  Large surface temperatures on the illuminated hemisphere of the Moon can lead
it to contribute a significant amount of flux to spatially unresolved infrared
(IR) observations of the Earth-Moon system, especially at wavelengths where
Earth's atmosphere is absorbing. We have paired the NASA Astrobiology
Institute's Virtual Planetary Laboratory three-dimensional spectral Earth model
with a model of the phase dependent IR spectrum of a Moonlike satellite to
investigate the effects of an unresolved companion on IR observations of
Earthlike extrasolar planets. For an extrasolar twin Earth-Moon system observed
at full phase at IR wavelengths, the Moon consistently comprises about 20% of
the total signal, approaches 30% of the signal in the 9.6 micron ozone band and
the 15 micron carbon dioxide band, makes up as much as 80% of the total signal
in the 6.3 micron water band, and more than 90% of the signal in the 4.3 micron
carbon dioxide band. These excesses translate to inferred brightness
temperatures for Earth that are too large by about 20-40 K, and demonstrate
that the presence of an undetected satellite can have a significant impact on
the spectroscopic characterization of terrestrial exoplanets. The thermal flux
contribution from an airless companion depends strongly on the
star-planet-observer angle (i.e., the phase angle), allowing moons to mimic or
mask seasonal variations in the host planet's IR spectrum, and implying that
observations of exoplanets should be taken when the phase angle is as small as
feasibly possible if contributions from airless companions are to be minimized.
We show that, by differencing IR observations of an Earth twin with a companion
taken at both gibbous phase and at crescent phase, Moonlike satellites may be
detectable by future exoplanet characterization missions for a wide range of
system inclinations.
