Final answer:
The gas that evolves, according to the passage, is Carbon dioxide (CO₂) due to the described transition of the terrestrial planets' atmospheres from hydrogen-dominated to oxygen-dominated.
Step-by-step explanation:
According to the information provided in the passage, the gas that evolves is Carbon dioxide (CO₂). The passage discusses the changes in the gas composition of the terrestrial planets' atmospheres over time, highlighting the presence of hydrogen-containing gases like methane (CH₄) and ammonia (NH₃) in the early solar system. However, it specifies that these gases were broken down by ultraviolet light, and the light hydrogen atoms escaped into space, leaving behind atmospheres dominated by oxidized gases, such as carbon dioxide, which is now the predominant volatile gas on terrestrial planets.