nai

Project Progress

We have been researching biosignatures from metabolisms other than oxygenic photosynthesis, which are more likely to be prevalent in early Earth type atmospheres, and for planets orbiting stars cooler than our Sun. We have also explored the potential for abiotic false positives for early Earth like planets orbiting cooler stars. These two projects are described in more detail below. In parallel, we have obtained and are now running simulators for Terrestrial Planet Finder Coronograph (S. Heap and D. Lindler) and Interferometer (T. Velusamy) mission concepts. We are using these simulators to explore the detectability of signs of habitability and biosignatures from oxygenic photosynthetic, as well as the other metabolisms described here (Evans et al., 2010).

Biosignatures in Anoxic Atmospheres

For planets like the early Earth, in which oxygenic photosynthesis has not yet evolved or has not yet produced detectable levels of oxygen and ozone, identification of biosignatures from alternative metabolisms is important. This year we used photochemical and radiative transfer modeling to explore the potential of various sulfur-bearing biogenic gases to act as biosignatures for anoxic planets similar to the early Earth, and in orbit around stars of different spectral type (Domagal-Goldman, et al., submitted). Our results (Figure 1) indicate that these gases – methyl mercaptan (CH₃SH), in particular – may build up to detectable concentrations on planets around quiescent M-dwarfs (bottom panel). On planets around G-dwarfs like the Sun (top panel) or active M-dwarfs like AD Leo (middle panel) the spectral features of CH₃SH and other biogenic sulfur species are not detectable. However, absorption features from ethane (C₂H₆) are much deeper due to the methyl (CH₃) groups attached to the biogenic sulfur gases. C₂H₆ could be a byproduct of photolysis of abiotic CH₄. However, these spectra can be distinguished from those that contain abiological C₂H₆ if methane (CH₄) concentrations can be constrained from the width of the CH₄ absorption features.

Abiotic Formation of Ozone On Planets Around M dwarfs

While studying the photochemistry of anoxic atmospheres around other star types, we discovered the unexpected presence of ozone (O₃) in anoxic atmospheres on planets around active M-dwarfs. This results from the production of O atoms from photolysis of O₂, H₂O, and CO₂ with short-wavelength photons that arise from stellar activity, and a lack of longer-wavelength photons that are responsible for O₃ photolysis. This allows for a detectable buildup of O₃ in an atmosphere, even if O₂ concentrations are at low, abiotic levels (Figure 2). These results are being prepared for publication.