Modeling the Infrared Spectrum of the Earth-Moon System: Implications for the Detection and Characterization of Earthlike Extrasolar Planets and their Moonlike Companions

Abstract

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.