Atmosphere of the Moon

Surveyor 7observes levitating dust, a phenomenon namedLunar horizon glowcan be seen

The thin lunar atmosphere is visible on the Moon's surface atsunriseandsunsetwith thelunar horizon glowand lunar twilight rays, like Earth'screpuscular rays.ThisApollo 17sketch depicts the glow and raysamong the generalzodiacal light.

Theatmosphere of the Moonis a very sparse layer of gases surrounding theMoon.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 surroundingEarthand mostplanetsof theSolar System.The pressure of this small mass is around 3×10⁻¹⁵atm(0.3nPa), varying throughout the day, and in total mass less than 10 metric tonnes.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 Boscovichwas the first modern astronomer to argue for the lack of atmosphere around the Moon in hisDe lunae atmosphaera(1753).

Sources

One source of the lunar atmosphere isoutgassing:the release of gases such asradonandheliumresulting fromradioactive decaywithin thecrustandmantle.Another important source is the bombardment of the lunar surface bymicrometeorites,thesolar wind,andsunlight,in a process known assputtering.

Escape velocity and atmospheric hold

Gases can:

• be re-implanted into theregolithas a result of theMoon's gravity;
• escape the Moon entirely if the particle is moving at or above the lunarescape velocityof 2.38 km/s (1.48 mi/s), or 5,328 mph (8,575 km/h);
• be lost to space either by solarradiation pressureor, if the gases areionized,by being swept away in the solar wind'smagnetic field.

Composition

What little atmosphere the Moon has consists of some unusual gases, includingsodiumandpotassium,which are not found in the atmospheres of Earth,Mars,orVenus.At sea level on Earth, each cubic centimeter of the atmosphere contains approximately 10¹⁹molecules; by comparison the lunar atmosphere contains fewer than 10⁶molecules 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 Stationorbits.

The elements sodium and potassium have been detected in the Moon's atmosphere using Earth-basedspectroscopic methods,whereas the isotopesradon-222andpolonium-210have been inferred from data obtained by theLunar ProspectorAlpha particlespectrometer.Argon-40,helium-4,oxygenand/ormethane(CH₄),nitrogen(N₂) and/orcarbon monoxide(CO), andcarbon dioxide(CO₂)) were detected by in-situdetectorsplaced by theApolloastronauts.

The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows:

• Argon:20,000–100,000
• Helium:5,000–30,000
• Neon:up to 20,000
• Sodium:70
• Potassium:17
• Hydrogen:fewer than 17

This yields approximately 80,000 total atoms per cubic centimeter, marginally higher than the quantity posited to exist in theatmosphere of Mercury.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 levitateddust. SeeLunar soilfor more details.

Ancient atmosphere

In October 2017,NASAscientists at theMarshall Space Flight Centerand theLunar and Planetary InstituteinHoustonannounced their finding, based on studies of Moon magma samples retrieved by theApollomissions, 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.The ancient lunar atmosphere was eventuallystripped awayby solar winds and dissipated into space.

See also

• Atmosphere of Mercury
• Exosphere
• Lunar Atmosphere and Dust Environment Explorer(LADEE)
• Orders of magnitude (pressure)
• Sodium tail of the Moon

References

This article incorporatespublic domain materialfromIs There an Atmosphere on the Moon?.National Aeronautics and Space Administration.