In Search of Subsurface Oceans Within the Uranian Moons

The Physical Oceanography of Ice-Covered Moons

In the outer solar system, a growing number of giant planet satellites are now known to be abodes for global oceans hidden below an outer layer of ice. These planetary oceans are a natural laboratory for studying physical oceanographic processes in settings that challenge traditional assumptions made for Earth's oceans. While some driving mechanisms are common to both systems, such as buoyancy-driven flows and tides, others, such as libration, precession, and electromagnetic pumping, are likely more significant for moons in orbit around a host planet. Here, we review these mechanisms and how they may operate across the solar system, including their implications for ice-ocean interactions. Future studies should continue to advance our understanding of each of these processes as well as how they may act together in concert. This interplay also has strong implications for habitability as well as testing oceanic hypotheses with future missions.

Investigating Europa's Habitability with the Europa Clipper

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

Exploring the Interior of Europa with the Europa Clipper

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa's interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

Ice Shell Structure and Composition of Ocean Worlds: Insights from Accreted Ice on Earth

Accreted ice retains and preserves traces of the ocean from which it formed. In this work, we study two classes of accreted ice found on Earth-frazil ice, which forms through crystallization within a supercooled water column, and congelation ice, which forms through directional freezing at an existing interface-and discuss where each might be found in the ice shells of ocean worlds. We focus our study on terrestrial ice formed in low temperature gradient environments (e.g., beneath ice shelves), consistent with conditions expected at the ice-ocean interfaces of Europa and Enceladus, and we highlight the juxtaposition of compositional trends in relation to ice formed in higher temperature gradient environments (e.g., at the ocean surface). Observations from Antarctic sub-ice-shelf congelation ice and marine ice show that the purity of frazil ice can be nearly two orders of magnitude higher than congelation ice formed in the same low temperature gradient environment (∼0.1% vs. ∼10% of the ocean salinity). In addition, where congelation ice can maintain a planar ice-water interface on a microstructural scale, the efficiency of salt rejection is enhanced (∼1% of the ocean salinity) and lattice soluble impurities such as chloride are preferentially incorporated. We conclude that an ice shell that forms by gradual thickening as its interior cools would be composed of congelation ice, whereas frazil ice will accumulate where the ice shell thins on local (rifts and basal fractures) or regional (latitudinal gradients) scales through the operation of an "ice pump."

How does salinity shape ocean circulation and ice geometry on Enceladus and other icy satellites?

Of profound astrobiological interest, Enceladus appears to have a global saline subsurface ocean, indicating water-rock reaction at present or in the past, an important mechanism in the moon's potential habitability. Here, we investigate how salinity and the partition of heat production between the silicate core and the ice shell affect ocean dynamics and the associated heat transport-a key factor determining equilibrium ice shell geometry. Assuming steady-state conditions, we show that the meridional overturning circulation of the ocean, driven by heat and salt exchange with the poleward-thinning ice shell, has opposing signs at very low and very high salinities. Regardless of these differing circulations, heat and fresh water converge toward the equator, where the ice is thick, acting to homogenize thickness variations. Among scenarios explored here, the pronounced ice thickness variations observed on Enceladus are most consistent with heating that is predominantly in the ice shell and a salinity of intermediate range.

A perturbation method for evaluating the magnetic field induced from an arbitrary, asymmetric ocean world analytically

Magnetic investigations of icy moons have provided some of the most compelling evidence available confirming the presence of subsurface, liquid water oceans. In the exploration of ocean moons, especially Europa, there is a need for mathematical models capable of predicting the magnetic fields induced under a variety of conditions, including in the case of asymmetric oceans. Existing models are limited to either spherical symmetry or assume an ocean with infinite conductivity. In this work, we use a perturbation method to derive a semi-analytic result capable of determining the induced magnetic moments for an arbitrary layered body, provided each layer is nearly spherical. Crucially, we find that degree-2 tidal deformation results in changes to the induced dipole moments. We demonstrate application of our results to models of plausible asymmetry from the literature within the oceans of Europa and Miranda and the ionospheres of Callisto and Triton. For the models we consider, we find that in the asymmetric case, the induced magnetic field differs by more than 2 nT near the surface of Europa, 0.25-0.5 nT at 1 R above Miranda and Triton, and is essentially unchanged for Callisto. For Miranda and Triton, this difference is as much as 20%-30% of the induced field magnitude. If measurements near the moons can be made precisely to better than a few tenths of a nT, these values may be used by future spacecraft investigations to characterize asymmetry within the interior of icy moons.

In Search of Subsurface Oceans Within the Uranian Moons

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden subsurface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter's strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager 2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with subsurface oceans. Future missions to the ice giants may therefore be capable of discovering subsurface oceans, thereby adding to the family of known "ocean worlds" in our Solar System. Here, we assess magnetic induction as a technique for investigating subsurface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.

In Search of Subsurface Oceans Within the Uranian Moons

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden subsurface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter's strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager 2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with subsurface oceans. Future missions to the ice giants may therefore be capable of discovering subsurface oceans, thereby adding to the family of known "ocean worlds" in our Solar System. Here, we assess magnetic induction as a technique for investigating subsurface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.