Long-term stability of a subsurface ocean on Enceladus
A heat flow anomaly of 4-7 GW is observed in the south polar region of Enceladus [Spencer et al., 2006]. Tidal dissipation has been suggested as the heat source for the south polar thermal anomaly on Enceladus. Under reasonable rheologic conditions, we find that tidal dissipation is only significant in the ice shell if it is decoupled from the silicate core by a subsurface ocean, suggesting the presence of such an ocean in order to explain the observed surface activity. We have modeled convection and conduction in the ice shell in 2D axisymmetric and 3D spherical geometry in which we include the spatially-variable tidal heat distribution for a Maxwellian body. In general, we find that more heat must be removed from the core than can be produced by radioactive decay in order to maintain the ocean at the melting point of water. Under likely conditions, the ocean would freeze solid on a timescale of order tens of Myr (depending on the initial thickness of the ice shell). This result does not preclude the existence of an ocean, only that it is not in long-term thermal equilibrium. This conclusion is consistent with studies of orbital dynamics which suggest that the long-term tidal heat production cannot exceed 1.1 GW, [Meyer and Wisdom, 2007], assuming the present-day orbit. If the eccentricity of Enceladus were higher (≥ 0.015) in the past, the increased dissipation in the ice shell would have been sufficient to maintain a liquid layer. A subsurface ocean may exist today as the relic of an earlier era of greater heating. If the eccentricity has been periodically pumped up, then the variations in tidal heating may have caused the ocean thickness to vary on the same timescale as for the orbital evolution, provided that this timescale is faster than the time required for the ocean to freeze completely. Prior to the current e-resonance with Dione, Enceladus has passed through several other higher-order 2:1 resonances [Meyer and Wisdom, 2008]. Using coupled models of thermal and orbital evolution of Dione and Enceladus, we find that the instantaneous heat production rate may reach the observed value, but that none of these resonances can maintain an average heat flow this high. It therefore seems likely that either Enceladus's eccentricity is not in steady-state, or that the heat currently being released was generated at an earlier time. Orbital observations place constraints on the interior structure of Enceladus (parameterized by k2/Q) to be 1.2 × 10-4 < k2/Q < 8 × 10-4. This is consistent either with a convective ice shell with no ocean or a conductive ice shell above an ocean. Only the latter scenario is physically plausible. However, even under a conductive ice shell, a water ocean is likely to freeze on a geologically short timescale. The freezing point of the ocean may be lowered if it is not pure water, e.g. it contains significant amounts of ammonia. However, chemistry alone cannot prevent the ocean from freezing, it can only delay it. Even the H20-NH3 peritectic temperature is too high to be maintained by tidal dissipation under present-day conditions. In order for the ocean on Enceladus to be in long-term thermal equilibrium, another thus far unidentified heat source may be required. Tidal heating is unlikely to be significant in the silicate core, but may be important in the ocean itself [Tyler, 2008].
Ammonia, radiogenic argon, organics, and deuterium measured in the plume of Saturn's icy moon Enceladus
Observations made with the Cassini Ion and Neutral Mass Spectrometer (INMS) during two close flybys of Enceladus on 12 March and 9 October 2008 reveal the presence of ammonia, complex organics such as benzene, and deuterium in the gas plume as well as the probable presence of radiogenic argon. The INMS data provide compelling evidence for the existence -- today or in the recent past -- of liquid water in Enceladus' interior and support a hybrid model for the source of the plume, with contributions from both degassing volatile- charged ice (in the form of clathrate hydrates) and material that is or recently was in a reservoir of liquid water. The measurement of the ratio of deuterium to hydrogen is the first such measurement at an icy satellite and is consistent with the accretion of Enceladus from planetesimals formed in the solar nebula in the region of the giant planets
No Sodium in Enceladus' Vapor Plumes
The discovery of water vapor and ice particles erupting from Saturn's moon Enceladus fueled speculation that an internal ocean was the source. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions. The presence or absence of sodium chloride salt, expected in a long-lived ocean in contact with a rocky core, offers clues. While sodium has been detected in particles escaping Enceladus (Postberg et al., submitted), by far the vast majority of mass escapes in gaseous form. Here we report results from a groundbased spectroscopic search for atomic sodium near Enceladus which places an upper limit on the mixing ratio in the vapor plumes orders of magnitude below the expected ocean salinity. The low sodium content of escaping vapor, plus the small fraction of salt-bearing particles, argues against a scenario in which a deep, salty ocean fuels a near-surface geyser through cracks in the crust. The observed low-sodium vapor is consistent with a wide variety of alternative eruption hypotheses from liquid water or from ice, and offers significant constraints on each. The combination of Cassini and groundbased data may be insufficient to distinguish between these hypotheses.
