, e.g.]. Thus, if vary in magnitude and direction as the inducing field varies, the gravity coefficients and physical libration amplitude are thus allowing it to be distinguished from a permanent in- known, the moment of inertia of the body can, in principle, ternal field. Because Jupiter’s magnetic field is tilted by 10◦ relative to its rotation axis, the Galilean satellites ex- be determined. perience a field that varies with Jupiter’s roughly ten-hour If the ice shell is decoupled from the interior, the analysis synodic rotation period. Induced fields have been defini- becomes more complicated. The shell permanent bulge may tively identified at Europa and Callisto [Zimmer et al., 2000]. only represent some fraction of the total bulge, but the mo- Interpretation of the magnetic data at Ganymede is more ment of inertia of the shell will certainly be less than that complicated [Kivelson et al., 2002], because that body also of the body as a whole. Additionally, if the shell is elastic apparently possesses a permanent internal field. but not infinitely rigid any libration will cause shell defor- For both Europa and Callisto, the induction response is mation, which reduces the libration amplitude [Goldreich almost as strong as that of a perfect conductor. This re- and Mitchell, 2010; Van Hoolst et al., 2013; Jara-Oru´eand quires that the conductor cannot be deeply buried - like an iron core - and is moderately to highly conductive (depend- Vermeersen, 2014]. As a result, the libration amplitude of ing on the thickness assumed). A salty ocean at least a few the shell can be either greater or less than that of a rigid km thick would produce the correct response, and is con- body, depending mainly on the shell thickness and rigidity sistent with other lines of evidence (see below). Europa’s [Jara-Oru´eand Vermeersen, 2014]. induction response is the main reason that the presence of Despite recent progress, the librational behavior of vis- a subsurface ocean is so widely accepted. coelastic bodies with subsurface oceans is not yet completely If the field only varies at a single period, then only the understood. Furthermore, measuring librations is compli- product of the conductive layer’s thickness and conductivity cated because the amplitudes are small (typically kilome- can be recovered [Zimmer et al., 2000, e.g.]. The Galilean ters or less), requiring high-resolution repeat imaging and a satellites will experience time-varying fields at Jupiter’s syn- very accurate control network (or radar observations, see be- odic rotation period, their own orbital period, and harmon- ics thereof. With multiple periods, it is in principle possible low). Additionally there are longer-period librations arising to recover both the conductivity and layer thickness [Khu- from orbital perturbations [Rambaux et al., 2011], and po- rana et al., 2002]; indeed, this is a major aim of the planned tentially also non-synchronous rotation[Greenberg and Wei- Europa Clipper mission [Pappalardo et al., 2014] and ESA’s denschilling, 1984], both of which make detection of libra- Jupiter Icy Moons Explorer (JUICE) mission to Ganymede tions at the orbital period more challenging. Nonetheless, [Grasset et al., 2013]. for two icy satellites libration amplitudes have now been Saturn’s magnetic field is almost perfectly aligned with its measured, as described next. rotation axis [Cao et al., 2011]. Thus, the Saturnian satel- The libration amplitude at Mimas is marginally larger lites do not experience a time-varying field as the Galilean than that expected for a solid body having Mimas’s shape satellites do. They will experience smaller variations due to their varying distance to Saturn - a consequence of ec- [Tajeddine et al., 2014]. While this could in principle be centric orbits. But even for Titan, with its relatively large due to a decoupling ocean, Mimas is so geologically inactive eccentricity, the resulting induction effect has not yet been that a more likely explanation is a slightly distorted rocky measured. The large tilt of Neptune’s magnetic field relative core. These alternatives could be distinguished if Mimas’s to its rotation axis [Connerney et al., 1991] offers promise degree-two gravity coefficients were measured. for future detection of an ocean at Triton via the induction At Enceladus, the libration amplitude is significantly method. larger than that expected for a solid body, based on its A recent extension of the induction technique [Saur et al., known gravity coefficients [Thomas et al., 2016]. The pres- 2015] has been to use Earth-based (HST) optical observa- ence of subsurface water was already strongly suspected be- tions of Ganymede’s aurora, which responds to both the external inducing field and to any induced field. This tech- cause of the salty nature of the plumes (see Section 4.4), but nique lacks the precision of spacecraft magnetometer obser- the libration measurements argue that the ocean is global, vations, but it inferred the presence of a subsurface ocean not simply regional (because only a global ocean can decou- and has the considerable advantage of not requiring a dedi- ple the shell from the interior), and that the ice shell is thin cated mission. relative to the satellite radius. 4.2.2. Obliquity 4.2. Geodesy A second geodetic approach is to measure the obliquity If a subsurface ocean exists, the ice shell will be mechan- of the satellite - the angle between its rotation axis and the ically decoupled from the deeper interior. As a result, mea- orbit pole. A simplified explanation is as follows [Bills and surements of the response of the shell to external forces (such Nimmo, 2008]: for a satellite with an inclined orbit, torques X - 4 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM will cause its rotation axis to precess, while its orbit will may exhibit a resonant response which significantly increases itself also exhibit precession. Dissipation in the satellite will the overall k2 and h2 beyond what would be expected for then drive it towards a so-called Cassini state, in which the a passive decoupling layer [Kamata et al., 2015]. Similarly, orbit pole and the spin pole remain coplanar as they circle such calculations usually assume that the various layers are a third (invariable) pole (Fig 6). In this state the obliquity spherically symmetric, which for somewhere like Enceladus then depends on the satellite’s orbital precession rate (which is likely a significant over-simplification. The tidal response depends mainly on the known C20 gravity coefficient of the of bodies with lateral variations in mechanical properties can planet), the satellite’s degree-two gravity moments (which be calculated [A et al., 2014] but doing so is computationally can be measured), and its moment of inertia. challenging and not likely to be testable with observations The analysis then proceeds in a very similar manner to unless an orbiter is involved. that for librations. For a solid body, given the orbital preces- sion rate and the degree-two gravity, the moment of inertia 4.3. Radar Sounding can be deduced directly. For a decoupled shell the analysis is much more complicated, and indeed a complete solution Cold ice is relatively radar-transparent. As a result, radar does not yet appear to have been derived. Nonetheless, if the sounding has been used to image the subsurface structure observed obliquity is much larger than that predicted for a of ice caps on both Earth [Peters et al., 2007] and Mars solid body, a decoupled shell is a reasonable inference. This [Phillips et al., 2008]. In principle, the same technique could is exactly the situation that applies at Titan: radar images be used to probe the structure of an icy satellite’s shell have been used to deduce an obliquity that is about three [Blankenship et al., 2009]. Echoes from off-nadir surface times larger than it should be for a solid body [Bills and reflectors produce “clutter” which complicate the interpre- Nimmo, 2011]. A plausible explanation for this discrepancy tation, but with sufficiently precise measurements of sur- is the presence of a subsurface ocean. face topography this problem can be eliminated [Holt et al., The biggest challenge that obliquity measurements face is 2006]. The more serious challenge is that radar absorption that the predicted obliquities are in many cases quite small. in ice is a very strong function of temperature [Chyba et al., Chen et al. [2014] tabulated predicted obliquities for all the 1998]. Thus, as the ice-ocean interface is approached, the icy satellites, finding values of 0.35◦ and 0.24◦ for Triton and Callisto, but 0.05◦ and 0.00014◦ for Europa and Ence- radar signals become significantly attenuated. As a result, ladus. The difference between obliquities with and without even in a conductive ice shell detection of a subsurface ocean an ocean would likely be even smaller. More complicated more than roughly 10 km deep is likely to be very challeng- analyses take into account the fact that spin and orbit poles ing. In a convecting shell (where a thick layer of warm ice ex- are forced at a variety of different frequencies [Baland et al., ists), detection of an ocean is likely to be impossible in most 2016], but the predicted obliquities are not very different places, except perhaps in localized cold, downwelling regions from the simple analysis. [McKinnon, 2005]. On the other hand, convection may de- As with the induction technique, there exists the possi- velop anisotropic fabrics in the ice which are also in principle bility of carrying out obliquity (or libration) observations detectable with radar [Barr and Stillman, 2011]. Although remotely. These would be accomplished using Earth-based not yet demonstrated at icy satellites, radar sounding is a radar, as has been done very successfully at Mercury [Mar- promising technique, and sounders form part of the payload got et al., 2007]. Although some preliminary measurements of both the Europa Clipper and the JUICE missions [Bruz- have been made for the Galilean satellites [Margot et al., zone et al., 2013, e.g.]. 2013] no complete analysis has been published to date. 4.2.3. Tidal Response 4.4. Compositional evidence The decoupling effect of a subsurface ocean can also be Prior to the libration observations, the strongest evidence manifested in its response to tides. A satellite in an eccen- tric orbit experiences a time-varying potential. Its response for a subsurface ocean on Enceladus came from composi- to this perturbation can be measured, and is described by tional measurements of its plume. The Cassini spacecraft sampled the plume material directly and found that some the dimensionless degree-two tidal Love numbers: k2, which describes the response of the satellite’s own gravity field, of the ice grains were Na-rich [Postberg et al., 2009]. These measurements constituted strong evidence for the presence and h2 and l2, which describe radial and lateral surface dis- placement, respectively. of subsurface liquid water that had interacted with silicates As one might expect, a shell which is decoupled from (see below), and effectively ruled out the possibility that the interior by a subsurface ocean generally shows a much the water vapor was being produced from solid ice via sub- larger tidal response i.e. larger h2 and k2 [Moore and Schu- limation [Nimmo et al., 2007]. Interestingly, spectroscopic bert, 2000, 2003]. Thus, measurement of either h2 or k2 can observations of the E-ring failed to detect any Na [Schneider in principle determine whether a subsuface ocean is present. et al., 2009], most likely because the salt-rich grains were too The quantity 1 + k2 − h2 is particularly dependent on the heavy to reach such distances from the source. thickness of the ice shell [Wahr et al., 2006], but the thick- In the absence of samples from the subsurface, compo- ness obtained depends on the rigidity assumed. sitional measurements can still be made via spectroscopy. In practice, determining k2 using Doppler tracking of Infrared spectra of Ganymede and Europa’s surfaces show spacecraft flybys is challenging, while measuring h2 requires features which are likely to be caused by hydrated salts [Mc- an altimeter. As a result, to date the only determination Cord et al., 1999, 2001; Fischer et al., 2015] and thus sug- of k2 in the outer solar system has been made at Titan. gestive of liquid water. Subsequent spectral mixing analy- The initial result yielded k2 ≈ 0.6 ± 0.2 [Iess et al., 2012]. ses based on laboratory data are consistent with this result The central value was larger than previously-published pre- [Shirley et al., 2010, e.g.]