P11F-01 INVITED 

Cold Fire: The Geology and Geophysics of Enceladus

* McKinnon, W B (mckinnon@wustl.edu), Dept. Earth Planet. Sci. and McDonnell Center for Space Sci., Washington Univ., Saint Louis, MO 63130, United States

The mythological Enceladus is buried beneath Mt. Etna, and is responsible for its tremors and volcanism. Enceladus, the 500-km-wide moon of Saturn, is no less active, but the source of its geological vigor is one of the great challenges of contemporary planetary science. Data from Cassini has revolutionized our view of this world. It is rock-rich, 56-59% rock by mass, yet its surface is nearly pure water ice. Most of Enceladus has been resurfaced, and active plumes of vapor and micron-sized ice particles are erupting from a set of semi- parallel fissures ("tiger stripes") at its south pole. Thermal emission from the south polar terrain (SPT) of 5.8 ± 1.9 GW is measured directly. The SPT is also ringed with distinctive compressional fold and/or thrust belts. Enceladus is thus the icy satellite equivalent to Io, in terms of phenomena and data type, i.e., traditional visual imaging and infrared spectroscopy are augmented by heat flow determination; in addition, Cassini can and has flown through the plume and measured its composition. The source of Enceladus' thermal output must be tidal heating associated with its 2:1 e-type mean-motion resonance with larger Dione. The magnitude of its heat output can be explained by a suitably dissipative interior, but one that in all likelihood is differentiated and contains an internal ocean ("aquasphere"), because solid-body dissipation is inadequate. Its present heat flow cannot be maintained over geologic time, however, which implies that Enceladus' eccentricity is time variable (presently decreasing) and/or that the source of Saturn's dissipation (its ` Q') has also varied (decreased) over geologic time. Where in Enceladus is tidal strain energy dissipated, and why is it concentrated in one geographic locale? Is it in the rock core, the overlying warm (in a homologous sense) ice mantle, or in the cold lithosphere, along vertical, "tiger-rific" shear zones, or some combination? Core dissipation is unlikely to be quantitatively sufficient, whereas models of mantle and/or lithospheric dissipation are under active development. And what is the relation between the present activity at the SPT and the rest of the surface, most of which appears geologically youthful and also betrays evidence of high heat flows (viscously relaxed craters, groove-like extensional terrain)? Ammonia and salts have been suggested as antifreezes in Enceladus' ocean, facilitating plume eruptions. To date neither have been detected in the plumes or in the related E-ring torus, and the limits on Na are particularly stringent. The latter sets a severe constraint on the extent of water-rock interaction within Enceladus, if the proximate plume source is ocean water. An alternate view posits that the plumes are driven by degassing clathrate; this hypothesis is attractive in terms of physical chemistry but a clathrate- dominated mantle is too stiff (as presently measured) to be compatible with Enceladus' active tectonics. Whatever the cause and path for Enceladus' activity, it promises to greatly clarify our theoretical understanding of how icy worlds work.

P11F-02 INVITED 

Models of the Enceladus Plumes: Is Liquid Water Required?

* Ingersoll, A P (api@gps.caltech.edu), California Institute of Technology, Mail Code 150-21 1200 E California Blvd, Pasadena, CA 91125, United States

What do the observations of the plumes and surface environment tell us about the interior of Enceladus? We have information about the gas composition, particle composition, particle size, the collimation and speed of the jets, the composition and size of crystals on the surface, and the surface temperatures, from which we can infer the surface heat flow. All of this is preliminary; new data are expected and the old data are still being analyzed. We also know a lot about the geologic setting around the south pole where the plumes originate. The basic questions are: What is Enceladus made of, especially in the icy upper crust? What is the internal temperature distribution? Is liquid water present, and how deep is it? How does the heat get to the surface? Two important end members in the spectrum of possibilities are the boiling liquid model and the cold condensing vapor model. They both produce jets of water vapor and ice, as observed, but they differ in many important respects. Accounting for the other gases observed in the plumes, many of which are more volatile than water, is a challenge for any theory. The gas-to-solid ratio is an important observable, but its value may depend more on the geometry of the vents than on the temperature of the sub-surface sources. A liquid transports heat more effectively than a low- density gas, but even the gas seems able to supply the observed heat. If this is true, any liquid water may be far below the surface. At most we can hope to rule out some hypotheses, but the range of possibilities remains large. The talk will review models that have been published at the time of the meeting. One can expect further progress in the months ahead.

