The ice-albedo feedback on rapidly-rotating terrestrial planets in the habitable zone can lead to abrupt transitions (bifurcations) between a warm and a snowball (ice-covered) state, bistability between these states, and hysteresis in planetary climate. This is important for planetary habitability because snowball events may trigger rises in the complexity of life, but could also endanger complex life that already exists. Recent work has shown that planets tidally locked in synchronous rotation states will transition smoothly into the snowball state rather than experiencing bifurcations. Here we investigate the structure of snowball bifurcations on planets that are tidally influenced, but not synchronously rotating, so that they experience long solar days. We use PlaSIM, an intermediatecomplexity global climate model, with a thermodynamic mixed layer ocean and the Sun's spectrum. We find that the amount of hysteresis (range in stellar flux for which there is bistability in climate) is significantly reduced for solar days with lengths of tens of Earth days, and disappears for solar days of hundreds of Earth days. These results suggest that tidally influenced planets orbiting M and K-stars that are not synchronously rotating could have much less hysteresis associated with the snowball bifurcations than they would if they were rapidly rotating. This implies that the amount of time it takes them to escape a snowball state via CO 2 outgassing would be greatly reduced, as would the period of cycling between the warm and snowball state if they have a low CO 2 outgassing rate.

Fluid recharge and discharge between the deep ocean and the porous upper layer of off‐axis oceanic crust tends to concentrate in small volumes of rock, such as seamounts and fractures, that are unimpeded by low‐permeability sediments. Basement structure, sediment burial, heat flow, and other regional characteristics of off‐axis hydrothermal systems appear to produce considerable diversity of circulation behaviors. Circulation of seawater and seawater‐derived fluids controls the extent of fluid‐rock interaction, resulting in significant geochemical impacts. However, the primary regional characteristics that control how seawater is distributed within upper oceanic crust are still poorly understood. In this paper we present the details of the two‐dimensional (2‐D) BASALT (Basement Activity Simulated At Low Temperatures) numerical model of heat and fluid transport in an off‐axis hydrothermal system. This model is designed to simulate a wide range of conditions in order to explore the dominant controls on circulation. We validate the BASALT model's ability to reproduce observations by configuring it to represent a thoroughly studied transect of the Juan de Fuca Ridge eastern flank. The results demonstrate that including series of narrow, ridge‐parallel fractures as subgrid features produces a realistic circulation scenario at the validation site. In future projects, a full reactive transport version of the validated BASALT model will be used to explore geochemical fluxes in a variety of off‐axis hydrothermal environments.