Sander Goossens

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 .

Seismic data for Mars has provided constraints on the thickness of its crust, but its density and how much it varies spatially, especially across the hemispherical dichotomy, is still undetermined. We apply a novel constraint in the analysis of spacecraft tracking data to determine Mars' gravity field that allows for the determination of the density of the north and south directly from the data. We find a density of kg for the north and kg for the south. This result is independent from the seismic results, and consistent with seismic constraints that indicate the difference between the north and south should not be greater than 200 kg . Accounting for the higher densities of volcanoes changes the difference slightly, still within the seismic constraint. Our density results provide additional, independent constraints for crustal thickness modeling and elucidating the origin of the dichotomy.

MESSENGER’s Mercury Laser Altimeter (MLA) provided high-resolution topographic models and geodetic insights into Mercury’s internal structure, though its data was limited to the northern hemisphere. Due to its less elliptical orbit, the BepiColombo Laser Altimeter (BELA) will soon be delivering global altimetry coverage. We present expected improvements on our knowledge of Mercury’s geodetic parameters, such as its obliquity, short- and long-period librations, rotation rate, and the amplitude of tidal deformations, by comparing crossover analyses of the MLA and BELA altimetry datasets within consistent closed-loop simulations. We discuss the potential of cross-dataset crossovers (consisting of a BELA track crossing with an MLA track) to refine the determination of key geodetic parameters by providing extended coverage at lower latitudes. Our simulations incorporate realistic altimetric ranges, orbital dynamics, and perturbations due to the imperfect knowledge of, e.g., MESSENGER orbit and attitude, and of Mercury’s orientation. We show the improvement of crossover-derived uncertainties of the geodetic parameters, calibrated using variance component estimation. In particular, rotation parameters, including long-period forced librations, significantly benefit from combining inter-instrument crossovers. These uncertainties are finally mapped through Markov Chain Monte Carlo to study BELA’s impact on constraining plausible models for Mercury’s internal structure.

On 27 December 2024, Juno's JIRAM (Jovian InfraRed Auroral Mapper) instrument observed an unprecedented volcanic event in Io's southern hemisphere, covering a vast region of ∼65,000 km ² , near 73°S, 140°E. Within the imaged region, only one hot spot was previously known (Pfd454). This feature was earlier estimated to cover an area of 300 km ² with a total power output of 34 GW. JIRAM results show that the region produces a power output of 140–260 TW, over 1,000 times higher than earlier estimates and likely exceeding the brightest eruption ever recorded on Io, that of Surt in 2001 (∼80 TW). Three adjacent hot spots also exhibited dramatic power increases: P139, PV18, and an unnamed feature south of the main one that surged to ∼1 TW, placing all of them among the top 10 most powerful hot spots observed on Io. A temperature analysis of the features supports the simultaneous onset of these brightenings and suggests a single eruptive event propagating beneath the surface across hundreds of kilometers; this is the first time this has been observed on Io. This in turn would imply a connection among the hotspots' magma reservoirs, while other nearby hotspots that have been known to be active in the recent past, such as Kurdalagon Patera, appear unaffected. The simultaneity supports models of massive, interconnected magma reservoirs. The topology of these regional magma systems may resemble that of a large‐scale sponge, in which the massive reservoirs are the pores, interconnected through a largely solid outer shell.

We document the first operational demonstration of two-way spacecraft-to-surface laser ranging using Laser Retroreflector Arrays (LRAs) for orbit determination (OD) of the Lunar Reconnaissance Orbiter (LRO). We detail how the Lunar Orbiter Laser Altimeter (LOLA) was operationally adapted to target and successfully range to several LRAs on the lunar surface. Although LOLA was not designed for this purpose, and despite many spacecraft constraints that limited the number of measurements we could collect, we demonstrate that incorporating these ranging observations into OD procedures yields tangible improvements in LRO’s orbital solution. Our analysis yields two key findings: (1) systematic ranging to different types of surface LRAs from orbit is operationally feasible, and (2) such measurements provide valuable geodetic data for spacecraft navigation. This first demonstration is particularly timely as more LRAs are being deployed across the lunar surface through various international and commercial missions, while LiDAR technology continues to advance. Combined with the growing number of missions requiring Position, Navigation, and Timing services in lunar orbit, our findings indicate that spacecraft-to-LRA ranging could become an essential technique for navigation in the near future, complementing traditional Earth-based tracking methods.