P11F-03 

Tidal Evolution of Mimas, Enceladus, and Dione

* Meyer, J A (meyerj@mit.edu), Massachusetts Institute of Technology, 54-411 77 Massachusetts Ave, Cambridge, MA 02139, United States Wisdom, J (wisdom@mit.edu), Massachusetts Institute of Technology, 54-411 77 Massachusetts Ave, Cambridge, MA 02139, United States

The tidal evolution through several resonances involving Mimas, Enceladus, and/or Dione is studied numerically with an averaged resonance model. We find that, in the Enceladus-Dione 2:1 e-Enceladus type resonance, Enceladus evolves chaotically in the future for some values of k₂/Q. Past evolution of the system is marked by temporary capture into the Enceladus-Dione 4:2 ee'-mixed resonance. We find that the free libration of the Enceladus-Dione 2:1 e-Enceladus resonance angle of 1.5° can be explained by a recent passage of the system through a secondary resonance. In simulations with passage through the secondary resonance, the system enters the current Enceladus-Dione resonance close to tidal equilibrium and thus the equilibrium value of tidal heating of 1.1 (18,000/Q_S) GW applies. We find that the current anomalously large eccentricity of Mimas can be explained by passage through several past resonances. In all cases, escape from the resonance occurs by unstable growth of the libration angle, sometimes with the help of a secondary resonance. Explanation of the current eccentricity of Mimas by evolution through these resonances implies that the Q of Saturn is below 100,000. Though the eccentricity of Enceladus can be excited to moderate values by capture in the Mimas-Enceladus 3:2 e-Enceladus resonance, the libration amplitude damps and the system does not escape. Thus past occupancy of this resonance and consequent tidal heating of Enceladus is excluded. The construction of a coherent history places constraints on the allowed values of k₂/Q for the satellites.

P11F-04 

Solid tidal friction above a liquid water reservoir as the origin of the South Pole Hotspot on Enceladus

* Tobie, G (gabriel.tobie@univ-nantes.fr), CNRS, University of Nantes, UFR Sciences et Techniques, 2 rue de la Houssinière BP 92208, Nantes, 44322, France Cadek, O (oc@karel.troja.mff.cuni.cz), Charles University, Department of Geophysics, Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, Prague, 180 00, Czech Republic Sotin, C (christophe.sotin@univ-nantes.fr), CNRS, University of Nantes, UFR Sciences et Techniques, 2 rue de la Houssinière BP 92208, Nantes, 44322, France

Earth, Jupiter's moon Io and Saturn's tiny moon Enceladus are the only solid objects in the solar system to be sufficiently geologically active for their internal heat to be detected by remote sensing. But in contrast to the Earth and Io, the endogenic activity on Enceladus is only located on a specific region at the South Pole, from which jets of water vapor and ice particles have been observed. The current polar location of the thermal anomaly can be explained by diapir-induced reorientation of the satellite, but the thermal anomaly triggering and the heat power required to sustain it over geological timescales remain problematic. Using a tri-dimensional viscoelastic numerical model simulating the response of Enceladus to tidal forcing, we demonstrate that only interior models with a liquid water layer at depth can explain the observed magnitude of dissipation rate and its particular location at the South Pole. Tidal dissipation in the ice shell is large enough to explain the observed heat power and can sustain a layer of liquid water at depth over geologic timescales. Spatial and temporal fluctuations of Enceladus' gravity field to be measured by future missions should confirm the presence of the liquid water zone and its depth below the surface.

P11F-05 

Geological Evidence That Enceladus Librates About Synchronous Rotation

* Hurford, T A (hurfordt@core2.gsfc.nasa.gov), Planetary Geodynamics Lab., NASA Goddard Space Flight Center, Greenbelt, MD 20771, Bills, B G (bbills@ucsd.edu), Planetary Geodynamics Lab., NASA Goddard Space Flight Center, Greenbelt, MD 20771, Bills, B G (bbills@ucsd.edu), SCRIPPS, Institute of Oceanography, La Jolla, CA 92093, Helfenstein, P (helfenst@astro.cornell.edu), CRSR, Cornell University, Ithaca, NY 14853, Hamilton, D P (hamilton@astro.umd.edu), Department of Astronomy, University of Maryland, College Park, MD 20742, Hoppa, G V (gvhoppa@raytheon.com), Raytheon, 235 Presidential Way, Woburn, MA 01801, Greenberg, R (greenberg@lpl.arizona.edu), Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, Purser, C M (carola@minnowpond.us), Eleanor Roosevelt High School, 7601 Hanover Parkway, Greenbelt, MD 20771,

