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Lunar Reconnaissance Orbiter WideAngle Camera image of ReinerGamma

Another view of Reiner Gammaswirls

Lunar swirlsFrom Wikipedia, the free encyclopedia

Lunar swirls are enigmatic features found across the Moon’ssurface, which are characterized by having a high albedo, appearingoptically immature (i.e. having the optical characteristics of arelatively young regolith), and (often) having a sinuous shape. Theircurvilinear shape is often accentuated by low albedo regions thatwind between the bright swirls. They appear to overlay the lunarsurface, superposed on top of craters and ejecta deposits, but impartno observable topography. Swirls have been identified on the lunarmaria and highlands ­ they are not associated with a specificlithologic composition. Swirls on the maria are characterized bystrong albedo contrasts and complex, sinuous morphology, whereasthose on highland terrain appear less prominent and exhibit simplershapes, such as single loops or diffuse bright spots.

Contents

1 Association with magnetic anomalies2 Formation models3 References4 External links

Association with magnetic anomalies

The lunar swirls are coincident with regions of relatively highmagnetic field strength on a planetary body that does not, and maynever have had, an active core dynamo with which to generate itsown magnetic field. Every swirl has an associated magneticanomaly, but not every magnetic anomaly has an identifiable swirl.Orbital magnetic field mapping by the Apollo 15 and 16 sub­satellites, Lunar Prospector, and Kaguya show regions with a local magnetic field. Because the Moon hasno currently active global magnetic field, these regional anomalies are regions of remanent magnetism;their origin remains controversial.

Formation models

There are three leading models for swirl formation. There are two characteristics of lunar swirls that anymodel of their formation must take into account:

1. the lunar swirls are optically immature, and2. the lunar swirls are associated with magnetic anomalies.

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Mare Ingenii

Another view of Ingenii

Models for creation of the magnetic anomalies relevant to swirlformation, although independent of the swirls' opticalcharacteristics, point to the observation that several of the magneticanomalies are antipodal to the younger, large impact basins.[1]Magnetization of these antipodal regions could occur in the presenceof an amplified magnetic field, such as would result from a plasmacloud generated by the basin forming impact interacting with a weakmagnetic field present at the Moon at the time of basin formation.[2]An unusually thick and/or strongly magnetized deposit of basinejecta has been invoked to explain swirls that are not antipodal to abasin, such as the Reiner Gamma Formation.[3] An alternativemodel to a plasma­induced magnetic anomaly, but which also drawsupon the basin­antipode correlation is by impact­induced currentsand seismic waves ringing the planet.[4]

Once the magnetic anomaly has been installed, the swirls are formedbecause they are protected from the solar wind. The solar windshielding model[5] proposes that the swirls represent exposedsilicate materials whose albedos have been selectively preservedover time from the effects of space weathering via deflection ofsolar wind ion bombardment. According to this model, opticalmaturation of exposed silicate surfaces in the inner solar system is atleast partly a function of the solar wind ion bombardment. Thismodel suggests that swirl formation is a continuing process, whichbegan after placement of the magnetic anomaly.

The cometary impact model[6] argues that the high albedo of theswirls is the result of scouring of the top­most surface regolith bythe coma’s turbulent flow of gas and dust, which exposed fresh material and redeposited the fine, scouredmaterial in discrete deposits.[7] According to this model, the associated strong magnetic anomalies are theresult of magnetization of near­surface materials heated above the Curie temperature through hyper­velocity gas collisions and micro­impacts as the coma impacts the surface. Proponents of the cometaryimpact model consider the occurrence of many swirls antipodal to relatively young, major basins to becoincidental or the result of incomplete mapping of swirl locations.

The dust transport model[8] proposes that weak electric fields created by interaction between the crustalmagnetic anomalies and the solar wind plasma could attract or repel electrically charged fine dust. Highalbedo, feldspathic material is the dominant component of the finest particles of lunar soil. Electrostaticmovement of dust lofted above the surface during terminator crossings could cause this material topreferentially accumulate and form the bright, looping swirl patterns.

References1. L. L. Hood, P. J. Coleman, & D. E. Wilhelms (1979). "The Moon: Sources of the crustal magnetic anomalies".Science 204: 53–57. Bibcode:1979Sci...204...53H. doi:10.1126/science.204.4388.53.

2. L. L. Hood & D. E. Williams (1989). "The lunar swirls ­ Distribution and possible origins". Proceedings of the

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Wikimedia Commons hasmedia related to Lunarswirls.

Lunar and Planetary Science Conference 19: 99–113.3. L. L. Hood & A. Zakharian, J. Halekas, D. L. Mitchell, R. P. Lin, M. H. Acuña, & A. B. Binder (2001). "Initial

mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data".Journal of Geophysical Research 106: 27825–27840. Bibcode:2001JGR...10627825H.doi:10.1029/2000JE001366.

4. G. Kletetschka, F. Freund, P. J. Wasilewski, V. Mikula, & T. Kohout (2005). "Antipodal Magnetic Anomalieson the Moon, Contributions from Impact Induced Currents Due to Positive Holes and Flexoelectric Phenominaand Dynamo". Lunar and Planetary Science Conference 36: 1854.

5. L. L. Hood & G. Schubert (1980). "The Moon: Lunar magnetic anomalies and surface optical properties".Science 208: 49–51. Bibcode:1980Sci...208...49H. doi:10.1126/science.208.4439.49.

6. P. H. Schultz & L. J. Srnka (1980). "Cometary collisions on the Moon and Mercury". Nature 284: 22–26.Bibcode:1980Natur.284...22S. doi:10.1038/284022a0.

7. P. C. Pinet, V. V. Shevchenko, S. D. Chevrel, Y. Daydou, & C. Rosemberg, (2000). "Local and regional lunarregolith characteristics at Reiner Gamma Formation: Optical and spectroscopic properties from Clementine andEarth­based data" 105. pp. 9457–9476. Bibcode:2000JGR...105.9457P. doi:10.1029/1999JE001086.

8. Garrick­Bethell, Ian; et al. (2011). "Spectral properties, magnetic fields, and dust transport at lunar swirls".Icarus 212 (2). Bibcode:2011Icar..212..480G. doi:10.1016/j.icarus.2010.11.036.

External links

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Categories: Geological features on the Moon

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