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The energy provided in the radioactive decay of thorium (Th) and uranium (U) isotopes, embedded in planetary mantles, sustains geodynamics important for surface habitability such as the generation of a planetary magnetic dynamo. In order to better understand the thermal evolution of nearby exoplanets, stellar photospheric abundances can be used to infer the material composition of orbiting planets. Here we constrain the intrinsic dispersion of the r-process element europium (Eu) (measured in relative abundance [Eu/H]) as a proxy for Th and U in local F, G, and K type dwarf stars. Adopting stellar-chemical data from two high quality spectroscopic surveys, we have determined a small intrinsic scatter of 0.025 dex in [Eu/H] within the disk. We further investigate the stellar anti-correlation in [Eu/$\alpha$] vs [$\alpha$/H] at late metallicities to probe in what regimes planetary radiogenic heating may lead to periods of extended dynamo collapse. We find that only near-solar metallicity stars in the disk have Eu inventories supportive of a persistent dynamo in attendant planets, supporting the notion of a ``metallicity Goldilocks zone'' in the galactic disk. The observed anti-correlation further provides novel evidence regarding the nature of r-processes injection by substantiating $\alpha$ element production is decoupled from Eu injection. This suggests either a metallicity-dependent r-process in massive core-collapse supernovae, or that neutron-star merger events dominate r-process production in the recent universe., Comment: 11 pages, 4 figures

The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario., Comment: In press as a Frontier paper in Earth and Planetary Science Letters

The nucleosynthetic isotope signatures of meteorites and the bulk silicate Earth (BSE) indicate that Earth consists of a mixture of "carbonaceous" (CC) and "non-carbonaceous" (NC) materials. We show that the fration of CC material recorded in the isotopic composition of the BSE varies for different elements, and depends on the element's tendency to partition into metal and its volatility. The observed behaviour indicates that the majority of material accreted to the Earth was NC-dominated, but that CC-dominated material enriched in moderately-volatile elements by a factor of ~10 was delivered during the last ~2-10% of Earth's acccretion. The late delivery of CC material to Earth contrasts with dynamical evidence for the early implantation of CC objects into the inner solar system during the growth and migration of the giant planets. This, together with the NC-dominated nature of both Earth's late veneer and bulk Mars, suggests that material scattered inwards had the bulk of its mass concentrated in a few, large CC embryos rather than in smaller planetesimals. We propose that Earth accreted a few of these CC embryos while Mars did not, and that at least one of the CC embryos impacted Earth relatively late (when accretion was 90-90% complete). This scenario is consistent with the subsequent Moon-formign impact of a large NC body, as long as this impact did not re-homogenize the entire Earth's mantle.

We present a novel python-based 1D sub-Neptune evolution model that emphasizes the thermal evolution and potential solidification of the rock/iron core and the structure of the radiative atmosphere. This model explores planetary structure from the molten center to nbar pressure levels. Treating the radiative atmosphere is crucial for sub-Neptunes, due to the large scale height and low gravity, which contributes up to 40\% of their observed radius, especially for low-mass, highly irradiated planets. Consequently, we generically find that lower H/He mass fractions are needed to match a given planetary radius, compared to previous work. While the presence of metal-enrichment in the H/He layers (here modeled as 50$\times$ solar) does not substantially influence the size of the convective envelope, it notably reduces the transit radius by shrinking the radiative atmospheric scale height. Sub-Neptunes cool differently from terrestrial planets, with the rock/iron core's cooling rate limited by the envelope, leading to longer solidification timescales. Complete solidification of the silicate mantle by 10 Gyr is found only for planets with very low masses ($\leq 1M_\oplus$) and small H/He envelopes ($\leq$ 0.1\%). Dynamo action in sub-Neptune iron cores persists as long as the mantle surface remains molten, often exceeding 10 Gyr, and becomes sensitive to core thermal conductivity after solidification. We examine aspects of ''boil-off,'' which sets the maximum allowed H/He mass and planetary radius for subsequent evolution. The rock/iron's cooling energy moderately decreases the post-boil-off H/He mass fraction in planets with large atmospheric scale heights only., Comment: 32 pages, 17 figures, submitted to ApJ

