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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 isostasy Tidal heat produced in a convecting aesthenosphere can reduce spatial variation in tidal heating, but requires prohibitively low viscosity Io’s topography is consistent with expected tidal heating spatial variations if thermal expansion drives crustal density variations

One of Venus' most enigmatic landforms is Baltis Vallis, the longest channel on the surface (∼7,000 km long). We identify a possible mid‐channel island that implies a south to north flow direction during formation. However, since the flow direction of Baltis Vallis is otherwise not well constrained, we analyze topographic conformity in both flow directions. In either case, topography appears to be altered across most analyzed wavelengths after the formation of Baltis Vallis. Fourier analysis shows two ranges of prominent wavelengths, 225 ± 15 km and ∼3,500 ± 1,200 km. The shorter wavelengths correspond to deformation belts that cross Venus' low plains. The longest is plausibly associated with the dynamic uplift wavelength of the crust by mantle plumes, but is less robustly detected. Higher resolution observations provided by the VERITAS and EnVision missions can help resolve the source location of Baltis Vallis and constrain if the longest wavelength postdated the canale's formation. Plain Language Summary Venus' surface is covered in a plethora of strange landforms, at least from the perspective of Earth. One of the longest is an about 7,000 km channel named Baltis Vallis, comparable to the Amazon and Nile rivers, but instead likely formed by volcanic processes. Baltis Vallis serves as a unique opportunity on Venus due to its length. The channel recorded surface altering processes in its topography, but we first check if the channel retained topographic information from when it initially formed. Our test shows that the topography has been altered by later processes and those processes should dominate the signal in analysis of the current topography. That analysis shows 2 length‐scales are overrepresented in the topography. The shorter length‐scale correspond to thin mountain range‐like features that cross Venus' low plains. The longest wavelength is plausibly associated with uplift of the crust by mantle plumes and this value will be useful when creating models of Venus' interior. Key Points A possible mid‐channel island in the longest channel on Venus implies a south to north flow direction We show that the topography and morphology of this channel was modified along most of its length Fourier analysis of the channel's topography shows a group of prominent wavelengths at ∼210–240 km, that we link to deformation belts

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 moons Combining librations with gravity measurements can yield comprehensive constraints on the interiors of the Uranian moons Thick oceans are easier to detect but finding thin oceans may require libration amplitudes to be measured to within better than 10 m

The isotopic dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites indicates that meteorite parent bodies derive from two genetically distinct reservoirs, which presumably were located inside (NC) and outside (CC) the orbit of Jupiter and remained isolated from each other for the first few million years of the solar system. Here we review the discovery of the NC–CC dichotomy and its implications for understanding the early history of the solar system, including the formation of Jupiter, the dynamics of terrestrial planet formation, and the origin and nature of Earth’s building blocks. The isotopic difference between the NC and CC reservoirs is probably inherited from the solar system’s parental molecular cloud and has been maintained through the rapid formation of Jupiter that prevented significant exchange of material from inside (NC) and outside (CC) its orbit. The growth and/or migration of Jupiter resulted in inward scattering of CC bodies, which accounts for the co-occurrence of NC and CC bodies in the present-day asteroid belt and the delivery of presumably volatile-rich CC bodies to the growing terrestrial planets. Earth’s primitive mantle, at least for siderophile elements like Mo, has a mixed NC–CC composition, indicating that Earth accreted CC bodies during the final stages of its growth, perhaps through the Moon-forming giant impactor. The late-stage accretion of CC bodies to Earth is sufficient to account for the entire budget of Earth’s water and highly volatile species.

The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

The conditions under which a dynamo can operate in the core of a small planetary body or asteroid are examined. Compositional convection driven by inner core growth is thermodynamically much more efficient than thermal convection at driving a dynamo, but whether asteroid cores crystallize in this fashion is currently uncertain. Inner core solidification will drive dynamo activity in cores larger than ≈50–150 km in radius. Dynamo activity requires core cooling rates exceeding ∼0.001–0.1 K/My if compositional convection occurs. In the absence of an inner core, cooling rates of ∼1–100 K/My or heating by 60Fe within 10–20 Myr of solar system formation are required to drive a dynamo. If inner core growth is important (as for the IVB iron meteorite parent body) then a dynamo should develop with magnetic paleointensities that depend on the core sulphur content. If 60Fe decay is dominant, the frequency of asteroid dynamo occurrence is predicted to decay with distance from the Sun.

