| − | The Miller-Urey experiment assumed an a primitive atmosphere was composed of ''reducing'' gases [e.g. methane (CH<sub>4</sub>), ammonia (NH<sub>3</sub>), hydrogen (H<sub>2</sub>), and water vapor (H<sub>2</sub>O)], but by the 1980s geochemists believes that early earth atmospheres probably had ''neutral'' gases like those emitted by volcanoes [e.g. water vapor (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), and nitrogen ((N<sub>2</sub>)] and perhaps some ''reducing' gases like carbon monoxide (CO). Since hydrogen was a light element, there probably would not have been any in early earth.<ref name="Wells" /> | + | The Miller-Urey experiment assumed an a primitive atmosphere was composed of ''reducing'' gases [e.g. methane (CH<sub>4</sub>), ammonia (NH<sub>3</sub>), hydrogen (H<sub>2</sub>), and water vapor (H<sub>2</sub>O)], but by the 1980s geochemists believes that early earth atmospheres probably had ''neutral'' gases like those emitted by volcanoes [e.g. water vapor (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), and nitrogen ((N<sub>2</sub>)] and perhaps some ''reducing'' gases like carbon monoxide (CO). Since hydrogen was a light element, there probably would not have been any in early earth.<ref name="Wells" /> |