. These studies also suggest the dictions and indicated a deformable interior. This result is presence of sulfuric acid hydrate, as originally suggested by certainly consistent with a deep global ocean, although it Carlson et al. [2005]. does not rule out a relatively rigid ice shell on top [Heming- way et al., 2013]. 4.5. Surface Features When calculating the tidal response of a body, any sub- surface ocean is usually treated as a passively-responding Surface observations of geological features have frequently layer. In reality, of course, the ocean response to tides may been used to argue for the presence of a subsurface ocean, be quite complex (Section 5.2). In particular, thin oceans but in general such arguments have not been regarded as NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 5 conclusive. For instance, global patterns of extensional tec- expected higher temperatures during accretion of Jupiter tonics (seen on many icy worlds; Fig 2a,2g) and possible ex- satellites compared with the satellites of Saturn [Lunine and amples of contractional tectonics (Fig 2f) could both be due Stevenson, 1982]. A lack of ammonia would make the Jo- to ice shell freezing/melting and the difference in specific vian satellites significantly more likely to lose their oceans volumes of water and ice, but this is not a unique expla- via freezing than more distant bodies. nation. Even current geological activity is not necessarily Ammonia is the most commonly-mentioned antifreeze, indicative of a subsurface ocean: unlike Enceladus (Fig 2b), but there are other possibilities. For instance, methanol the geysers at Triton (Fig 2e) may be the result of solar may play a role in the subsurface ocean of Titan [Deschamps heating of solid, near-surface ice [Kirk et al., 1990]. et al., 2010]. Simple salts (e.g. NaCl) typically result in A survey of Galileo-era images of Europa concluded that, small freezing-point depressions. while many landforms could be indicative of a subsurface ocean, none provided conclusive evidence [Pappalardo et al., 5.2. Heat production 1999]. Some of the strongest geological evidence for subsur- face oceans based on surface features comes from impact For the icy satellites, there are three main sources of heat: craters. Craters and multi-ringed structures on Europa, accretion; radioactive decay; and tidal heating. Ganymede and Callisto all show anomalous transitions in Even for Ganymede-size satellites, the gravitational en- morphology with increasing diameter [Schenk, 2002]. This ergy released during accretion is rather modest, so that was ascribed to the effect of fluid subsurface material, pre- initial differentiation is not guaranteed [Barr and Canup, sumably oceans. At that time only limited numerical studies 2008]. If accretion happens sufficiently rapidly, some melting of crater formation on icy bodies had been performed [Tur- will take place [Lunine and Stevenson, 1982, e.g.] but the tle and Pierazzo, 2001]; more recently, this topic has seen overall contribution to the existence of present-day oceans renewed interest [Bray et al., 2014; Cox and Bauer, 2015, is minor to negligible. e.g.]. For many bodies, radioactive decay is the main source of Europa’s cycloidal features (Fig 2d) also provide indirect heat. Unless satellites formed before 26Al became extinct evidence for a subsurface ocean on that moon. These unique (at about 3.5 Myr after solar system formation), the heat- patterns are most plausibly related to propagation of cracks ing will have been dominated by decay of K, U and Th. For in the rotating and fluctuating stress environment [Hoppa differentiated bodies the heat builds up in the silicate inte- et al., 1999]. Indirectly, these cycloidal features suggest a rior and is then transferred to the ice above by conduction, subsurface ocean, because the tidal stresses thought to form convection or advection. For a silicate core >1000 km in them are marginally sufficient to crack the ice shell if Eu- radius the heat diffusion timescale is longer than the age of ropa does have an ocean, but far too small if Europa has no the solar system, so large silicate cores provide a long-term ocean. reservoir of energy which can potentially maintain a subsur- face ocean. Conversely, for bodies with small silicate core 5. How are oceans maintained? radii like Enceladus or Tethys, ancient heat cannot be stored in this manner. Whether or not a satellite can develop and maintain a Chondritic silicates generate heat at a rate of about subsurface ocean depends on the rate of internal heating 3.4 × 10−12Wkg−1 at the present day, with the rate almost compared with the rate at which heat is removed [Lewis, an order of magnitude higher at 4.5 Ga. For a satellite with 1971], and the freezing point of the liquid. The means of re- a silicate mass fraction f the surface heat flux F due to moval of heat, in turn, depends primarily on whether the present-day decay is ice shell is conductive or convecting, while heat produc- tion occurs mainly due to radioactive decay and/or tidal 1 ρ HR˙ F = c (1) heating. An imbalance between heat production and heat “ 1 ” ρ −2 1 + − 1 c 3 loss of 1 mWm results in a shell melting/freezing rate of f ρi about 100 km/Gyr. In comparison, the diffusion timescale for a 100 km thick ice shell is about 0.3 Gyr. These long where ρi and ρc are the density of ice and rock, respectively, timescales suggest that oceans, particularly beneath thick H˙ is the heat production rate in the silicates (in W/kg) conductive ice shells, could have lifetimes comparable to the and R is the radius. Thus, for example, a Europa-like body age of the Solar System. Useful reviews of this topic include 1500 km in radius which is 90% rock by mass will have a sur- Hussmann et al. [2006] , Matson et al. [2009] and Spohn and face radiogenic heat flux of about 4 mWm−2. Storage and Schubert [2003]. subsequent release of ancient heat (from whatever source) in the silicate core could increase this value. 5.1. Antifreeze The other main source of energy to maintain an ocean is The temperature to which the ocean must be cooled prior tidal heating. The principle is relatively straightforward: a to freezing depends strongly on its composition, consider- body in an eccentric orbit and/or with a finite obliquity ex- ing that the antifreeze effect of certain species can be dra- periences time-variable tidal stresses and deformation; some matic. One important example is ammonia, which can de- of this mechanical work will be converted into heat. For a press the freezing point of water by almost 100 K (Figure 4) synchronous satellite the total rate of heat production (in [Choukroun and Grasset, 2010, e.g.]. Note, however, that if W) is given by [Wisdom, 2008]: more than about 15% NH3 is present by weight, the result- 3 n5R5 k ing liquid will be less dense than ice I [Haynes, 2014], which H˙ = 2 (7e2 + sin2 θ) (2) will therefore tend to founder. Because NH3 is excluded 2 G Q during solidification of an NH3-H2O melt, an ocean that is slowly freezing will become progressively more enriched where n is the mean motion, G is the gravitational constant, in NH3 (and thus becomes more difficult to freeze further). e is the eccentricity, θ is the obliquity, k2 the tidal Love num- Ammonia has been detected directly at Enceladus [Waite ber (see Section 4.2) and Q is the dissipation factor which is et al., 2009], and may be the ultimate source of the N2 in related to the phase lag between the applied potential and Titan’s atmosphere [Glein, 2015]. Ammonia ice has been the body response. A high Q implies a small phase lag and identified spectroscopically at Charon and some of the Sat- low total heat production. Tidal heating depends on semi- urnian satellites, but not in the Jovian moons [Clark et al., major axis raised to the power of 15/2, so that inner satel- 2014]. This apparent absence of NH3 is consistent with the lites are much more likely to be heated than more distant X - 6 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM bodies (Fig 3). The radial distribution of heat within the (which increases eccentricity) and dissipation in each satel- satellite depends on the local mechanical properties and can lite (which decreases it). As a result, there is a maximum be calculated e.g. using the approach of Tobie et al. [2005]. long-term average satellite heating rate which cannot be ex- The longitudinal and latitudinal variations are documented ceeded, and which depends on the Q of the primary but in Beuthe [2013]; in thin shells heating is maximized at the poles and at a minimum at the tidal axis. not the Q of the satellite [Meyer and Wisdom, 2007]. This For a generic satellite tidal heating may occur in the sil- situation may well be relevant to Enceladus (see below). icate core, the intervening ocean and/or the overlying ice 5.2.1. Applications shell. Laboratory experiments suggest that solid geological Ganymede and Callisto are not experiencing significant materials like ice respond to periodic stresses in a complex tidal heating at the present day, while tidal heating on Ti- fashion [McCarthy and Castillo-Rogez, 2013]. But some in- tan is modest at best. The high current eccentricities of sight can be gained by assuming that the response is that of a viscoelastic (Maxwell) body [Ross and Schubert, 1986, e.g.]. Titan and Mimas limit how much tidal heating they could In this approach, there are only two timescales of interest: have experienced over their history [Sohl et al., 1995, e.g.], the forcing period τ, and the characteristic response time of unless the eccentricities were excited by some unknown re- the body. This is known as the Maxwell time and is given by cent event [Cuk´ et al., 2016]. Titan’s apparently deformable τM = η/µ, where η is the viscosity and µ the shear modulus. interior (Section 4.2.3) makes the high eccentricity partic- For τ τM the material responds as a nearly inviscid fluid, ◦ ularly puzzling. Ganymede may have passed through an the phase lag approaches 90 and heat production is small. ancient orbital resonance which caused a pulse of heating For τ τ the material responds elastically, the phase lag M - perhaps responsible for the generation of bright terrain approaches zero and heat production is also small. Heat (Fig 2g) - prior to its attaining its current orbital configu- production is maximized when τ ≈ τM . The viscosity and rigidity of ice near its melting point are roughly 1014Pas and ration [Showman et al., 1997]. Although none of the Ura- 3 GPa, respectively, yielding a Maxwell time of about a day. nian satellites are in a resonance at the present day, they Since this is comparable to the orbital periods of many satel- may have encountered resonances in the past which could lites, it is clear that tidal heating in ice shells can potentially help explain the tectonic deformation seen at Miranda and be an important process. Furthermore, since η is strongly Ariel [Dermott et al., 1988; Tittemore and Wisdom, 1990]; temperature-dependent, the potential for feedbacks between something similar may have happened at Tethys [Zhang and heating and temperature is significant. Silicates tend to have much larger Maxwell times, but because viscosity is again Nimmo, 2012]. Triton represents an unusual case because, strongly temperature-dependent, runaway feedbacks leading as a captured object [Agnor and Hamilton, 2006], its pri- to large heating can occur (as appears to be the case at Io). mordial eccentricity was extremely high and will have led to For silicates that reach the melting point, an additional com- extreme early tidal heating and stresses [Ross and Schubert, plicating factor is that the main heat transport mechanism 1990]. Charon probably also had a high initial eccentricity is advection (by melt) rather than convection [Moore, 2001]. and potentially significant ancient heating [Rhoden et al., It has been suggested that tides, and particularly obliq- 2015, e.g.], but its orbit is now circular. uity tides, may result in heating in subsurface oceans [Tyler, 2014, e.g.]