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We detected surface waves from two meteorite impacts on Mars. By measuring group velocity dispersion along the impact-lander path, we obtained a direct constraint on crustal structure away from the InSight lander. The crust north of the equatorial dichotomy had a shear wave velocity of approximately 3.2 kilometers per second in the 5- to 30-kilometer depth range, with little depth variation. This implies a higher crustal density than inferred beneath the lander, suggesting either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves. The lower velocities and the crustal layering observed beneath the landing site down to a 10-kilometer depth are not a global feature. Structural variations revealed by surface waves hold implications for models of the formation and thickness of the martian crust.

Several of the icy moons in the Jupiter and Saturn systems appear to possess internal liquid water oceans. Our knowledge of the Uranian moons is more limited but a future tour of the system has the potential to detect subsurface oceans. Planning for this requires an understanding of how the moons' internal structures—with and without oceans—relate to observable quantities. Here, we show that the amplitude of forced physical librations could be diagnostic of the presence or absence of subsurface oceans within the Uranian moons. In the presence of a decoupling global ocean, ice shell libration amplitudes at Miranda, Ariel, and Umbriel will exceed 100 m if the shells are <30 ${< } 30$km $\mathrm{k}\mathrm{m}$ thick. The presence of oceans could also imply significant tidal heating within the last few hundred million years. Combining librations with the quadrupole gravity field could provide comprehensive constraints on the internal structures and histories of the Uranian moons. Plain Language Summary: Several of the icy moons in the Jupiter and Saturn systems appear to possess internal liquid water oceans. Our knowledge of the Uranian moons is more limited but a future tour of the Uranus system has the potential to detect subsurface oceans. Planning for this requires an understanding of how the moons' internal structures—with and without oceans—relate to observable quantities. Here, we show that certain aspects of their rotational states could be diagnostic of the presence or absence of internal liquid water oceans within several of the Uranian moons and that combining this with measurements of the gravity field could provide comprehensive constraints on the internal structures and histories of the Uranian moons. Key Points: Measuring physical libration amplitudes can be used to detect subsurface liquid water oceans within the Uranian moonsCombining librations with gravity measurements can yield comprehensive constraints on the interiors of the Uranian moonsThick oceans are easier to detect but finding thin oceans may require libration amplitudes to be measured to within better than 10 m [ABSTRACT FROM AUTHOR]

We investigated how spatial variations in tidal heating affect Io's isostatic topography at long wavelengths. The long‐wavelength relief is less than the 0.3 km uncertainty in Io's global shape. Assuming Airy isostasy, degree‐2 topography <0.3 km amplitude is only possible if surface heat flux varies spatially by <19% of the mean value. This is consistent with Io's volcano distribution and is possible if tidal heat is generated within and redistributed by a convecting layer underneath the lithosphere. However, that layer would require a viscosity <1010 Pa s. A magma ocean would have low enough viscosity but would not generate enough tidal heat internally. Conversely, assuming Pratt isostasy, we found ∼0.15 km degree‐2 topography is easily achievable. If a magma ocean was present, Airy isostasy would dominate; we therefore conclude that Io is unlikely to possess a magma ocean. Plain Language Summary: As it orbits Jupiter elliptically, the difference in gravitational pull experienced by the moon Io results in tidal heating due to internal friction. Some evidence suggests this heat forms a magma ocean beneath Io's crust. If so, there would be a difference in the amount of heat generated at Io's equator versus its poles and would alter the thickness of Io's crust between the two locales. Assuming the crust has a uniform density, its thickness would be inversely proportional to the tidal heat beneath the crust, which in turn affects the difference in Io's radius at the equator versus at its poles. However, reasonable variation in tidal heating across Io would result in a greater difference in radius than is observed. The difference in observed radius is more likely if variation in tidal heat across Io affects crustal density rather than crustal thickness. Then, it is more likely that Io does not have a magma ocean. Key Points: Long‐wavelength relief implies low spatial variation in Io's tidal heating when assuming Airy isostasyTidal heat produced in a convecting aesthenosphere can reduce spatial variation in tidal heating, but requires prohibitively low viscosityIo's topography is consistent with expected tidal heating spatial variations if thermal expansion drives crustal density variations [ABSTRACT FROM AUTHOR]

