Magnetic Field Of The Moon
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300px, Total magnetic field strength at the surface of the Moon as derived from the Lunar Prospector electron reflectometer experiment. The magnetic field of the Moon is very weak in comparison to that of the Earth; the major difference is the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
does not have a
dipolar In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
magnetic field currently (as would be generated by a
geodynamo In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...
in its core), so that the magnetization present is varied (see picture) and its origin is almost entirely crustal in location; so it's difficult to compare as a percentage to Earth. But, one experiment discovered that lunar rocks formed 1 - 2.5 billion years ago were created in a field of about 5
microtesla The tesla (symbol: T) is the unit of magnetic flux density (also called magnetic B-field strength) in the International System of Units (SI). One tesla is equal to one weber per square metre. The unit was announced during the General Confer ...
(μT), compared to present day Earth's 50 μT. During the Apollo program several magnetic field strength readings were taken with readings ranging from a low of 6γ (6nT) at the
Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the United States' Apollo program and the fourth to Moon landing, land on the Moon. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a ...
site to a maximum of 313γ (0.31μT) at the
Apollo 16 Apollo 16 (April 1627, 1972) was the tenth crewed mission in the United States Apollo space program, administered by NASA, and the fifth and penultimate to land on the Moon. It was the second of Apollo's " J missions", with an extended sta ...
site, note these readings were recorded in gammas(γ) a now outdated unit of
magnetic flux density A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
equivalent to 1nT. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. An analysis of magnetized moon rocks brought to Earth by Apollo astronauts showed that the Moon must have had a strong (above 110 μT) magnetic field at least 4.25 billion years ago, which then fell to 20 μT level in the 3.6 - 3.1 billion years BP period. The small size of the lunar core, however, is a potential obstacle to promoting that hypothesis to the status of
theory A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
. However, single silicate grains with magnetic inclusions from Apollo rocks formed at 3.9, 3.6, 3.3, and 3.2 billion years ago have been shown to be capable of recording strong magnetic fields but do not. This supports the alternative hypothesis that the Moon never had a long-lasting core dynamo, consistent with the lack of energy needed to sustain a field. It is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s. In particular, study of Apollo impact glass associated with a young, 2 million-year-old crater has yielded a strong magnetization comparable in strength to Earth's magnetic field /sup>. This magnetization could not have originated in the lunar core, but is instead consistent with predictions from fields associated with impact plasmas /sup>. These observations have led to the hypothesis that prior reports of high paleofield strengths from Apollo samples record impacts, not a core dynamo ,6/sup>. Importantly, the lack of a long-lasting lunar dynamo and paleomagnetosphere should have allowed 3He, water, and other volatile resources acquired from solar winds and Earth’s magnetosphere over some 4 billion years to accumulate in lunar soils ,6/sup>. It has also been noted that the largest crustal magnetizations appear to be located near the
antipodes In geography, the antipode () of any spot on Earth is the point on Earth's surface diametrically opposite to it. A pair of points ''antipodal'' () to each other are situated such that a straight line connecting the two would pass through Ear ...
of the giant
impact basin An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
s. It has been proposed that such a phenomenon could result from the free expansion of an impact-generated plasma cloud around the Moon in the presence of an ambient magnetic field. For example, the
Chandrayaan-1 Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impacto ...
spacecraft mapped a "mini-magnetosphere" at the Crisium antipode on the Moon's far side, using its Sub-keV Atom Reflecting Analyzer (SARA) instrument. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. There is growing evidence that fine particles of
moondust Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disintegra ...
might actually float, ejected from the lunar surface by
electrostatic repulsion Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ambe ...
. This could create a temporary nighttime "atmosphere" of dust. The moondust atmosphere might also gather itself into a sort of diaphanous wind. Drawn by differences in global charge accumulation, floating dust would naturally fly from the strongly negative nightside to the weakly negative dayside. This "dust storm" effect would be strongest at the Moon's
terminator Terminator may refer to: Science and technology Genetics * Terminator (genetics), the end of a gene for transcription * Terminator technology, proposed methods for restricting the use of genetically modified plants by causing second generation s ...
. Much of these details are still speculative, but the
Lunar Prospector ''Lunar Prospector'' was the third mission selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, ...
spacecraft detected changes in the lunar nightside voltage during magnetotail crossings, jumping from -200 V to -1000 V. Further characterization was done by the
Lunar Atmosphere and Dust Environment Explorer The Lunar Atmosphere and Dust Environment Explorer (LADEE; ) was a NASA lunar exploration and technology demonstration mission. It was launched on a Minotaur V rocket from the Mid-Atlantic Regional Spaceport on September 7, 2013. During its sev ...
orbiter in late 2013. The
plasma sheet In the magnetosphere, the plasma sheet is a sheet-like region of denser (0.3-0.5 ions/cm3 versus 0.01-0.02 in the lobes) hot plasma and lower magnetic field near the equatorial plane, between the magnetosphere's north and south lobes. A magneto ...
is a very dynamic structure, in a constant state of motion, so as the Moon orbits through the magnetotail the plasma sheet can sweep across it many times with encounters lasting anywhere from minutes to hours or even days.


In fiction

In the ''
Space Odyssey The ''Space Odyssey'' series is a series of science fiction novels by the writer Arthur C. Clarke. Two of the novels have been made into feature films, released in 1968 and 1984 respectively. Two of Clarke's early short stories may also be c ...
'' series by Arthur C. Clarke, a
monolith A monolith is a geological feature consisting of a single massive rock (geology), stone or rock, such as some mountains. For instance, Savandurga, Savandurga mountain is a monolith mountain in India. Erosion usually exposes the geological for ...
is found on the Moon near the crater Tycho by its unnaturally powerful magnetic field and named ''Tycho
Magnetic Anomaly In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...
1'' (TMA-1).


See also

* Gravity field of the Moon *
Topography of the Moon Selenography is the study of the surface and physical features of the Moon (also known as geography of the Moon, or selenodesy). Like geography and areography, selenography is a subdiscipline within the field of planetary science. Historical ...


References

{{Magnetospherics Lunar science Geomagnetism