. Other work, however, suggests that this effect Tidal heating is responsible for Io’s volcanic activity and is likely to be important only in unusual circumstances, for is likely important in maintaining Europa’s ocean. Because three reasons. First, the effective Q of subsurface oceans, of thermal-orbital feedbacks, these bodies may have un- based on studies of terrestrial ocean dissipation, is much dergone oscillatory behaviour in the past [Ojakangas and larger (less dissipative) than was originally proposed [Chen Stevenson, 1986; Hussmann and Spohn, 2004], with epochs et al., 2014]. Second, heating is most effective in oceans during which tidal heating in Europa was not much smaller which are so thin (<1 km) that the assumption of a global, than that at Io (Figure 7). In fact, it is not clear whether uniform layer is most likely violated [Matsuyama, 2014]. Europa’s silicate interior is currently hot and dissipative - in Third, the original suggestion neglected the role of any over- −2 lying lid, which can significantly decrease the tidal ampli- which case the heat flux could be something like 100mWm tude [Hay and Matsuyama, 2015]. With the possible ex- - or cold and non-dissipative [Moore and Hussmann, 2009]. ception of Triton, which has a high inclination and thus a Similarly, tidal heating must play a role in the mainte- high predicted obliquity [Chen et al., 2014], tidal heating in nance of a subsurface ocean on Enceladus, although here oceans is unlikely to play a significant role in icy satellite it is generally assumed that dissipation in the silicate inte- evolution. rior is small [Roberts and Nimmo, 2008]. Enceladus may The effects of tidal heating are further complicated by the also have undergone oscillatory behaviour [Ojakangas and fact that dissipation tends to reduce the orbital eccentric- ity, and thus the tidal heating rate, over time [Murray and Stevenson, 1986; Shoji et al., 2014]. In fact, Enceladus ap- Dermott, 1999; Peale, 1999]. One would therefore generally pears to be losing more heat at present than the expected expect circular orbits and no tidal heating at the present equilibrium heat production rate of 1.1 GW for a Saturnian day, as is the case for Tethys and Charon. But in many Q of 18,000 [Meyer and Wisdom, 2007]. Either its eccentric- cases, satellites are in orbital resonances with near-by bod- ity was higher in the past and is now decreasing, or it is now ies which excite their eccentricities and prolong tidal heat- releasing heat that was generated in the past [O’Neill and ing; in this case energy is ultimately coming from rotational Nimmo, 2010], or dissipation in Saturn (which ultimately energy of the primary, which represents a large reservoir. controls heating in the satellites) is larger than expected (Q Since dissipation depends on internal structure and thus temperature, these resonant situations can result in com- is smaller), or some combination of these scenarios. Pre- plex and non-monotonic thermal histories. Enceladus and cise astrometric measurements of the orbital position of the Dione are currently in an eccentricity-type resonance, as are satellites over decades suggest that the last of these expla- Io, Europa and Ganymede, while Mimas and Tethys are in nations is the correct one [Lainey et al., 2012], but further an inclination-type resonance. Because dissipation in the work is needed. primary drives outwards satellite migration, other satellites The thermal-orbital coupling of satellites is a complex may have encountered and then escaped from other reso- problem that is imperfectly understood. It has also become nances in the past [Showman et al., 1997; Meyer and Wis- dom, 2007; Zhang and Nimmo, 2009, e.g.], giving rise to the clear that any investigation of this problem also needs to possibility of ancient heating events. take the tidal response of the primary into account. Tidal If two satellites are in a mean-motion resonance, then dissipation in Saturn appears to be a very strong function there is competition between dissipation in the primary of frequency, so that the Q of Saturn for one satellite at one NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 7 particular semi-major axis may be very different from that of to penetrate the lid to potentially be advected down to the another satellite at a different distance (or the same satellite ocean, and vice versa. This barrier is thus relevant both at a different epoch) [Fuller et al., 2016]. One consequence to the potential habitability of any subsurface ocean (Sec- of this picture is that the rate of outwards evolution of the tion 9), and to the possibility of detecting products of the satellites may be governed by the overall evolution timescale ocean at the surface. Clearly, there are mechanisms which of Saturn. Thus, high present-day dissipation in Saturn does can overcome this difficulty, notably the cryovolcanic plumes not necessarily imply young satellites, as is sometimes as- of Enceladus, the chaos areas and spots [Pappalardo et al., sumed [Charnoz et al., 2011, e.g.]. Equally, requiring only small degrees of outwards migration does not necessarily re- 1998] and perhaps regions of apparent plate divergence [Sul- quire that the present-day Q of Saturn is large [Cuk´ et al., livan et al., 1998] or convergence [Kattenhorn and Prockter, 2016, cf.]. 2014] on Europa. Nonetheless, the presence of a stagnant lid is likely to limit any surface-subsurface transport. 5.3. Heat removal The viscosity of ice is complicated: it is non-Newtonian, grain-size dependent and affected by several different defor- Water ice has a thermal conductivity k that varies in- mation mechanisms [Goldsby and Kohlstedt, 2001]. Near the versely with temperature, k ≈ 651/T in SI units [Petrenko melting point, large viscosity reductions occur, probably be- and Whitworth, 1999]. For a thin shell the resulting conduc- tive heat flux F is then given by cause of localized melting at the grain-scale. However, as a rule of thumb, ice at 260 K is expected to have a viscos- 651 ity ∼ 1014 Pa s while γ ≈ 0.1K−1. Applying equations (4) F = ln(T /T ) (3) −4 −1 d b s and (5) to the Europa case and taking α ≈ 10 K and κ ≈ 10−6m2s−1, the minimum shell thickness for convec- 14 where d is the shell thickness and Tb and Ts are the temper- tion to occur is about 18 km for ηb = 10 Pa s. A higher atures at the bottom and surface of the shell. In a thicker basal viscosity (e.g. if the ocean is cold due to the presence shell sphericity will slightly modify this equation. Contin- of ammonia) would require a thicker shell for convection to uing our Europa example, with a radiogenic heat flux of occur. 4mWm−2 the equilibrium conductive shell thickness would be approximately 180 km. Tidal heating or leftover heat 5.3.1. Applications from an earlier epoch would modify the temperature struc- The above analysis makes it clear that the thick ice shells ture and yield a thinner shell. of Ganymede and Callisto are most likely undergoing convec- If the shell is sufficiently thick, it will start to convect. tion [McKinnon, 2006, e.g.], albeit not vigorously enough to Whether convection occurs depends on the Rayleigh num- cause complete ocean freezing. Callisto in particular places ber, Ra, which is defined as limits on how much heat can have been transferred across its ice shell, because it does not appear to have experienced 3 ρgα(Tb − Ts)d tidal heating (see above). At Titan the situation is less clear. Ra = (4) κηb There is some evidence that Titan’s shell is rigid and conduc- tive [Hemingway et al., 2013], in which case suppression of where g is the acceleration due to gravity, α is the ther- convection probably occurs because the ocean is ammonia- mal expansivity, κ is the thermal diffusivity and ηb is the rich and therefore cold. The Jovian moons, in contrast, may viscosity at Tb. Here the viscosity is assumed to be Newto- have formed in a region where ammonia was rare or absent nian; an equivalent definition exists for non-Newtonian cases [Lunine and Stevenson, 1982]. [Solomatov, 1995]. Convection occurs if Ra exceeds the crit- Europa’s ice shell is most likely a few to a few tens of ical Rayleigh number Rac, which for strongly temperature- dependent Newtonian viscosities and a Cartesian geometry km thick, so that it could be either conductive or convec- is given by tive. Investigations of the equilibrium shell thickness aris- 4 Rac = 20.9(γ[Tb − Ts]) (5) ing from a balance between tidal heating and heat trans- port have yielded a wide range of answers [Ojakangas and Here γ describes the sensitivity of the viscosity to temper- Stevenson, 1989; Hussmann et al., 2002; Moore, 2006, e.g.], ature, γ = d ln η/dT , and is related to the activation energy because the (uncertain) ice viscosity has a strong effect on and Tb. Because of the strong temperature dependence of ice vis- both tidal heating and heat transport. Some Europan land- cosity and the large temperature contrast between surface forms (Fig 2c) resemble the surface expression of diapirs, and interior, a convecting ice shell will consist of a relatively and may thus be indicative of convection [Pappalardo et al., thick, stagnant lid in which heat transfer occurs via conduc- 1998], though this interpretation has been debated [Green- tion, with an isothermal convecting layer beneath [Soloma- berg et al., 2003]. tov, 1995]. Heat flow is thus regulated by the stagnant lid The ice shell of Enceladus is perhaps 20-40 km [Iess et al., thickness δ, not the total shell thickness d. The convective 2014; McKinnon, 2015; Cadek et al., 2016], and g is small, heat flux is then given by so that conduction is likely favoured over convection. Local- ized convection might help to explain the anomalous south 1/3 (Tb − Ts) Ra pole [Bˇehounkov´aet al., 2015], though the local shell thick- F ≈ k = k(Tb − Ts) 4/3 (6) δ 2d(γ[Tb − Ts]) ness there may be as small as 5 km [Cadek et al., 2016]. The real puzzle at Enceladus is the apparent survival of a global Here k is taken to be constant for convenience and the ocean. The large surface area to volume ratio and thin ice second equality arises because the stagnant lid thickness δ shell means that even a conductive ice shell should cause the is controlled by the vigour of convection (Ra) and the de- ocean to freeze in a few tens of Myr [Roberts and Nimmo, pendence of viscosity on temperature (γ). Note that equa- 2008; Cadek et al., 2016]. Tidal heating, especially if it oc- tion (6) implies that the convective heat flux is indepen- curs in or beneath the ocean, might help to resolve this puz- dent of both Tb − Ts and the layer thickness d. For thick ice shells, sphericity will modify both the critical Rayleigh zle, and as noted above the total tidal heating available may number [Robuchon and Nimmo, 2011, e.g.] and the rate be larger than was previously thought. Nonetheless, as yet of convective heat transfer [Yao et al., 2014], although the there is no generally accepted explanation for the apparent underlying physics does not change. long-term survival of a global ocean on Enceladus. One important aspect of the stagnant lid is that it Pluto presents another interesting