| アブストラクト |
Earth is an aqua planet that holds liquid water on the surface and can support life. For centuries, people have been wondering whether there are any other aqua planets in space. Until now, spacecraft explorations have revealed that there is no aqua planet in our solar system today, other than Earth. In our galaxy, however, billions aqua planets are predicted to orbit in the habitable zone around stars. In fact, the challenge for finding aqua planets has started using space telescopes.
If aqua planets are discovered, people would then wonder the next questions. Is there life there? If so, what types of life? What are the signs of life there? Answering this question requires research at the interactions of life sciences, chemistry, physics, and earth sciences. Here, I will discuss how the atmosphere, oceans, and life have evolved and interacted over Earth’s history, especially focused on the first and largest rise in atmospheric oxygen in 2.5–2.2 billion years ago.
Recent geochemical studies suggested that before the rise in oxygen, Earth’s atmosphere had been composed mainly of nitrogen, carbon dioxide, and methane. In such a relatively reducing atmosphere, photochemical reactions of methane would have generated organic aerosols, as occurred in the atmosphere of Titan, Saturn’s largest moon. The organic aerosols could have affected the atmospheric structure and surface temperature. Here I will discuss the climatic stability of an Earth-like planet with a methane-rich atmosphere and some possible reasons why Earth’s reducing atmosphere had suddenly oxidized around 2.5 billion years ago.
It has been suggested the rise in oxygen would have occurred in the climatic recovery immediately after a global-scale glaciation (called “Snowball Earth event“). These studies suggest that a drastic climate change have promoted the atmospheric evolution, which in turn has strongly affected the evolution of life. In this talk, I also will discuss the mechanism how the climate change has promoted transition of atmospheric composition based on geochemical records. Knowledge on interactions between life and the Earth system would help us to consider alternative bio- and geochemistries that might occur on aqua planets beyond the solar system. |
