Alexander Berne

Mars undergoes seasonal tidal forcing as a result of its eccentric orbit and the tilt of its rotation axis relative to the Sun ¹ . Response to this forcing produces temporal variations in the Martian gravity field and is sensitive to the internal structure of the planet ^2–4 . Using tracking data from the Mars Global Surveyor (MGS), Mars Odyssey (ODY) and Mars Reconnaissance Orbiter (MRO) spacecraft ^5–7 , we demonstrate that degree-3 components of the time-variable gravity field of Mars differ by up to 300% from predictions for a spherically symmetric planet ^8–10 . These deviations can be explained if the effective shear modulus of the mantle varies by >20% over a roughly north–south pattern that closely aligns with the surface expression of the Martian crustal dichotomy ¹¹ . On the basis of this correlation, we infer preservation of a 200–400 °C thermal anomaly in the present-day mantle below the southern highlands of Mars. This temperature variation could reflect regional mantle convection ^12,13 or insulation by the thick southern highlands crust of Mars that has persisted over several billion years ^14,15 .

The lunar nearside and farside exhibit striking geological and geophysical differences. Although a variety of mechanisms, including asymmetric crystallization, tidal heating, mantle overturn, and moonlet accretion, have been proposed to explain this dichotomy, its origin remains a fundamental and open question. The extent to which these asymmetries penetrate into the lunar interior likewise remains unresolved. Here we jointly invert the latest static and time-variable (tidal) gravity measurements to construct a three-dimensional tomographic model of the lunar mantle. We find that the nearside mantle is up to 1.5% denser and 4% less rigid than the farside, defining a hemispheric-scale interior compositional asymmetry that extends throughout the mantle rather than being confined to the crust and shallow interior as previously assumed. The inferred density and rigidity contrasts are most consistent with a nearside mantle that is comparatively iron-rich, warmer, and wetter than the farside, in agreement with geochemical constraints from the Chang'e-5 and Chang'e-6 sample return missions and surface heatflow measurements from Apollo 15 and 17 and Blue Ghost Mission 1. The thermochemical anomaly is strongest in the upper mantle and provides geophysical evidence that the Moon’s primordial differentiation generated a long-lived hemispheric mantle dichotomy through asymmetric magma-ocean crystallization and nearside sequestration of KREEP-rich cumulates through some degree of lunar magma ocean overturn. These results imply that relics of early lunar differentiation remain preserved throughout the present-day mantle and may also explain the Moon’s long-wavelength degree-2 gravity structure without requiring a fossil rotational–tidal bulge.

Icy satellites host topography at many length scales, from rifts and craters on the small end to equatorial‐pole shell thickness differences that are comparable to these bodies' circumference. The current paradigm is that icy satellites should not host stable small‐scale topography. This idea comes from previous work using a “shallow”‐shell model (i.e., ice shell circumference much larger than shell thickness) with a rigid outer crust. In this limit, large‐scale topography relaxes over a longer time scale than small‐scale features. Here we revisit this paradigm and analyze relaxation of topography starting from the Stokes equations for viscous fluid flow. For a shell with a viscosity that decreases exponentially with depth, we show numerically that there is a regime where the larger viscosity outer crust acts as a nearly rigid boundary. In this case, the relaxation time scale depends on the wavelength. For the largest spatial scales, however, the time scale becomes independent of wavelength and the value is set by the average shell viscosity. However, the spatial scale that this transition occurs at becomes larger as the viscosity contrast increases, limiting the applicability of the scale‐independent relaxation rate. These results for the relaxation of topography have implications for interpreting relaxed crater profiles, inferences of ice shell thickness from topography, and upcoming observations from missions to the outer solar system.

Enceladus is among the most intriguing bodies in the solar system due to its astrobiological potential. Determining the extent and duration of habitability (i.e., sustained habitability ) requires characterizing the interior properties and the level and distribution of tidal heating in Enceladus. Inferring the intensity of geophysical activity in the core has direct implications for the potential hydrothermal activity and supply of chemical species important for habitability to the ocean. We build a statistical framework to constrain the interior using estimates of libration, shape, heat flux, gravity, and total mass. We use this framework to examine the extent that geodetic measurements can improve our understanding of the interior structure, with an emphasis on partitioning of dissipation between the shell and the core. We quantify plausible ranges of gravitational ( k ₂ ) and displacement ( h ₂ , l ₂ ) tidal Love numbers consistent with existing observations. We demonstrate that measuring k ₂ alone can only constrain the total tidally dissipated energy, not its radial distribution. However, measuring the amplitude and phase of h ₂ or l ₂ facilitates determining the extent of tidal dissipation in the shell and the core. We provide the precisions required for measuring k ₂ , h ₂ , and l ₂ that enable distinguishing between the main tidal heating scenarios, i.e., in the shell versus the core. We also explore the effect of the structural heterogeneities of the shell on the tidal response. Lastly, we evaluate the efficacy of future geodetic measurements to constrain key interior properties essential to understand the present-day ( instantaneous ) and long-term ( sustained ) habitability at Enceladus.