The Surface Composition of Enceladus: Ultraviolet Constraints
Enceladus' reflectance spectrum, while being very bright at VNIR wavelengths and consistent with a surface composed primarily of H2O ice, is darker at far-UV wavelengths than predicted by pure H2O ice spectral models. The visible spectrum of Enceladus is bright and featureless, like pure water ice, and the near-IR spectrum has also been compared to pure water ice (Cruikshank et al., 2005) or pure water ice plus a small amount of NH3 hydrate (Verbiscer et al., 2006) or NH3 (Emery et al., 2005). We investigate the darkness of the FUV spectrum by examining existing laboratory measurements of the optical constants and reflectance spectra of H2O and other candidate species, and by comparing with spectral models. We find that the FUV darkness of Enceladus can be explained by the presence of a small amount of NH3 and a small amount of a tholin in addition to H2O ice. (Optical constants for NH3 hydrate in the UV are not available but we expect that the gross spectral properties are similar to those of NH3 and cannot rule out the presence of NH3 hydrate rather than NH3.) The presence of these three species (H2O, NH3 and a tholin) appears to satisfy not only the FUV darkness and spectral shape, but also the visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume condenses in the E-ring, grains of which accumulate on and coat the surface Enceladus throughout its orbit, constantly enhancing the visible brightness of the moon and resupplying a small amount of NH3 to the surface.
How the Enceladus Dust Jets Form Saturn's E Ring
Pre--Cassini models of Saturn's E ring failed to reproduce its peculiar vertical structure inferred from earth-bound observations. After the discovery of an active ice- volcanism of Saturn's icy moon Enceladus the relevance of the directed injection of particles for the vertical ring structure of the E ring was swiftly recognised. However, simple models for the delivery of particles from the plume to the ring predict a too small vertical ring thickness and overestimate the amount of the injected dust. Here we report on numerical simulations of grains leaving the plume and populating the dust torus of Enceladus. We run a large number of dynamical simulations including gravity and Lorentz force to investigate the earliest phase of the ring particle life span. The evolution of the electrostatic charge carried by the initially uncharged grains is treated selfconsistently. Freshly ejected plume particles are moving in almost circular orbits because the Enceladus orbital speed exceeds the particles' ejection speeds by far. Only a small fraction of grains that leave the Hill sphere of Enceladus survive the next encounter with the moon. The flux and the size distribution of the surviving grains, replenishing the ring particle reservoir, differs significantly from the flux and the size distribution of the ejected plume particles. Our numerical simulations reproduce the vertical ring profile measured by the Cassini dust instrument CDA. From our simulations we calculate the deposition rates of plume particles hitting Enceladus' surface. We find that at a distance of 100 m from a jet a 10 m sized ice boulder should be covered by plume particles in 105 to 106 years.
Evidence for Enceladus Link to Saturn Ionosphere: Does the Plume Have an Auroral Footprint?
It is increasingly well accepted that, despite its diminutive size, the tiny icy moon Enceladus is the dominant source of water group neutrals and charged particles throughout Saturn's magnetosphere through the copious gas and dust emanations from its South pole. During two recent Cassini flybys the spacecraft plasma instruments were oriented such that they looked along a magnetic flux tube nominally connecting the Enceladus plume to Saturn's ionosphere. Two of the remarkable discoveries from these observational campaigns were, 1) high energy (10s-100s of keV) field aligned ion beams propagating from Saturn toward the plume and 2) lower energy field aligned electron beams which were observed to 'flicker' in energy from 10s of eV to several 100 eV. Initial speculation was that this is evidence of an Alfven wing type interaction, such as exists at Io due to significant mass loading in the wake of the moon. It was subsequently realised that the magnetic field signature is not consistent with this simple picture, leading us to speculate that there exists a more filamentary Birkeland current system with the observed variability linked to the highly dynamic and variable nature of the Enceladus outgassing. Ions could be accelerated by wave activity or field-aligned potential drops just above the ionosphere, but we have yet to ascertain if either is sufficient to explain the observed very high energy ion beams. Additionally we will show that similar phenomena exist near the L-value of Enceladus, but away from the moon - implying the existence of a significant extended Enceladus plasma torus.
MHD Study of the Force Balance in the Plasma Disk of the Saturnian Inner Magnetosphere
The dynamics of the saturnian magnetosphere is largely controlled by the planetary spin at a rate of about 10.5 hours. In addition, in the inner magnetosphere, Enceladus creates neutrals that is picked up at a rate of a few kilograms per second and spun up to the corotational velocity to form a torus. The gas and plasma density peaks at the Enceladus orbit. In this torus, the majority of the gas particles travel at their keplerian speed of 14km/s, while the bulk of the plasma rotates at 40km/s as a response to the rigid spinning of the saturnian magnetic field. The corotating plasma torus feels a centrifugal force that is balanced by the magnetic tension force. The frozen-in saturnian magnetic field is in turn moved outward to contain the plasma near the spin equator. As an essential step to understand the role that Enceladus plays in the inner Saturnian system, we investigate the force balance in the torus plasma and the possible effect of charge exchange with the Enceladus neutral plume, using our Saturn-centered MHD model and the recent Cassini observations.