Detectability of biosignature gases in the atmospheres of terrestrial exoplanets

Abstract

Biosignature gases in the atmosphere of an exoplanet provide a means by which we can deduce the possible existence of life on that planet. As the list of possible biosignature gases is ever growing, the need to determine which molecules provide the best opportunities for detection grows as well. One way to explore these systems is through modeling radiative transfer via transmissivity as light travels from the parent star, through the atmosphere of the planet, and then impacts a detector located at Earth. As the light travels through the planetary atmosphere, it acquires molecular features from the planet due to the composition, temperature, and pressure structure of the atmosphere. By adding synthetic noise to the modeled transmissivity spectra, I determine the detectability of a range of atmospheric mixing ratios for ten biosignature gases from the HIgh-resolution TRANsmission molecular absorption (HITRAN) database: oxygen, ozone, methane, nitrous oxide, methyl bromide, methyl chloride, hydrogen sulfide, carbonyl sulfide, phosphine, and sulfur dioxide. The deep investigation of the HITRAN biosignature gases in this study is possible due to the ability to properly map their absorption cross sections to varying temperatures and pressures. For each of the above HITRAN molecules, I analyze alternative spectral features for detection in order to emphasize the importance of and determine the ability for multiple band detection of biosignature gases. Water vapor (though not a biosignature gas) is included in order to study its potential for spectral masking. Though I nd that each of the above HITRAN gases could be detected in exoplanet atmospheres if that molecule has a large enough atmospheric mixing ratio, an Earthsize planet with an Earth-like atmosphere located at 35.45 parsecs would only allow for discernible biosignature features from ozone, nitrous oxide, and methane in the infrared wavelength region. Sixteen additional (and non-standard) biosignature gases included in this study do not have absorption cross sections that are currently mapable to alternative temperatures and pressures. These sixteen biosignature gases are acetaldehyde, acetone, benzene, carbon disulfide, dimethyl disulfide, dimethyl sulfide, dimethyl sulfoxide, ethanol, ethyl mercaptan, fluoroacetone, isoprene, methyl ethyl ketone, methyl mercaptan, methyl vinyl ketone, thioglycol, and toluene. To circumvent the nonmapability of the absorption cross sections to dierent temperatures and pressures, I use the detectivity calculations and the absorption cross sections from ozone, methane, and nitrous oxide to estimate the threshold atmospheric mixing ratios for the detection of the sixteen non-standard biosignature gases with a 35 m telescope, 100 hours of observation, and a target distance of 35.45 parsecs. The combination of the threshold atmospheric mixing ratios calculated for these sixteen non-standard biosignature gases with the results from the HITRAN biosignature gases investigated in this study demonstrate that an atmospheric gas will require a mixing ratio in the tens to hundreds of ppm to be detectable above a 5[sigma] level with a 35 m telescope, an observation time of 100 hours, and a target distance of 35.45 parsecs. Keeping with the theme of multi-wavelength detection, I end the analysis of the sixteen non-standard biosignature gases by proposing potential spectral feature wavelengths for each gas based on their molecular absorption cross section spectral profiles. As many biosignature gases have molecular features at longer wavelengths than the traditional IR region, I investigated the technological requirements for detecting biosignature gas spectral features in one of the low-signal long-wavelength regions, the millimeter. Though the investigation into the millimeter region reveals unrealistic technological demands for the successful detection of the case study, oxygen, I use the analysis as a platform to introduce the theoretical concept of observing future targets with multiple next-generation telescopes stationed in a matrix in order to produce the same observational ability of a larger (and more distant future) telescope. While interferometric investigations into millimeter spectral features are improbable in the near future, the use of interferometry with next generation instruments may allow for investigations in the 10 - 30 [mu]m region, thereby opening alternative wavelengths for biosignature gas detection. Since this theoretical interferometry idea relies on the ability to increase the signal-to-noise ratio (SNR) of the observations, I investigated the interaction between telescope aperture size and observation duration on the detectability (i.e. SNR) of biosignature gases in reference to finding a middle ground between these two system parameters for both a 6 m and 35 m telescope. Unfortunately, a 6 m telescope does not provide a large enough collecting area to increase the SNR sufficiently enough to detect atmospheric gases. For futuristic telescope designs, though a 20 m telescope (or nine JWSTs working together to achieve the same collecting area) would begin to discern some biosignature gas features from the continuum (for high biosignature gas atmospheric abundances), a 35 m class telescope (or equivalent interferometric telescope array) should be the minimum aperture size considered for biosignature gas detection in transmissivity spectroscopy.