Extrapolating the Rules for Life: Determining Alternative Photosynthetic Biosignatures Evolved With Other Stars and Atmospheres
Nancy Y. Kiang (NASA Goddard Institute for Space Studies)
Photosynthesis – broadly, the utilization of light by life to drive biochemical processes – is so successful a process that it provides the foundation for virtually all life on Earth. Its presence is visible at the global scale, in our abundant atmospheric oxygen and in the wide distribution of chlorophyll over the Earth's surface. Since stellar radiation is such a ubiquitous energy source for a planet, we expect photosynthesis to be successful also on other habitable planets. However, to detect alien photosynthetic life, should we look for the same gaseous and pigment biosignatures as on Earth? We can obtain some clues as to what sort of extrasolar photosynthesis might dominate on another planet from the diversity of photosynthetic organisms on Earth and their path of evolution with our changing atmosphere: the constraints are both environmental resources (stellar radiation spectrum and intensity, carbon source, nutrients, and electron donors) and molecular mechanisms (light harvesting, redox requirements), and how the one is bounded by the other. This talk aims to summarize the biology for the astronomers to help inform strategies for detection of photosynthetic life. What are the likely rules for adaptation of photosynthesis to other stars and atmospheres? What has been the range of biosphere productivity from the Archaen to modern times on Earth? How much can the spectral characteristics of pigments vary? What do we need to know to determine is the long wavelength limit of photosynthesis, and of oxygenic photosynthesis in particular?