Multiple moist climate equilibrium states on arid rocky M-dwarf planets: A last-saturation tracer analysis

Abstract

Terrestrial-type exoplanets orbiting nearby red dwarf stars (M dwarfs) are the first potentially habitable exoplanets suitable for atmospheric characterization in the near future. Understanding the stability of water in cold-trap regions on such planets is critical because it directly impacts transmission spectroscopy observations, the global energy budget, and long-term surface water evolution. Here we diagnose the humidity distribution in idealized general circulation model (GCM) simulations of terrestrial-type exoplanets. We use the `tracer of last saturation' technique to study the saturation statistics of air parcels. We find that on synchronously rotating planets, the water vapor abundance in the nightside upper troposphere depends weakly on planetary rotation, while more water vapor builds up in the nightside lower troposphere on fast-rotating planets. We then discuss how last-saturation statistics can elucidate the multiple moist climate equilibrium states on synchronously and asynchronously rotating arid planets. We show that the multiple moist climate states arise from the cold-trapping competition between the substellar upper atmosphere and cold surface regions. We find that fast synchronously rotating planets tend to trap surface water on the nightside as a result of their weak atmospheric and strong surface cold traps compared to the slow rotating case. These results elucidate the nature of the water cycle on arid rocky exoplanets and will aid interpretation of atmospheric observations in the future.

28 pages, 10 figures, 2 tables, 1 movie, accepted for publication in The Planetary Science Journal

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Cited by in corpus (6)

• Terminator Habitability: the Case for Limited Water Availability on M-dwarf Planets
• Prospects for water vapor detection in the atmospheres of temperate and arid rocky exoplanets around M-dwarf stars
• Lorenz Energy Cycle: Another Way to Understand the Atmospheric Circulation on Tidally Locked Terrestrial Planets
• Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
• Retention of surface water on tidally locked rocky planets in the Venus zone around M dwarfs
• An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley