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
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 18 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk ...composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 15 of 18 KBOs within their 2σ errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early 26Al would have removed too much porosity in small (∼ 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Ocean worlds in the outer solar system Nimmo, F.; Pappalardo, R. T.
Journal of geophysical research. Planets,
August 2016, Volume:
121, Issue:
8
Journal Article
Peer reviewed
Open access
Many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells. This article first reviews how such oceans are detected. We then discuss how they are maintained, ...when they formed, and what the oceans' likely characteristics are. We focus in particular on Europa, Ganymede, Callisto, Titan, and Enceladus, bodies for which there is direct evidence of subsurface oceans. We also consider candidate ocean worlds such as Pluto and Triton.
Key Points
Ocean‐bearing worlds are common in the outer solar system
We review how these oceans are detected and maintained
We also summarize the likely characteristics of these oceans
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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 controlled 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 m 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.
•Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments attributed to evaporation of molten planetesimals.•We evaluate the mechanisms of planetesimal evaporation and the conditions that controled attendant isotope fractionations.•A steady-state rock vapor forms within minutes to hours from a balance between rates of magma evaporation and atmospheric escape.•Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation.•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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
•We argue that Triton is heated by obliquity tidal dissipation in a subsurface ocean.•This heating is sufficient to cause convective yielding and the observed young surface age.•Pluto does not ...experience significant tidal heating and is predicted to show no signs of recent resurfacing.
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 300km thick ice shell, the estimated ocean heat production rate (≈0.3TW) 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 ≈240K 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The measured inward motion of Phobos provides a constraint on the tidal dissipation factor, Q, within Mars. We model viscoelastic dissipation inside a convective Mars using a modified Burgers model ...based on laboratory experiments on anhydrous, melt‐free olivine. The model tidal Q is highly sensitive to the mantle potential temperature and grain size assumed but relatively insensitive to the bulk density and rigidity structure. Q thus provides a tight constraint on the Martian interior temperature. By fitting the observed tidal Q and tidal Love number (k2) values and requiring present‐day melt generation, we estimate that for a grain size of 1 cm the current mantle potential temperature is 1625±75 K, similar to that of the Earth. This estimate is consistent with recent petrologically derived determinations of mantle potential temperature but lower than estimates in some thermal evolution models. The presence of water in the Martian mantle would reduce our estimated temperature. Our preferred mantle grain size of ≈1 cm is somewhat larger than that of the Earth's upper mantle. The predicted mantle seismic Q is about 130 and is almost independent of depth. The Martian lithosphere represents a high seismic velocity lid, which should be readily detectable with future seismological observations.
Key Points
Tidal dissipation on Mars constrains its mantle temperature
We find the Martian mantle temperature is the same as the Earth's
The Martian mantle grain size is larger than Earth's
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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