Reiner Gamma (γ) is an albedo feature that is located on the Oceanus Procellarum, to the west of the crater Reiner on the Moon. The center of the formation is located at selenographic coordinates . It has an overall dimension of about 70 kilometres. The feature has a higher albedo than the relatively dark mare surface, with a diffuse appearance and a distinctive swirling, concentric oval shape. Related albedo features continue across the surface to the east and southwest, forming loop-like patterns over the mare.
Reiner Gamma is not associated with any particular irregularities in the surface, and so the cause was a mystery until similar features were discovered in Mare Ingenii and Mare Marginis on photographs taken by orbiting spacecraft. The feature on Mare Ingenii is located at the lunar opposite point from the center of Mare Imbrium. Likewise the feature on Mare Marginis is opposite the midpoint of Mare Orientale. Thus it is believed that the feature resulted from seismic energies generated by the impacts that created these maria. Unfortunately there is no such lunar mare formation located precisely on the opposite surface of the Moon, although the large crater Tsiolkovskiy lies within one crater diameter.
The central feature of Reiner Gamma does bear a resemblance to the dipolar formation created by iron filings on a surface with a bar magnet on the underside. Low-orbiting spacecraft have observed a relatively strong magnetic field associated with each of these albedo markings. Some have speculated that this magnetic field and the patterns were created by cometary impacts. However the true cause remains uncertain.
Reiner Gamma's magnetic field strength is approximately 15 nT, measured from an altitude of 28 km. This is one of the strongest localized magnetic anomalies on the Moon. The surface field strength of this feature is sufficient to form a mini-magnetosphere that spans 360 km at the surface, forming a 300 km thick region of enhanced plasma where the solar wind flows around the field. As the particles in the solar wind are known to darken the lunar surface, the magnetic field at this site may account for the survival of this albedo feature.
In early lunar maps by Francesco Maria Grimaldi, this feature was incorrectly identified as a crater. His colleague Giovanni Riccioli then named it Galilaeus, after Galileo Galilei. The name was later transferred northwest to the current crater Galilaei.
- Huddleston, Marvin W. "Lunar Swirls, Magnetic Anomalies, and the Reiner Gamma Formation". Association of Lunar & Planetary Observers. Retrieved 2010-04-19.
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- Wieser, Martin et al. (March 2010). "First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms". Geophysical Research Letters 37 (5): L05103. arXiv:1011.4442. Bibcode:2010GeoRL..3705103W. doi:10.1029/2009GL041721.
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- Ewen A. Whitaker, Mapping and Naming the Moon: A History of Lunar Cartography and Nomenclature, Cambridge University Press, 1999, ISBN 0-521-62248-4.
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- Overexposed 35mm Nikon camera image of Reiner Gamma from Apollo 17: AS17-158-23895
- Lunar Orbiter 4 image showing Reiner Gamma in upper left: IV-157-H1
- Oblique Lunar Orbiter 2 image showing part of Reiner Gamma: II-215