Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects

Hußmann, H.; Sohl, F.; Spohn, Tilman

doi:10.1016/j.icarus.2006.06.005

Cited by 262 publications

(292 citation statements)

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“…We considered the shell thickness variations between 40 km and 120 km except for Europa where it varies between 5 km and 45 km. The simple interior models developed with homogenous density layers are consistent with previous studies (Schubert et al 2004;Spohn & Schubert 2003;Sohl et al 2002;Van Hoolst et al 2008;Hussmann et al 2006). A large number (more than 20 for each satellite) of models is constructed to obtain plausible ranges of moments of inertia, equatorial flattenings and associated torques.…”

Section: Interior Modelssupporting

confidence: 53%

Librational response of Europa, Ganymede, and Callisto with an ocean for a non-Keplerian orbit

Rambaux

Hoolst

Karatekin

2011

A&A

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Context. The Galilean satellites Europa, Ganymede, and Callisto are thought to harbor a subsurface ocean beneath an ice shell but its properties, such as the depth beneath the surface, have not been well constrained. Future geodetic observations with, for example, space missions like the Europa Jupiter System Mission (EJSM) of NASA and ESA may refine our knowledge about the shell and ocean. Aims. Measurement of librational motion is a useful tool for detecting an ocean and characterizing the interior parameters of the moons. The objective of this paper is to investigate the librational response of Galilean satellites, Europa, Ganymede, and Callisto assumed to have a subsurface ocean by taking the perturbations of the Keplerian orbit into account. Perturbations from a purely Keplerian orbit are caused by gravitational attraction of the other Galilean satellites, the Sun, and the oblateness of Jupiter. Methods. We use the librational equations developed for a satellite with a subsurface ocean in synchronous spin-orbit resonance. The orbital perturbations were obtained from recent ephemerides of the Galilean satellites. Results. We identify the wide frequency spectrum in the librational response for each Galilean moon. The librations can be separated into two groups, one with short periods close to the orbital period, and a second group of long-period librations related to the gravitational interactions with the other moons and the Sun. Long-period librations can have amplitudes as large as or even larger than the amplitude of the main libration at orbital period for the Keplerian problem, implying the need to introduce them in analyses of observations linked to the rotation. The amplitude of the short-period librations contains information on the interior of the moons, but the amplitude associated with long periods is almost independent of the interior at first order in the low frequency. For Europa, we identified a short-period libration with period close to twice the orbital period, which could have been resonantly amplified in the history of Europa. For Ganymede, we also found a possible resonance between a proper period and a forced period when the icy shell thickness is around 50 km. The librations of Callisto are dominated by solar perturbations.

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Section: Interior Modelssupporting

confidence: 53%

Librational response of Europa, Ganymede, and Callisto with an ocean for a non-Keplerian orbit

Rambaux

Hoolst

Karatekin

2011

A&A

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“…The interior structure of icy bodies is considered to be mainly composed of rock (silicates, iron alloys) and nonporous ice (e.g., Sotin & Tobie 2004;Hussmann et al 2006Hussmann et al , 2007. This structure can be homogeneous, i.e., undifferentiated, or differentiated into several layers.…”

Section: Mechanical Model Of the Icy Bodymentioning

confidence: 99%

“…This structure can be homogeneous, i.e., undifferentiated, or differentiated into several layers. Although two-layer structures composed of an external layer of ice-I and an internal rocky core are possible interior models for these bodies (Hussmann et al 2006(Hussmann et al , 2007, icy bodies with an internal ocean require at least three layers. Hence, a three-layer icy body is the simplest structure compatible with the presence of a subsurface ocean.…”

Section: Mechanical Model Of the Icy Bodymentioning

confidence: 99%

“…A global theoretical model for this kind of structure was developed by Hussmann et al (2006), who considered a midsized icy body whose interior was differentiated into three constituents with constant density and spherical symmetry: an ice-I external shell, a subsurface ammonia-water ocean, and a rocky inner core (Figure 1). The existence of the internal ocean is established through the balance between the rates of heat generated by the radiogenic components of the rocky core, assuming that the H 2 O layer contains some ammonia.…”

Section: Mechanical Model Of the Icy Bodymentioning

confidence: 99%

“…The existence of the internal ocean is established through the balance between the rates of heat generated by the radiogenic components of the rocky core, assuming that the H 2 O layer contains some ammonia. The presence of ammonia, even in small amounts, would reduce the melting point of water considerably, and therefore make it easier for a subsurface ocean to exist (Hussmann et al 2006). Specifically, the consideration of an equilibrium condition in the heat balance equations of a differentiated three-layer body leads to solutions that would allow the actual presence of an internal ocean layer for Europa, Rhea, Titania, Oberon, Triton, Pluto, and the trans-Neptunian objects Eris (formerly known as 2003 UB 313 ), Sedna, and 2004 DW.…”

Section: Mechanical Model Of the Icy Bodymentioning

confidence: 99%

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Free Translational Oscillations of Icy Bodies With a Subsurface Ocean Using a Variational Approach

Escapa

Fukushima

2011

The Astronomical Journal

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We analyze the influence of the interior structure of an icy body with an internal ocean on the relative translational motions of its solid constituents. We consider an isolated body differentiated into three homogeneous layers with spherical symmetry: an external ice-I layer, a subsurface ammonia-water ocean, and a rocky inner core. This composition represents icy bodies such as Europa, Titania, Oberon, and Triton, as well as Pluto, Eris, Sedna, and 2004 DW. We construct the equations of motion by assuming that the solid constituents are rigid and that the ocean is an ideal fluid, the internal motion being characterized by the relative translations of the solids and the induced flow in the fluid. Then we determine the dynamics of the icy body using the methods of analytical mechanics, that is, we compute the kinetic energy and the gravitational potential energy, and obtain the Lagrangian function. The resulting solution of the Lagrange equations shows that the solid layers perform translational oscillations of different amplitudes with respect to the barycenter of the body. We derive the dependence of the frequency of the free oscillations of the system on the characteristics of each layer, expressing the period of the oscillations as a function of the densities and masses of the ocean and the rocky inner core, and the mass of the icy body. We apply these results to previously developed subsurface models and obtain numerical values for the period and the ratio between the amplitudes of the translational oscillations of the solid components. The features obtained are quite different from the cases of Earth and Mercury. Our analytical formulas satisfactorily explain the source of these differences. When models of the same icy body, compatible with the existence of an internal ocean, differ in the thickness of the ice-I layer, their associated periods experience a relative variation of at least 10%. In particular, the different models for Titania and Oberon exhibit a larger variation of about 37% and 30%. This indicates an absolute difference of the order of three and two hours, respectively. This suggests that the free period of the internal oscillations might provide a new procedure to constrain the internal structure of icy bodies with a subsurface ocean.

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The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals

et al. 2015

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We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiter's time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0• ± 1.3 • . Our model calculations predict a significantly stronger oscillation by 5.8Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymede's dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.

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Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects

Cited by 262 publications

References 59 publications

Librational response of Europa, Ganymede, and Callisto with an ocean for a non-Keplerian orbit

Librational response of Europa, Ganymede, and Callisto with an ocean for a non-Keplerian orbit

Free Translational Oscillations of Icy Bodies With a Subsurface Ocean Using a Variational Approach

The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals

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