case. If Pluto’s ice presents a barrier to transport. Surface material would have shell is conductive, radiogenic heat is sufficient to maintain X - 8 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM an ocean to the present day, while if the shell is convec- stresses in the shell. Conversely, if an ocean freezes com- tive, removal of heat is rapid enough to prevent an ocean pletely, the tidal amplitude decreases markedly, so that it is ever forming [Robuchon and Nimmo, 2011]. The observa- then much harder to heat the body and regenerate an ocean. tion of wide-spread extensional tectonics on Pluto [Moore Ocean survival and tidal heating are thus intimately linked, et al., 2016] (Fig 2a) suggests that shell thickening has oc- while the presence of an ocean makes the generation of tec- curred and thus that an ocean was present for at least part tonic features more likely. As noted above, however, linking of its history. Triton could likewise possess a global ocean at the present day [Gaeman et al., 2012]. Smaller bodies specific geological features to the presence of a subsurface like Charon or Rhea have too little radiogenic heat to main- ocean is difficult, and necessarily indirect. tain an ocean at present, unless very severe melting-point Another important consequence of oceans is that decou- depression is invoked [Hussmann et al., 2006]. pled ice shells are more likely to undergo reorientation (true Satellite poles are colder than their equators, while in a polar wander). There is possible geological evidence for re- thin shell tidal heating is maximized at the poles [Ojakangas orientation at Europa [Schenk et al., 2008] which had pre- and Stevenson, 1989]. As a result, the balance between heat viously been suggested on theoretical grounds [Ojakangas production and heat transport may result in spatial varia- and Stevenson, 1989], while the south polar location of the tions in shell thickness. Such variations will only arise if the Enceladus hot-spot has also been attributed to reorientation ice shell is conductive - flow in convective shells will rapidly [Nimmo and Pappalardo, 2006]. In a similar fashion, float- smooth out any such variations. Titan’s long-wavelength ing ice shells can potentially undergo non-synchronous ro- surface topography and gravity suggest shell thickness vari- ations [Nimmo and Bills, 2010], which necessarily implies a tation (NSR) [Greenberg and Weidenschilling, 1984], which conductive shell. Similar variations may also arise at Eu- can lead to significant tectonic stresses. Direct evidence for ropa, though existing topography data cannot confirm this NSR is elusive; a claimed detection of NSR at Titan [Lorenz [Nimmo et al., 2007]. et al., 2008] was later found to be incorrect [Meriggiola et al., 2016]. However, the absence of predicted leading-trailing 6. When did oceans form? hemisphere asymmetries in crater densities may be due to long-term NSR [Zahnle et al., 2001]. As discussed above, ocean formation requires a heat Because ice I occupies more volume than water, progres- source, either radioactive decay, tidal heating, the energy sive freezing of an ocean (thickening of the ice shell) results associated with satellite accretion or the early luminosity in global expansion. The isotropic extensional stresses in- of the parent body. Unfortunately, the initial conditions volved are large [Nimmo, 2004] and may be the explanation following accretion are quite uncertain. First, the tim- for the pervasive extensional features seen on many icy satel- ing of satellite accretion relative to solar system formation lites. Note, however, that this mechanism is only effective is unclear: rapid enough formation could result in strong in the absence of higher pressure ice phases; if these are heating from 26Al (half-life 0.7 Myr) [Castillo-Rogez et al., present, the effect is reduced or even reversed. Ice I volume 2009]. Second, the amount of energy delivered by accretion is sensitive to poorly-known parameters such as the size- changes can also result in pressurization of the underlying distribution and flux of the accreting material [Barr and ocean, which could help drive “cryovolcanic” eruptions es- Canup, 2008; Barr et al., 2010]. Third, the background pecially on small bodies like Enceladus [Manga and Wang, temperature evolution (set by the luminosity of the parent 2007]. planet) is not entirely understood [Lunine and Stevenson, The extent to which “cryovolcanism” - by which mate- 1982; Fortney and Nettelmann, 2010]. As a result, it is far rial originating in a subsurface ocean erupts onto a satellite from obvious whether subsurface oceans formed promptly, surface - really occurs is a subject of some debate [Moore or only later due to longer-lived heat sources (either tidal or and Pappalardo, 2011, e.g.]. Water, being denser than ice radioactive). by about 8%, has difficulty erupting under normal circum- For cases in which tidal heating is important in maintain- stances. Exsolution of dissolved gases can potentially over- ing an ocean, it is often argued that if the ocean ever freezes come this problem [Crawford and Stevenson, 1988] as may completely, it will never re-melt [Roberts and Nimmo, 2008, e.g.]. This is because once the ocean disappears, the tidal the ocean-pressurization mechanism described above. At deformation is greatly reduced (see Section 4.2.3) and the Enceladus, it appears that cryovolcanism occurs because tidal heating is correspondingly much smaller. In detail, water-filled cracks are periodically exposed to vacuum, the however, this argument may be an oversimplification: for a cracks being opened and closed by tidal stresses [Hurford satellite in resonance, without tidal dissipation the eccen- et al., 2007; Hedman et al., 2013]. Other bodies where tricity (and thus the tidal heating) will grow until either the cryovolcanism might play a significant role include Europa resonance breaks or the tidal heating rate reaches the equi- [Fagents, 2003], Ganymede [Schenk et al., 2001] and Pluto librium value set by the properties of the parent planet (see [Moore et al., 2016]. Section 5.2 above). Alternatively, as the ocean becomes very thin, further freezing may be shut off, because a thin ocean 7.2. Silicates will have a high concentration of antifreeze (Section 5.1), and/or because tidal heating in the ocean itself is amplified An ocean can also have significant consequences for un- (Section 5.2). derlying silicates. In particular, if the silicates are perme- able then water-rock interactions will occur. Reviews of this 7. What are the consequences of oceans? topic include Vance et al. [2007] and Sohl et al. [2010]. Pore closure is more rapid at high pressures and tem- The presence of an ocean can affect the behaviour and peratures, so water-rock interactions are expected to be evolution of both the ice shell above and the deeper inte- more widespread for small bodies than for large ones. On rior. the other hand, reaction kinetics are strongly temperature- dependent so that the rates on small, cold bodies may be 7.1. Ice shell too low to be important. The process of hydrothermal al- The decoupling effect of the ocean on the ice shell has teration may also be self-limiting, as pore spaces are likely been discussed above (Section 4.2). One particularly impor- to be progressively filled by alteration products. tant consequence is that this decoupling allows much greater Hydrothermal processes on icy bodies have received sig- ice shell deflection, and thus enhanced tidal heating and nificant attention from both theoretical and experimental NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 9 standpoints. For instance, water-rock interactions will result et al. [2004] argue that convective stirring is likely to de- in the water acquiring dissolved species such as Na and ex- stroy at least thermal stratification, while Vance and Brown periencing changes in pH [Zolotov, 2007; Glein et al., 2008]. [2005] suggest compositional stratification may be signifi- Serpentinization can result in changes in silicate bulk density [Castillo-Rogez and Lunine, 2010; Malamud and Prialnik, cant. Ocean density stratification due to the peculiar ther- 2015, e.g.]. Depending on the temperature and initial rock mal properties of pure water [Melosh et al., 2004] has been composition, generation of gases can result which may be proposed, but the relevance to the Europan ocean is un- compared with measured atmospheric constituents [Shock clear, given its uncertain salinity [Hand and Chyba, 2007]. and McKinnon, 1993; Glein et al., 2008; Matson et al., 2007]. These gases (or other primordial species, such as The ocean on Enceladus is also likely quite saline [Zolotov, noble gases) could also be transported to shallower regions 2007]. via dissolution in water [Glein, 2015]. In addition to thermal or compositional effects, ocean cir- Unfortunately, observational constraints on these pro- culation may also be driven by external forcing. The tidal posed processes are sparse, with the exception of Enceladus, effects discussed in Section 5.2 are one example; another is where in situ measurements are available. In this particu- lar case, predicted ocean compositions [Zolotov, 2007] were librations of the shell, which may cause turbulence in the found to be in quite good agreement with the measurements underlying ocean [Noir et al., 2009]. Conversely, ocean flow [Postberg et al., 2009]. The low inferred bulk density of the might cause torques on the overlying shell, which would be silicate core of Enceladus could also be explained by porosity potentially detectable as rotation rate variations (similar to and/or hydrothermal alteration. length-of-day variations on Earth). Water-rock interactions will also have thermal effects. In particular, fluid flow is an efficient way of advecting heat, Most of the studies referred to above have assumed very so that permeable parts of the silicate core may remain rel- simplified ocean dynamics, but more recent work has started atively cold [Vance et al., 2007; Travis et al., 2012]. An to make use of global circulation models (GCMs). For in- important corollary is that the water will be heated, driv- stance, Goodman and Lenferink [2012] applied one such ing hydrothermal circulation and (potentially) buoyant jets or plumes in the ocean (see below). Fluid flow can also GCM to study hydrothermal plumes, while a study by leach soluble species such as potassium, thus decreasing the Soderlund et al. [2014] examined how a global ocean could amount of radioactive heating in the silicates. Serpentiniza- deliver more heat to the equator, potentially explaining the tion is an exothermic reaction, though its potential as a observed distribution of chaos terrain. Similar studies in- heat source is likely limited compared to radioactive decay corporating ocean-shell coupling and tidal effects are likely [Malamud and Prialnik, 2015]. More speculatively, the re- duction in strength caused by serpentinization and/or the to become more common in future. presence of interstitial H2O may enhance silicate tidal heat- ing [Roberts, 2015]. 9. Are the oceans habitable? 8. What are the characteristics of the Although not primarily a geophysical topic, the habit- oceans themselves? ability of subsurface oceans is a subject of some interest. As typically defined, life requires liquid water, a source of The physical and chemical characteristics of the oceans are in general very poorly constrained. Single-frequency in- energy, and an array of elements usable for biological pur- duction is insufficient to determine the thickness of the ocean poses[Chyba and Hand, 2005, e.g.]