2015ApJ...810...57S

Characterizing the bulk atmosphere of a terrestrial planet is important for determining surface pressure and potential habitability. Molecular nitrogen (N₂) constitutes the largest fraction of Earth's atmosphere and is likely to be a major constituent of many terrestrial exoplanet atmospheres. Due to its lack of significant absorption features, N₂ is extremely difficult to remotely detect. However, N₂produces an N₂-N₂collisional pair, (N₂)₂, which is spectrally active. Here we report the detection of (N₂)₂in Earth's disk-integrated spectrum. By comparing spectra from NASA's EPOXI mission to synthetic spectra from the NASA Astrobiology Institute's Virtual Planetary Laboratory three-dimensional spectral Earth model, we find that (N₂)₂absorption produces a ∼35% decrease in flux at 4.15 µm. Quantifying N₂could provide a means of determining bulk atmospheric composition for terrestrial exoplanets and could rule out abiotic O₂generation, which is possible in rarefied atmospheres. To explore the potential effects of (N₂)₂ in exoplanet spectra, we used radiative transfer models to generate synthetic emission and transit transmission spectra of self-consistent N₂-CO₂-H₂O atmospheres, and analytic N₂-H₂and N₂-H₂-CO₂atmospheres. We show that (N₂)₂absorption in the wings of the 4.3 µm CO₂band is strongly dependent on N₂partial pressures above 0.5 bar and can significantly widen this band in thick N₂atmospheres. The (N₂)₂transit transmission signal is up to 10 ppm for an Earth-size planet with an N₂-dominated atmosphere orbiting within the habitable zone of an M5V star and could be substantially larger for planets with significant H₂mixing ratios.