Mimas has a high eccentricity and an anomalously high physical libration like its neighbor, Enceladus, but does not appear to have a geologically active surface. We investigate Mimas' interior with a technique that infers spatial variations in tidal heating from its global shape. To account for its hydrostatic shape, we find Mimas' normalized moment of inertia is 0.375 ± 0.0025, indicating a relatively undifferentiated world. Its remaining topography is consistent with a ∼30 km thick conductive ice shell in Airy isostasy atop a weakly convecting ∼30 km thick layer that itself mantles a ∼140 km radius ice‐rock interior. The convective shell's density must be closer to the interior density to satisfy our moment of inertia and provide a denser compensating layer for Airy isostasy. This ice‐rock interior is elongated along the Mimas‐Saturn axis, which can match Mimas' observed physical libration without appealing to an ocean. The inferred ice shell thickness variations indicate a high obliquity (≈1.7°). We suggest that the obliquity damped rapidly, after which topography froze in when internal heat was conducted out of Mimas quicker than isostatic ice shell thickness variations could relax. We speculate on several possible explanations for this transient high obliquity, including excitation by ring‐forming material following the recent tidal disruption of an eccentric satellite. We cannot rule out a young Mimantean ocean, but our inferred moment of inertia favors a Mimas that was solid when it experienced a period of high obliquity, did not significantly melt during a recent resonance with Enceladus, and is solid today. Plain Language Summary: While Saturn's moon Mimas appears to have an ancient surface covered in craters, it has a highly elliptical orbit, as well as a wobble in its rotation that is higher than expected for a solid, spherically symmetric moon. This wobble could indicate Mimas has either an ocean or an elongated core. To investigate the possibility of an ocean, we examine differences from sphericity in Mimas' shape. Some of this shape is from Mimas' tidal bulge and rotational flattening. Assuming the rest is topography due to differences in heat that arise due to tidal forces, we can relate Mimas' topography to its tidal heating pattern. Because a moon's tidal heating pattern is influenced by its interior structure, we can then make inferences on Mimas' interior. What we find indicates that the tidal heating conditions that produced Mimas' shape indicate its spin pole was once tilted much higher than one would expect at present day. This could have occurred long ago, after which the topography froze in when Mimas rapidly cooled. The tidal heating pattern we infer also indicates that Mimas did not have an ocean at the time of this strong tidal heating, and likely remains solid to this day. Key Points: We infer spatial variations in tidal heating and ice‐shell thickness from Mimas' non‐hydrostatic shapeOur preferred Mimas interior has no sub‐surface ocean but requires high obliquity tides in Mimas' pastMimas' past obliquity may have been excited by gravitational interaction with an inward‐migrating debris disk that formed Saturn's rings [ABSTRACT FROM AUTHOR]

Triton, the largest satellite of Neptune, is in a retrograde orbit and is likely a captured Kuiper Belt Object (KBO). Triton has a mean density of only 2.061 gm/cm 3 and is therefore believed to have a 250-400 km thick hydrosphere. Triton is also one of the few planetary satellites to possess a thick ionosphere whose height-integrated Pedersen conductivity exceeds 10 4 S, complicating the sounding of Triton's subsurface using electromagnetic induction. Triton experiences a time-varying magnetic field dominated by two periods, one at 14.4 h, at the synodic rotation period of Neptune (from Neptune's tilted field) and one at 141 h, at the orbital period of Triton (from large inclination of Triton's orbit). We show that for most models of ionospheric conductivity, the 14.4 h wave creates a large response from the ionosphere itself and is unable to sound the putative ocean below. However, the 141 h wave penetrates the ionosphere easily and provides information on Triton's ocean. We introduce a technique that allows us to determine the complex magnetic moments generated at the two key periods from the magnetic data from a single flyby, allowing us to infer the presence of a subsurface ocean.This article is part of the theme issue 'Magnetometric remote sensing of Earth and planetary oceans'.

