  Sagan et al. (1993) used the Galileo space probe data and first principles to
find evidence of life on Earth. Here we ask whether Sagan et al. (1993) could
also have detected whether life on Earth had three-dimensional structure, based
on the Galileo space probe data. We reanalyse the data from this probe to see
if structured vegetation could have been detected in regions with abundant
photosynthetic pigments through the anisotropy of reflected shortwave
radiation. We compare changing brightness of the Amazon forest (a region where
Sagan et al. (1993) noted a red edge in the reflectance spectrum, indicative of
photosynthesis) as the planet rotates to a common model of reflectance
anisotropy and found measured increase of surface reflectance of 0.019 versus a
0.007 predicted from only anisotropic effects. We hypothesize the difference
was due to minor cloud contamination. However, the Galileo dataset had only a
small change in phase angle (sun-satellite position) which reduced the observed
anisotropy signal and we demonstrate that theoretically if the probe had a
variable phase angle between 0-20, there would have been a much larger
predicted change in surface reflectance of 0.06 and under such a scenario
three-dimensional vegetation structure on Earth could possibly have been
detected. These results suggest that anisotropic effects may be useful to help
determine whether exoplanets have three-dimensional vegetation structure in the
future but that further comparisons between empirical and theoretical results
are first necessary.