Enceladus is thermally active even though radiation from its surface was expected to have cooled it long ago. Radiogenic heating (Schubert et al., 1986) and tidal heating (Porco et al., 2006), resulting from its small orbital eccentricity, appear inadequate to explain observations of heat flow from its surface (Spencer et al., 2006). Additional heat can be generated if Enceladus librates, or departs from a uniform rate of rotation, due to resonant enhancement of its response to gravitational torques from Saturn (Wisdom, 2004). This forced libration has not yet been directly observed, but an upper limit of 1.5^circ has been placed on its amplitude (Porco et al., 2006). Libration would affect the diurnal stresses produced by tides on its surface. Tidal stresses due to orbital eccentricity should generate strike-slip displacements, which, in the absence of a forced libration, would be right- lateral in sense for all faults in the south polar region according to the tidal walking theory (Hoppa et al., 1999). Strike-slip displacement generated in the presence of a forced libration allows for left-lateral strike slip displacement there, which is consistent with our observations of left-lateral strike-slip displacements, thus providing evidence that Enceladus may librate. In order to produce the observed left-lateral displacement in the south polar region the librational amplitude must be at least 0.54^circ. Tidal stress, incorporating Enceladus' libration, may have impacted the formation and subsequent evolution of other features in the tectonic record. Moreover, tidal stresses due to both orbital eccentricity and the libration may control the timing and location of jets from rifts near the south pole (Hurford et al., 2007), and may generate at least some heat along these faults (Nimmo et al., 2007). Until direct observations of Enceladus' libration are possible, geologic observations combined with tidal theory provide the strongest evidence for its existence.

P11F-06 

Tidally Driven Stress Accumulation and Fault Displacements of Enceladus's Tiger Stripes

* Smith-Konter, B R (bkonter@jpl.nasa.gov AF: AF:

Cassini observations of the south polar region of Saturn's moon Enceladus revealed four large linear fractures, or "tiger stripes," associated with anomalous heat flow and active plumes. These features are thought to be active faults along which tidally induced strike-slip and/or open-close tectonic motions occur, similar to motions inferred for some fractures on Europa. These motions are likely a result of tidally induced stresses exerted on a satellite during its daily elliptical orbital cycle around its parent body. When resolved onto potentially active fault planes, tidal shear stresses drive strike-slip motions, while normal stresses control a fault's frictional resistance to failure. Accounting for both stress contributions, the Coulomb failure criterion holds that shear failure will occur on optimally oriented fault planes when the applied shear stress exceeds the frictional resistance of a fault. Thus, Coulomb stress is a measure of a fault's potential to store stress in the form of fault locking (when normal stresses dominate), or to release stress in the form of fault slip (when shear stresses dominate). We resolve shear and normal tidal stresses onto the tiger stripe fault system and also account for normal stress at depth due to the overburden pressure. We compute Coulomb stress failure conditions to assess failure direction, frequency, and location throughout the Enceladus orbital cycle and find that the entire tiger stripe system is capable of sustaining periods of fault locking (stress accumulation) near periapse and fault displacement (shear slip) near apoapse. We integrate these stresses into a 3D time-dependent fault dislocation model to evaluate tectonic displacements and stress variations at depth. Depending on the sequence of imposed stress accumulation and release, which varies as a function of fault location and orientation, frictional coefficient, and fault depth, we estimate that approximately 0.5 m of horizontal strike-slip displacement (both right and left-lateral) are possible during a tiger stripe fault slip episode. These analyses of tidal stress accumulation and subsequent fault displacement may help explain observed plume activity and temperature anomalies at Enceladus's south polar region as related to shear heating and vapor release.

P11F-07 

The Role of Ammonia in the Evolution of Enceladus

Freeman, J (freeman.justin@gmail.com), RSES, Australian National University, Acton, ACT 0200, Australia * Stegman, D (dave.stegman@sci.monash.edu), School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia May, D (david.may@sci.monash.edu), School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia

