DETECTING and CONSTRAINING N₂ ABUNDANCES in PLANETARY ATMOSPHERES USING COLLISIONAL PAIRS

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, N₂ produces an N₂-N₂ collisional pair, (N₂)₂, which is spectrally active. Here we report the detection of (N₂)₂ in 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)₂ absorption produces a ∼35% decrease in flux at 4.15 μm. Quantifying N₂ could provide a means of determining bulk atmospheric composition for terrestrial exoplanets and could rule out abiotic O₂ generation, 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-H₂ and N₂-H₂-CO₂ atmospheres. We show that (N₂)₂ absorption in the wings of the 4.3 μm CO₂ band is strongly dependent on N₂ partial pressures above 0.5 bar and can significantly widen this band in thick N₂ atmospheres. The (N₂)₂ transit 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 H₂ mixing ratios.