In a paper published in the journal Geophysical Research Letters, researchers analyzed the potential presence of subsurface oceans within the Uranian moons by studying their internal structures. They proposed that forced physical libration amplitudes exceeding 100 meters at Miranda, Ariel, and Umbriel could indicate the decoupling of global oceans beneath thick ice shells.
Study: Looking for Subsurface Oceans Within the Moons of Uranus Using Librations and Gravity. Image Credit: buradaki/Shutterstock.com
Such oceans may have caused significant tidal heating within the last few hundred million years. Combining libration data with the quadrupole gravity field could offer insights into the moons' internal structures and histories.
Related Work
Past work revealed that several icy moons of Jupiter and Saturn host subsurface oceans, raising interest in similar possibilities for Uranian moons. Techniques like geodetic measurements and magnetic induction have been used to detect such oceans, but small tidal responses in Uranian moons require new approaches.
Researchers focused on forced physical librations, which vary with internal structures, and proposed combining these with quadrupole gravity field data for insights. Understanding present structures could also illuminate Uranian moons' thermal and orbital evolution.
Modeling Uranian Moon Structures
The methods employed for the calculations were derived from established literature, with key equations summarized in the appendix for convenience. The study modeled the Uranian satellites as concentric shells of uniform density, adhering to the known radius and bulk density values.
For models without an ocean, only the densities of the icy mantle and rocky core were specified, with the core radius determined through relevant calculations. For three-layer models, the densities of ice, ocean, core, and ice shell thickness were defined, and the core radius was calculated accordingly.
All of the Uranian satellites were deemed small enough to require consideration of high-pressure ice phases, simplifying the models to three layers: ice, ocean, and core. The bodies were assumed to be in hydrostatic equilibrium, and their hydrostatic figures were determined using a methodology established by Tricarico. These hydrostatic figures were the basis for calculating gravitational field asymmetries and libration amplitudes.
Gravitational field asymmetries and libration amplitudes were calculated based on whether a subsurface ocean decoupled the ice shell. The study accounted for the ice shells' finite rigidity and modeled their deformation potential under Maxwell rheology, particularly in response to dynamical tidal forces.
This approach considered the possibility of deformation reducing libration amplitudes, especially in thin ice shells in larger satellites. The results aligned with previous work on tidal dynamics and offered insights into the structural configurations of the Uranian moons.
Libration and Gravity Insights
The study calculated the forced physical libration amplitudes for the five large Uranian satellites under varying conditions and compared them to measurements from Enceladus. Libration amplitude increased with radius and orbital eccentricity but decreased with bulk density and orbital period. Titania and Oberon's outer satellites exhibited smaller libration amplitudes due to their larger semi-major axes.
Meanwhile, Miranda, Ariel, and Umbriel showed similar amplitudes, with opposing effects of proximity to Uranus, eccentricity, and size approximately canceling out. Enceladus, comparable in size to Miranda, had a much larger eccentricity, resulting in greater libration amplitude.
The results illustrated the influence of ice shell thickness on libration amplitude, particularly when a global subsurface ocean mechanically decoupled the ice shell from the rocky interior. In such cases, libration amplitude became inversely proportional to shell thickness.
Variations in core density, modeled to reflect porous or hydrated silicate rock, also affected the calculations. Due to updated eccentricity data, the findings for the 'no-ocean' case were smaller than those of previous studies. Allowing for elastic deformation of ice shells further reduced libration amplitudes compared to the assumption of infinite rigidity.
Measurements of libration amplitudes were found to be useful for detecting global oceans and estimating ice shell thickness but were less sensitive to other structural details like ocean thickness or core properties. However, combining libration measurements with quadrupole gravity field data provided a more comprehensive picture. These combined observations constrained key parameters such as ice shell thickness, ocean thickness, and core density, enabling insights into the internal structure of the Uranian satellites.
The analysis highlighted the synergy between libration amplitude and gravity field data, showing how these measurements could simultaneously constrain the moment of inertia, hydrosphere thickness, and core density. These results underscored the value of integrating libration and gravitational observations to enhance understanding of icy moons' internal configurations.
Conclusion
To sum up, the study demonstrated that measuring the forced physical libration amplitudes of the Uranian satellites could reveal subsurface liquid water oceans and provide constraints on ice shell thickness, ocean thickness, and rocky core properties. Thin ice shells required significant tidal heating to prevent ocean freezing, or the oceans might have been freezing in recent geological times.
Precise libration amplitudes and gravitational field measurements could detect subsurface oceans if sufficiently thick, offering insights into the moons' internal structures and energy dynamics. However, the low libration amplitudes posed measurement challenges for Titania and Oberon. A future Uranus system mission could enhance understanding the moons' thermal and orbital evolution.
Journal Reference
Hemingway, D. J., & Nimmo, F. (2024). Looking for Subsurface Oceans Within the Moons of Uranus Using Librations and Gravity. Geophysical Research Letters, 51:18, e2024GL110409. DOI:10.1029/2024GL110409, https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL110409
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