Io experiences tidal deformation due to its eccentric orbit around Jupiter, which provides a primary energy source for Io's ongoing volcanic activity and infrared emission 1 . The amount of tidal energy dissipated within Io is enormous and has been hypothesized to support the large-scale melting of Io's interior and the formation of a global subsurface magma ocean. If Io has a shallow global magma ocean, its tidal deformation would be much larger than in the case of a more rigid, mostly solid interior 2 . Here we report the measurement of Io's tidal deformation, quantified by the gravitational tidal Love number k 2 , enabled by two recent flybys of the Juno spacecraft. By combining Juno 3,4 and Galileo 5-7 Doppler data from the Deep Space Network and astrometric observations, we recover Re(k 2 ) of 0.125±0.047 (1σ) and the tidal dissipation parameter Q of 11.4±3.6 (1σ). These measurements confirm that a shallow global magma ocean in Io does not exist and are consistent with Io having a mostly solid mantle 2 . Our results indicate that tidal forces do not universally create global magma oceans, which may be prevented from forming due to rapid melt ascent, intrusion, and eruption 8,9 , so even strong tidal heating - like that expected on several known exoplanets and super-Earths 10 - may not guarantee the formation of magma oceans on moons or planetary bodies., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The last giant impact on Earth is thought to have formed the Moon 1 . The timing of this event can be determined by dating the different rocks assumed to have crystallized from the lunar magma ocean (LMO). This has led to a wide range of estimates for the age of the Moon between 4.35 and 4.51 billion years ago (Ga), depending on whether ages for lunar whole-rock samples 2-4 or individual zircon grains 5-7 are used. Here we argue that the frequent occurrence of approximately 4.35-Ga ages among lunar rocks and a spike in zircon ages at about the same time 8 is indicative of a remelting event driven by the Moon's orbital evolution rather than the original crystallization of the LMO. We show that during passage through the Laplace plane transition 9 , the Moon experienced sufficient tidal heating and melting to reset the formation ages of most lunar samples, while retaining an earlier frozen-in shape 10 and rare, earlier-formed zircons. This paradigm reconciles existing discrepancies in estimates for the crystallization time of the LMO, and permits formation of the Moon within a few tens of million years of Solar System formation, consistent with dynamical models of terrestrial planet formation 11 . Remelting of the Moon also explains the lower number of lunar impact basins than expected 12,13 , and allows metal from planetesimals accreted to the Moon after its formation to be removed to the lunar core, explaining the apparent deficit of such materials in the Moon compared with Earth 14 ., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

Jupiter's moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io's inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass-loss history and thus records information about its evolution. We used submillimeter observations of Io's atmosphere to measure sulfur isotopes in gaseous sulfur dioxide and sulfur monoxide, and chlorine isotopes in gaseous sodium chloride and potassium chloride. We find 34 S/ 32 S = 0.0595 ± 0.0038 (equivalent to δ 34 S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of 37 Cl/ 35 Cl = 0.403 ± 0.028 (δ 37 Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times.

Single crystal paleointensity (SCP) reveals that the Moon lacked a long-lived core dynamo, though mysteries remain. An episodic dynamo, seemingly recorded by some Apollo basalts, is temporally and energetically problematic. We evaluate this enigma through study of ~3.7 billion-year-old (Ga) Apollo basalts 70035 and 75035. Whole rock analyses show unrealistically high nominal magnetizations, whereas SCP indicate null fields, illustrating that the former do not record an episodic dynamo. However, deep crustal magnetic anomalies might record an early lunar dynamo. SCP studies of 3.97 Ga Apollo breccia 61016 and 4.36 Ga ferroan anorthosite 60025 also yield null values, constraining any core dynamo to the Moon's first 140 million years. These findings suggest that traces of Earth's Hadean atmosphere, transferred to the Moon lacking a magnetosphere, could be trapped in the buried lunar regolith, presenting an exceptional target for future exploration., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2024.)

Here we present the current state of knowledge on the long-term evolution of Saturn's moon system due to tides within Saturn. First we provide some background on tidal evolution, orbital resonances and satellite tides. Then we address in detail some of the present and past orbital resonances between Saturn's moons (including the Enceladus-Dione and Titan-Hyperion resonances) and what they can tell us about the evolution of the system. We also present the current state of knowledge on the spin-axis dynamics of Saturn: we discuss arguments for a (past or current) secular resonance of Saturn's spin precession with planetary orbits, and explain the links of this resonance to the tidal evolution of Titan and a possible recent cataclysm in the Saturnian system. We also address how the moons' orbital evolution, including resonances, affects the evolution of their interiors. Finally, we summarize the state of knowledge about the Saturnian system's long-term evolution and discuss prospects for future progress., Competing Interests: Competing InterestsThe authors have no relevant financial or non-financial interests to disclose., (© The Author(s) 2024.)