Jupiter's moon Io is a highly compelling target for future exploration that offers critical insight into tidal dissipation processes and the geology of high heat flux worlds, including primitive planetary bodies, such as the early Earth, that are shaped by enhanced rates of volcanism. Io is also important for understanding the development of volcanogenic atmospheres and mass-exchange within the Jupiter System. However, fundamental questions remain about the state of Io's interior, surface, and atmosphere, as well as its role in the evolution of the Galilean satellites. The Io Volcano Observer (IVO) would address these questions by achieving the following three key goals: (A) Determine how and where tidal heat is generated inside Io; (B) Understand how tidal heat is transported to the surface of Io; and (C) Understand how Io is evolving. IVO was selected for Phase A study through the NASA Discovery program in 2020 and, in anticipation of a New Frontiers 5 opportunity, an enhanced IVO-NF mission concept was advanced that would increase the Baseline mission from 10 flybys to 20, with an improved radiation design; employ a Ka-band communications to double IVO's total data downlink; add a wide angle camera for color and stereo mapping; add a dust mass spectrometer; and lower the altitude of later flybys to enable new science. This study compares and contrasts the mission architecture, instrument suite, and science objectives for Discovery (IVO) and New Frontiers (IVO-NF) missions to Io, and advocates for continued prioritization of Io as an exploration target for New Frontiers., Comment: Submitted to The Planetary Science Journal for peer-review on 14 August 2024

Jupiter's moon Io hosts extensive volcanism driven by tidal heating. The isotopic composition of Io's inventory of volatile elements, including sulfur and chlorine, reflects its outgassing and mass loss history and provides an avenue for exploring its evolution. We used millimeter observations of Io's atmosphere to measure sulfur isotopes in gaseous SO2 and SO, and chlorine isotopes in gaseous NaCl and KCl. We find $^{34}$S/$^{32}$S=0.0595$\pm$0.0038 ($\delta^{34}$S=+347$\pm$86 per mille), 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$\pm$0.028 ($\delta^{37}$Cl=+263$\pm$88 per mille) shows chlorine is similarly enriched. These measurements 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., Comment: This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on May 10, 2024, DOI: 10.1126/science.adj0625

Tidal dissipation in a celestial body can be used to probe its internal structure. Tides govern the orbital evolution of binary systems and therefore constraints on the interior of binary system members can be derived by knowing the age and tidal state of the binary system. For asteroids, age estimates are challenging due to a lack of direct observation of their surface. However, the age of asteroid pairs formed by rotational fission of a parent body can be derived from dynamical modeling, and as such can be used to constrain the age of binary systems existing within asteroid pairs.We study 13 binary asteroid systems existing in asteroid pairs by modeling their tidal locking and eccentricity damping timescales from tidal dissipation in the primaries and secondaries. We consider the impact of thermal torques on these timescales from the YORP and BYORP effects. The resulting constraints on the tidal dissipation ratio Q/k2 are compared to monolithic and rubble pile asteroid theories, showing that all secondaries are consistent with rubble piles with regolith layers greater than 3m and suggest that Q/k2 for rubble piles increases with radius. A particular case is the first bound secondary of asteroid (3749) Balam, whose Q/k2 is constrained to be between 2.7x10^4 and 1.4x10^6, consistent with a rubble-pile with a regolith thickness between 30m and 100m., Comment: 24 pages, 5 figures, 1 appendix Published in Icarus: Pou L, Nimmo F. Tidal dissipation of binaries in asteroid pairs. Icarus. 2024 Mar 15;411:115919. https://doi.org/10.1016/j.icarus.2023.115919

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., Comment: Accepted for Space Science Reviews. Chapter in the book based on the ISSI workshop "New Vision of the Saturnian System in the Context of a Highly Dissipative Saturn" (9-13 May 2022)