We map surface eruptive activity in Cassini images of Enceladus' south-polar terrain (SPT) at fifteen epochs spanning late 2009 to late 2015 using a refined curtain approach derived from that of our earlier work [1]. This refined approach gives a better representation of the uncertainties in identifying source fractures by allowing for multiple candidate sources to be identified for an observed eruption. Our analysis includes the five epochs studied in [1], which were independently re-examined here. Our results are summarized in Figure 1. About 80% of the currently active fracture system in the SPT (by length) was observed to be erupting at a level detectable in Cassini images during every epoch. On/off variability is observed almost exclusively at the fringes of the fracture system, possibly reflecting a thickening ice shell away from the pole. No definitive connection between surface activity and the roughly factor-of-three plume optical depth variation with mean anomaly [2,3] is made here, but our results do not rule out a scenario where the pattern of eruptive activity on the surface is correlated with mean anomaly, provided only the fractures at the fringe of the SPT ever turn completely off during the tidal cycle. Our results are best explained by eruptions sourced directly from the ocean.Figure 1. a) Averaged eruptive activity. At each point, the intensity represents the number of times the point was determined to be active divided by the number of times it was observed (i.e., determined to be either active or inactive). Indeterminate and degenerate points are not counted, except in cases where two fractures are so close to one another as to be effectively indistinguishable. b) Number of observations used for each average in (a). c) Standard deviation at each point in (a). Blue points have zero standard deviation. (from [4])References:[1] Spitale et al. 2015; Nature 521[2] Hedman et al. 2013; Nature 500[3] Nimmo et al. 2014; AJ 148[4] Spitale et al. 2025; PSJ 6:67

Juice (JUpiter ICy moons Explorer) 3GM Radio Science Experiment will map the gravitational field of Ganymede with unprecedented accuracy and measure tidally‐induced variations. These measurements will allow the characterization of its putative ocean and may resolve lateral variations in internal structure. Lateral variations cause an additional tidal signal that depends on their wavelength and amplitude. We show that shell thickness variations of the mean thickness produce an additional tidal signal times smaller than the main tidal signal, detectable given the accuracy of Juice. Using a Bayesian framework, we show that measuring differences between and constrain equator to pole shell thickness differences. Also measuring the degree‐3 spherical harmonic signal due to degree 2 forcing constrain degree‐1 and degree‐3 structure. This demonstrates tidal tomography's potential to map three dimensional structure and supports its consideration for future missions.

The Moon undergoes periodic tidal forcing due to its eccentric and oblique orbit around the Earth¹. The response to this tidal interaction drives temporal changes in the lunar gravity field and is sensitive to the satellite’s internal structure^2–4. We use data from the NASA GRAIL spacecraft^5–9 to recover the time-varying lunar gravity field, including a degree-3 gravitational tidal Love number, k ₃. Here, we report our estimated value of k ₃ = 0.0163 ± 0.0007, which is about 72% higher than that expected for a spherically symmetric moon¹⁰. Such a large k ₃ can be explained if the elastic shear modulus of the mantle varies by about 2–3% between the nearside and farside⁴, providing an observational demonstration of lateral heterogeneities in the deep lunar interior. This asymmetric structure suggests preservation of a predominantly thermal anomaly of roughly 100–200 K in the nearside mantle that formed surface mare regions 3–4 billion years ago¹¹ and could influence the spatial distribution of deep moonquakes¹².

Developing a comprehensive understanding of global water resources – and their responses to extreme events and variations in climate – requires integrating diverse modeling and observational approaches across disciplines, with geodesy playing an increasingly integral role. Geodetic measurements and models, including those tracking solid Earth deformation caused by mass redistribution in the hydrosphere, provide key insights into water-cycle processes and systems. This study focuses on Global Navigation Satellite System (GNSS) data from the western United States to examine recurring cycles of severe drought and rapid recovery over the past two decades. Interdisciplinary evidence from hydrology, meteorology, and geodesy suggests that these cycles are strongly associated with variability in the frequency and intensity of seasonal atmospheric rivers (ARs). During Water Year 2023, GNSS data revealed record-breaking water-storage gains in California’s Sierra Nevada mountains and Sacramento-San Joaquin-Tulare (SST) river basins, driven largely by an exceptional series of powerful ARs. In the six-month period between October 2022 and March 2023, water-storage gains in these regions surpassed those of any prior year in the analysis, which began in 2006, with an estimated 80% of the gains delivered by ARs. By early spring 2023, we infer that approximately half of the water-storage gains had infiltrated the subsurface, providing a critical water resource for downstream communities through processes such as mountain block recharge. Our analysis further shows that hydrological drought and recovery, based on GNSS estimates of total water-storage changes, respond more slowly to precipitation patterns than meteorological drought and recovery, highlighting the insulation of subsurface pools from surface fluxes. We find that years with heavy precipitation can help to sustain storage levels into subsequent years with less precipitation. Moreover, as geodetic observational accuracy improves, a deeper understanding of the assumptions, limitations, and opportunities inherent in our models is necessary. To assess the precision of GNSS-informed water-storage estimates, we compare results derived from independent GNSS position estimates and inversion techniques. Additionally, we provide updates on recent progress in developing community-available modeling tools and investigating the effects of 3-D heterogeneities in Earth structure on deformation responses to surface mass loading.

Body tides reveal information about planetary interiors and affect their evolution. Most models to compute body tides rely on the assumption of a spherically symmetric interior. However, several processes can lead to lateral variations of interior properties. We present a new spectral method to compute the tidal response of laterally heterogeneous bodies. Compared to previous spectral methods, our approach is not limited to small-amplitude lateral variations; compared to finite element codes, this approach is more computationally efficient. While the tidal response of a spherically symmetric body has the same wavelength as the tidal force; lateral heterogeneities produce an additional tidal response with a spectra that depends on the spatial pattern of such variations. For Mercury, the Moon, and Io, the amplitude of this signal is as high as 1%–10% of the main tidal response for long-wavelength shear modulus variations higher than ∼10% of the mean shear modulus. For Europa, Ganymede, and Enceladus, shell-thickness variations of 50% of the mean shell thickness can cause an additional signal of ∼1% and ∼10% for the Jovian moons and Encelaudus, respectively. Future missions, such as BepiColombo and JUICE, might measure these signals. Lateral variations of viscosity affect the distribution of tidal heating. This can drive the thermal evolution of tidally active bodies and affect the distribution of active regions.