The long-term carbon cycle is vital for maintaining liquid water oceans on rocky planets due to the negative climate feedbacks involved in silicate weathering. Plate tectonics plays a crucial role in driving the long-term carbon cycle because it is responsible for CO 2 degassing at ridges and arcs, the return of CO 2 to the mantle through subduction, and supplying fresh, weatherable rock to the surface via uplift and orogeny. However, the presence of plate tectonics itself may depend on climate according to recent geodynamical studies showing that cool surface temperatures are important for maintaining vigorous plate tectonics. Using a simple carbon cycle model, I show that the negative climate feedbacks inherent in the long-term carbon cycle are uninhibited by climate's effect on plate tectonics. Furthermore, initial atmospheric CO 2 conditions do not impact the final climate state reached when the carbon cycle comes to equilibrium, as long as liquid water is present and silicate weathering can occur. Thus an initially hot, CO 2 rich atmosphere does not prevent the development of a temperate climate and plate tectonics on a planet. However, globally supply-limited weathering does prevent the development of temperate climates on planets with small subaerial land areas and large total CO 2 budgets because supply-limited weathering lacks stabilizing climate feedbacks. Planets in the supply-limited regime may become inhospitable for life and could experience significant water loss. Supply-limited weathering is less likely on plate tectonic planets, because plate tectonics promotes high erosion rates and thus a greater supply of bedrock to the surface.Subject headings: astrobiology -planets and satellites: physical evolution -planets and satellites: terrestrial planets 1

Models of thermal evolution, crustal production, and CO cycling are used to constrain the prospects for habitability of rocky planets, with Earth-like size and composition, in the stagnant lid regime. Specifically, we determine the conditions under which such planets can maintain rates of CO degassing large enough to prevent global surface glaciation but small enough so as not to exceed the upper limit on weathering rates provided by the supply of fresh rock, a situation which would lead to runaway atmospheric CO accumulation and an inhospitably hot climate. The models show that stagnant lid planets with initial radiogenic heating rates of 100-250 TW, and with total CO budgets ranging from ∼10 to 1 times Earth's estimated CO budget, can maintain volcanic outgassing rates suitable for habitability for ≈1-5 Gyr; larger CO budgets result in uninhabitably hot climates, while smaller budgets result in global glaciation. High radiogenic heat production rates favor habitability by sustaining volcanism and CO outgassing longer. Thus, the results suggest that plate tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable climate. The model is necessarily highly simplified, as the uncertainties with exoplanet thermal evolution and outgassing are large. Nevertheless, the results provide some first-order guidance for future exoplanet missions, by predicting the age at which habitability becomes unlikely for a stagnant lid planet as a function of initial radiogenic heat budget. This prediction is powerful because both planet heat budget and age can potentially be constrained from stellar observations. Key Words: Exoplanets-Habitability-Stagnant lid tectonics-Carbon cycle-Volcanism. Astrobiology 18, 873-896.

Measurements of source‐side splitting in S waves from events within the Tonga slab reveal anisotropy in the upper and mid‐mantle beneath the slab. We observed splitting for events originating at both upper mantle and transition zone depths. Anisotropic fast directions (ϕ) are trench parallel or sub‐parallel for both upper mantle and transition zone events. Delay times (δt) decrease with depth for upper mantle events. The source of anisotropy for the upper mantle events is likely in the sub‐slab mantle, and is likely indicative of trench parallel flow due to slab rollback. The source of anisotropy for the deeper earthquakes is more difficult to constrain, but the pattern of splitting measurements argues for an uppermost lower mantle anisotropic source, and slab induced deformation may be responsible for this anisotropy as well. Additional constraints from mineral physics studies are necessary to interpret the mid‐mantle anisotropy signal in terms of geodynamical processes.

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[1] How plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical, viscoplastic computations. We present mainly internally heated convection results with temperature-dependent viscosity and explore parts of the Rayleigh number (Ra)-yield stress (s y ) phase space, as well as the effects of depth-dependent s y , bottom heating, and a lowviscosity asthenosphere. Convective planform and toroidal-poloidal velocity field ratio (TPR) are affected by near-surface viscosity variations, and TPR values are close to observed values for our most plate-like models. At the relatively low convective vigor that is accessible at present, most models favor spherical harmonic degree one convection, though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra. An asthenospheric viscosity reduction improves plate-like nature, as expected. For our incompressible computations, pure bottom heating produces strong plumes that tend to destroy plates at the surface. This implies that significant internal heating may be required, both to reduce the role of active upwellings and to form a low-viscosity zone beneath the upper boundary layer.Components: 8735 words, 12 figures, 1 table.