Cloudy with a Chance of Spin Evolution: Transient Habitability on early Venus and Mars

Clouds can either warm or cool a planet depending on their water content and the altitude at which they form. Reflective clouds have been proposed as a way for early Venus to remain habitable for billions of years, while high-altitude water ice clouds may have warmed early Mars. However, cloud cover also depends on a planet’s spin (rotation rate and obliquity). In this talk, I will use Venus and Mars to explore how spin and climate evolve together on the edges of the habitable zone. On Venus, the Sun’s gravity deforms the solid planet, raising a tide that competes with an atmospheric thermal tide driven by uneven solar heating. The torques associated with these tides drive spin evolution. Because the strength of the atmospheric thermal tide depends strongly on climate state, the planet’s rotation and climate influence one another. Using a coupled spin–climate model, I find that the strong thermal tide associated with a habitable Venus accelerates its rotation, weakens reflective cloud cover, and ends habitable conditions within approximately 300 million years. On an arid early Mars, the temperature of polar ice reservoirs controls how much water enters the atmosphere. As the planet’s obliquity increases, warmer polar summers drive transitions among three climate states: dry and nearly cloud-free, warm with high-altitude clouds, and cold with low-altitude clouds. Abrupt transitions may briefly permit liquid water at low latitudes, potentially helping to explain ancient valley networks. Together, these cases show how evolving planetary spins can alter cloud feedbacks and reshape the edges of the habitable zone.