  Can multicellular life be distinguished from single cellular life on an
exoplanet? We hypothesize that abundant upright photosynthetic multicellular
life (trees) will cast shadows at high sun angles that will distinguish them
from single cellular life and test this using Earth as an exoplanet. We first
test the concept using Unmanned Arial Vehicles (UAVs) at a replica moon landing
site near Flagstaff, Arizona and show trees have both a distinctive reflectance
signature (red edge) and geometric signature (shadows at high sun angles) that
can distinguish them from replica moon craters. Next, we calculate reflectance
signatures for Earth at several phase angles with POLDER (Polarization and
Directionality of Earth's reflectance) satellite directional reflectance
measurements and then reduce Earth to a single pixel. We compare Earth to other
planetary bodies (Mars, the Moon, Venus, and Uranus) and hypothesize that
Earths directional reflectance will be between strongly backscattering rocky
bodies with no weathering (like Mars and the Moon) and cloudy bodies with more
isotropic scattering (like Venus and Uranus). Our modelling results put Earth
in line with strongly backscattering Mars, while our empirical results put
Earth in line with more isotropic scattering Venus. We identify potential
weaknesses in both the modeled and empirical results and suggest additional
steps to determine whether this technique could distinguish upright
multicellular life on exoplanets.