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018 and fully deployed its seismometer by the end of February 2019. The mission aims to detect, characterize and locate seismic activity on Mars, and to further constrain the internal structure, composition and dynamics of the planet. Here, we present seismometer data recorded until 30 September 2019, which reveal that Mars is seismically active. We identify 174 marsquakes, comprising two distinct populations: 150 small-magnitude, high-frequency events with waves propagating at crustal depths and 24 low-frequency, subcrustal events of magnitude Mw 3–4 with waves propagating at various depths in the mantle. These marsquakes have spectral characteristics similar to the seismicity observed on the Earth and Moon. We determine that two of the largest detected marsquakes were located near the Cerberus Fossae fracture system. From the recorded seismicity, we constrain attenuation in the crust and mantle, and find indications of a potential low-S-wave-velocity layer in the upper mantle.Mars is seismically active: 24 subcrustal magnitude 3–4 marsquakes and 150 smaller events have been identified up to 30 September 2019, by an analysis of seismometer data from the InSight lander.

The flyby of Pluto and its moon Charon by the New Horizons spacecraft generated news coverage around the world. Now Stern et al. report the first scientific results from the high-speed encounter. The surface of Pluto is surprisingly diverse, with large regions of differing brightness and composition. There is ample evidence for ongoing rich geological processes that act to sculpt its surface. Charon's surface is similarly complex, with numerous relief structures and varied coloration. Pluto's atmosphere is extensive but less dense than expected, whereas Charon has no detectable atmosphere. Science , this issue p. 10.1126/science.aad1815 The first scientific results from New Horizon’s flyby of Pluto reveal a rich and diverse geology. The Pluto system was recently explored by NASA’s New Horizons spacecraft, making closest approach on 14 July 2015. Pluto’s surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto’s atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto’s diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto’s large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

We use limb profiles to quantify the long‐wavelength topography of the Saturnian satellites. The degree 2 shapes of Mimas, Enceladus, and Tethys are not consistent with hydrostatic equilibrium. We derive 2‐D topographic maps out to spherical harmonic degree 8. There is a good correlation with topography derived from stereo techniques. If uncompensated, topography at degree 3 and higher is large enough to be detectable during close spacecraft flybys. If not properly accounted for, this topography may bias estimates of a satellite's degree 2 gravity coefficients (which are used to determine the moment of inertia). We also derive a one‐dimensional variance spectrum (a measure of how roughness varies with wavelength) for each body. The short‐wavelength spectral slope is −2 to −2.5, similar to silicate bodies. However, unlike the terrestrial planets, each satellite spectrum shows a reduction in slope at longer wavelengths. If this break in slope is due to a transition from flexural to isostatic support, the globally averaged elastic thickness Te of each satellite may be derived. We obtain Te values of ≥5 km, 1.5–5 km, ≈5 km, and ≥5 km for Tethys, Dione, Rhea, and Iapetus, respectively. For Europa, we obtain Te ≈ 1.5 km. These estimates are generally consistent with estimates made using other techniques. For Enceladus, intermediate wavelengths imply Te ≥ 0.5 km, but the variance spectrum at wavelengths greater than 150 km is probably influenced by long‐wavelength processes such as convection or shell thickness variations. Impact cratering may also play a role in determining the variance spectra of some bodies. Key Points Satellite topographic variance spectra show a break in slope This break in slope can be used to derive the elastic thickness Uncompensated long‐wavelength topography may bias gravity estimates

A strong inverse correlation between gravity and topography leads to the conclusion that Saturn’s largest moon, Titan, must have a rigid ice shell with an elastic thickness exceeding 40 kilometres. Titan's rigid ice shell Titan, Saturn's largest moon, may have a stronger ice shell than previously thought. Several lines of evidence suggest that Titan has a global subsurface ocean beneath an outer ice shell 50–200 km thick, with a rigid portion that is thin and weak. Here Hemingway et al . report a strong inverse correlation between gravity and topography at long wavelengths that are not dominated by tides and rotation. This finding is not compatible with a geologically active, low-rigidity ice shell, suggesting that Titan's ice shell must be substantially rigid with an elastic thickness of greater than 40 km. Several lines of evidence suggest that Saturn’s largest moon, Titan, has a global subsurface ocean beneath an outer ice shell 50 to 200 kilometres thick 1 , 2 , 3 , 4 . If convection 5 , 6 is occurring, the rigid portion of the shell is expected to be thin; similarly, a weak, isostatically compensated shell has been proposed 7 , 8 to explain the observed topography. Here we report a strong inverse correlation between gravity 3 and topography 9 at long wavelengths that are not dominated by tides and rotation. We argue that negative gravity anomalies (mass deficits) produced by crustal thickening at the base of the ice shell overwhelm positive gravity anomalies (mass excesses) produced by the small surface topography, giving rise to this inverse correlation. We show that this situation requires a substantially rigid ice shell with an elastic thickness exceeding 40 kilometres, and hundreds of metres of surface erosion and deposition, consistent with recent estimates from local features 10 , 11 . Our results are therefore not compatible with a geologically active, low-rigidity ice shell. After extrapolating to wavelengths that are controlled by tides and rotation, we suggest that Titan’s moment of inertia may be even higher (that is, Titan may be even less centrally condensed) than is currently thought 12 .