Characterizing the Atmospheres of Rocky Exoplanets Using Entropy Budgets

Terrestrial planets are extremely common in the galaxy. To understand the potential atmospheres of these planets, it is important to identify general principles that govern their atmospheric circulations. For Earth, one well-known principle is that its atmosphere resembles a heat engine — the atmosphere absorbs heat near the surface, at a hot temperature, and emits heat to space in the upper troposphere, at a cold temperature, which allows it to perform work and balance dissipative processes such as friction. Previous studies quantified this balance using entropy budgets and showed that, for present-day Earth, the dominant sources of entropy are all associated with the hydrological cycle.

Here we seek to extend the heat engine principle from Earth towards other rocky planets. We explore both dry and moist atmospheres in an idealized general circulation model (GCM) in which we diagnose all sources and sinks of entropy.

First, we show that convection and turbulent heat diffusion are an important entropy source in dry atmospheres, and we develop a scaling that accounts for its effects. Our scaling allows us to predict the entropy generated by friction acting on the large-scale atmospheric circulation, analogous to how potential intensity theory predicts the strength of tropical cyclones on Earth. For example, in dry Earth-like atmospheres (e.g., present-day Mars or Snowball Earth), our scaling captures the strength of near-surface winds in the mid-latitude storm tracks. Similarly, for dry tidally locked exoplanets, our scaling yields a thermodynamic speed limit for the overturning circulation between day- and nightside, which is a prediction that can be tested using exoplanet observations.

Second, we address moisture. Moisture affects large-scale circulations both via the diffusion of water vapor as well as the frictional dissipation associated with raindrops. We explore the impact of both effects and use numerical simulations to explore the difference between dry and moist atmospheric circulations across a wide range of climates.