GRE Reading Comprehension: PPO-GRE阅读PPO - 26T8LR16JB1TCI13S

One way to determine which planets outside the solar system might harbor life is to look for telltale signs in their light. Living organisms create a chemical disequilibrium in their environment, typically indicated by a molecule whose presence can be explained only if some process is continually regenerating it. For example, Earth's atmosphere contains oxygen because photosynthesis produces it faster than it is lost through processes such as the rusting of iron in rocks. However, chemical disequilibrium does not necessarily indicate the presence of life because, as in the case of methane on Mars, the imbalance could arise through nonbiological processes.