Titan's ice shell floats on top of a global ocean revealed by the large tidal Love number $k_2 = 0.616\pm0.067$ registered by Cassini. The Cassini observation exceeds the predicted $k_2$ by one order of magnitude in the absence of an ocean, and is 3-$\sigma$ away from the predicted $k_2$ if the ocean is pure water resting on top of a rigid ocean floor. Previous studies demonstrate that an ocean heavily enriched in salts (salinity $S\gtrsim200$ g/kg) can explain the 3-$\sigma$ signal in $k_2$. Here we revisit previous interpretations of Titan's large $k_2$ using simple physical arguments and propose a new interpretation based on the dynamic tidal response of a stably stratified ocean in resonance with eccentricity tides raised by Saturn. Our models include inertial effects from a full consideration of the Coriolis force and the radial stratification of the ocean, typically neglected or approximated elsewhere. The stratification of the ocean emerges from a salinity profile where salt concentration linearly increases with depth. We find multiple salinity profiles that lead to the $k_2$ required by Cassini. In contrast with previous interpretations that neglect stratification, resonant stratification reduces the bulk salinity required by observations by an order of magnitude, reaching a salinity for Titan's ocean that is compatible with that of Earth's oceans and close to Enceladus' plumes. Consequently, no special process is required to enrich Titan's ocean to a high salinity as previously suggested., Comment: 29 pages, 8 figures, accepted to PSJ

As one of the most detectable types of terrestrial planets, lava worlds are highly prioritized targets for exoplanet atmosphere characterization since their atmospheres may reveal what they are made of and how. Our work examines the possibility of true polar wander (TPW) occurring on these ultra-hot tidally-locked planets, powered by mass redistribution from atmospheric flow between the hot permanent day-side and the cold permanent night-side. We find that lava planets within a certain mass and temperature range may undergo TPW, and this likelihood increases with star mass. As a result of TPW, the magma ocean and atmospheric compositions may be less evolved (refractory-enriched) than previously thought and may be validated by exoplanet demographic surveys.

It has been suggested that the ice shell of Jupiter's moon Europa may drift non-synchronously due to tidal torques. Here we argue that torques applied by the underlying ocean are also important and can result in non-synchronous rotation (NSR). The resulting spin rate can be slightly slower than the synchronous angular rate that would have kept the same point of the ice shell facing Jupiter. We develop an ice shell rotation model, driven by ocean stress calculated using a high-resolution state-of-the-art ocean general circulation model, and take into account the viscoelastic deformation of the ice shell. We use the ice shell model results together with observed limits on the ice shell drift speed to constrain ice shell parameters such as effective viscosity, which is currently uncertain by at least four orders of magnitude. Our results suggest, at best, sluggish ice shell convection. Depending on the relaxation time scale of the ice shell and on the ocean currents, the ice shell may exhibit negligible drift, constant drift, or oscillatory drift superimposed on random fluctuations. The expected rotation rate exceeds $\sim$30~m/yr; future spacecraft observations can be used to test these predictions and yield insight into the properties of the ice shell and underlying ocean.

Various theories have been proposed to explain the Moon's current inclined orbit. We test the viability of these theories by reconstructing the thermal-orbital history of the Moon. We build on past thermal-orbital models and incorporate the evolution of the lunar figure including a fossil figure component. Obliquity tidal heating in the lunar magma ocean would have produced rapid inclination damping, making it difficult for an early inclination to survive to the present-day. An early inclination is preserved only if the solid-body of the early Moon were less dissipative than at present. If instabilities at the Laplace plane transition were the source of the inclination, then the Moon had to recede slowly, which is consistent with previous findings of a weakly dissipative early Earth. If collisionless encounters with planetesimals up to 140 Myr after Moon formation excited the inclination, then the Moon had to migrate quickly to pass through the Cassini state transition at 33 Earth radii and reach a period of limited inclination damping. The fossil figure was likely established before 16 Earth radii to match the present-day degree-2 gravity field observations., Comment: 18 pages, 6 figures

