  For the first time in human history the possibility of detecting and studying
Earth-like planets is on the horizon. Terrestrial Planet Finder (TPF), with a
launch date in the 2015 timeframe, is being planned by NASA to find and
characterize planets in the habitable zones of nearby stars. The mission Darwin
from ESA has similar goals. The motivation for both of these space missions is
the detection and spectroscopic characterization of extrasolar terrestrial
planet atmospheres. Of special interest are atmospheric biomarkers--such as O2,
O3, H2O, CO and CH4--which are either indicative of life as we know it,
essential to life, or can provide clues to a planet's habitability. A mission
capable of measuring these spectral features would also obtain sufficient
signal-to-noise to characterize other terrestrial planet properties. For
example, physical characteristics such as temperature and planetary radius can
be constrained from low- resolution spectra. In addition, planet
characteristics such as weather, rotation rate, presence of large oceans or
surface ice, and existence of seasons could be derived from photometric
measurements of the planet's variability. We will review the potential to
characterize terrestrial planets beyond their spectral signatures. We will also
discuss the possibility to detect strong surface biomarkers--such as Earth's
vegetation red edge near 700 nm--that are different from any known atomic,
molecular, or mineralogical signature.
