2019ApJ...887..166H

Giant exoplanets located >1 au away from their parent stars have atmospheric environments cold enough for water or ammonia clouds. We have developed a new equilibrium cloud and reflected-light spectrum model, ExoREL, for widely separated giant exoplanets. The model includes the dissolution of ammonia in liquid-water cloud droplets, an effect studied for the first time for exoplanets. While preserving the causal relationship between temperature and cloud condensation, ExoREL is simple and fast to enable efficient exploration of parameter space. Using the model, we find that the mixing ratio of methane and the cloud top pressure of a giant exoplanet can be uniquely determined from a single observation of its reflected-light spectrum at wavelengths less than 1 µm if it has a cloud deck deeper than ∼0.3 bar. This measurement is enabled by the weak and strong bands of methane and requires a signal-to-noise ratio of 20. The cloud pressure, once derived, provides information about the internal heat flux of the planet. Importantly, we find that for a low, Uranus-like internal heat flux, the planet can have a deep liquid-water cloud, which will sequester ammonia and prevent the formation of the ammonia cloud that would otherwise be the uppermost cloud layer. This newly identified phenomenon causes a strong sensitivity of the cloud top pressure to the internal heat flux. Reflected-light spectroscopy from future direct-imaging missions should therefore not only measure the atmospheric abundances but also characterize the thermal evolution of giant exoplanets.