Enceladus is a primary target for astrobiology due to the $\rm H_2O$ plume ejecta measured by the Cassini spacecraft and the inferred subsurface ocean sustained by tidal heating. Sourcing the plumes via a direct connection from the ocean to the surface requires a fracture through the entire ice shell ($\sim$10 km). Here we explore an alternative mechanism in which shear heating within shallower tiger stripe fractures produces partial melting in the ice shell and interstitial convection allows fluid to be ejected as geysers. We use an idealized two-dimensional multiphase reactive transport model to simulate the thermomechanics of a mushy region generated by an upper bound estimate for the localized shear heating rate in a salty ice shell. From our simulations, we predict the temperature, porosity, salt content, melting rate, and liquid volume of an intrashell mushy zone surrounding a fracture. We find that the rate of internal melting can match the observed $\rm H_2O$ eruption rate and that there is sufficient brine volume within the mushy zone to sustain the geysers for $\sim350$ kyr without additional melting. The composition of the liquid brine is, however, distinct from that of the ocean, due to partial melting. This shear heating mechanism for geyser formation applies to Enceladus and other icy moons and has implications for our understanding of the geophysical processes and astrobiological potential of icy satellites., Comment: Main text (11 pages, 3 figures), supplement (16 pages, 7 figures, 1 table). Added new figures and discussion to the supplement

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 (>104 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., Comment: 22 pages, including both main paper and supplement as one pdf

Tidal dissipation is thought to be responsible for the observed high heat loss on Enceladus. Forced librations can enhance tidal dissipation in the ice shell, but how such librations affect the thermal state of Enceladus has not been investigated. Here we investigate the heating effect of forced librations using the model of Van Hoolst et al. (2013), which includes the elasticity of the ice shell. We find that libration heating in the ice shell is insufficient to match the inferred conductive heat loss of Enceladus. This suggests that either Enceladus is not in a thermal steady state, or additional heating mechanisms beneath the ice shell are contributing the bulk of the power. In the presence of such an additional heat source, Enceladus resides in a stable thermal equilibrium, resisting small perturbations to the shell thickness. Our results do not support the occurrence of a runaway melting process proposed by Luan and Goldreich (2017). In our study, the strong dependence of conductive loss on shell thickness stabilizes the thermal state of Enceladus's ice shell. Our study implies that thermal runaway (if it occurred) or episodic heating on Enceladus is unlikely to originate from librations of the ice shell., Comment: 22 Pages, 5 figures, published in Icarus

Venus's past climate evolution is uncertain. General circulation model simulations permit a habitable climate as late as ~0.7 Ga, and there is suggestive-albeit inconclusive-evidence for previous liquid water from surface geomorphology and mineralogy. However, it is unclear whether a habitable past can be reconciled with Venus's inferred atmospheric evolution. In particular, the lack of leftover atmospheric oxygen argues against recent water loss. Here, we apply a fully coupled model of Venus's atmospheric-interior-climate evolution from post-accretion magma ocean to present. The model self-consistently tracks C-, H-, and O-bearing volatiles and surface climate through the entirety of Venus's history. Atmospheric escape, mantle convection, melt production, outgassing, deep water cycling, and carbon cycling are explicitly coupled to climate and redox evolution. Plate tectonic and stagnant lid histories are considered. Using this coupled model, we conclude that both a habitable Venusian past and one where Venus never possessed liquid surface water can be reconciled with known constraints. Specifically, either scenario can reproduce bulk atmospheric composition, inferred surface heat flow, and observed $^{40}$Ar and $^{4}$He. Moreover, the model suggests that Venus could have been habitable with a ~100 m global ocean as late as 1 Ga, without violating any known constraints. In fact, if diffusion-limited water loss is throttled by a cool, CO$_2$-dominated upper atmosphere, then a habitable past is tentatively favored by our model. This escape throttling makes it difficult to simultaneously recover negligible water vapor and ~90 bar CO$_2$ in the modern atmosphere without temporarily sequestering carbon in the interior via silicate weathering to enhance H escape., Comment: Complete text 27 pages (main text 16 pages), including 9 figures and 7 supplementary figures

