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The solidification of the Lunar Magma Ocean (LMO) and formation of impact basins are important events that took place on the early Moon. The relative timing of these events, however, is poorly constrained. The aim of this study is to constrain the formation ages of old impact basins based on inferences of their thermal state. Most proposed basins formed before Pre-Nectarian (PN) 5 stage do not exhibit clear concentric features in either topography or gravity, suggesting substantial viscous lateral flow in the crust. Recent geodetic measurements reveal that the lunar crust is thinner than previously estimated, indicating that an extremely high crustal temperature is required for lateral flow to occur. In this study, we calculate lunar thermal evolution and viscoelastic deformation of basins and investigate the thermal state at the time of basin formation using recent crustal thickness models. We find that a Moho temperature >1300–1400K at the time of basin formation is required for substantial viscous relaxation of topography to occur; the implied elastic thickness at the time of loading is

We infer the elastic thickness of regions of the Moon by performing an admittance analysis on GRAIL gravity and LOLA topography data. Reliable results are generally confined to the lunar farside, away from large mare deposits. We find that elastic thickness varies between 13 km and 72 km, with a mean elastic thickness (T e) of 29 km. The highest reliable elastic thicknesses are found to the northwest of South Pole-Aitken (SPA) basin while the lowest elastic thicknesses are found in the equatorial farside and within SPA. Our findings suggest that much of the loading on the Moon must have taken place within the first 200 Myr after lunar magma ocean solidification, though some areas (T e > 40 km) may be recording events as late at 3.5 Ga. We find no correlation between elastic thickness and crater density, crustal thickness, or mean elevation in areas where reliable estimates of elastic thickness can be made, though we do note that regions with the highest crustal thickness and mean elevation are associated with lower elastic thickness values. We also demonstrate that smaller localization windows can lead to systematically lower elastic thickness estimates. [ABSTRACT FROM AUTHOR]

Some large impact basins on the Moon are covered with younger basalts, making it hard to determine their age by crater counting. Evans et al. (2018, https://doi.org/10.1029/2017JE005421) use Gravity Recovery and Interior Laboratory gravity data to identify buried craters larger than 90 km and construct a relative chronology for the impact basins. This relative chronology is broadly consistent with the degree to which the basins have relaxed: Older basins tend to be more relaxed. Pre‐Nectarian basins are likely saturated, so the roughly constant measured crater density does not require a brief formation interval. Some of the results of Evans et al. (2018) are at odds with other determinations; for example, they suggest that Imbrium could be older than Crisium, and Serenitatis as young as Imbrium. Plain Language Summary: To tell the age of an impact basin on the Moon, you count the number of craters. But some basins are flooded by later lavas, making crater counting difficult. Evans et al. (2018) identify buried craters by looking for small changes in the Moon's gravity. This allows them to decide how old each impact basin is. The older basins are also generally shallower. This is because the older basins formed when the Moon was hotter and the lunar rocks were able to flow and partly fill the basin in. Key Points: Evans et al. (2018) use gravity data to identify buried craters within large basinsThey reinterpret the relative ages of lunar basins using these observationsThe relative ages correlate quite well with the degree of basin relaxation [ABSTRACT FROM AUTHOR]

Highlights: [•] Impact-related mass loss cannot explain volatile-poor Io and Europa. [•] Radial mixing efficiently smooths out initial compositional gradients. [•] Tidal heating may help to explain volatile loss of inner moons. [ABSTRACT FROM AUTHOR]

Abstract: We couple the results of N-body simulations of late-stage accretion (O''Brien et al., 2006) to a hafnium–tungsten (Hf–W) isotopic evolution code to investigate the evolution of planetary bodies in the inner solar system. Simulations can simultaneously produce planets having Earth- and Mars-like masses and Hf–W systematics by assuming that the tungsten partition coefficient decreases with increasing semi-major axis (e.g. due to increasing oxidation). Simulations assuming that Jupiter and Saturn occupy circular orbits are more successful at reproducing the Hf–W systematics than those assuming present-day Jupiter and Saturn orbits. To generate Earth-like tungsten anomalies, 30–80% of each impactor core is required to re-equilibrate with the target mantle. Some model outcomes yield a target and final impactor having similar (Earth- and Moon-like) tungsten anomalies. However, in no case can the inferred lunar Hf/W ratio be simultaneously matched. This result suggests that the Moon isotopically equilibrated with the Earth''s mantle in the aftermath of the giant impact (cf. Pahlevan and Stevenson, 2007). Alternatively, either the dynamical models which show the Moon being derived primarily from the impactor mantle, or the accretion timescales obtained by the N-body simulations, are incorrect. [Copyright &y& Elsevier]

Lunar rocks contain a record of an ancient magnetic field that seems to have persisted for more than 400 million years and which has been attributed to a lunar dynamo. Models of conventional dynamos driven by thermal or compositional convection have had difficulty reproducing the existence and apparently long duration of the lunar dynamo. Here we investigate an alternative mechanism of dynamo generation: continuous mechanical stirring arising from the differential motion, due to Earth-driven precession of the lunar spin axis, between the solid silicate mantle and the liquid core beneath. We show that the fluid motions and the power required to drive a dynamo operating continuously for more than one billion years and generating a magnetic field that had an intensity of more than one microtesla 4.2 billion years ago are readily obtained by mechanical stirring. The magnetic field is predicted to decrease with time and to shut off naturally when the Moon recedes far enough from Earth that the dissipated power is insufficient to drive a dynamo; in our nominal model, this occurred at about 48 Earth radii (2.7 billion years ago). Thus, lunar palaeomagnetic measurements may be able to constrain the poorly known early orbital evolution of the Moon. This mechanism may also be applicable to dynamos in other bodies, such as large asteroids. [ABSTRACT FROM AUTHOR]

Enceladus, a small icy satellite of Saturn, has active plumes jetting from localized fractures (‘tiger stripes’) within an area of high heat flux near the south pole. The plume characteristics and local high heat flux have been ascribed either to the presence of liquid water within a few tens of metres of the surface, or the decomposition of clathrates. Neither model addresses how delivery of internal heat to the near-surface is sustained. Here we show that the most likely explanation for the heat and vapour production is shear heating by tidally driven lateral (strike-slip) fault motion with displacement of ∼0.5 m over a tidal period. Vapour produced by this heating may escape as plumes through cracks reopened by the tidal stresses. The ice shell thickness needed to produce the observed heat flux is at least 5 km. The tidal displacements required imply a Love number of h2 > 0.01, suggesting that the ice shell is decoupled from the silicate interior by a subsurface ocean. We predict that the tiger-stripe regions with highest relative temperatures will be the lower-latitude branch of Damascus, Cairo around 60° W longitude and Alexandria around 150° W longitude. [ABSTRACT FROM AUTHOR]

Abstract: Using stereo generated topography of Dione, a moon of Saturn, we infer flexure across a prominent ridge on the leading hemisphere to estimate local elastic thickness and place constraints on Dione’s thermal evolution. Assuming topography is related to the flexing of a broken elastic plate, we estimate an effective elastic thickness of 3.5±1km. This local estimate is in good agreement with global values derived from long wavelength topography. Corresponding heat fluxes of 25–60mW/m2 are much greater than those expected for a body heated solely from radioactive decay. It would be possible to generate our inferred heat flux values via tidal heating at close to the current orbital eccentricity of Dione, but only if a subsurface ocean were present at the time of flexural deformation. [Copyright &y& Elsevier]