内行星大气海洋的成因和初期演化

Abstract

只要太阳系的内行星(水星、金星、地球、月亮和火星)是由含碳酸盐矿物和含水矿物的微星物质所依附会聚而来的，内行星大气和海洋的物质主要是与生俱来的，而不是在成长后再附加上去的。在早期会聚成长中，金星、地球、火星体内应已埋藏了相当于今日地球表面的海洋水量，而它们成长后的原始大气是由大于95%的CO2所组成。金星和火星至今保存着它们的原始大气，而介于两者之间的地球却因造月大撞击改变了命运。大撞击后的地球获得了月亮，大撞击也把地球体内大部分的H2O气化进入原始CO2大气而结合成超临界H2O-CO2流体。此物在地表温度冷却到摄氏450~300度之间应会凝聚依附地表形成原始海洋，高温原始海洋会与地表最多的长石作用形成碳酸盐，从而把地球原始大气中的CO2悉数移入海底。火星的引力小到无法抓获气态H2O分子成为它大气的一员，但火星体内和表层物质中绝对不欠缺H2O的。 For as long as the terrestrial planets (Mercury, Venus, Earth, Moon and Mars) were accreted from planetesimals containing some carbonates and hydrous minerals, the majority of the materials that form today’s terrestrial planet atmospheres and oceans should be inherent. An amount of H2O that is roughly equivalent to today’s Earth oceans should be buried inside Venus, Earth and Mars during the early stage of accretion. After accretion, the proto-atmospheres of Venus, Earth and Mars likely consisted of more than 95% CO2 which are probably kept by both Venus and Mars till today. The fate of the Earth, on the other hand, had been changed by the Moon-forming giant impact. After the giant impact, the Earth gained its Moon and released most of its H2O contents into the CO2 proto-atmosphere. At such temperature and pressure conditions, H2O and CO2 would form a supercritical H2O-CO2 fluid which would later precipitate to yield the indigenous oceans when Earth’s surface temperature cooled down to 450˚C - 300˚C. The hot indigenous ocean reacted with feldspars, the most abundant surface minerals, to form carbonates, thus removing all CO2 from Earth’s proto-atmosphere eventually. The gravity force of Mars is too small to hold gaseous H2O in its atmosphere, but this does not necessarily imply that Mars’ interior or its surface materials do not contain H2O.