The 182Hf-182W chronology of iron meteorites provides crucial information on the timescales of accretion and differentiation of some of the oldest planetesimals of the Solar System. Determining accurate Hf-W model ages of iron meteorites requires correction for cosmic ray expo-sure (CRE) induced modifications of W isotope compositions, which can be achieved using in-situ neutron dosimeters such as Pt isotopes. Until now it has been assumed that all Pt isotope variations in meteorites reflect CRE, but here we show that some ungrouped iron meteorites display small nucleosynthetic Pt isotope anomalies. These provide the most appropriate starting composition for the correction of CRE-induced W isotope variations in iron meteorites from all major chemical groups, which leads to a ~1 Ma upward revision of previously reported Hf-W model ages. The revised ages indicate that core formation in non-carbonaceous (NC) iron meteorite parent bodies occurred at ~1-2 Ma after CAI formation, whereas most carbonaceous (CC) iron meteorite parent bodies underwent core formation ~2 Ma later. We show that the younger CC cores have lower Fe/Ni ratios than the earlier-formed NC cores, indicating that core formation under more oxidizing conditions occurred over a more protracted timescale. Thermal modeling of planetesimals heated by 26Al-decay reveals that this protracted core formation timescale is consistent with a higher fraction of water ice in CC compared to NC planetesimals, implying that in spite of distinct core formation timescales, NC and CC iron meteorite parent bodies accreted about contemporaneously within ~1 Ma after CAI formation, but at different radial locations in the disk., Comment: Accepted for publication in Earth and Planetary Science Letters

Oxygen is a promising exoplanet biosignature due to the evolutionary advantage conferred by harnessing starlight for photosynthesis, and the apparent low likelihood of maintaining oxygen-rich atmospheres without life. Hypothetical scenarios have been proposed for non-biological oxygen accumulation on planets around late M-dwarfs, where the extended pre-main sequence may favor abiotic O$_2$ accumulation. In contrast, abiotic oxygen accumulation on planets around F, G, and K-type stars is seemingly less likely, provided they possess substantial non-condensable gas inventories. The comparative robustness of oxygen biosignatures around larger stars has motivated plans for next-generation telescopes capable of oxygen detection on planets around sun-like stars. However, the general tendency of terrestrial planets to develop oxygen-rich atmospheres across a broad range of initial conditions and evolutionary scenarios has not been explored. Here, we use a coupled thermal-geochemical-climate model of terrestrial planet evolution to illustrate three scenarios whereby significant abiotic oxygen can accumulate around sun-like stars, even when significant non-condensable gas inventories are present. For Earth-mass planets, we find abiotic oxygen can accumulate to modern levels if (1) the CO$_2$:H$_2$O ratio of the initial volatile inventory is high, (2) the initial water inventory exceeds ~50 Earth oceans, or (3) the initial water inventory is very low. Fortunately, these three abiotic oxygen scenarios could be distinguished from biological oxygen with observations of other atmospheric constituents or characterizing the planetary surface. This highlights the need for broadly capable next-generation telescopes that are equipped to constrain surface water inventories via time-resolved photometry and search for temporal biosignatures or disequilibrium biosignatures to assess whether oxygen is biogenic., Comment: Main text 20 pages, 7 figures. Supporting materials 45 pages

Persephone is a NASA concept mission study that addresses key questions raised by New Horizons' encounters with Kuiper Belt objects (KBOs), with arguably the most important being "Does Pluto have a subsurface ocean?". More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 years long, with launch in 2031 on a Space Launch System (SLS) Block 2 rocket with a Centaur kick stage, followed by a 27.6 year cruise powered by existing radioisotope electric propulsion (REP) and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50- to 100-km-class KBO encounter before starting a 3.1 Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8-year extended mission, which would enable the exploration of another KBO in the 100- to 150-km-size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager; Low-Light Camera; Ultra-Violet Spectrometer; Near-Infrared (IR) Spectrometer; Thermal IR Camera; Radio Frequency Spectrometer; Mass Spectrometer; Altimeter; Sounding Radar; Magnetometer; and Plasma Spectrometer. The nominal cost of this mission is $3.0B, making it a large strategic science mission.