. Ocean worlds by defi- (Section 4.1). For Europa, the inferred moment of inertia nition satisfy the first requirement, and are likely to have and presumption that the shell is not more than a few tens thermal (Section 5.2) and perhaps chemical sources of en- of km thick implies an ocean thickness of about 100 km ergy available. Small bodies such as Europa and Ence- [Schubert et al., 2004]. For Ganymede, Callisto and Titan the ocean thickness is essentially unknown, though models ladus are regarded as particularly habitable environments suggest thicknesses of order 100 km [Spohn and Schubert, because their oceans are in direct contact with the underly- 2003, e.g.]. At Enceladus the ocean is perhaps 10 km thick ing silicates, a potential source of both heat and biologically- on average, but greater at the south pole [McKinnon, 2015; important elements. The surface geological activity of these Cadek et al., 2016]. Except for Enceladus (see Section 7.2 above), the com- bodies may also lead to the transport to the oceans of positions of the subsurface oceans are also unclear. Be- biologically-important species implanted at the surface (e.g. cause pure water has a very low conductivity, dissolved ions organic molecules from comets or the products of irradi- must be present at some level in the oceans of the three ation). Conversely, bodies like Callisto which have devel- icy Galilean satellites where induction signals have been de- oped a thick, stagnant lid (Section 5.3) are regarded as less tected. At Titan some of the species now present in the at- mosphere may have originally been dissolved in the subsur- promising for habitability, because of the difficulty of mate- face ocean [Tobie et al., 2012] while there is limited evidence rial transport across this lid. A long-lived ocean, rather than that the ocean is cold (perhaps ammonia-rich) [Hemingway one which is present only briefly or episodically, is regarded et al., 2013]. as more likely to be habitable. Compared with the Earth, satellite oceans are not only poorly characterized but are also driven by a different set of forces. On Earth, wind stresses and salinity variations play 10. The Future a major role in ocean dynamics [Schmitz and McCartney, 1993, e.g.]; for ice-covered satellite oceans, the former are Major advances in understanding usually occur when certainly negligible and the role of the latter is uncertain new spacecraft data are acquired. Analysis of the recently- (see below). Much of the existing dynamical work focuses on Europa’s acquired New Horizons data has only just begun [Stern ocean. Local hydrothermal plumes on Europa were inves- et al., 2015; Moore et al., 2016] and will undoubtedly provide tigated by Thomson and Delaney [2001], who assumed a further insight into the likelihood (or not) of a subsurface weakly-stratified ocean; Goodman et al. [2004] and Low- ocean on Pluto. Two missions currently under development ell and DuBose [2005] performed similar studies but as- sumed an unstratified environment. Compositional stratifi- are especially relevant to ocean worlds: the NASA Europa cation plays an important role in the Earth’s ocean dynam- Clipper mission, and ESA’s JUICE mission to Ganymede. ics, and may also be relevant to satellite oceans. Goodman According to current plans, these spacecraft will arrive at X - 10 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM the Jovian system in the period 2025-2030, perhaps fol- a contract with the National Aeronautics and Space Adminis- lowed by a Europa lander. Each will carry an ice-penetrating tration. Parts of FN’s work were supported by NASA grants radar, and JUICE would be the first spacecraft to orbit an NNX13AG02G and NNX15AQ88G. This is a review article. It outer solar system satellite. If successful, these missions will contains no novel data. undoubtedly revolutionize our understanding of the Galilean satellites and the broader array of ocean worlds. A Europa mission was the second-highest priority iden- tified by the most recent Planetary Decadal Survey (after References Mars sample return). The third-highest priority mission A, G., J. Wahr, and S. Zhong, The effects of laterally varying icy was an ice giant mission, specifically a Uranus orbiter, so shell structure on the tidal response of Ganymede and Europa, it is conceivable that the Uranian moons will be much bet- Journal of Geophysical Research (Planets), 119, 659–678, doi: ter understood twenty-five years from now. Because of their 10.1002/2013JE004570, 2014. astrobiological potential, Titan and Enceladus continue to Agnor, C. B., and D. P. Hamilton, Neptune’s capture of its moon be of great interest. For Enceladus the possibility exists Triton in a binary-planet gravitational encounter, Nature, 441, to take advantage of the eruptive activity and carry out a 192–194, doi:10.1038/nature04792, 2006. sample-return mission. At Titan various combinations of Anderson, J. D., R. A. Jacobson, T. P. McElrath, W. B. Moore, a lander (to float on one of the hydrocarbon seas) and an G. Schubert, and P. C. Thomas, Shape, Mean Radius, Gravity orbiter have been proposed. Field, and Interior Structure of Callisto, Icarus, 153, 157–161, Theoretical developments and ground-based observations doi:10.1006/icar.2001.6664, 2001. will also continue to play a role. For the former, there are Asphaug, E., and A. Reufer, Late origin of the Saturn system, several areas which are likely to prove fruitful in the next Icarus, 223, 544–565, doi:10.1016/j.icarus.2012.12.009, 2013. Bagenal, F., T. E. Dowling, and W. B. McKinnon, Jupiter : the decade. The first is impact modelling: large impact fea- planet, satellites and magnetosphere, 2004. tures provide a powerful probe of ice shell subsurface struc- Baland, R.-M., G. Tobie, A. Lef`evre, and T. Van Hoolst, ture. Recent computational advances have allowed a so- Titans internal structure inferred from its gravity field, phisticated understanding of lunar impact basins to be de- shape, and rotation state, Icarus, 237, 29–41, doi: veloped [Melosh et al., 2013, e.g.]; now that there is a good 10.1016/j.icarus.2014.04.007, 2014. equation of state for water ice [Stewart and Ahrens, 2005], Baland, R.-M., M. Yseboodt, and T. Van Hoolst, The similarly sophisticated modeling is beginning to be carried obliquity of Enceladus, Icarus, 268, 12–31, doi: out for icy bodies [Bray et al., 2014; Cox and Bauer, 2015; 10.1016/j.icarus.2015.11.039, 2016. Bray and Schenk, 2015]. The second is the dynamics of Barr, A. C., and R. M. Canup, Constraints on gas gi- decoupled shells. As described in Section 4.2, measuring ant satellite formation from the interior states of par- the rotational state of the surface is a potentially powerful tially differentiated satellites, Icarus, 198, 163–177, doi: probe of internal structure, but our theoretical understand- 10.1016/j.icarus.2008.07.004, 2008. Barr, A. C., and D. E. Stillman, Strain history of ice shells ing is incomplete. For example, the role of viscoelasticity, of the Galilean satellites from radar detection of crystal and the obliquity behaviour of decoupled shells both repre- orientation fabric, Geophys. Res. Lett., 38, L06203, doi: sent unsolved problems. The third topic is ocean dynam- 10.1029/2010GL046616, 2011. ics. Very sophisticated models of conductive fluid dynamics Barr, A. C., R. I. Citron, and R. M. Canup, Origin of a in rotating spheres have been developed to study dynamos partially differentiated Titan, Icarus, 209, 858–862, doi: [Jones et al., 2011], but almost no work has been done on the 10.1016/j.icarus.2010.05.028, 2010. equivalent problem of subsurface oceans. The tidal response Bergstralh, J. T., E. D. Miner, and M. S. Matthews, Uranus, of such oceans in particular represents a topic of interest. 1991. Last, the issue of the coupled thermal-orbital evolution of Beuthe, M., Spatial patterns of tidal heating, Icarus, 223, 308– satellites has received periodic attention over the years (Sec- 329, doi:10.1016/j.icarus.2012.11.020, 2013. tion 5.2), but more could certainly be done, especially with Bills, B. G., and F. Nimmo, Forced obliquity and mo- regard to dissipation in the primary. For instance, the ap- ments of inertia of Titan, Icarus, 196, 293–297, doi: 10.1016/j.icarus.2008.03.002, 2008. parent persistence of a global ocean on Enceladus represents Bills, B. G., and F. Nimmo, Rotational dynamics and in- a major unsolved problem. ternal structure of Titan, Icarus, 214, 351–355, doi: As far as ground-based observations go, there are several 10.1016/j.icarus.2011.04.028, 2011. areas of promise. HST observations have proved powerful, Blankenship, D. D., D. A. Young, W. B. Moore, and J. C. Moore, especially for long-term activity monitoring, and the same Radar Sounding of Europa’s Subsurface Properties and Pro- is likely to be true of its successor (James Webb Space Tele- cesses: The View from Earth, p. 631, 2009. scope). Earth-based radar observations are a potentially Bray, V. J., and P. M. Schenk, Pristine impact crater morphol- very useful geodetic tool (Section 4.2), although progress to ogy on Pluto - Expectations for New Horizons, Icarus, 246, date has been slow. Astrometry (Section 5.2) has shown 156–164, doi:10.1016/j.icarus.2014.05.005, 2015. notable advances recently, although these have been at least Bray, V. J., G. S. Collins, J. V. Morgan, H. J. Melosh, and P. M. in part due to modern ephemerides derived from spacecraft Schenk, Hydrocode simulation of Ganymede and Europa cra- observations. Continuing surveys of Kuiper Belt Objects tering trends - How thick is Europa’s crust?, Icarus, 231, 394– 406, doi:10.1016/j.icarus.2013.12.009, 2014. are likely to both reveal new objects and provide improved Brown, M. E., The Compositions of Kuiper Belt Objects, An- characterizations of existing ones, some of which may turn nual Review of Earth and Planetary Sciences, 40, 467–494, out to be ocean worlds. And, lastly, it is very likely that the doi:10.1146/annurev-earth-042711-105352, 2012. next few years will see the detection of the first “exo-moon” Brown, R. H., J.-P. Lebreton, and J. H. Waite, Titan from in a neighboring planetary system. Cassini-Huygens, doi:10.1007/978-1-4020-9215-2, 2010. The last quarter-century has seen an enormous increase Bruzzone, L., J. J. Plaut, G. Alberti, D. D. Blankenship, F. Bo- in our understanding of the structures and histories of icy volo, B. A. Campbell, A. Ferro, Y. Gim, W. Kofman, G. Ko- worlds in general, and ocean-bearing moons in particular. matsu, W. McKinnon, G. Mitri, R. Orosei, G. W. Patterson, Whether any of these bodies actually harbor life remains to D. Plettemeier, and R. Seu, RIME: Radar for icy moon ex- be seen; in the mean time, many challenges remain to keep ploration, IEEE international geoscience and remote sensing forthcoming generations of planetary scientists occupied. symposium (IGARSS), pp. 3907–3910, 2013. Burns, J. A., and M. S. Matthews, Satellites, 1986. Acknowledgments. We thank Hauke Hussmann and an Bˇehounkov´a,M., G. Tobie, O. Cadek,ˇ G. Choblet, C. Porco, and anonymous reviewer for careful comments on the MS. The por- F. Nimmo, Timing of water plume eruptions on Enceladus ex- tion of this work performed by RTP was carried out at the Jet plained by interior viscosity structure, Nature Geoscience, 8, Propulsion Laboratory, California Institute of Technology, under 601–604, doi:10.1038/ngeo2475, 2015. NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 11
