[1] Numerical simulations show that depth-dependent viscosity can increase the wavelength of mantle convection. The physical mechanism behind this phenomenon and its robustness with respect to model parameters remain to be fully elucidated. Toward this end, we develop theoretical heat flow scalings for a convecting fluid layer with depthdependent viscosity. Bottom and internally heated end-members are considered. For the former, the viscosity structure consists of a high-viscosity central region bounded from above and below by horizontal low-viscosity channels. For internally heated cases, only a surface low-viscosity channel is present. Theoretical scalings derived from boundary layer theory show that depth-dependent rheology lowers the lateral dissipation associated with steady state convective rolls, allowing longer aspect ratio cells to form as the viscosity contrast between the channels and the central region is increased. The maximum cell aspect ratio is estimated from the condition that the pressure gradients that drive lateral flow in the channels do not become so large as to inhibit vertical flow into the channels. Scaling predictions compare favorably to results of numerical simulations for steady state cells. As the Rayleigh number driving convection is increased, small-scale boundary layer instabilities begin to form. This increases lateral dissipation within the channels and the preferred cell aspect ratio decreases as a result. Internally heated simulations show that a near-surface high-viscosity layer, an analog to tectonic plates, can suppress these small-scale instabilities. This allows a low-viscosity channel to maintain large aspect ratio cells for Rayleigh numbers approaching that of the present-day Earth.

S U M M A R YMixed heated 3-D mantle convection simulations with a low-viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere-asthenosphere region is a Poiseuille-Couette flow. The Poiseuille/Couette velocity magnitude ratio, D/U , allows us to characterize solid-state flow in the asthenosphere and to predict the regime transition. The transition from dominantly pressure-driven Poiseuille flow at shorter wavelengths to dominantly shear-driven Couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion. In the Poiseuille regime significant resistance is provided by platebending, whereas in the Couette regime most resistance is due to vertical shear in the bulk mantle. The Couette case corresponds to classical scaling ideas for mantle convection whereas the Poiseuille case, with asthenospheric velocities exceeding surface velocities, is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the Earth. Our simulations show that both modes can exist for the same level of convective vigour (i.e. Rayleigh number) but at different convective wavelengths. Additional simulations with temperature-and yield-stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins. Our simulations establish a connection between the strength of plate margins, solid-state flow in the asthenosphere and the wavelength of mantle convection. This connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned.

Plate tectonics is a particular mode of tectonic activity that characterizes the present-day Earth. It is directly linked to not only tectonic deformation but also magmatic/volcanic activity and all aspects of the rock cycle. Other terrestrial planets in our Solar System do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation. This indicates the existence of tectonic modes different from plate tectonics. This article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date. A categorization of tectonic modes relates to the issue of when plate tectonics initiated on Earth as it provides insights into possible pre-plate tectonic behaviour. The final focus of this contribution relates to transitions between tectonic modes. Different transition scenarios are discussed. One follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet. The other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions. The latter scenario allows for the existence of multiple stable tectonic modes under the same physical/chemical conditions. The different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents. Under the classic regime transition view, the tectonic mode of a planet is a state variable (akin to temperature). Under the multiple stable modes view, the tectonic mode of a planet is a process variable. That is, something that flows through the system (akin to heat). The different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on Earth and why does Earth have plate tectonics. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

The current goals of the astrobiology community are focused on developing a framework for the detection of biosignatures, or evidence thereof, on objects inside and outside of our solar system. A fundamental aspect of understanding the limits of habitable environments (surface liquid water) and detectable signatures thereof is the study of where the boundaries of such environments can occur. Such studies provide the basis for understanding how a once inhabitable planet might come to be uninhabitable. The archetype of such a planet is arguably Earth's sibling planet, Venus. Given the need to define the conditions that can rule out bio‐related signatures of exoplanets, Venus provides a unique opportunity to explore the processes that led to a completely uninhabitable environment by our current definition of the term. Here we review the current state of knowledge regarding Venus, particularly in the context of remote‐sensing techniques that are being or will be employed in the search for and characterization of exoplanets. We discuss candidate Venus analogs identified by the Kepler and TESS exoplanet missions and provide an update to exoplanet demographics that can be placed in the potential runaway greenhouse regime where Venus analogs are thought to reside. We list several major outstanding questions regarding the Venus environment and the relevance of those questions to understanding the atmospheres and interior structure of exoplanets. Finally, we outline the path toward a deeper analysis of our sibling planet and the synergy to exoplanetary science.

SUMMARY Boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low‐viscosity asthenosphere. The theory assumes a plate tectonic like mode of mantle convection with flow driven by an active upper boundary layer. The theory predicts that the confinement of horizontal mantle flow within a low‐viscosity, sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel (even if the absolute viscosity of the sublithosphere mantle does not change between the two cases). The theory further predicts increasing shear stress with decreasing low‐viscosity channel thickness. If the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere, then the plate normal stress is predicted to also increase with decreasing channel thickness. We use 3‐D spherical shell simulations of mantle convection with temperature‐, depth‐ and stress dependent rheology to test scaling trends. The simulations and theoretical scalings demonstrate that a low‐viscosity layer (asthenosphere) can amplify convective stresses. If the level of convective stress plays a role in maintaining and/or reactivating plate boundaries, this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics. The numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active‐lid mode of convection to a stagnant lid state. Collectively, the simulations and theoretical scalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect, associated with a low absolute asthenosphere viscosity, but, instead, from a mantle flow channelization effect, associated with a high viscosity contrast from the asthenosphere to the mantle below.