  In our solar system, the densely cloud-covered atmosphere of Venus stands out
as an example of how polarimetry can be used to gain information on cloud
composition and particle mean radius. With current interest running high on
discovering and characterizing extrasolar planets in the habitable zone where
water exists in the liquid state, making use of spectropolarimetric
measurements of directly-imaged exoplanets could provide key information
unobtainable through other means. In principle, spectropolarimetric
measurements can determine if acidity causes water activities in the clouds to
be too low for life. To this end, we show that a spectropolarimeter measurement
over the range 400 nm - 1000 nm would need to resolve linear polarization to a
precision of about 1% or better for reflected starlight from an optically thick
cloud-enshrouded exoplanet. We assess the likelihood of achieving this goal by
simulating measurements from a notional spectropolarimeter as part of a
starshade configuration for a large space telescope (a HabEx design, but for a
6 m diameter primary mirror). Our simulations include consideration of noise
from a variety of sources. We provide guidance on limits that would need to be
levied on instrumental polarization to address the science issues we discuss.
For photon-limited noise, integration times would need to be of order one hour
for a large radius (10 Earth radii) planet to more than 100 hours for smaller
exoplanets depending on the star-planet separation, planet radius, phase angle
and desired uncertainty. We discuss implications for surface chemistry and
habitability.