Cadek, O., G. Tobie, T. Van Hoolst, M. Mass´e, G. Choblet, Fagents, S. A., Considerations for effusive cryovolcanism on Eu- A. Lef`evre,G. Mitri, R.-M. Baland, M. Behounkova, O. Bour- ropa: The post-Galileo perspective, Journal of Geophysi- geois, and A. Trinh, Enceladus’s internal ocean and ice shell cal Research (Planets), 108, 5139, doi:10.1029/2003JE002128, constrained from Cassini gravity, shape and libration data, 2003. Geophys. Res. Lett., doi:10.1002/2016GL068634, 2016. Fischer, P. D., M. E. Brown, and K. P. Hand, Spatially Re- Canup, R. M., On a Giant Impact Origin of Charon, Nix, and solved Spectroscopy of Europa: The Distinct Spectrum of Hydra, Astron. J., 141, 35, doi:10.1088/0004-6256/141/2/35, Large-scale Chaos, Astron. J., 150, 164, doi:10.1088/0004- 2011. 6256/150/5/164, 2015. Cao, H., C. T. Russell, U. R. Christensen, M. K. Dougherty, and Fortney, J. J., and N. Nettelmann, The Interior Structure, Com- M. E. Burton, Saturn’s very axisymmetric magnetic field: No position, and Evolution of Giant Planets, Space Sci. Rev., 152, detectable secular variation or tilt, Earth and Planetary Sci- 423–447, doi:10.1007/s11214-009-9582-x, 2010. ence Letters, 304, 22–28, doi:10.1016/j.epsl.2011.02.035, 2011. Fuller, J., J. Luan, and E. Quataert, Resonance locking as the Carlson, R. W., M. S. Anderson, R. Mehlman, and R. E. source of rapid tidal migration in the Jupiter and Saturn Johnson, Distribution of hydrate on Europa: Further evi- moon systems, Mon. Not. R. Astr. Soc., 458, 3867–3879, doi: dence for sulfuric acid hydrate, Icarus, 177, 461–471, doi: 10.1093/mnras/stw609, 2016. 10.1016/j.icarus.2005.03.026, 2005. Gaeman, J., S. Hier-Majumder, and J. H. Roberts, Sustainabil- Cassen, P., R. T. Reynolds, and S. J. Peale, Is there liq- ity of a subsurface ocean within Triton’s interior, Icarus, 220, uid water on Europa, Geophys. Res. Lett., 6, 731–734, doi: 339–347, 2012. 10.1029/GL006i009p00731, 1979. Gao, P., and D. J. Stevenson, Nonhydrostatic effects and the de- Castillo-Rogez, J., T. V. Johnson, M. H. Lee, N. J. Turner, termination of icy satellites moment of inertia, Icarus, 226, D. L. Matson, and J. Lunine, 26Al decay: Heat production 1185–1191, doi:10.1016/j.icarus.2013.07.034, 2013. and a revised age for Iapetus, Icarus, 204, 658–662, doi: Glein, C. R., Noble gases, nitrogen, and methane from the deep 10.1016/j.icarus.2009.07.025, 2009. interior to the atmosphere of Titan, Icarus, 250, 570–586, doi: Castillo-Rogez, J. C., and J. I. Lunine, Evolution of Titan’s rocky 10.1016/j.icarus.2015.01.001, 2015. core constrained by Cassini observations, Geophys. Res. Lett., Glein, C. R., M. Y. Zolotov, and E. L. Shock, The oxidation state 37, L20205, doi:10.1029/2010GL044398, 2010. of hydrothermal systems on early Enceladus, Icarus, 197, 157– Charnoz, S., A. Morbidelli, L. Dones, and J. Salmon, Did Saturn’s 163, doi:10.1016/j.icarus.2008.03.021, 2008. rings form during the Late Heavy Bombardment?, Icarus, 199, Goldreich, P., and S. Soter, Q in the Solar System, Icarus, 5, 413–428, doi:10.1016/j.icarus.2008.10.019, 2009. 375–389, 1966. Charnoz, S., A. Crida, J. C. Castillo-Rogez, V. Lainey, L. Dones, Goldreich, P. M., and J. L. Mitchell, Elastic ice shells of syn- O.¨ Karatekin, G. Tobie, S. Mathis, C. Le Poncin-Lafitte, and chronous moons: Implications for cracks on Europa and non- J. Salmon, Accretion of Saturns mid-sized moons during the synchronous rotation of Titan, Icarus, 209, 631–638, doi: viscous spreading of young massive rings: Solving the paradox 10.1016/j.icarus.2010.04.013, 2010. of silicate-poor rings versus silicate-rich moons, Icarus, 216, Goldsby, D. L., and D. L. Kohlstedt, Superplastic deformation 535–550, doi:10.1016/j.icarus.2011.09.017, 2011. of ice: Experimental observations, J. Geophys. Res., 106, 11, Chen, E. M. A., F. Nimmo, and G. A. Glatzmaier, Tidal doi:10.1029/2000JB900336, 2001. heating in icy satellite oceans, Icarus, 229, 11–30, doi: Goodman, J. C., and E. Lenferink, Numerical simulations of ma- rine hydrothermal plumes for Europa and other icy worlds, 10.1016/j.icarus.2013.10.024, 2014. Icarus, 221, 970–983, doi:10.1016/j.icarus.2012.08.027, 2012. Choukroun, M., and O. Grasset, Thermodynamic data and mod- Goodman, J. C., G. C. Collins, J. Marshall, and R. T. Pierrehum- eling of the water and ammonia-water phase diagrams up to bert, Hydrothermal plume dynamics on Europa: Implications 2.2 GPa for planetary geophysics, J. Chem. Physics, 133 (14), for chaos formation, Journal of Geophysical Research (Plan- 144,502–144,502, doi:10.1063/1.3487520, 2010. ets), 109, E03008, doi:10.1029/2003JE002073, 2004. Chyba, C. F., and K. P. Hand, ASTROBIOLOGY: The Study of Grasset, O., M. K. Dougherty, A. Coustenis, E. J. Bunce, the Living Universe, Ann. Rev. Astron. Astrophys., 43, 31–74, C. Erd, D. Titov, M. Blanc, A. Coates, P. Drossart, L. N. doi:10.1146/annurev.astro.43.051804.102202, 2005. Fletcher, H. Hussmann, R. Jaumann, N. Krupp, J.-P. Lebre- Chyba, C. F., S. J. Ostro, and B. C. Edwards, Radar Detectabil- ton, O. Prieto-Ballesteros, P. Tortora, F. Tosi, and T. Van ity of a Subsurface Ocean on Europa, Icarus, 134, 292–302, Hoolst, JUpiter ICy moons Explorer (JUICE): An ESA mis- doi:10.1006/icar.1998.5961, 1998. sion to orbit Ganymede and to characterise the Jupiter system, Clark, R. N., G. A. Swayze, R. Carlson, W. Grundy, and K. Noll, Planet. Space Sci., 78, 1–21, doi:10.1016/j.pss.2012.12.002, Spectroscopy from Space, Reviews in Mineralogy and Geo- 2013. chemistry, Vol. 78, No. 1, p. 399-446, 78, 399–446, doi: Greenberg, R., and S. J. Weidenschilling, How fast do Galilean 10.2138/rmg.2014.78.10, 2014. satellites spin?, Icarus, 58, 186–196, doi:10.1016/0019- Connerney, J. E. P., M. H. Acuna, and N. F. Ness, The magnetic 1035(84)90038-1, 1984. field of Neptune, J. Geophys. Res., 96, 19, 1991. Greenberg, R., M. A. Leake, G. V. Hoppa, and B. R. Tufts, Cox, R., and A. W. Bauer, Impact breaching of Eu- Pits and uplifts on Europa, Icarus, 161, 102–126, doi: ropa’s ice: Constraints from numerical modeling, Journal 10.1016/S0019-1035(02)00013-1, 2003. of Geophysical Research (Planets), 120, 1708–1719, doi: Hand, K. P., and C. F. Chyba, Empirical constraints on the salin- 10.1002/2015JE004877, 2015. ity of the europan ocean and implications for a thin ice shell, Crawford, G. D., and D. J. Stevenson, Gas-driven water vol- Icarus, 189, 424–438, doi:10.1016/j.icarus.2007.02.002, 2007. canism in the resurfacing of Europa, Icarus, 73, 66–79, doi: Hay, H. C. F. C., and I. Matsuyama, Numerically Simulating 10.1016/0019-1035(88)90085-1, 1988. Tidal Dissipation in the Icy Satellites, in Lunar and Planetary Cruikshank, D. P., Neptune and Triton, 1996. Science Conference, Lunar and Planetary Science Conference, Cuk,´ M., L. Dones, and D. Nesvorn´y,Dynamical Evidence for vol. 46, p. 1329, 2015. a Late Formation of Saturn’s Moons, Astrophys. J., 820, 97, Haynes, W. M., CRC Handbook of chemistry and physics, Taylor doi:10.3847/0004-637X/820/2/97, 2016. and Francis, 2014. Dermott, S. F., R. Malhotra, and C. D. Murray, Dynamics of Head, J., R. Pappalardo, G. Collins, M. J. S. Belton, B. Giese, the Uranian and Saturnian satellite systems - A chaotic route R. Wagner, H. Breneman, N. Spaun, B. Nixon, G. Neukum, to melting Miranda?, Icarus, 76, 295–334, doi:10.1016/0019- and J. Moore, Evidence for Europa-like tectonic resurfac- 1035(88)90074-7, 1988. ing styles on Ganymede, Geophys. Res. Lett., 29, 2151, doi: Deschamps, F., O. Mousis, C. Sanchez-Valle, and J. I. Lunine, 10.1029/2002GL015961, 2002. The Role of Methanol in the Crystallization of Titan’s Pri- Hedman, M. M., C. M. Gosmeyer, P. D. Nicholson, C. Sotin, mordial Ocean, Astrophys. J., 724, 887–894, 2010. R. H. Brown, R. N. Clark, K. H. Baines, B. J. Buratti, and Dougherty, M. K., L. W. Esposito, and S. M. Krimigis, Saturn M. R. Showalter, An observed correlation between plume ac- from Cassini-Huygens, doi:10.1007/978-1-4020-9217-6, 2009. tivity and tidal stresses on Enceladus, Nature, 500, 182–184, Dwyer, C. A., F. Nimmo, M. Ogihara, and S. Ida, The in- doi:10.1038/nature12371, 2013. fluence of imperfect accretion and radial mixing on ice:rock Hemingway, D., F. Nimmo, H. Zebker, and L. Iess, A rigid ratios in the Galilean satellites, Icarus, 225, 390–402, doi: and weathered ice shell on Titan, Nature, 500, 550–552, doi: 10.1016/j.icarus.2013.03.025, 2013. 10.1038/nature12400, 2013. X - 12 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM
Holt, J. W., M. E. Peters, S. D. Kempf, D. L. Morse, and Lewis, J. S., Satellites of the Outer Planets: Thermal Models, D. D. Blankenship, Echo source discrimination in single- Science, 172, 1127–1128, doi:10.1126/science.172.3988.1127, pass airborne radar sounding data from the Dry Valleys, 1971. Antarctica: Implications for orbital sounding of Mars, Jour- Lorenz, R. D., B. W. Stiles, R. L. Kirk, M. D. Allison, P. Persi del nal of Geophysical Research (Planets), 111, E06S24, doi: Marmo, L. Iess, J. I. Lunine, S. J. Ostro, and S. Hensley, Ti- 10.1029/2005JE002525, 2006. tans Rotation Reveals an Internal Ocean and Changing Zonal Hoppa, G. V., B. R. Tufts, R. Greenberg, and P. E. Geissler, For- Winds, Science, 319, 1649, doi:10.1126/science.1151639, 2008. mation of cycloidal features on Europa, Science, 285, 1899– Lowell, R. P., and M. DuBose, Hydrothermal systems on Europa, 1902, doi:10.1126/science.285.5435.1899, 1999. Geophys. Res. Lett., 32, L05202, doi:10.1029/2005GL022375, Hurford, T. A., P. Helfenstein, G. V. Hoppa, R. Greenberg, and 2005. B. G. Bills, Eruptions arising from tidally controlled periodic Lucchitta, B. K., and L. A. Soderblom, The geology of Europa, openings of rifts on Enceladus, Nature, 447, 292–294, doi: in Satellites of Jupiter, edited by D. Morrison, pp. 521–555, 10.1038/nature05821, 2007. 1982. Hussmann, H., and T. Spohn, Thermal-orbital evolu- Lunine, J. I., and D. J. Stevenson, Formation of the Galilean satel- tion of Io and Europa, Icarus, 171, 391–410, doi: lites in a gaseous nebula, Icarus, 52, 14–39, doi:10.1016/0019- 10.1016/j.icarus.2004.05.020, 2004. 1035(82)90166-X, 1982. Hussmann, H., T. Spohn, and K. Wieczerkowski, Thermal Equi- Lunine, J. I., D. J. Stevenson, and Y. L. Yung, Ethane ocean on librium States of Europa’s Ice Shell: Implications for Internal Titan, Science, 222, 1229, doi:10.1126/science.222.4629.1229, Ocean Thickness and Surface Heat Flow, Icarus, 156, 143–151, 1983. Malamud, U., and D. Prialnik, Modeling Kuiper belt objects doi:10.1006/icar.2001.6776, 2002. Charon, Orcus and Salacia by means of a new equation Hussmann, H., F. Sohl, and T. Spohn, Subsurface oceans and of state for porous icy bodies, Icarus, 246, 21–36, doi: deep interiors of medium-sized outer planet satellites and large 10.1016/j.icarus.2014.02.027, 2015. trans-neptunian objects, Icarus, 185, 258–273, 2006. Malin, M. C., and D. C. Pieri, Europa, pp. 689–717, 1986. Hussmann, H., C. Sotin, and J. I. Lunine, Interiors and evolution Manga, M., and C.-Y. Wang, Pressurized oceans and the erup- of icy satellites, in Treatise Geophys. Vol. 10 (2nd ed), edited tion of liquid water on Europa and Enceladus, Geophys. Res. by G. Schubert, pp. 605–635, Elsevier, 2015. Lett., 34, L07202, doi:10.1029/2007GL029297, 2007. Iess, L., N. J. Rappaport, R. A. Jacobson, P. Racioppa, D. J. Margot, J. L., S. J. Peale, R. F. Jurgens, M. A. Slade, and I. V. Stevenson, P. Tortora, J. W. Armstrong, and S. W. Asmar, Holin, Large Longitude Libration of Mercury Reveals a Molten Gravity Field, Shape, and Moment of Inertia of Titan, Sci- Core, Science, 316, 710, doi:10.1126/science.1140514, 2007. ence, 327, 1367, doi:10.1126/science.1182583, 2010. Margot, J.-L., S. Padovan, D. Campbell, S. Peale, and F. Ghigo, Iess, L., R. A. Jacobson, M. Ducci, D. J. Stevenson, J. I. Lunine, Measurements of the spin states of Europa and Ganymede, J. W. Armstrong, S. W. Asmar, P. Racioppa, N. J. Rappaport, in AAS/Division for Planetary Sciences Meeting Abstracts, and P. Tortora, The Tides of Titan, Science, 337, 457–459, AAS/Division for Planetary Sciences Meeting Abstracts, 2012. vol. 45, p. 504.02, 2013. Iess, L., D. J. Stevenson, M. Parisi, D. Hemingway, R. A. Ja- Matson, D. L., J. C. Castillo, J. Lunine, and T. V. Johnson, cobson, J. I. Lunine, F. Nimmo, J. W. Armstrong, S. W. Enceladus’ plume: Compositional evidence for a hot interior, Asmar, M. Ducci, and P. Tortora, The Gravity Field and Icarus, 187, 569–573, doi:10.1016/j.icarus.2006.10.016, 2007. Interior Structure of Enceladus, Science, 344, 78–80, doi: Matson, D. L., J. C. Castillo-Rogez, G. Schubert, C. Sotin, and 10.1126/science.1250551, 2014. W. B. McKinnon, The Thermal Evolution and Internal Struc- Jara-Oru´e,H. M., and B. L. A. Vermeersen, The forced libration ture of Saturn’s Mid-Sized Icy Satellites, p. 577, 2009. of Europas deformable shell and its dependence on interior pa- Matsuyama, I., Tidal dissipation in the oceans of icy satellites, rameters, Icarus, 229, 31–44, doi:10.1016/j.icarus.2013.10.027, Icarus, 242, 11–18, doi:10.1016/j.icarus.2014.07.005, 2014. 