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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.

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥ 0.1% M ⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10-8 bar and a vapor pressure at the surface of 10-4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H2 on timescales of 104 to 105 years.

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]

The surface of Ariel, an icy satellite orbiting Uranus, shows extensional tectonic features suggesting an episode of endogenic heating in the satellite's past. Using topography derived from stereo-photoclinometry, we identified flexural uplift at a rift zone suggesting elastic thickness values in the range 3.8-4.4 km. We estimate the temperature at the base of the lithosphere to be in the range 99 to 146 K, depending on the strain rate assumed, with corresponding heat fluxes of 28-92 mW/m2. Neither tidal heating, assuming Ariel's current eccentricity, nor radiogenic heat production from the silicate core are enough to cause the inferred heat fluxes. None of three proposed ancient mean-motion resonances produce equilibrium tidal heating values in excess of 4.3 mW/m2. Thus, the origin of the inferred high heat fluxes is currently mysterious.

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 investigate the problem of satellite survival during a hypothetical late heavy bombardment in the outer solar system, as predicted by the Nice Model (Tsiganis, Gomes, Morbidelli, & Levison 2005, Nature 435). Using a Monte-Carlo approach we calculate, for satellites of Jupiter, Saturn, and Uranus, the probability of experiencing a catastrophic collision during the LHB. We find that Mimas, Enceladus, Tethys, and Miranda experience at least one catastrophic impact in every simulation. Because re-accretion is expected to be rapid, these bodies will have emerged as scrambled mixtures of rock and ice. Tidal heating may have subsequently modified the latter three, but in the nominal LHB model Mimas should be a largely undifferentiated, homo geneous body. A differentiated Mimas would imply either that this body formed late, or that the Nice model requires significant modification.

We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in solar system history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant ( 0.7) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature 240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as NH3. In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton.

We investigate viscoelastic impact basin relaxation on Pluto for a variety of thermal evolution scenarios encompassing both convective and conductive ice shells. Basins smaller than 200 km in diameter do not relax appreciably, while relaxation fractions can be up to ∼60% for large impact basins. The main control on basin relaxation is the amount of radiogenic heat produced in the rocky core; our results are insensitive to the formation time of the basin, the ice reference viscosity adopted, and the presence/absence of a subsurface ocean. Other volatiles, such as CO2 or NH3, if present in the ice shell in sufficient quantities could increase the predicted relaxation fraction of basins. Relaxation causes extensional stresses interior to the basin; the orientation of the resulting tectonic features is controlled by the effective elastic thickness beneath the basin. Future observations of the relaxation states and tectonics of impact basins are therefore likely to provide a key constraint on Pluto’s thermal history.

The origin of the Moon's large-scale topography is important for understanding lunar geology, lunar orbital evolution, and the Moon's orientation in the sky.  Previous hypotheses for its origin have included late accretion events, large impacts, tidal effects, and convection processes.  However, testing these hypotheses and quantifying the Moon's topography is complicated by the large basins that have formed since the crust crystallized.  Here we estimate the low-order lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon.  The remainder of the degree-2 topography is consistent with a frozen tidal-rotational bulge that formed later, at a semi-major axis of »32 Earth radii.  The probability of the degree-2 shape having these two separate tidal characteristics by chance is less than 1%.  We also infer that internal density contrasts eventually reoriented the Moon's polar axis 36 ± 4°, to the present configuration we observe today.  Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon's low-order shape, gravity, and history of polar wander.

Tidal heating plays a significant role in the evolution of many satellites in the outer Solar System; however, it is unclear whether tidal dissipation in a global liquid ocean can represent a significant additional heat source. Tyler (Tyler, R.H. [2008]. Nature 456, 770-772; Tyler, R.H. [2009]. Geophys. Res. Lett. 36, doi:10.1029/2009GL038300) suggested that obliquity tides could drive large-scale flow in the oceans of Europa and Enceladus, leading to significant heating. A critical unknown in this previous work is what the tidal quality factor, Q, of such an ocean should be. The corresponding tidal dissipation spans orders of magnitude depending on the value of Q assumed. To address this issue we adopt an approach employed in terrestrial ocean modeling, where a significant portion of tidal dissipation arises due to bottom drag, with the drag coefficient O (0.001) being relatively well-established. From numerical solutions to the shallow-water equations including nonlinear bottom drag, we obtain scalings for the equivalent value of Q as a function of this drag coefficient. In addition, we provide new scaling relations appropriate for the inclusion of ocean tidal heating in thermal-orbital evolution models. Our approach is appropriate for situations in which the ocean bottom topography is much smaller than the ocean thickness. Using these novel scalings, we calculate the ocean contribution to the overall thermal energy budgets for many of the outer Solar System satellites. Although uncertainties such as ocean thickness and satellite obliquity remain, we find that for most satellites it is unlikely that ocean tidal dissipation is important when compared to either radiogenic or solid-body tidal heating. Of known satellites, Triton is the most likely icy satellite to have ocean tidal heating play a role in its present day thermal budget and long-term thermal evolution. © 2013 Elsevier Inc.

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]