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Project Progress

In this task, we have attempted to understand the nature and detectability of biosignatures and planetary characteristics for Earth-like planets around other stars. In our initial experiment, we used a coupled climate-photochemical model to simulate Earth-like planets with different amounts of atmospheric oxygen, in orbit around the Sun (a G2V star), an F2V star, and a K2V star. For the solar case, Proterozoic Earth was also included in the simulations. Three significant results were derived from this research: 1) O₂ can be detected at visible wavelengths (~0.765 microns) in disk-averaged spectra at atmospheric concentrations as small as 10^-2 present atmospheric levels (PAL) and O₃ can be detected at O₂ concentrations as low as 10^-3 PAL at millimeter imaging radiometer (MIR) wavelengths; 2) Mid-Proterozoic Earth-like atmospheres show strong signatures from both CH₄ and O₃, and it is potentially easier to detect life remotely in a mid-Proterozoic-type atmosphere than a modern Earth; and 3) The critical threshold for atmospheric oxygen to provide a planetary ultraviolet (UV) shield for host stars of different spectral types is typically ~10^-2 PAL. We also discovered that planets with high-O₂ atmospheres orbiting K2V and F2V stars were better protected from surface UV radiation than Earth, but O₂ levels below ~10^-2 PAL were insufficient to provide adequate surface protection. (Segura et al., Astrobiology, 2003, vol. 3, pp. 689-708).

Our second, ongoing, experiment is focused on detectability of characteristics for planets orbiting M dwarf stars, which are typically highly variable. As a first step, the observed time-averaged spectra of two M stars and a quiescent, modeled M star were used (Figure 1). The planets modeled in this case contained 1PAL Earth-like atmospheres. Our results show that methane could be an excellent biosignature, along with ozone (Figures 2 and 3). We are currently developing a time-dependent climate-photochemical model to determine how the flux variability of M stars would affect the lifetime and detectability of planetary biosignatures. We also show that CH₄ is potentially observable along with O₂ (or O₃). The CH₄ abundance is relatively high in these systems because the enhanced ozone shield at near-UV wavelengths between 200-300 nm produces reduced tropospheric OH densities. Hence, methane is long-lived even in O₂-rich atmospheres of M-star planets.

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