  Launching in 2028, ESA's Atmospheric Remote-sensing Exoplanet Large-survey
(ARIEL) survey of $\sim$1000 transiting exoplanets will build on the legacies
of Kepler and TESS and complement JWST by placing its high precision exoplanet
observations into a large, statistically-significant planetary population
context. With continuous 0.5--7.8~$\mu$m coverage from both FGS (0.50--0.55,
0.8--1.0, and 1.0--1.2~$\mu$m photometry; 1.25--1.95~$\mu$m spectroscopy) and
AIRS (1.95--7.80~$\mu$m spectroscopy), ARIEL will determine atmospheric
compositions and probe planetary formation histories during its 3.5-year
mission. NASA's proposed Contribution to ARIEL Spectroscopy of Exoplanets
(CASE) would be a subsystem of ARIEL's FGS instrument consisting of two
visible-to-infrared detectors, associated readout electronics, and thermal
control hardware. FGS, to be built by the Polish Academy of Sciences' Space
Research Centre, will provide both fine guiding and visible to near-infrared
photometry and spectroscopy, providing powerful diagnostics of atmospheric
aerosol contribution and planetary albedo, which play a crucial role in
establishing planetary energy balance. The CASE team presents here an
independent study of the capabilities of ARIEL to measure exoplanetary
metallicities, which probe the conditions of planet formation, and FGS to
measure scattering spectral slopes, which indicate if an exoplanet has
atmospheric aerosols (clouds and hazes), and geometric albedos, which help
establish planetary climate. Our design reference mission simulations show that
ARIEL could measure the mass-metallicity relationship of its 1000-planet
single-visit sample to $>7.5\sigma$ and that FGS could distinguish between
clear, cloudy, and hazy skies and constrain an exoplanet's atmospheric aerosol
composition to $>5\sigma$ for hundreds of targets, providing
statistically-transformative science for exoplanet atmospheres.