2014. McCarthy, C., and J. C. Castillo-Rogez, Planetary Ices Attenu- Jones, C. A., P. Boronski, A. S. Brun, G. A. Glatzmaier, ation Properties, in Astrophysics and Space Science Library, T. Gastine, M. S. Miesch, and J. Wicht, Anelastic convection- Astrophysics and Space Science Library, vol. 356, edited by driven dynamo benchmarks, Icarus, 216, 120–135, doi: M. S. Gudipati and J. Castillo-Rogez, p. 183, doi:10.1007/978- 10.1016/j.icarus.2011.08.014, 2011. 1-4614-3076-6-7, 2013. Kamata, S., I. Matsuyama, and F. Nimmo, Tidal resonance in icy McCord, T. B., G. B. Hansen, D. L. Matson, T. V. Jonhson, J. K. satellites with subsurface oceans, Journal of Geophysical Re- Crowley, F. P. Fanale, R. W. Carlson, W. D. Smythe, P. D. search (Planets), 120, 1528–1542, doi:10.1002/2015JE004821, Martin, C. A. Hibbitts, J. C. Granahan, and A. Ocampo, Hy- 2015. drated salt minerals on Europa’s surface from the Galileo near- Kargel, J. S., and S. Pozio, The Volcanic and Tectonic History of infrared mapping spectrometer (NIMS) investigation, J. Geo- Enceladus, Icarus, 119, 385–404, doi:10.1006/icar.1996.0026, phys. Res., 104, 11,827–11,852, doi:10.1029/1999JE900005, 1996. 1999. Kattenhorn, S. A., and L. M. Prockter, Evidence for subduction McCord, T. B., G. B. Hansen, and C. A. Hibbitts, Hydrated in the ice shell of Europa, Nature Geoscience, 7, 762–767, doi: Salt Minerals on Ganymede’s Surface: Evidence of an Ocean 10.1038/ngeo2245, 2014. Below, Science, 292, 1523–1525, doi:10.1126/science.1059916, Khurana, K. K., M. G. Kivelson, and C. T. Russell, Searching for 2001. Liquid Water in Europa by Using Surface Observatories, As- McKinnon, W. B., NOTE: Mystery of Callisto: Is It Undifferenti- trobiology, 2, 93–103, doi:10.1089/153110702753621376, 2002. ated?, Icarus, 130, 540–543, doi:10.1006/icar.1997.5826, 1997. McKinnon, W. B., Radar Sounding of Convecting Ice Shells in the Kirk, R. L., and D. J. Stevenson, Thermal evolution of a differen- Presence of Convection: Application to Europa, Ganymede, tiated Ganymede and implications for surface features, Icarus, and Callisto, in Workshop on Radar Investigations of Plane- 69, 91–134, doi:10.1016/0019-1035(87)90009-1, 1987. tary and Terrestrial Environments, edited by E. Heggy, S. Clif- Kirk, R. L., L. A. Soderblom, and R. H. Brown, Subsurface en- ford, T. Farr, C. Dinwiddie, and B. Grimm, 2005. ergy storage and transport for solar-powered geysers on Triton, McKinnon, W. B., On convection in ice I shells of outer Solar Science, 250, 424–429, doi:10.1126/science.250.4979.424, 1990. System bodies, with detailed application to Callisto, Icarus, Kivelson, M. G., K. K. Khurana, and M. Volwerk, The Perma- 183, 435–450, doi:10.1016/j.icarus.2006.03.004, 2006. nent and Inductive Magnetic Moments of Ganymede, Icarus, McKinnon, W. B., Effect of Enceladus’s rapid synchronous spin 157, 507–522, doi:10.1006/icar.2002.6834, 2002. on interpretation of Cassini gravity, Geophys. Res. Lett., 42, ¨ Lainey, V., J.-E. Arlot, O. Karatekin, and T. van Hoolst, Strong 2137–2143, doi:10.1002/2015GL063384, 2015. tidal dissipation in Io and Jupiter from astrometric observa- Melosh, H. J., A. G. Ekholm, A. P. Showman, and R. D. Lorenz, tions, Nature, 459, 957–959, doi:10.1038/nature08108, 2009. The temperature of Europa’s subsurface water ocean, Icarus, Lainey, V., O.¨ Karatekin, J. Desmars, S. Charnoz, J.-E. Arlot, 168, 498–502, doi:10.1016/j.icarus.2003.11.026, 2004. N. Emelyanov, C. Le Poncin-Lafitte, S. Mathis, F. Remus, Melosh, H. J., A. M. Freed, B. C. Johnson, D. M. Blair, J. C. G. Tobie, and J.-P. Zahn, Strong Tidal Dissipation in Saturn Andrews-Hanna, G. A. Neumann, R. J. Phillips, D. E. Smith, and Constraints on Enceladus’ Thermal State from Astrome- S. C. Solomon, M. A. Wieczorek, and M. T. Zuber, The Ori- try, Astrophys. J., 752, 14, doi:10.1088/0004-637X/752/1/14, gin of Lunar Mascon Basins, Science, 340, 1552–1555, doi: 2012. 10.1126/science.1235768, 2013. NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 13
Meriggiola, R., L. Iess, B. W. Stiles, J. I. Lunine, and G. Mitri, Pappalardo, R. T., M. J. S. Belton, H. H. Breneman, M. H. Carr, The rotational dynamics of Titan from Cassini RADAR im- C. R. Chapman, G. C. Collins, T. Denk, S. Fagents, P. E. ages, Icarus, 275, 183–192, doi:10.1016/j.icarus.2016.01.019, Geissler, B. Giese, R. Greeley, R. Greenberg, J. W. Head, 2016. P. Helfenstein, G. Hoppa, S. D. Kadel, K. P. Klaasen, J. E. Meyer, J., and J. Wisdom, Tidal heating in Enceladus, Icarus, Klemaszewski, K. Magee, A. S. McEwen, J. M. Moore, W. B. 188, 535–539, 2007. Moore, G. Neukum, C. B. Phillips, L. M. Prockter, G. Schu- bert, D. A. Senske, R. J. Sullivan, B. R. Tufts, E. P. Turtle, Moore, J. M., and R. T. Pappalardo, Titan: An exogenic world?, R. Wagner, and K. K. Williams, Does Europa have a subsur- Icarus, 212, 790–806, doi:10.1016/j.icarus.2011.01.019, 2011. face ocean? Evaluation of the geological evidence, J. Geophys. Moore, J. M., et al., The geology of Pluto and Charon through Res., 104, 24,015–24,056, doi:10.1029/1998JE000628, 1999. the eyes of New Horizons, Science, 351, 1284–1293, doi: Pappalardo, R. T., W. B. McKinnon, and K. K. Khurana, Eu- 10.1126/science.aad7055, 2016. ropa, 3 pp., 2009. Moore, W. B., NOTE: The Thermal State of Io, Icarus, 154, Peale, S. J., Origin and Evolution of the Natural Satel- 548–550, doi:10.1006/icar.2001.6739, 2001. lites, Ann. Rev. Astron. Astrophys., 37, 533–602, doi: Moore, W. B., Thermal equilibrium in Europa’s ice shell, Icarus, 10.1146/annurev.astro.37.1.533, 1999. 180, 141–146, doi:10.1016/j.icarus.2005.09.005, 2006. Peale, S. J., P. Cassen, and R. T. Reynolds, Melting of Io by Moore, W. B., and H. Hussmann, Thermal Evolution of Europa’s Tidal Dissipation, Science, 203, 892–894, 1979. Silicate Interior, p. 369, 2009. Peters, M. E., D. D. Blankenship, S. P. Carter, S. D. Kempf, D. A. Young, and J. W. Holt, Along-Track Focusing of Air- Moore, W. B., and G. Schubert, NOTE: The Tidal Response borne Radar Sounding Data From West Antarctica for Im- of Europa, Icarus, 147, 317–319, doi:10.1006/icar.2000.6460, proving Basal Reflection Analysis and Layer Detection, IEEE 2000. Transactions on Geoscience and Remote Sensing, 45, 2725– Moore, W. B., and G. Schubert, The tidal response of Ganymede 2736, doi:10.1109/TGRS.2007.897416, 2007. and Callisto with and without liquid water oceans, Icarus, 166, Petrenko, V. F., and R. W. Whitworth, Physics of Ice, 1999. 223–226, doi:10.1016/j.icarus.2003.07.001, 2003. Phillips, R. J., M. T. Zuber, S. E. Smrekar, M. T. Mellon, Morrison, D. (Ed.), Satellites of Jupiter, 1982. J. W. Head, K. L. Tanaka, N. E. Putzig, S. M. Milkovich, Movshovitz, N., F. Nimmo, D. G. Korycansky, E. Asphaug, and B. A. Campbell, J. J. Plaut, A. Safaeinili, R. Seu, D. Bic- J. M. Owen, Disruption and reaccretion of midsized moons cari, L. M. Carter, G. Picardi, R. Orosei, P. S. Mohit, during an outer solar system Late Heavy Bombardment, Geo- E. Heggy, R. W. Zurek, A. F. Egan, E. Giacomoni, F. Russo, phys. Res. Lett., 42, 256–263, doi:10.1002/2014GL062133, M. Cutigni, E. Pettinelli, J. W. Holt, C. J. Leuschen, and 2015. L. Marinangeli, Mars North Polar Deposits: Stratigraphy, Age, and Geodynamical Response, Science, 320, 1182, doi: M¨uller-Wodarg, I., C. A. Griffith, E. Lellouch, and T. E. Cravens, 10.1126/science.1157546, 2008. Titan, 2014. Postberg, F., S. Kempf, J. Schmidt, N. Brilliantov, A. Beinsen, Murray, C. D., and S. F. Dermott, Solar System Dynamics, Cam- B. Abel, U. Buck, and R. Srama, Sodium salts in E-ring ice bridge University Press, New York, 1999. grains from an ocean below the surface of Enceladus, Nature, Nagel, K., D. Breuer, and T. Spohn, A model for the interior 459, 1098–1101, doi:10.1038/nature08046, 2009. structure, evolution, and differentiation of Callisto, Icarus, Rambaux, N., T. van Hoolst, and O.¨ Karatekin, Librational re- 169, 402–412, doi:10.1016/j.icarus.2003.12.019, 2004. sponse of Europa, Ganymede, and Callisto with an ocean for Nimmo, F., Stresses generated in cooling viscoelastic ice shells: a non-Keplerian orbit, Astron. Astrophys., , 527, A118, doi: Application to Europa, Journal of Geophysical Research 10.1051/0004-6361/201015304, 2011. (Planets), 109, E12001, doi:10.1029/2004JE002347, 2004. Rhoden, A. R., W. Henning, T. A. Hurford, and D. P. Nimmo, F., and B. G. Bills, Shell thickness variations and the Hamilton, The interior and orbital evolution of Charon as long-wavelength topography of Titan, Icarus, 208, 896–904, preserved in its geologic record, Icarus, 246, 11–20, doi: doi:10.1016/j.icarus.2010.02.020, 2010. 10.1016/j.icarus.2014.04.030, 2015. Roberts, J. H., The fluffy core of Enceladus, Icarus, 258, 54–66, Nimmo, F., and R. T. Pappalardo, Diapir-induced reorienta- doi:10.1016/j.icarus.2015.05.033, 2015. tion of Saturn’s moon Enceladus, Nature, 441, 614–616, doi: Roberts, J. H., and F. Nimmo, Tidal heating and the long-term 10.1038/nature04821, 2006. stability of a subsurface ocean on Enceladus, Icarus, 194, 675– Nimmo, F., J. R. Spencer, R. T. Pappalardo, and M. E. Mullen, 689, 2008. Shear heating as the origin of the plumes and heat flux on Robuchon, G., and F. Nimmo, Thermal evolution of Pluto and Enceladus, Nature, 447, 289–291, doi:10.1038/nature05783, implications for surface tectonics and a subsurface ocean, 2007. Icarus, 216, 426–439, doi:10.1016/j.icarus.2011.08.015, 2011. Noir, J., F. Hemmerlin, J. Wicht, S. M. Baca, and J. M. Au- Ross, M., and G. Schubert, Tidal dissipation in a vis- rnou, An experimental and numerical study of librationally coelastic planet, J. Geophys. Res., 91, D447–D452, doi: driven flow in planetary cores and subsurface oceans, Physics 10.1029/JB091iB04p0D447, 1986. of the Earth and Planetary Interiors, 173, 141–152, doi: Ross, M. N., and G. Schubert, The coupled orbital and thermal 10.1016/j.pepi.2008.11.012, 2009. evolution of Triton, Geophys. Res. Lett., 17, 1749–1752, doi: 10.1029/GL017i010p01749, 1990. Ojakangas, G. W., and D. J. Stevenson, Episodic Volcanism of Roth, L., J. Saur, K. D. Retherford, D. F. Strobel, P. D. Tidally Heated Satellites with Application to Io, Icarus, 66, Feldman, M. A. McGrath, and F. Nimmo, Transient Water 341–358, 1986. Vapor at Europa’s South Pole, Science, 343, 171–174, doi: Ojakangas, G. W., and D. J. Stevenson, Thermal State of an Ice 10.1126/science.1247051, 2014. Shell on Europa, Icarus, 81, 220–241, 1989. Sagan, C., and S. F. Dermott, The tide in the seas of Titan, O’Neill, C., and F. Nimmo, The role of episodic overturn in gen- Nature, 300, 731–733, doi:10.1038/300731a0, 1982. erating the surface geology and heat flow on Enceladus, Nature Saur, J., S. Duling, L. Roth, X. Jia, D. F. Strobel, P. D. Feld- Geoscience, 3, 88–91, doi:10.1038/ngeo731, 2010. man, U. R. Christensen, K. D. Retherford, M. A. McGrath, Pang, K. D., C. C. Voge, J. W. Rhoads, and J. M. Ajello, The E F. Musacchio, A. Wennmacher, F. M. Neubauer, S. Simon, and ring of Saturn and satellite Enceladus, J. Geophys. Res., 89, O. Hartkorn, The search for a subsurface ocean in Ganymede 9459–9470, doi:10.1029/JB089iB11p09459, 1984. with Hubble Space Telescope observations of its auroral ovals, Pappalardo, R., L. Prockter, D. Senske, B. Paczkowski, S. Vance, Journal of Geophysical Research (Space Physics), 120, 1715– 1737, doi:10.1002/2014JA020778, 2015. G. W. Patterson, B. Goldstein, T. Magner, and B. Cooke, Schenk, P., I. Matsuyama, and F. Nimmo, True polar wander on Science and Reconnaissance from the Europa Clipper Mission Europa from global-scale small-circle depressions, Nature, 453, Concept, in Lunar and Planetary Science Conference, Lunar 368–371, doi:10.1038/nature06911, 2008. and Planetary Science Conference, vol. 45, p. 1655, 2014. Schenk, P. M., Thickness constraints on the icy shells of the Pappalardo, R. T., J. W. Head, R. Greeley, R. J. Sullivan, galilean satellites from a comparison of crater shapes, Nature, C. Pilcher, G. Schubert, W. B. Moore, M. H. Carr, J. M. 417, 419–421, 2002. Moore, M. J. S. Belton, and D. L. Goldsby, Geological evi- Schenk, P. M., W. B. McKinnon, D. Gwynn, and J. M. Moore, dence for solid-state convection in Europa’s ice shell, Nature, Flooding of Ganymede’s bright terrains by low-viscosity water- 391, 365, doi:10.1038/34862, 1998. ice lavas, Nature, 410, 57–60, 2001. X - 14 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM
