Theatmosphere of the Moon is a very sparse layer of gases surrounding theMoon, consisting only of anexosphere.[5] For most practical purposes, the Moon is considered to be surrounded byvacuum. The elevated presence of atomic and molecular particles in its vicinity compared tointerplanetary medium, referred to as "lunar atmosphere" for scientific objectives, is negligible in comparison with the gaseous envelopes surroundingEarth and mostplanets of theSolar System, and comparable to their exospheres. The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and has a total mass of less than 10 metric tonnes.[6][7] 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 gases are lost into space.
Roger Joseph Boscovich was the first modern astronomer to argue for the lack of atmosphere around the Moon in hisDe lunae atmosphaera (1753).
One source of the lunar atmosphere isoutgassing: the release of gases such asradon andhelium resulting fromradioactive decay within thecrust andmantle. Another important source is the bombardment of the lunar surface bymicrometeorites, thesolar wind, andsunlight, in a process known assputtering.[8]
Gases can:
What little atmosphere the Moon has consists of some unusual gases, includingsodium andpotassium, which are not found in the atmospheres of Earth,Mars, orVenus. At sea level on Earth, each cubic centimeter of the atmosphere contains approximately 1019 molecules; by comparison the lunar atmosphere contains fewer than 106 molecules in the same volume. On Earth, this is considered to be a very good vacuum. In fact, the density of the atmosphere at the Moon's surface is comparable to the density of some of the outermost fringes of Earth's atmosphere, where theInternational Space Station orbits.[9]
The elements sodium and potassium have been detected in the Moon's atmosphere using Earth-basedspectroscopic methods, whereas the isotopesradon-222 andpolonium-210 have been inferred from data obtained by theLunar Prospectoralpha particlespectrometer.[10]Argon-40,helium-4,oxygen and/ormethane (CH4),nitrogen (N2) and/orcarbon monoxide (CO), andcarbon dioxide (CO2)) were detected by in-situdetectors placed by theApollo astronauts.[11]
The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows:
This yields approximately 80,000 total atoms per cubic centimeter, marginally higher than the quantity posited to exist in theatmosphere of Mercury.[11] While this greatly exceeds the density of the solar wind, which is usually on the order of just a few protons per cubic centimeter, it is virtually a vacuum in comparison with the atmosphere of the Earth.
The Moon may also have a tenuous "atmosphere" ofelectrostatically levitated dust. SeeLunar soil for more details.
In October 2017,NASA scientists at theMarshall Space Flight Center and theLunar and Planetary Institute inHouston announced their finding, based on studies of Moon magma samples retrieved by theApollo missions, that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected fromlunar volcanic eruptions, was twice the thickness of that of present-dayMars. It has been theorized that this ancient atmosphere could have supported life, though no evidence of life has been found.[14] The ancient lunar atmosphere was eventuallystripped away by solar winds and dissipated into space.[15]
Neon was detected over the nightside at levels comparable to He and was found to exhibit the spatial distribution of a surface accommodated noncondensable gas.
This article incorporatespublic domain material fromIs There an Atmosphere on the Moon?.National Aeronautics and Space Administration.
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Atmospheres in boldface are significant atmospheres; atmospheres in italics are unconfirmed atmospheres. | ||
