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Register a new userOcean tidal heating in icy satellites with solid shells
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As a long-term energy source, tidal heating in subsurface oceans of icy satellites can influence their thermal, rotational, and orbital evolution, and the sustainability of oceans. We present a new theoretical treatment for tidal heating in thin subsurface oceans with overlying incompressible elastic shells of arbitrary thickness. The stabilizing effect of an overlying shell damps ocean tides, reducing tidal heating. This effect is more pronounced on Enceladus than on Europa because the effective rigidity on a small body like Enceladus is larger. For the range of likely shell and ocean thicknesses of Enceladus and Europa, the thin shell approximation of Beuthe (2016) is generally accurate to less than about 4%.The time-averaged surface distribution of ocean tidal heating is distinct from that due to dissipation in the solid shell, with higher dissipation near the equator and poles for eccentricity and obliquity forcing respectively. This can lead to unique horizontal shell thickness variations if the shell is conductive. The surface displacement driven by eccentricity and obliquity forcing can have a phase lag relative to the forcing tidal potential due to the delayed ocean response. For Europa and Enceladus, eccentricity forcing generally produces greater tidal amplitudes due to the large eccentricity values relative to the obliquity values. Despite the small obliquity values, obliquity forcing generally produces larger phase lags due to the generation of Rossby-Haurwitz waves. If Europas shell and ocean are respectively 10 and 100 km thick, the tide amplitude and phase lag are 26.5 m and $<1$ degree for eccentricity forcing, and $<2.5$ m and $<18$ degrees for obliquity forcing. Measurement of the obliquity phase lag (e.g. by Europa Clipper) would provide a probe of ocean thickness
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Oceanic tides are a major source of tidal dissipation. They are a key actor for the orbital and rotational evolution of planetary systems, and contribute to the heating of icy satellites hosting a subsurface ocean. Oceanic tides are characterized by a highly frequency-resonant behavior, which is mainly due to the propagation of surface gravity waves in the case of thin oceans, and internal waves when they are deeper. In this work, we derive self-consistent ab initio expressions of the oceanic tidal torque as a function of the key physical parameters of the system (the ocean depth, the Brunt-Vaisala stratification frequency, the rotation rate, the tidal frequency, the Rayleigh friction). These solutions include the coupled mechanisms of internal and surface gravito-inertial waves, which allows us to study the case of planets hosting deep oceans and offer interesting prospects for the coupling between subsurface oceans and ice shells in the case of icy satellites.
Of profound astrobiological interest is that not only does Enceladus have a water ocean, but it also appears to be salty, important for its likely habitability. Here, we investigate how salinity affects ocean dynamics and equilibrium ice shell geometry and use knowledge of ice shell geometry and tidal heating rates to help constrain ocean salinity. We show that the vertical overturning circulation of the ocean, driven from above by melting and freezing and the temperature dependence of the freezing point of water on pressure, has opposing signs at very low and very high salinities. In both cases, heat and freshwater converges toward the equator, where the ice is thick, acting to homogenize thickness variations. In order to maintain observed ice thickness variations, ocean heat convergence should not overwhelm heat loss rates through the equatorial ice sheet. This can only happen when the oceans salinity has intermediate values, order $20$~psu. In this case polar-sinking driven by meridional temperature variations is largely canceled by equatorial-sinking circulation driven by salinity variations and a consistent ocean circulation, ice shell geometry and tidal heating rate can be achieved.
The structure of the icy shells of ocean worlds is important for understanding the stability of their underlying oceans as it controls the rate at which heat can be transported outward and radiated to space. Future spacecraft exploration of the ocean worlds (e.g., by NASAs Europa Clipper mission) will allow for higher-resolution measurements of gravity and shape than currently available. In this paper, we study the sensitivity of gravity-topography admittance to the structure of icy shells in preparation for future data analysis. An analytical viscous relaxation model is used to predict admittance spectra given different shell structures determined by the temperature-dependent viscosity of a tidally heated, conductive shell. We apply these methods to the ocean worlds of Europa and Enceladus. We find that admittance is sensitive to the mechanisms of topography support at different wavelengths and estimate the required gravity performance to resolve transitions between these mechanisms. We find that the Airy isostatic model is unable to accurately describe admittance universally across all wavelengths when the shell thickness is a significant fraction of bodys radius. Our models suggest that measurements of admittance at low spherical harmonic degrees are more sensitive to thick shells with high tidal dissipation, and may complement ice-penetrating radar measurements in constraining shell thickness. Finally, we find that admittance may be used to constrain the tidal dissipation within the icy shell, which would be complementary to a more demanding measurement of the tidal phase lag.
Beneath the icy shell encasing Enceladus, a small icy moon of Saturn, a global ocean of liquid water ejects geyser-like plumes into space through fissures in the ice, making it an attractive place to investigate habitability and to search for extraterrestrial life. The existence of an ocean on Enceladus has been attributed to the heat generated in dissipative processes associated with deformation by tidal forcing. However, it remains unclear whether that heat is mostly generated in its ice shell or silicate core. Answering this question is crucial if we are to unravel patterns of ocean circulation and tracer transport that will impact both the habitability of Enceladus and our ability to interpret putative evidence of any habitability and/or life. Using a nonhydrostatic ocean circulation model, we describe and contrast the differing circulation patterns and implied ice shell geometries to be expected as a result of heating in the ice shell above and heating in the core below Enceladus ocean layer. If heat is generated primarily in the silicate core we would predict enhanced melting rates at the equator. In contrast, if heat is primarily generated in the ice shell we would infer a poleward-thinning ice geometry consistent with Cassini Mission observations.
The spectral position of the 3.6 micron continuum peak measured on Cassini-VIMS I/F spectra is used as a marker to infer the temperature of the regolith particles covering the surfaces of Saturns icy satellites. This feature is characterizing the crystalline water ice spectrum which is the dominant compositional endmember of the satellites surfaces. Laboratory measurements indicate that the position of the 3.6 micron peak of pure water ice is temperature-dependent, shifting towards shorter wavelengths when the sample is cooled, from about 3.65 micron at T=123 K to about 3.55 micron at T=88 K. A similar method was already applied to VIMS Saturns rings mosaics to retrieve ring particles temperature (Filacchione et al., 2014). We report here about the daytime temperature variations observed on the icy satellites as derived from three different VIMS observation types. Temperature maps are built by mining the complete VIMS dataset collected in years 2004-2009 (pre-equinox) and in 2009-2012 (post equinox) by selecting pixels with max 150 km/pixel resolution. VIMS-derived temperature maps allow to identify thermal anomalies across the equatorial lens of Mimas and Tethys.
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