Schmitz, W. J., and M. S. McCartney, On the North At- Tittemore, W. C., and J. Wisdom, Tidal evolution of the Ura- lantic Circulation, Reviews of Geophysics, 31, 29–49, doi: nian satellites. III - Evolution through the Miranda-Umbriel 10.1029/92RG02583, 1993. 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion Schneider, N. M., M. H. Burger, E. L. Schaller, M. E. Brown, commensurabilities, Icarus, 85, 394–443, doi:10.1016/0019- R. E. Johnson, J. S. Kargel, M. K. Dougherty, and N. A. 1035(90)90125-S, 1990. Achilleos, No sodium in the vapour plumes of Enceladus, Na- Tobie, G., O. Grasset, J. I. Lunine, A. Mocquet, and C. Sotin, Ti- ture, 459, 1102–1104, doi:10.1038/nature08070, 2009. tan’s internal structure inferred from a coupled thermal-orbital Schubert, G., J. D. Anderson, T. Spohn, and W. B. McKinnon, model, Icarus, 175, 496–502, 2005. Interior Composition, Structure and Dynamics of the Galilean Tobie, G., D. Gautier, and F. Hersant, Titan’s Bulk Composi- Satellites, in Jupiter: The Planet, Satellites and Magneto- tion Constrained by Cassini-Huygens: Implication for Inter- sphere, edited by F. Bagenal, T. Dowling, and W. McKinnon, nal Outgassing, Astrophys. J., 752, 125, doi:10.1088/0004- pp. 281–306, Cambridge University Press, 2004. 637X/752/2/125, 2012. Tortora, P., M. Zannoni, D. Hemingway, F. Nimmo, R. A. Ja- Shirley, J. H., J. B. Dalton, L. M. Prockter, and L. W. Kamp, cobson, L. Iess, and M. Parisi, Rhea gravity field and interior Europa’s ridged plains and smooth low albedo plains: Dis- modeling from Cassini data analysis, Icarus, 264, 264–273, tinctive compositions and compositional gradients at the lead- doi:10.1016/j.icarus.2015.09.022, 2016. ing side-trailing side boundary, Icarus, 210, 358–384, doi: Travis, B. J., J. Palguta, and G. Schubert, A whole-moon 10.1016/j.icarus.2010.06.018, 2010. thermal history model of Europa: Impact of hydrothermal Shock, E. L., and W. B. McKinnon, Hydrothermal processing of circulation and salt transport, Icarus, 218, 1006–1019, doi: cometary volatiles - Applications to Triton, Icarus, 106, 464, 10.1016/j.icarus.2012.02.008, 2012. doi:10.1006/icar.1993.1185, 1993. Turtle, E. P., and E. Pierazzo, Thickness of a Europan Ice Shell Shoemaker, E. M., B. K. Lucchitta, D. E. Wilhelms, J. B. Plescia, from Impact Crater Simulations, Science, 294, 1326–1328, doi: and S. W. Squyres, The geology of Ganymede, in Satellites of 10.1126/science.1062492, 2001. Jupiter, edited by D. Morrison, pp. 435–520, 1982. Tyler, R., Comparative estimates of the heat generated by ocean Shoji, D., H. Hussmann, F. Sohl, and K. Kurita, Non-steady tides on icy satellites in the outer Solar System, Icarus, 243, state tidal heating of Enceladus, Icarus, 235, 75–85, doi: 358–385, doi:10.1016/j.icarus.2014.08.037, 2014. 10.1016/j.icarus.2014.03.006, 2014. Van Hoolst, T., R.-M. Baland, and A. Trinh, On the librations Showman, A. P., D. J. Stevenson, and R. Malhotra, Coupled and tides of large icy satellites, Icarus, 226, 299–315, doi: orbital and thermal evolution of Ganymede, Icarus, 129, 367– 10.1016/j.icarus.2013.05.036, 2013. 383, 1997. Vance, S., and J. M. Brown, Layering and double-diffusion Soderlund, K. M., B. E. Schmidt, J. Wicht, and D. D. Blanken- style convection in Europa’s ocean, Icarus, 177, 506–514, doi: ship, Ocean-driven heating of Europa’s icy shell at low lat- 10.1016/j.icarus.2005.06.005, 2005. itudes, Nature Geoscience, 7, 16–19, doi:10.1038/ngeo2021, Vance, S., J. Harnmeijer, J. Kimura, H. Hussmann, B. deMartin, 2014. and J. M. Brown, Hydrothermal Systems in Small Ocean Sohl, F., W. D. Sears, and R. D. Lorenz, Tidal Dissipation on Planets, Astrobiology, 7, 987–1005, doi:10.1089/ast.2007.0075, Titan, Icarus, 115, 278–294, 1995. 2007. Sohl, F., M. Choukroun, J. Kargel, J. Kimura, R. Pappalardo, Wahr, J. M., M. T. Zuber, D. E. Smith, and J. I. Lunine, S. Vance, and M. Zolotov, Subsurface Water Oceans on Icy Tides on Europa, and the thickness of Europa’s icy shell, Journal of Geophysical Research (Planets), 111, E12005, doi: Satellites: Chemical Composition and Exchange Processes, 10.1029/2006JE002729, 2006. Space Sci. Rev., 153, 485–510, doi:10.1007/s11214-010-9646- Waite, J. H., Jr., W. S. Lewis, B. A. Magee, J. I. Lunine, W. B. y, 2010. McKinnon, C. R. Glein, O. Mousis, D. T. Young, T. Brockwell, Solomatov, V. S., Scaling of temperature- and stress-dependent J. Westlake, M.-J. Nguyen, B. D. Teolis, H. B. Niemann, R. L. viscosity convection, Physics of Fluids, 7, 266–274, doi: McNutt, M. Perry, and W.-H. Ip, Liquid water on Enceladus 10.1063/1.868624, 1995. from observations of ammonia and 40Ar in the plume, Nature, Sotin, C., O. Grasset, and S. Beauchesne, Thermodynamical 460, 487–490, doi:10.1038/nature08153, 2009. Properties of High Pressure Ices. Implications for the Dynam- Wisdom, J., Tidal dissipation at arbitrary eccentricity and obliq- ics and Internal Structure of Large Icy Satellites, in Solar Sys- uity, Icarus, 193, 637–640, doi:10.1016/j.icarus.2007.09.002, tem Ices, Astrophysics and Space Science Library, vol. 227, 2008. edited by B. Schmitt, C. de Bergh, and M. Festou, p. 79, 1998. Yao, C., F. Deschamps, J. P. Lowman, C. Sanchez-Valle, and Spohn, T., and G. Schubert, Oceans in the icy Galilean satel- P. J. Tackley, Stagnant lid convection in bottom-heated thin lites of Jupiter?, Icarus, 161, 456–467, doi:10.1016/S0019- 3-D spherical shells: Influence of curvature and implications 1035(02)00048-9, 2003. for dwarf planets and icy moons, Journal of Geophysical Re- Stern, S. A., and D. J. Tholen, Pluto and Charon, 1997. search (Planets), 119, 1895–1913, doi:10.1002/2014JE004653, Stern, S. A., et al., The Pluto system: Initial results from its 2014. exploration by New Horizons, Science, 350, aad1815, doi: Zahnle, K., P. Schenk, S. Sobieszczyk, L. Dones, and H. F. 10.1126/science.aad1815, 2015. Levison, Differential Cratering of Synchronously Rotating Stewart, S. T., and T. J. Ahrens, Shock properties of H2O ice, Satellites by Ecliptic Comets, Icarus, 153, 111–129, doi: Journal of Geophysical Research (Planets), 110, E03005, doi: 10.1006/icar.2001.6668, 2001. 10.1029/2004JE002305, 2005. Zahnle, K., P. Schenk, H. Levison, and L. Dones, Cratering Sullivan, R., R. Greeley, K. Homan, J. Klemaszewski, M. J. S. rates in the outer Solar System, Icarus, 163, 263–289, doi: Belton, M. H. Carr, C. R. Chapman, R. Tufts, J. W. Head, 10.1016/S0019-1035(03)00048-4, 2003. R. Pappalardo, J. Moore, and P. Thomas, Episodic plate sep- Zhang, K., and F. Nimmo, Recent orbital evolution and the inter- aration and fracture infill on the surface of Europa, Nature, nal structures of Enceladus and Dione, Icarus, 204, 597–609, 2009. 391, 371, doi:10.1038/34874, 1998. Zhang, K., and F. Nimmo, Late-stage impacts and the orbital Tajeddine, R., N. Rambaux, V. Lainey, S. Charnoz, A. Richard, and thermal evolution of Tethys, Icarus, 218, 348–355, doi: A. Rivoldini, and B. Noyelles, Constraints on Mimas interior 10.1016/j.icarus.2011.12.013, 2012. from Cassini ISS libration measurements, Science, 346, 322– Zimmer, C., K. K. Khurana, and M. G. Kivelson, Subsur- 324, doi:10.1126/science.1255299, 2014. face Oceans on Europa and Callisto: Constraints from Thomas, P. C., R. Tajeddine, M. S. Tiscareno, J. A. Burns, Galileo Magnetometer Observations, Icarus, 147, 329–347, doi: J. Joseph, T. J. Loredo, P. Helfenstein, and C. Porco, Ence- 10.1006/icar.2000.6456, 2000. ladus’s measured physical libration requires a global subsurface Zolotov, M. Y., An oceanic composition on early and to- ocean, Icarus, 264, 37–47, doi:10.1016/j.icarus.2015.08.037, day’s Enceladus, Geophys. Res. Lett., 34, L23203, doi: 2016. 10.1029/2007GL031234, 2007. Thomson, R. E., and J. R. Delaney, Evidence for a weakly strati- fied Europan ocean sustained by seafloor heat flux, J. Geophys. F. Nimmo, Department of Earth and Planetary Sciences, Uni- Res., 106, 12,355–12,365, doi:10.1029/2000JE001332, 2001. versity of California, 1156 High Street, Santa Cruz, CA 95064, Tiscareno, M. S., P. C. Thomas, and J. A. Burns, The rota- USA. ([email protected]) tion of Janus and Epimetheus, Icarus, 204, 254–261, doi: R.T. Pappalardo, Jet Propulsion Laboratory, Pasadena, CA 10.1016/j.icarus.2009.06.023, 2009. 91109, USA (pappalardo@jpl.nasa.gov) NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 15
Figure 1. Montage of (mostly) icy worlds, courtesy of Emily Lakdawalla/Planetary Society. For satellites la- belled in blue there are geophysical measurements im- plying the presence of a subsurface ocean (see text). X - 16 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM
Figure 2. Surfaces of possible ocean worlds. a) Pluto, radius 1188 km. Note the extensional faults at lower left. NASA planetary image atlas (PIA) 19937. b) Gey- sers at Enceladus, radius 252 km. PIA11688. c) Pits spots and domes on Europa. PIA03878. d) Cycloidal ridges on Europa. From [Hoppa et al., 1999]. e) Tri- ton, radius 1353 km. The dark streaks are deposits from geysers; also note the curvilinear tectonic features at far right. PIA00059. f) Radar image of mountains on Ti- tan. Radar look direction is indicated by the arrow. PIA10654. g) Bright (highly tectonized) and older dark (less tectonized) terrain on Ganymede. PIA01618. NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 17
Figure 3. Rock mass fraction vs. semi-major axis for outer solar system bodies. Semi-major axis is given in terms of primary radii (Rp) and rock mass fraction is calculated based on measured bulk density assuming rock and ice densities of 3500 kgm−3 and 950 kgm−3, respectively. Abbreviations are as fol- lows: Ar(iel), Ca(llisto), Ch(aron), Di(one), En(celadus), Eu(ropa), Ga(nymede), Mi(randa) [Uranus], Mi(mas) [Saturn], Ob(eron), Pl(uto), Rh(ea), Te(thys), Ti(tan) [Saturn], Ti(tania) [Uranus], Tr(iton), Um(briel). Error bars for Uranian satellites reflect uncertain masses. X - 18 NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM
Figure 4. Ice melting behavior and phases, after Huss- mann et al. [2015]. Solid lines represent pure H2O phase diagram, with different phases marked with Roman nu- merals. Dashed lines represent melting curve of H2O plus 5% or 15% ammonia. Thin solid red and blue dashed lines show hypothetical thermal profiles, illustrat- ing how a liquid layer may develop between low- and high-pressure solid phases. The inset shows the generic structure of a large icy satellite such as Ganymede.
Figure 5. Cartoon of physical libration (view from above the orbit plane). In the reference frame set by the mean rotation rate of satellite with a non-circular orbit, the planet executes an ellipse. In general, any per- manent tidal bulge of the satellite will not point towards the planet but librate (oscillate) around a mean posi- tion. The amplitude of this physical libration is γ. Figure adapted from [Tiscareno et al., 2009]. NIMMO AND PAPPALARDO: OCEAN WORLDS IN THE OUTER SOLAR SYSTEM X - 19
Figure 6. Cartoon of Cassini state. The instantaneous orbit pole precesses around an invariable pole. The ro- tation pole of the satellite precesses around the instan- taneous orbit pole. In a Cassini state, the rotation pole, orbit pole and invariable pole remain coplanar.
Figure 7. Modeled time-variable heat production in Io and Europa resulting from thermal-orbital feedbacks, from Hussmann and Spohn [2004]. The oscillations result from the comparable timescales of eccentricity growth and changes in the interior temperature structure.