Detecting and Constraining N$_2$ Abundances in Planetary Atmospheres Using Collisional Pairs

Edward W. Schwieterman; Tyler D. Robinson; Victoria S. Meadows; Amit Misra; Shawn Domagal-Goldman

doi:10.1088/0004-637X/810/1/57

paper

Detecting and Constraining N $_{2}$ Abundances in Planetary Atmospheres Using Collisional Pairs

arXiv:1507.07945 · doi:10.1088/0004-637X/810/1/57

Abstract

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 EPOXI mission to synthetic spectra from the NASA Astrobiology Institute $^{'}$ s Virtual Planetary Laboratory three-dimensional spectral Earth model, we find that (N $_{2}$ ) $_{2}$ absorption produces a ~35 $%$ decrease in flux at 4.15 $μ$ m. 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 $_{2}$ -CO $_{2}$ -H $_{2}$ O atmospheres, and analytic N $_{2}$ -H $_{2}$ and N $_{2}$ -H $_{2}$ -CO $_{2}$ atmospheres. We show that (N $_{2}$ ) $_{2}$ absorption in the wings of the 4.3 $μ$ m CO $_{2}$ 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 HZ of an M5V star and could be substantially larger for planets with significant H $_{2}$ mixing ratios.

Accepted for publication in The Astrophysical Journal. 46 pages, 12 figures, 3 tables

Detecting and Constraining N $_{2}$ Abundances in Planetary Atmospheres Using Collisional Pairs

Abstract

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Cited by in corpus (53)

Detecting and Constraining N2​ Abundances in Planetary Atmospheres Using Collisional Pairs

Abstract

References in corpus (11)

Cited by in corpus (53)

Detecting and Constraining N $_{2}$ Abundances in Planetary Atmospheres Using Collisional Pairs