DETECTING AND CONSTRAINING N₂ABUNDANCES IN PLANETARY ATMOSPHERES USING COLLISIONAL PAIRS

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 µm. 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 µm 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.

Dim Prospects for Transmission Spectra of Ocean Earths around M Stars

“…This is due to the dramatic difference of the cloud deck heights between the planets orbiting the 2600 K star and the planets orbiting higher temperature stars, as seen in Figure 4; the spectral feature achieves significant depth only when the cloud-based continuum is lower than 30 km, as see in Figure 3. The maximum depth of the spectral feature for the 3000 K models is ∼3 ppm, similar to results by Schwieterman et al (2015) for a similar stellar type. However, for even cooler stars, the effective continuum occurs at even lower altitudes, and the lowest-flux models produce a signal up to 11 ppm.…”

“…In the synthesized spectrum, there are four major water vapor signatures in the infrared: the 1.4 µm, the 1.8 µm, the 2.7 µm, and the 6 µm, with the latter two being the most prominent. The small feature at 4.15 µm visible for the 2600 K lower-flux cases is a N 2 -N 2 collision-induced absorption feature, which was examined in detail by Schwieterman et al (2015). No features caused by other molecules are present, as the GCMs of Kopparapu et al (2017) only include H 2 O and N 2 in the atmospheres of the planets.…”

The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets

“…For modern Earth, the largest overall disequilibrium is caused by the simultaneous presence of oxygen (O 2 ), atmospheric nitrogen (N 2 ), and liquid water (H 2 O), which would react to form nitrate and hydrogen ions in equilibrium (Krissansen-Totton et al 2016). Unfortunately, N 2 may be challenging to observe in direct spectral observations (Schwieterman et al 2015), so other directly delectable biosignatures should be sought.…”

Observing Isotopologue Bands in Terrestrial Exoplanet Atmospheres with the James Webb Space Telescope: Implications for Identifying Past Atmospheric and Ocean Loss

“…We include collision-induced absorption for CO 2 -CO 2 (Moore 1972;Kasting et al 1984;Gruszka & Borysow 1997;Baranov et al 2004;Wordsworth et al 2010;Lee et al 2016), N 2 -N 2 (Schwieterman et al 2015based on Lafferty et al 1996, and O 2 -O 2 (Greenblatt et al 1990;Hermans et al 1999;Maté et al 1999).…”