  A milestone in understanding life in the universe is the detection of
biosignature gases in the atmospheres of habitable exoplanets. Future mission
concepts under study by the 2020 decadal survey, e.g., HabEx and LUVOIR, have
the potential of achieving this goal. We investigate the baseline requirements
for detecting four molecular species, H$_2$O, O$_2$, CH$_4$, and CO$_2$,
assuming concentrations of these species equal to that of modern Earth. These
molecules are highly relevant to habitability and life on Earth and other
planets. Through numerical simulations, we find the minimum requirements of
spectral resolution, starlight suppression, and exposure time for detecting
biosignature and habitability marker gases. The results are highly dependent on
cloud conditions. A low-cloud case is more favorable because of deeper and
denser lines whereas a no-cloud case is the pessimistic case for its low
albedo. The minimum exposure time for detecting a certain molecule species can
vary by a large factor ($\sim$10) between the low-cloud case and the no-cloud
case. For all cases, we provide baseline requirements for HabEx and LUVOIR. The
impact of exo-zodiacal contamination and thermal background is also discussed
and will be included in future studies.
