• A Very Young Age for True Polar Wander on Europa From Related Fracturing

      Schenk, Paul; Matsuyama, Isamu; Nimmo, Francis; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2020-09)
      En echelon fissures 100-300 km long on Europa are found to be concentric and external to arcuate troughs previously attributed to true polar wander (TPW) of Europa's ice shell, strengthening the case for TPW. Fissures are composed of parallel faults distributed over 10-to-20-km-wide zones, with deformation focused in a main fissure 1-2 km wide and up to 200 m deep. Fissures crosscut all known terrains, including (apparently) ejecta of bright ray crater Manannan, establishing that fissures and by inference TPW are among the most recent geologic events on Europa. Very late similar to 70 degrees of TPW shell rotation requires that most observed structures on Europa are not in their original configuration with respect to other stress regimes, requiring complete reanalysis of Europa's strain history. If reorientation happened recently, we predict that any crater distribution asymmetries and shell thickness variations measured by Europa Clipper will be offset from expected equilibrium patterns. Plain Language Summary The large icy ocean world of Europa has a very young surface that has been highly deformed. Recent evidence for "polar wander," or reorientation of the floating outer ice shell away from its original orientation, has been confirmed by the recognition that long fissures are part of the polar wander tectonic pattern and arc among the youngest features on the planet. This means that polar wander occurred very recently and that older features are no longer in their original locations and will require a complete reassessment of Europa's tectonic history.
    • The NASA Roadmap to Ocean Worlds

      Hendrix, Amanda R; Hurford, Terry A; Barge, Laura M; Bland, Michael T; Bowman, Jeff S; Brinckerhoff, William; Buratti, Bonnie J; Cable, Morgan L; Castillo-Rogez, Julie; Collins, Geoffrey C; et al. (MARY ANN LIEBERT, INC, 2019-01-01)
      In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to "identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find." The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.
    • Nonlinear tidal dissipation in the subsurface oceans of Enceladus and other icy satellites

      Hay, Hamish C.F.C.; Matsuyama, Isamu; Univ Arizona, Lunar & Planetary Lab (ACADEMIC PRESS INC ELSEVIER SCIENCE, 2019-02)
      Subsurface ocean tides act as a mechanism to dissipate tidal energy in icy satellite interiors. We numerically model the effect of an ice shell on ocean tides using non-linear bottom drag for the first time. We demonstrate that subsurface oceans experience tidal pressurization due to the confining nature of the ice shell, and find that Enceladus' eccentricity forcing can generate up to 2.2 kPa of pressure excess at the ocean surface. Existing free surface oceanic energy dissipation scaling laws are extended to subsurface oceans, and are benchmarked against our numerical results to within 10 %. We show that for the large bodies Ganymede, Europa and Titan, an ice shell increases eccentricity tidal heating due to self-gravity, whereas the shell's suppressive mechanical forcing reduces eccentricity tide dissipation on Enceladus and Dione by several orders of magnitude due to their high effective rigidities. In contrast, the ice shell enhances obliquity-forced dissipation in all satellites investigated, except Triton, because the largely divergence-free ocean response is unaffected by the shell's rigidity but is still enhanced by self-gravity. We conclude that the fundamental difference in ocean response to obliquity and eccentricity forcing, combined with self-gravity, results in increased obliquity heating and suppressed eccentricity heating in small satellites. For large satellites with low effective rigidities, the type of ocean response is less important because the shell's mechanical forcing has little impact on the flow, whereas self-gravity will enhance the response, and consequently dissipation, regardless of the forcing. Overall, obliquity tides are likely to dominate the tidal heating budget of icy satellite oceans, remaining the most prominent source of fluid dissipation in subsurface barotropic ocean tides.
    • Ocean tidal heating in icy satellites with solid shells

      Matsuyama, Isamu; Beuthe, Mikael; Hay, Hamish C.F.C.; Nimmo, Francis; Kamata, Shunichi; Univ Arizona, Lunar & Planetary Lab (ACADEMIC PRESS INC ELSEVIER SCIENCE, 2018-09-15)
      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%. Explaining Enceladus' endogenic power radiated from the south polar terrain by ocean tidal heating requires ocean and shell thicknesses that are significantly smaller than the values inferred from gravity and topography constraints. 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 Europa's shell and ocean are, respectively, 10 and 100 km thick, the tide amplitude and phase lag are 26.5 m and <1 degrees 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 (C) 2018 Elsevier Inc. All rights reserved.
    • The High-Frequency Tidal Response of Ocean Worlds: Application to Europa and Ganymede

      Hay, H.C.F.C.; Matsuyama, I.; Pappalardo, R.T.; Lunar and Planetary Laboratory, University of Arizona (John Wiley and Sons Inc, 2022)
      Europa and Ganymede, whose liquid water oceans are of uncertain thickness, are subject to tidal forces across a broad frequency spectrum. Tidal deformation is inherently frequency dependent, an effect which is enhanced when a subsurface ocean is present. We model the tidal response of Europa and Ganymede, taking into account ocean dynamics and the viscoelastic coupling to the ice shell. Tidal deformation at high frequencies - a result of moon-moon interactions - is resonantly amplified by ocean dynamics. We find the corresponding tidal Love numbers to be extremely sensitive to ocean thickness and weakly sensitive to ice shell thickness, shear modulus, and viscosity. Measuring these high-frequency deformations would provide a unique determination of ocean thickness, though the minimum sensitivity required to detect the relevant deformation (0.1 mm, 2 nGal) makes this an extreme challenge. Detection of a large signal on the order of centimeters would only be possible if the ocean was tuned to a particular thickness, which would suggest that moon-moon tides play a role in the thermal/orbital evolution of the moon. Scaling laws are also derived that predict the resonant enhancement of tidal Love numbers and associated tidal dissipation in the ocean and ice shell. © 2022 American Geophysical Union. All Rights Reserved.