| Abstract from DBPedia | The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collisionless. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space. It is located directly above the thermosphere. Very little is known about it due to lack of research. Mercury, the Moon, Ceres, Europa, and Ganymede have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly hydrogen and helium, with some heavier atoms and molecules near the base.外気圏（がいきけん、exosphere）は、大気層のうち、最も外側の層である。さらにその外側の宇宙空間との境界部分は、Surface boundary exosphere：SBE、表面境界外気圏という（こちらは月などの厚い大気層をもたない天体にも存在する）。地球では、その下層の熱圏との境界は高度500 - 1,000 kmで、高さは約10,000 kmに及ぶ。外気圏からは大気の気体分子や原子が宇宙空間に大量に流出している。人工衛星の軌道では低軌道の上半分と中軌道の下半分に相当する。 (Source: http://dbpedia.org/resource/Exosphere) |