The thermal evolution of rocky planets on geological timescales (Gyr) depends on the heat input from the long-lived radiogenic elements potassium, thorium, and uranium. Concentrations of the latter two in rocky planet mantles are likely to vary by up to an order of magnitude between different planetary systems because Th and U, like other heavy r-process elements, are produced by rare stellar processes. Here we discuss the effects of these variations on the thermal evolution of an Earth-size planet, using a 1D parameterized convection model. Assuming Th and U abundances consistent with geochemical models of the Bulk Silicate Earth based on chondritic meteorites, we find that Earth had just enough radiogenic heating to maintain a persistent dynamo. According to this model, Earth-like planets of stars with higher abundances of heavy r-process elements, indicated by the relative abundance of europium in their spectra, are likely to have lacked a dynamo for a significant fraction of their lifetimes, with potentially negative consequences for hosting a biosphere. Because the qualitative outcomes of our 1D model are strongly dependent on the treatment of viscosity, further investigations using fully 3D convection models are desirable.

Callisto is thought to possess a subsurface ocean, which will dissipate energy due to obliquity tides. This dissipation should have damped any primordial inclination within 1 Gyr - and yet Callisto retains a present-day inclination. We argue that Callisto's inclination and eccentricity were both excited in the relatively recent past (~0.3 Gyr). This excitation occurred as Callisto migrated outwards according to the "resonance-locking" model and passed through a 2:1 mean-motion resonance with Ganymede. Ganymede's orbital elements were likewise excited by the same event. To explain the present-day orbital elements we deduce a solid-body tidal k2/Q~0.05 for Callisto and a significantly lower value for Ganymede., Comment: 12 pages, 9 figures, accepted in MNRAS

Orbital geophysical investigations of Enceladus are critical to understanding its energy balance. We identified key science questions for the geophysical exploration of Enceladus, answering which would support future assessment of Enceladus' astrobiological potential. Using a Bayesian framework, we explored how science requirements map to measurement requirements. We performed mission simulations to study the sensitivity of a single spacecraft and dual spacecraft configurations to static gravity and tidal Love numbers of Enceladus. We find that mapping Enceladus' gravity field, improving the accuracy of the physical libration amplitude, and measuring Enceladus' tidal response would provide critical constraints on the internal structure, and establish a framework for assessing Enceladus' long-term habitability. This kind of investigation could be carried out as part of a life search mission at little additional resource requirements., Comment: 21 pages, 6 figures. A paper submitted to PSJ, which is an extension of the white paper previously submitted to the decadal survey

We examine 141 N-body simulations of terrestrial planet late-stage accretion that use the Grand Tack scenario, coupling the collisional results with a hafnium-tungsten (Hf-W) isotopic evolution model. Accretion in the Grand Tack scenario results in faster planet formation than classical accretion models because of higher planetesimal surface density induced by a migrating Jupiter. Planetary embryos that grow rapidly experience radiogenic ingrowth of mantle $^{182}$W that is inconsistent with the measured terrestrial composition, unless much of the tungsten is removed by an impactor core that mixes thoroughly with the target mantle. For physically Earth-like surviving planets, we find that the fraction of equilibrating impactor core $k_\text{core} \geq 0.6$ is required to produce results agreeing with observed terrestrial tungsten anomalies (assuming equilibration with relatively large volumes of target mantle material; smaller equilibrating mantle volumes would require even larger $k_\text{core}$). This requirement of substantial core re-equilibration may be difficult to reconcile with fluid dynamical predictions and hydrocode simulations of mixing during large impacts, and hence this result does not favor the rapid planet building that results from Grand Tack accretion., Comment: 34 pages, 5 figures, published in EPSL

The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's geological activity[1,2]. Composed of molecular nitrogen, methane, and carbon monoxide ices[3], but dominated by N2-ice, this ice layer is organized into cells or polygons, typically ~10-40 km across, that resemble the surface manifestation of solid state convection[1,2]. Here we report, based on available rheological measurements[4], that solid layers of N2 ice approximately greater than 1 km thick should convect for estimated present-day heat flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-km-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of N2-ice viscosity implies that the SP ice layer convects in the so-called sluggish lid regime[5], a unique convective mode heretofore not definitively observed in the Solar System. Average surface horizontal velocities of a few cm/yr imply surface transport or renewal times of ~500,000 years, well under the 10 Myr upper limit crater retention age for Sputnik Planum[2]. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help explain the high albedos of some of them.

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%.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 $<1$ degree 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, Comment: Icarus, accepted for publication