Numerous space agencies are planning sustainable human presence at the lunar south pole. Understanding the nature of the lunar crust, particularly its three-dimensional density structure, is important, for example, for the search for water ice predicted to be present. The paucity of in situ measurements requires that we exploit the knowledge obtained through orbital measurements. Two data sets, gravity and topography, in combination with local geology, can provide constraints on the density of the crust. We analyzed the radial component of the gravity field to determine the density of the crust, which we relate to depth using a point-mass spherical harmonic approach. Our investigation provides estimates of density and density gradient at 20 sites in the south polar region: the original 13 Artemis sites, 5 permanently shadowed regions (PSRs), the south pole, and a site on the floor of the Amundsen crater. We find that the mean density in the top 2–10 km for the six PSRs (2475 ± 108 kg m⁻³) is comparable to that (2483 ± 147 kg m⁻³) for the 14 non-PSRs. The weighted mean gradient for the five PSRs and the Amundsen floor is 3 times greater than that for the 14 non-PSRs in the 2–4 km depth range; the average density increases with depth at 25 kg m⁻³ km⁻¹. To investigate whether a relationship exists between inferred areal coverage of ice and our density results, we compared densities with results for reflectance in several common PSR localities. We find lower densities and higher gradients for areas characterized by greater areal coverage of ice.

We present a constraint matrix to be used in the determination of gravity field models from spacecraft tracking data that uses information from topography and that allows for the direct determination of lateral variations in crustal density. Our approach is based on a matrix formulation of Bouguer corrections using a spherical harmonics expansion for density. This formulation also allows us to directly determine the spherical harmonics coefficients for density in the crust from a set of gravity coefficients. We validate this by comparison with an approach using localization and show that both methods result in comparable density maps. We then apply the constraint matrix on a test case using lunar data from before the Gravity Recovery and Interior Laboratory (GRAIL) mission and compare this with results from GRAIL, showing that we can obtain density results similar to those from GRAIL data. We also apply this constraint to the determination of the gravity field of Mars. While our results for Mars tentatively suggest a hemispherical density contrast separated by the north–south dichotomy, the resolution of Martian gravity is currently too low to robustly determine large-scale density variations.

The outer solar system beyond Saturn remains unexplored by dedicated orbital missions. With a launch window opening in 2029, the Uranus Orbiter and Probe (UOP) mission has been prioritized as a NASA Flagship mission for the next decade (2023–2032) to comprehensively study Uranus and its major moons—Miranda, Ariel, Umbriel, Titania, and Oberon. We define and apply novel mission design principles centered on scientific objectives to UOP's gravity science (GS) experiment. Using a combination of Bayesian and Precise Orbit Determination inversions, it is possible to determine mission requirements ensuring the achievement of scientific goals. Our methodology involves building measurement-to-interior parameter maps via extensive Markov Chain Monte Carlo simulations, linking geodetic measurements’ precisions to uncertainties in key interior parameters of the Uranian moons. We show how this mapping approach allows for the rapid evaluation of the ability of a GS experiment design to constrain interior parameters. We conduct a covariance analysis of two orbital tours, multiple measurement strategies, and inversion settings. The tested cases enable the satisfactory determination of Ariel's ice shell thickness (to about 16%), as well as its rock-to-ice mass ratio (≈28%). None of the solutions were able to constrain its ocean thickness. This reverse approach allows for the rapid and scientifically informed adjustment of mission design, thereby demonstrating its potential applicability to other planetary science experiments.

The Moon's frequency‐dependent tidal response, expressed as temporal variations in its gravity field through the Love number and as dissipation through the quality factor , provides information about its interior structure. Lunar laser ranging has provided measurements for , but so far no frequency‐dependent values for have been determined. We provide the first spacecraft measurements of and at two frequencies, monthly and yearly, from an analysis of Gravity Recovery and Interior Laboratory and Lunar Reconnaissance Orbiter radio tracking data. Interior modeling indicates that these values can be matched only with a low‐viscosity zone at the base of the lunar mantle, even when using complex rheological laws to model the mantle's response. The existence of this zone has profound implications for the Moon's thermal state and evolution.