Internal-current Lorentz-force Heating of Astrophysical Objects

Two forms of ohmic heating of astrophysical secondaries have received particular attention: unipolar-generator heating with currents running between the primary and secondary, and magnetic induction heating due to the primary’s time-varying field. Neither appears to cause significant dissipation in the contemporary solar system. But these discussions have overlooked heating derived from the spatial variation of the primary’s field across the interior of the secondary. This leads to Lorentz-force-driven currents around paths entirely internal to the secondary, with resulting ohmic heating. We examine three ways to drive such currents, by the cross product of (1) the secondary’s azimuthal orbital velocity with the nonaxially symmetric field of the primary, (2) the radial velocity (due to nonzero eccentricity) of the secondary with the primary’s field, or (3) the out-of-plane velocity (due to nonzero inclination) with the primary’s field. The first of these operates even for a spin-locked secondary whose orbit has zero eccentricity, in strong contrast to tidal dissipation. We show that Jupiter’s moon Io today could dissipate about 600 GW (more than likely current radiogenic heating) in the outer 100 m of its metallic core by this mechanism. Had Io ever been at 3 Jovian radii instead of its current 5.9, it could have been dissipating 15,000 GW. Ohmic dissipation provides a mechanism that could operate in any solar system to drive inward migration of secondaries that then necessarily comes to a halt upon reaching a sufficiently close distance to the primary.

Desiccation of the TRAPPIST-1 Planets During Their Magma Ocean Phase

<pre class="moz-quote-pre">The potentially habitable planets in the TRAPPIST-1 system (e,f,g) may have experienced a prolonged magma ocean phase during which volatiles were partitioned between the molten interior and the atmosphere. The duration of the magma ocean phase is determined by 1) the incident stellar radiation, 2) atmospheric heating due to the greenhouse effect, 3) water photolysis and hydrogen escape, 4) tidal heating, 5) radiogenic heating, and 6) the magma ocean’s initial temperature. We simulate these phenomena simultaneously with the VPLanet software package, including a new module called MagmOc that treats the thermal and geochemical evolution (water, O2, and CO2) of the magma ocean. We find the TRAPPIST-1 planets’ evolution depends on initial water content and distance from the host star. In a “dry” scenario (initial water content < 5TO, for planet g), the atmosphere after magma ocean solidification is desiccated and devoid of abiotically generated O2. In an “intermediate” scenario (initial water content between 5 and 50TO), the post magma ocean atmosphere still contains water. XUV photolysis of this water leads to abiotic O2 build-up. For “extremely wet” cases (initial water content > 50 TO) or extreme internal heating, the magma ocean lifetime can be extended and quench oxygen build up. The currently inferred high water content of the planets favors the extremely wet scenario for TRAPPIST-1 g and f, i.e. they likely ended their magma ocean state with large amounts of water vapor in their atmospheres but potentially avoid the build-up of large amounts of oxygen. TRAPPIST-1 e, on the other hand, could have experienced the intermediate scenario and is therefore even less likely to possess large amounts of abiotically created atmospheric O2.</pre>

Effect of grid resolution on tectonic regimes in global-scale convection models

<p>A key ingredient to reproduce plate-tectonics in numerical models is a viscoplastic rheology. Strongly temperature-dependent rheology generates a rigid lid at the surface, whereas plastic rheology allows for the formation of plate boundaries. The yield stress limiter  controls the strength of the lithosphere.</p><p>Depending on the value used for  different tectonics regimes can be observed: (i) dripping behaviour (low , (ii) plate-like behaviour (intermediate-low ), (iii) Episodic behaviour (intermediate-high ) and (iv) Stagnant lid behaviour (high ).</p><p>Each lid behaviour can be distinguished by comparing the evolution profile of several parameters: temperature, viscosity, surface Nusselt number and mobility (Tackley, 2000a.).</p><p>Despite the great importance of physical parameters, the outcome of geodynamical models is also affected by the grid resolution as it has been shown that the critical that separates each lid behavior is resolution dependent (Tosi et al., 2015).</p><p>Here we use the code StagYY (Tackley, 2008) in a 2D spherical annulus geometry (Hernlund & Tackley, 2008) to determine the resolution-dependent tectonic regime in a global-scale convection setting. We tested 12 grid resolutions (ranging from 128x32 to 1024x128 nodal points) and 9 different  (ranging from 10 to 90 MPa), keeping all the remaining physical parameters unchanged.</p><p>For these simplified models we assume isothermal free slip boundaries, constant radiogenic heating, no melting, endothermic (410) and exothermic (660) phase transitions. Each simulation was run for 15 Gyrs with a Rayleigh number of ≈8*10^7 to make sure that steady-state conditions were reached.</p><p>Our resolution tests show that the observed tectonic regime is affected by grid resolution as this parameter controls how well the lithosphere is resolved. Low radial resolutions favour weak lid regimes (dripping and plate-like) as the lithosphere is defined by few thick cells, that propagate basal stress to shallower depths. On the other hand low azimuthal resolutions favour strong lid regimes (episodic and stagnant) since plate boundaries remain unresolved. In conclusion, only at high grid resolutions (512x128 and higher) the numerical influence on the observed tectonic regime is low.</p>

