  Giant exoplanets located >1 AU away from their parent stars have atmospheric
environments cold enough for water and/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
micron if it has a cloud deck deeper than ~0.3 bars. 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 on the
internal heat flux. Reflected light spectroscopy from future direct-imaging
missions therefore not only measure the atmospheric abundances but also
characterize the thermal evolution of giant exoplanets.
