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 (N-2) 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, N-2 is extremely difficult to remotely detect. However, N-2 produces an N-2 N-2 collisional Pair, (N-2)(2), which is spectrally active. Here we report the detection of (N-2)(2) in Earth's disk-integrated spectrum. By comparing spectra from NASA's EPDXI mission to synthetic spectra from the NASA Astrobiology Institute's Virtual Planetary Laboratory threedimensional spectral Earth model, we find that (N-2)(2) absorption produces a 35% decrease in flux at 4.15 pm. Quantifying N-2 could provide a means of determining bulk atmospheric composition for terrestrial exoplanets and could rule out abiotic O-2 generation, which is possible in rarefied atmospheres. To explore the potential effects of (N-2)(2) in exoplanet spectra, we used radiative transfer models to generate synthetic emission and transit transmission spectra of self-consistent N-2-CO2 H2O atmospheres, and analytic N-2-H-2 and N-2-H-2-CO2 atmospheres. We show that (N-2)(2) absorption in the wings of the 4.3 pm CO2 band is strongly dependent on N-2 partial pressures above 0.5 bar and can significantly widen this band in thick N-2 atmospheres. The (N-2)(2) transit transmission signal is up to 10 ppm for an Earth-size planet with an N-2-dominated atmosphere orbiting within the habitable zone of an M5V star and could be substantially larger for planets with significant H-2 mixing ratios.