Stability of the subsurface ocean of Pluto

<p>We explore the long-term evolution of Pluto’s subsurface ocean in the absence of an insulating clathrate hydrate layer. Numerical simulations of the thermal history of the interior are performed using a 1D model assuming Pluto was initially differentiated into an outer hydrosphere (H<sub>2</sub>O shell) and an inner rocky core. We consider two endmember initial conditions: the hydrosphere was either entirely molten or frozen. We also consider different radiogenic heating rates, core sizes, ice reference viscosities, and ammonia concentrations. Our results indicate that the present-day Pluto can possess a subsurface ocean if the ice shell is purely conductive or only weakly convective. Our results also indicate that the initial state affects only little on the evolution scenario. These results strengthen previous conclusions obtained based on thermal evolution studies with limited calculation conditions. The thickness of the present-day ocean can be up to ~130 km, depending on the radiogenic heating rate and ice reference viscosity. The reference viscosity of ice required to maintain an ocean until today for the case of a CI chondritic core is approximately an order of magnitude higher than that for the case of an ordinary chondritic core. We also find that a thick subsurface ocean can be maintained until relatively recently for a dense small core case, which allows the formation of high-pressure ice at the seafloor. An inclusion of ammonia in the ocean increases the possibility of the current presence of a subsurface ocean even in the case of 1 wt% NH<sub>3</sub> at the initial.</p>

Ba1.2-xCsxM1.2-x/2Ti6.8+x/2O16 (M = Ni, Zn) hollandites for the immobilisation of radiocaesium

ABSTRACTImproved budgeting of heat loads arising from radiogenic heating in high level wastes (HLW) could allow enhanced usage of geological disposal facility space. Separation of high heat generating nuclides from HLW, such as Cs, would simplify management of heat loads. A potential host matrix for Cs-disposal is hollandite. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the structure; these ions may include Ni and Zn. Two series of hollandites, Ni-substituted and Zn-substituted, were synthesised via an alkoxide-nitrate route and consolidated by cold uniaxial pressing and sintering or by hot isostatic pressing. Characterisation of the resultant material by X-ray diffraction and scanning electron microscopy found that hollandite was formed for all levels of substitution. Materials produced via HIP were found to be denser indicating lower Cs loss. HIPed Ni hollandites were found to contain fewer secondary phases and it was concluded that they were the most suitable candidates

Quantification and Modeling of the Interactions of Gamma Radiation with Concrete from Bulk-Scale Observations

The exposure of concrete to gamma radiation gives rise to a set of physical and chemical processes over multiple length scales, from molecular to bulk. The literature includes a number of bulk-scale studies which report the radiogenic heating of concrete and the loss of water (unbound, physically-bound, and/or chemically-bound) due to irradiation. This paper mechanistically quantifies observations by these studies, and presents a continuum framework to model the effects of gamma photons on concrete. A basis is presented for comparing otherwise disparate results in the literature for radiolysis rates. The Stefan–Boltzmann Law, adapted to include a gamma heat source term, reasonably describes radiogenic heating in concrete specimens. In multiple studies, the primary mechanism for dehydration is the loss of liquid water in the pore network of the cement product, rather than of water which is physically or chemically bound in a solid state.

Cratonic basins and the Wilson cycle: a perspective from the Parnaíba Basin, Brazil

Cratonic basins appear to occupy a specific place in the Wilson cycle, initiating after continental collision and supercontinent development, but before rifting and continental break-up. They do not result directly from the horizontal plate motions characteristic of the Wilson cycle, but from localized, long-lived subsidence. Covering c. 10% of the Earth's continental crust, most of the preserved cratonic basins developed in the Early Paleozoic after the formation of Gondwana and Laurentia. Recent investigation of the Parnaíba cratonic basin of Brazil has shown that this basin, and potentially cratonic basins in general, are characterized by six features: (1) formation on thickened lithosphere (>150 km); (2) a pronounced basal unconformity; (3) a sub-circular outline and large area of 0.5 × 105 to 2 × 106 km2; (4) long-lived (100–300 myr) quasi-exponential tectonic subsidence of shallow marine and terrestrial sediments; (5) no major extensional strain features, such as rifts, crustal or lithospheric thinning or Moho elevation; and (6) dense, high velocity and conductive lower crust and upper mantle. These characteristics indicate basin initiation and development by purely vertical subsidence of the lithosphere, either thermally or mechanically driven. Thermal subsidence may be related to orogenic thickening, radiogenic heating and erosion associated with supercontinent assembly, whereas mechanical subsidence may be a result of the emplacement in the lower crust or upper mantle of a dense igneous body related to plume activity during the lifetime of a supercontinent.