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1. Spin states of Europa and Ganymede

   https://doi.org/10.5194/egusphere-egu25-14788  

   Margot, Jean-Luc (March 2025, EGU)

   Radar speckle tracking observations of Europa and Ganymede with the Goldstone Solar System Radar and the Green Bank Telescope in 2011-2023 yield estimates of their spin axis orientations to ~0.01 degrees. These measurements conform to the expected 30-year precessional cycle and provide insights into the moons' Cassini States. I will describe the latest results and discuss new scientific prospects associated with these observations. First, the spin state can reveal the presence of a subsurface ocean: a decoupling between the icy shell and the interior results in a different obliquity than that of a solid body. Second, an angular deviation from the strict Cassini state enables estimates of energy dissipation. Third, a measurement of librations, if detectable, would enable a measurement of the shell's moment of inertia and provide bounds on the rheology and thickness of the shell. Fourth, the obliquity may explain remarkable surface features, such as the distribution and orientation of cycloids, strike-slip faults, and lineaments on Europa. Fifth, knowledge of the obliquity is required to enable tidal heating calculations. Finally, these measurements are expected to facilitate Clipper and JUICE operations and prevent initial, large mapping errors in spacecraft data products. 

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2. Double ridge formation over shallow water sills on Jupiter’s moon Europa

   https://doi.org/10.1038/s41467-022-29458-3  

   Culberg, Riley; Schroeder, Dustin M.; Steinbrügge, Gregor (April 2022, Nature Communications)

   Abstract Jupiter’s moon Europa is a prime candidate for extraterrestrial habitability in our solar system. The surface landforms of its ice shell express the subsurface structure, dynamics, and exchange governing this potential. Double ridges are the most common surface feature on Europa and occur across every sector of the moon, but their formation is poorly understood, with current hypotheses providing competing and incomplete mechanisms for the development of their distinct morphology. Here we present the discovery and analysis of a double ridge in Northwest Greenland with the same gravity-scaled geometry as those found on Europa. Using surface elevation and radar sounding data, we show that this double ridge was formed by successive refreezing, pressurization, and fracture of a shallow water sill within the ice sheet. If the same process is responsible for Europa’s double ridges, our results suggest that shallow liquid water is spatially and temporally ubiquitous across Europa’s ice shell. 

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3. Tidal Evolution of the Earth–Moon System with a High Initial Obliquity

   https://doi.org/10.3847/PSJ/ac12d1  

   Ćuk, Matija; Lock, Simon J.; Stewart, Sarah T.; Hamilton, Douglas P. (August 2021, The Planetary Science Journal)

   Abstract A giant-impact origin for the Moon is generally accepted, but many aspects of lunar formation remain poorly understood and debated. Ćuk et al. proposed that an impact that left the Earth–Moon system with high obliquity and angular momentum could explain the Moon’s orbital inclination and isotopic similarity to Earth. In this scenario, instability during the Laplace Plane transition, when the Moon’s orbit transitions from the gravitational influence of Earth’s figure to that of the Sun, would both lower the system’s angular momentum to its present-day value and generate the Moon’s orbital inclination. Recently, Tian & Wisdom discovered new dynamical constraints on the Laplace Plane transition and concluded that the Earth–Moon system could not have evolved from an initial state with high obliquity. Here we demonstrate that the Earth–Moon system with an initially high obliquity can evolve into the present state, and we identify a spin–orbit secular resonance as a key dynamical mechanism in the later stages of the Laplace Plane transition. Some of the simulations by Tian & Wisdom did not encounter this late secular resonance, as their model suppressed obliquity tides and the resulting inclination damping. Our results demonstrate that a giant impact that left Earth with high angular momentum and high obliquity (θ> 61°) is a promising scenario for explaining many properties of the Earth–Moon system, including its angular momentum and obliquity, the geochemistry of Earth and the Moon, and the lunar inclination. 

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4. Large planets may not form fractionally large moons

   https://doi.org/10.1038/s41467-022-28063-8  

   Nakajima, Miki; Genda, Hidenori; Asphaug, Erik; Ida, Shigeru (December 2022, Nature Communications)

   Abstract One of the unique aspects of Earth is that it has a fractionally large Moon, which is thought to have formed from a Moon-forming disk generated by a giant impact. The Moon stabilizes the Earth’s spin axis at least by several degrees and contributes to Earth’s stable climate. Given that impacts are common during planet formation, exomoons, which are moons around planets in extrasolar systems, should be common as well, but no exomoon has been confirmed. Here we propose that an initially vapor-rich moon-forming disk is not capable of forming a moon that is large with respect to the size of the planet because growing moonlets, which are building blocks of a moon, experience strong gas drag and quickly fall toward the planet. Our impact simulations show that terrestrial and icy planets that are larger than ~1.3−1.6 R ⊕ produce entirely vapor disks, which fail to form a fractionally large moon. This indicates that (1) our model supports the Moon-formation models that produce vapor-poor disks and (2) rocky and icy exoplanets whose radii are smaller than ~1.6 R ⊕ are ideal candidates for hosting fractionally large exomoons. 

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5. Completion of lunar magma ocean solidification at 4.43 Ga

   https://doi.org/10.1073/pnas.2413802121  

   Dauphas, Nicolas; Zhang, Zhe J; Chen, Xi; Barboni, Mélanie; Szymanowski, Dawid; Schoene, Blair; Leya, Ingo; McKeegan, Kevin D (January 2025, Proceedings of the National Academy of Sciences)

   Crystallization of the lunar magma ocean yielded a chemically unique liquid residuum named KREEP. This component is expressed as a large patch on the near side of the Moon and a possible smaller patch in the northwest portion of the Moon’s South Pole-Aitken basin on the far side. Thermal models estimate that the crystallization of the lunar magma ocean (LMO) could have spanned from 10 and 200 My, while studies of radioactive decay systems have yielded inconsistent ages for the completion of LMO crystallization covering over 160 My. Here, we show that the Moon achieved >99% crystallization at 4,429 ± 76 Ma, indicating a lunar formation age of ~4,450 Ma or possibly older. Using the¹⁷⁶Lu–¹⁷⁶Hf decay system (t_1/2= 37 Gy), we found that the initial¹⁷⁶Hf/¹⁷⁷Hf ratios of lunar zircons with varied U–Pb ages are consistent with their crystallization from a KREEP-rich reservoir with a consistently low¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.0167 that emerged ~140 My after solar system formation. The previously proposed younger model age of ~4.33 Ga for the source of mare basalts (240 My after solar system formation) might reflect the timing of a large impact. Our results demonstrate that lunar magma ocean crystallization took place while the Moon was still battered by planetary embryos and planetesimals leftover from the main stage of planetary accretion. The study of Lu–Hf model ages for samples brought back from the South Pole-Aitken basin will help to assess the lateral continuity of KREEP and further understand its significance in the early history of the Moon. 

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