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| Atmosphere of the Moon ： ウィキペディア英語版 |
Atmosphere of the Moon
For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity (compared to interplanetary medium), referred to as 'lunar atmosphere' for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar system—less than one hundred trillionth (10−14) of Earth's atmospheric density at sea level. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its atmosphere is lost to space.
== Sources ==
One source of the lunar atmosphere is outgassing the release of gases such as radon and helium resulting from radioactive decay within the crust and mantle. Another important source is the bombardment of the lunar surface by micrometeorites, the solar wind, and sunlight, in a process known as sputtering.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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