The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) is a dual-frequency ice-penetrating radar (9 and 60 MHz) onboard the Europa Clipper mission. REASON is designed to probe Europa from exosphere to subsurface ocean, contributing the third dimension to observations of this enigmatic world. The hypotheses REASON will test are that (1) the ice shell of Europa hosts liquid water, (2) the ice shell overlies an ocean and is subject to tidal flexing, and (3) the exosphere, near-surface, ice shell, and ocean participate in material exchange essential to the habitability of this moon. REASON will investigate processes governing this material exchange by characterizing the distribution of putative non-ice material (e.g., brines, salts) in the subsurface, searching for an ice-ocean interface, characterizing the ice shell's global structure, and constraining the amplitude of Europa's radial tidal deformations. REASON will accomplish these science objectives using a combination of radar measurement techniques including altimetry , reflectometry , sounding , interferometry , plasma characterization , and ranging . Building on a rich heritage from Earth, the moon, and Mars, REASON will be the first ice-penetrating radar to explore the outer solar system. Because these radars are untested for the icy worlds in the outer solar system, a novel approach to measurement quality assessment was developed to represent uncertainties in key properties of Europa that affect REASON performance and ensure robustness across a range of plausible parameters suggested for the icy moon. REASON will shed light on a never-before-seen dimension of Europa and - in concert with other instruments on Europa Clipper - help to investigate whether Europa is a habitable world., Competing Interests: Competing InterestsThe authors have no competing interests to declare that are relevant to the content of this article., (© The Author(s) 2024.)

The large Kuiper Belt object Eris is tidally locked to its small companion Dysnomia. Recently obtained bounds on the mass of Dysnomia demonstrate that Eris must be unexpectedly dissipative for it to have despun over the age of the solar system. Here, we show that Eris must have differentiated into an ice shell and rocky core to explain the dissipation. We further demonstrate that Eris's ice shell must be convecting to be sufficiently dissipative, which distinguishes it from Pluto's conductive shell. The difference is likely due to Eris's apparent depletion in volatiles compared with Pluto, perhaps as the result of a more energetic impact.

Plate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadaean to Proterozoic eons 1-3 . Plate motion is a key diagnostic to distinguish between plate and stagnant-lid tectonics, but palaeomagnetic tests have been thwarted because the planet's oldest extant rocks have been metamorphosed and/or deformed 4 . Herein, we report palaeointensity data from Hadaean-age to Mesoarchaean-age single detrital zircons bearing primary magnetite inclusions from the Barberton Greenstone Belt of South Africa 5 . These reveal a pattern of palaeointensities from the Eoarchaean (about 3.9 billion years ago (Ga)) to Mesoarchaean (about 3.3 Ga) eras that is nearly identical to that defined by primary magnetizations from the Jack Hills (JH; Western Australia) 6,7 , further demonstrating the recording fidelity of select detrital zircons. Moreover, palaeofield values are nearly constant between about 3.9 Ga and about 3.4 Ga. This indicates unvarying latitudes, an observation distinct from plate tectonics of the past 600 million years (Myr) but predicted by stagnant-lid convection. If life originated by the Eoarchaean 8 , and persisted to the occurrence of stromatolites half a billion years later 9 , it did so when Earth was in a stagnant-lid regime, without plate-tectonics-driven geochemical cycling., (© 2023. The Author(s).)

It has recently been suggested that deformed crustal plateaus on Venus may be composed of felsic (silica-rich) rocks, possibly supporting the idea of an ancient ocean there. However, these plateaus have a tendency to collapse owing to flow of the viscous lower crust. Felsic minerals, especially water-bearing ones, are much weaker and thus lead to more rapid collapse, than more mafic minerals. We model plateau topographic evolution using a non-Newtonian viscous relaxation code. Despite uncertainties in the likely crustal thickness and surface heat flux, we find that quartz-dominated rheologies relax too rapidly to be plausible plateau-forming material. For plateaus dominated by a dry anorthite rheology, survival is possible only if the background crustal thickness is less than 29 km, unless the heat flux on Venus is less than the radiogenic lower bound of 34 [Formula: see text]. Future spacecraft determinations of plateau crustal thickness and mineralogy will place firmer constraints on Venus's heat flux.

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB)., Competing Interests: Competing InterestsThe authors have no competing interests to declare that are relevant to the content of this article., (© The Author(s) 2023.)

