Lunar swirls are enigmatic features found across 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 ...

's surface, which are characterized by having a high

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...

, appearing optically immature (i.e. having the optical characteristics of a relatively young

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...

), and (often) having a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls. They appear to overlay the lunar surface, superposed on craters and ejecta deposits, but impart no observable topography. Swirls have been identified on the

lunar maria The lunar maria (; singular: mare ) are large, dark, basaltic plains on Earth's Moon, formed by ancient asteroid impacts on the far side on the Moon that triggered volcanic activity on the opposite (near) side. They were dubbed , Latin for 'seas' ...

and on

highlands Highland is a broad term for areas of higher elevation, such as a mountain range or mountainous plateau. Highland, Highlands, or The Highlands, may also refer to: Places Albania * Dukagjin Highlands Armenia * Armenian Highlands Australia *Sou ...

- they are not associated with a specific lithologic composition. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain appear less prominent and exhibit simpler shapes, such as single loops or diffuse bright spots.

Association with magnetic anomalies

The lunar swirls are coincident with regions of the

magnetic field of the Moon 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 Mo ...

with relatively high strength on a planetary body that lacks, and may never have had, an active core dynamo with which to generate its own magnetic field. Every swirl has an associated magnetic anomaly, but not every magnetic anomaly has an identifiable swirl. Orbital magnetic field mapping by the Apollo 15 and 16 sub-satellites, Lunar Prospector, and Kaguya show regions with a local magnetic field. Because the Moon has no currently active global magnetic field, these regional anomalies are regions of remnant magnetism; their origin remains controversial.

Formation models

There are three leading models for swirl formation. Each model must address two characteristics of lunar swirls formation, namely that a swirl is optically immature, and that it is associated with magnetic anomaly. Models for creation of the magnetic anomalies associated with lunar swirls point to the observation that several of the magnetic anomalies are antipodal to the younger, large impact basins on the Moon.

Cometary impact model

This theory argues that the high albedo of the swirls is the result of an impact with a comet. The impact would cause scouring of the top-most surface regolith by the coma's turbulent flow of gas and dust, which exposed fresh material and redeposited the fine, scoured material in discrete deposits. According to this model, the associated strong magnetic anomalies are the result of magnetization of near-surface materials heated above the

Curie temperature In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...

through hyper-velocity gas collisions and micro-impacts as the coma impacted the surface. Proponents of the cometary impact model consider the occurrence of many swirls antipodal to the major basins to be coincidental or the result of incomplete mapping of swirl locations.

Solar wind shielding model

This theory argues that swirls are formed because lighter-colored regolith is protected from the

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

due to a magnetic anomaly. The swirls represent exposed silicate materials whose albedos have been selectively preserved over time from the effects of space weathering via deflection of solar wind ion bombardment. According to this model, optical maturation of exposed silicate surfaces is a result of solar wind ion bombardment. This model suggests that swirl formation is a continuing process, which began after creation of the magnetic anomaly. Mathematical simulations conducted in 2018 showed that

lava tubes A lava tube, or pyroduct, is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave. Formation A lava tube is a type of lava ca ...

could have become magnetic as they cooled, which would provide a magnetic field consistent with the observations near the lunar swirls.

Dust transport model

This theory argues that weak electric fields created by interaction between the crustal magnetic anomalies and the solar wind plasma could attract or repel electrically charged fine dust. High–albedo feldspathic material is the dominant component of the finest particles of lunar soil. Electrostatic movement of dust lofted above the surface during terminator crossings could cause this material to preferentially accumulate and form the bright, looping swirl patterns.

Satellite measurements

Direct magnetic observations of the lunar swirls have been conducted by several lunar spacecraft, including ''

Clementine A clementine (''Citrus × clementina'') is a tangor, a citrus fruit hybrid between a willowleaf mandarin orange ( ''C.'' × ''deliciosa'') and a sweet orange (''C. × sinensis''), named in honor of Clément Rodier, a French missionary who fir ...

'' and '' Lunar Prospector''. The results of these observations are inconsistent with the Cometary impact model. Further observations by the '' Lunar Reconnaissance Orbiter'' support the theory that solar wind is being deflected by a magnetic field. Spectral observations by the

Moon Mineralogy Mapper The Moon Mineralogy Mapper (M3) is one of two instruments that NASA contributed to India's first mission to the Moon, Chandrayaan-1, launched October 22, 2008. It is an imaging spectrometer, and the team is led by Principal investigator Carle Piet ...

instrument on ''

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 impact ...

'' confirmed that the lighter-colored regions are deficient in

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

, which also supports the hypothesis that solar wind is being deflected in the pale areas. , a

CubeSat A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSats ...

mission concept is under study at NASA, with the goal of understanding the formation of the lunar swirls. The proposed '' Bi-sat Observations of the Lunar Atmosphere above Swirls'', or ''BOLAS'' mission would involve two small satellites connected with a space tether. The lower CubeSat would orbit at an altitude of six miles above the surface.

Onsite investigations

NASA has proposed sending a rover to Reiner Gamma to obtain in-situ observations of the surface materials there. Run by the

Johns Hopkins Applied Physics Laboratory The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated with Johns Hopkins University and emplo ...

, the Lunar Vertex project projects a 2024 mission with a lander and a rover. The rover, carrying a multispectral microscope, will determine coarseness and brightness of surface particles, and transmit its data to the lander, which will communicate with Earth-based handlers.