A large internal density anomaly, most likely an ice diapir, is inferred to play a central role in a sequence of globally significant events 1) true polar wander induced by the ice diapir so that the region of anomalous activity has become situated at the south pole (Nimmo et al., 2006), 2) formation of large fractures due to the tectonic stresses generated as a consequence of the elastic lithosphere's reorientation (Melosh, 1980), 3) subsequent motion along the fractures producing frictional heating and water vapor which is deposited on the surface of the surrounding region where it recondenses (Nimmo et al., 2007). Recent models of shear heating along the tiger stripes (Nimmo et al., 2007) not only appear to account for nearly all of observed surface heat flow 5.8±1.9 GW (Spencer et al., 2006) but provide a good match to the surface distribution of temperature. Models of purely thermal convection that require between 3-5 GW of internal heating in order to successfully develop degree-one features (Grott et al., 2007) are problematic for two reasons: 1) nearly all of the observed 6 GW coming out of the south pole is produced near the surface (Nimmo et al., 2007) and 2) several independent analyses estimate the maximum tidal dissipation available for internal heating in the range between 0.1-0.5 GW. There are at least three notable features of Enceladus which remain unexplained: 1) origin of the ice diapir within the interior, 2) origin of a subsurface ocean beneath the south polar region which, by inference, must exist to allow sufficient shear velocities along the fractures to produce the observed amount of surface heat flow, and 3) the origin of the asymmetry in Enceladus' surface deformation (that is, IF the tectonic fractures were indeed generated by the reorientation of the satellite, which changes the stress pattern globally, why did these fractures only form in one place?) We propose that a compositional diapir of pure water ice generated by the differentiation of an ammonia-water ice mantle can reconcile these aspects into a self-consistent geodynamic evolution of Enceladus. Ammonia has been observed on numerous other icy bodies in the outer solar system, including Jovian satellites (Spohn and Schubert, 2003; Nagel et al., 2004), Charon (Cook et al., 2007), and even Kuiper Belt objects such as Quaoar (Jewitt and Luu, 2005), so it's quite conceiveable ammonia is present in Enceladus as well. We demonstrate that the relatively small amount of available tidal dissipation is adequete for a differentiated layer to form at the core-mantle boundary and present 2-D and 3-D numerical models and scaling analysis for different scenarios of how the diapir might rise to the surface. We show that the creation of a regional subsurface ocean is a natural consequence of a pure water ice diapir reaching the surface. The subsequent assymetry of large tectonic fractures on the surface will be more likely as stresses concentrate over the subsurface ocean where the elastic layer is decoupled from the underlying viscous ice shell. Several lines of evidence are addressed such as how processing of the vapor plume by magnetospheric ion irradiation may reconcile the apparent lack of observed ammonia components in the plumes vapour content or on the surface (Loeffler et al., 2006). These chemical species (NH₃, N₂ and N⁺) have been observed (Smith et al., 2005) in small quantites, but there is some indication ammonia is an impurity present in larger amounts increasing with depth as inferred from radar albedo measurements (Ostro et al., 2007).

P11F-08 

No Ocean Source for Enceladus' Plumes

* Schneider, N M (nick.schneider@lasp.colorado.edu), LASP, U Colorado, 392 UCB, Boulder, CO 80309, Burger, M H (Matthew.Burger@gsfc.nasa.gov), GSFC, Code 612.2 8800 Greenbelt Rd., Greenbelt, MD 20771, Johnson, R E (rej@virginia.edu), U. Virginia, Engineering Physics, Charlottesville, VA 22904, Kargel, J S (jkargel1054@earthlink.net), U. Arizona, Dept. of Hydrology & Water Resources, Tucson, AZ 85721, Schaller, E L (emily@gps.caltech.edu), CalTech, Dept. of Geological & Planetary Sci. 1200 E. California Blvd., Pasadena, CA 91125, Brown, M E (mbrown@caltech.edu), CalTech, Dept. of Geological & Planetary Sci. 1200 E. California Blvd., Pasadena, CA 91125,

A groundbased telescopic search for sodium emission near Saturn's moon Enceladus places a firm upper limit on the possible amount of sodium released by eruptions there. Independent observations at the Keck and Anglo- Australian Telescopes using high resolution spectroscopy failed to detect any sodium emission near Enceladus, despite the high sensitivity of such instruments to minute amounts of sodium originating at Jupiter's moons Io and Europa. Large amounts of sodium would be expected if Enceladus' plume material were derived directly from a long-lived ocean (or more confined "sea") in contact with rocky material. Chemical models predict that sodium would dissolve into such an ocean at mixing ratios relative to water of 10^-4 to 10^-1 (Zolotov et. al, 2007). Our numerical plumes models show that such high sodium concentrations would result in a long-lived torus of sodium encircling Saturn. Our detection upper limits fall orders of magnitude below these models, leading us to conclude that the Enceladus plumes do not originate in an ocean or sea. These observations support the alternative theories that Enceladus' plumes are generated by shear heating of the icy crust - resulting in sublimation or melting - or the decomposition of clathrates. These results do not rule out the possibility that a deep ocean exists at depth that is not directly responsible for the plumes. Plume sampling by Cassini or potential future missions, however, would not be probing this potentially habitable environment. This work has been supported by NSF's Planetary Astronomy Program.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.