Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres

Early Earth may have hosted a biologically-mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3SCH3), photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1x the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form haze decreases by a full order of magnitude. Detection of haze at an anomalously low CH4/CO2 ratio could suggest the influence of these biogenic sulfur gases, and therefore imply biological activity on an exoplanet. When these organic sulfur gases are not readily detectable in the spectrum of an Earth-like exoplanet, the thick organic haze they can help produce creates a very strong absorption feature at UV-blue wavelengths detectable in reflected light at a spectral resolution as low as 10. In direct imaging, constraining CH4 and CO2 concentrations will require higher spectral resolution, and R > 170 is needed to accurately resolve the structure of the CO2 feature at 1.57 {\mu}m, likely, the most accessible CO2 feature on an Archean-like exoplanet.Comment: accepted for publication in Astrobiolog

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets

We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large amplitude, high frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restrict our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verify that these systems are stable for $10^8$ years with N-body simulations, and calculate the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We run a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculate differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculate the outer edge of habitability for two conditions: 1) the full evolution of the planetary spin and orbit, and 2) the eccentricity and obliquity fixed at their average values. We recover previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but also find that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes.Comment: 46 pages, 12 Figures, 5 Table

Detecting and Constraining N2_2 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 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 $\mu$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 $\mu$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.Comment: Accepted for publication in The Astrophysical Journal. 46 pages, 12 figures, 3 table

Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production

O2 and O3 have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O2/O3: CO and O4. We produce the first explicit self-consistent simulations of these spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/or NIRSpec observe CO (2.35, 4.6 um) in conjunction with CO2 (1.6, 2.0, 4.3 um) in the transmission spectrum of a terrestrial planet it could indicate robust CO2 photolysis and suggest that a future detection of O2 or O3 might not be biogenic. Strong O4 bands seen in transmission at 1.06 and 1.27 um could be diagnostic of a post-runaway O2-dominated atmosphere from massive H-escape. We find that for these false positive scenarios, CO at 2.35 um, CO2 at 2.0 and 4.3 um, and O4 at 1.27 um are all stronger features in transmission than O2/O3 and could be detected with SNRs $\gtrsim$ 3 for an Earth-size planet orbiting a nearby M dwarf star with as few as 10 transits, assuming photon-limited noise. O4 bands could also be sought in UV/VIS/NIR reflected light (at 0.345, 0.36, 0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by a next generation direct-imaging telescope such as LUVOIR/HDST or HabEx and would indicate an oxygen atmosphere too massive to be biologically produced.Comment: 7 pages, 4 figures, accepted to the Astrophysical Journal Letter

Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth

The search for life on planets outside our solar system will use spectroscopic identification of atmospheric biosignatures. The most robust remotely-detectable potential biosignature is considered to be the detection of oxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicating fluxes from the planetary surface in excess of those that could be produced abiotically. Here, we use an altitude-dependent photochemical model with the enhanced lower boundary conditions necessary to carefully explore abiotic O_2 and O_3 production on lifeless planets with a wide variety of volcanic gas fluxes and stellar energy distributions. On some of these worlds, we predict limited O_2 and O_3 build up, caused by fast chemical production of these gases. This results in detectable abiotic O_3 and CH_4 features in the UV-visible, but no detectable abiotic O_2 features. Thus, simultaneous detection of O_3 and CH_4 by a UV-visible mission is not a strong biosignature without proper contextual information. Discrimination between biological and abiotic sources of O_2 and O_3 is possible through analysis of the stellar and atmospheric context - particularly redox state and O atom inventory - of the planet in question. Specifically, understanding the spectral characteristics of the star and obtaining a broad wavelength range for planetary spectra should allow more robust identification of false positives for life. This highlights the importance of wide spectral coverage for future exoplanet characterization missions. Specifically, discrimination between true- and false-positives may require spectral observations that extend into infrared wavelengths, and provide contextual information on the planet's atmospheric chemistry.Comment: Accepted for publication in The Astrophysical Journal. 43 pages, 6 figure

ATLAST detector needs for direct spectroscopic biosignature characterization in the visible and near-IR

Are we alone? Answering this ageless question will be a major focus for astrophysics in coming decades. Our tools will include unprecedentedly large UV-Optical-IR space telescopes working with advanced coronagraphs and starshades. Yet, these facilities will not live up to their full potential without better detectors than we have today. To inform detector development, this paper provides an overview of visible and near-IR (VISIR; $\lambda=0.4-1.8~\mu\textrm{m}$) detector needs for the Advanced Technology Large Aperture Space Telescope (ATLAST), specifically for spectroscopic characterization of atmospheric biosignature gasses. We also provide a brief status update on some promising detector technologies for meeting these needs in the context of a passively cooled ATLAST.Comment: 8 pages, Presented 9 August 2015 at SPIE Optics + Photonics, San Diego, C

Abiotic O_2 Levels on Planets around F, G, K, and M Stars: Effects of Lightning-produced Catalysts in Eliminating Oxygen False Positives

Over the last few years, a number of authors have suggested that, under certain circumstances, molecular oxygen (O_2) or ozone (O_3) generated by abiotic processes may accumulate to detectable concentrations in a habitable terrestrial planet's atmosphere, producing so-called "false positives" for life. But the models have occasionally disagreed with each other, with some predicting false positives, and some not, for the same apparent set of circumstances. We show here that photochemical false positives derive either from inconsistencies in the treatment of atmospheric and global redox balance or from the treatment (or lack thereof) of lightning. For habitable terrestrial planets with even trace amounts of atmospheric N_2, NO produced by lightning catalyzes the recombination of CO and O derived from CO_2 photolysis and should be sufficient to eliminate all reported false positives. Molecular oxygen thus remains a useful biosignature gas for Earth-like extrasolar planets, provided that the planet resides within the conventional liquid water habitable zone and has not experienced distinctly non-Earth-like, irrecoverable water loss