Enceladus possesses a subsurface ocean beneath a conductive ice shell. Based on shell thickness models, the estimated total conductive heat loss from Enceladus is 25-40 GW; the measured heat output from the South Polar Terrain (SPT) is 4-19 GW. The present-day SPT heat flux is of order 100  mW m - 2 , comparable to estimated paleo-heat fluxes for other regions of Enceladus. These regions have nominal ages of about 2 Ga, but the estimates are uncertain because the impactor flux in the Saturnian system may not resemble that elsewhere. Enceladus's measured rate of orbital expansion implies a low dissipation factor Q p for Saturn, with Q p ≈ 3 × 10 3 (neglecting the role of Dione). This value implies that Enceladus's present-day equilibrium tidal heat production (roughly 50 GW, but with large uncertainties) is in approximate balance with its heat loss. If Q p is constant, Enceladus cannot be older than 1.5 Gyr (because otherwise it would have migrated more than is permissible). However, Saturn's dissipation may be better described by the "resonance-locking" theory, in which case Enceladus's orbit may have only evolved outwards by about 35% over the age of the Solar System. In the constant- Q p scenario, any ancient tidal heating events would have been too energetic to be consistent with the observations. Because resonance-locking makes capture into earlier mean-motion orbital resonances less likely, the inferred ancient heating episodes probably took place when the current orbital resonance was already established. In the resonance-locking scenario, tidal heating did not change significantly over time, allowing for a long-lived ocean and a relatively stable ice shell. If so, Enceladus is an attractive target for future exploration from a habitability standpoint., Competing Interests: Competing InterestsThe authors declare that they have no competing interests., (© The Author(s) 2023.)

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa's interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world., Competing Interests: Competing InterestsThe authors declare that they have no conflicts of interest., (© The Author(s) 2023.)

The origin of Saturn's ~26.7° obliquity and ~100-million-year-old rings is unknown. The observed rapid outward migration of Saturn's largest satellite, Titan, could have raised Saturn's obliquity through a spin-orbit precession resonance with Neptune. We use Cassini data to refine estimates of Saturn's moment of inertia, finding that it is just outside the range required for the resonance. We propose that Saturn previously had an additional satellite, which we name Chrysalis, that caused Saturn's obliquity to increase through the Neptune resonance. Destabilization of Chrysalis's orbit ~100 million years ago can then explain the proximity of the system to the resonance and the formation of the rings through a grazing encounter with Saturn.

Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core., (© 2022. The Author(s).)

The inferred density of Enceladus' core, together with evidence of hydrothermal activity within the moon, suggests that the core is porous. Tidal dissipation in an unconsolidated core has been proposed as the main source of Enceladus' geological activity. However, the tidal response of its core has generally been modeled assuming it behaves viscoelastically rather than poroviscoelastically. In this work, we analyze the poroviscoelastic response to better constrain the distribution of tidal dissipation within Enceladus. A poroviscoelastic body has a different tidal response than a viscoelastic one; pressure within the pores alters the stress field and induces a Darcian porous flow. This flow represents an additional pathway for energy dissipation. Using Biot's theory of poroviscoelasticity, we develop a new framework to obtain the tidal response of a spherically symmetric, self-gravitating moon with porous layers and apply it to Enceladus. We show that the boundary conditions at the interface of the core and overlying ocean play a key role in the tidal response. The ocean hinders the development of a large-amplitude Darcian flow, making negligible the Darcian contribution to the dissipation budget. We therefore infer that Enceladus' core can be the source of its geological activity only if it has a low rigidity and a very low viscosity. A future mission to Enceladus could test this hypothesis by measuring the phase lags of tidally induced changes of gravitational potential and surface displacements., (© 2022. The Authors.)

The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>10 4 km 3 ) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto's interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto's history., (© 2022. The Author(s).)

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium-tungsten (Hf-W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth's protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N -body simulations with calculations of metal-silicate partitioning and Hf-W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182 W anomalies, and that it is readily possible to produce Mars-like Hf-W isotopic compositions for a variety of accretionary conditions. The Hf-W signature of Mars is very sensitive to the oxygen fugacity ( f O 2 ) of accreted material because the metal-silicate partitioning behavior of W is strongly dependent on redox conditions. The average f O 2 of Martian building blocks must fall in the range of 1.3-1.6 log units below the iron-wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182 W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf-W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf-W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.