Paleoatmospheric Chemistry

地球早期大气的组成将使用一维光化学模型进行研究。将讨论三个具体问题。第一个问题是，在地球历史的最初几亿年里，撞击是否将以二氧化碳为主的大气转变为以一氧化碳为主的大气。大气的氧化状态，特别是碳氧比，是生命起源的一个重要边界条件。第二个问题涉及早期大气中氨、甲烷和硫之间的耦合。如果早期的地球比现在更温暖，单质硫蒸气可能会提供一层有效的屏障，抵御太阳近紫外线辐射。通过屏蔽氨和甲烷的光解作用，这样的屏障可能允许这些气体在生命起源前的大气中存在浓度更高的气体。这也可能影响了生命的起源。第三个问题涉及从还原性气氛到氧化性气氛的转变。光化学模型将用于模拟含有低水平O2的大气（大约是现在的10-10到10-5倍），并确定一旦光合作用超过还原性气体的供应，氧气浓度将在哪里稳定下来。

The composition of the earth's early atmospheric will be studied using a one-dimensional photochemical model. Three specific questions will be addressed. The first is whether impacts could have converted a CO2-dominated atmosphere into a CO-dominated one during the first several hundred million years of the earth's history. The oxidation state of the atmosphere, and the C/O ratio in particular, is an important boundary condition on the origin of life. The second question deals with the coupling between ammonia, methane, and sulfur in the early atmosphere. Elemental sulfur vapor could have provided an effective screen against solar near-UV radiation if the early earth was warmer than today. By shielding ammonia and methane from photolysis, such a screen may have allowed much higher concentrations of these gases to exist in the prebiotic atmosphere. This, too, could have affected the origin of life. The third question concerns the transition from a reducing to an oxidizing atmosphere. The photochemical model will be used to simulate atmospheres containing low levels of O2 (about 10-10 to 10-5 times present) and to determine where the oxygen concentration would have stabilized once photosynthesis had overwhelmed the supply of reduced gases.