AbstractsEarth & Environmental Science

Detectability of biosignature gases in the atmospheres of terrestrial exoplanets

by Stephen Joseph Messenger

Institution: MIT
Department: Department of Earth, Atmospheric, and Planetary Sciences
Degree: MS
Year: 2013
Keywords: Earth, Atmospheric, and Planetary Sciences.
Record ID: 2009601
Full text PDF: http://hdl.handle.net/1721.1/79156


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…