August’s top lunar feature to observe
Reiner Gamma
Type: Lunar swirl
Size: 40x30km
Longitude/Latitude: 59˚ W, 7.4˚ N
Age: Less than 1.1 billion years
Best time to see: Full Moon until three days after last quarter (11–22 August)
Minimum equipment: 50mm refractor
Reiner Gamma is an enigmatic albedo feature visible in the Oceanus Procellarum. It’s classed as a lunar swirl: a high albedo, youthful feature which typically has a curving or sinuous shape. Lunar features divide into two categories, relief and albedo. Relief features have height, and examples include mountains, hills and crater rims, but the height doesn’t need to be upward: deep crater floors, valleys and rilles also count as relief features. The chief characteristic of a relief feature is that it will cast shadows when lighting is oblique. At times when the lighting is directly from above, such features may virtually disappear from view.
Albedo features are visible due to the reflective characteristics of the material which forms them. The most obvious albedo features visible from Earth are the lunar seas, the lava of their surfaces seeming darker than the surrounding lunar highlands.
The young material which forms the swirl known as Reiner Gamma is visible because it has a higher reflectivity (albedo) than the dark lava of Oceanus Procellarum on which it sits.
Theories include the swirls being formed when a comet impacts the lunar surface
It’s a curious feature which lacks any height, and this is very evident when the Sun is low in its sky, because it casts no shadows. It appears like a stain on Procellarum’s surface, the central portion (the part that’s named Reiner Gamma) being eye-shaped with swirling patterns surrounding it.
The patterns continue east for a distance of around 60km before heading in a pseudo-linear fashion to the north-northeast. A similar extension emanates to the southwest of the ‘eye’ but is less well defined than the northnortheast streak.
Various explanations have been put forward for the formation of lunar swirls, which always tend to be located in regions showing magnetic field anomalies. Theories include the swirls being formed when a comet impacts the lunar surface, or perhaps young and bright surface material being shielded by the darkening effects of solar wind space weathering. Another theory describes fine, high-albedo dust particles being lifted above the lunar surface electrostatically, to be deposited within the magnetic anomaly regions.
Measurements of the magnetic field from a height of 28km above Reiner Gamma report a strength of 15nT (15 nanoteslas), one of the strongest magnetic anomalies known on the Moon. This field strength is sufficient to create a localised magnetosphere which would act like a 360km diameter shield over the swirl, perhaps explaining why it looks so fresh in definition.
The actual formation mechanism remains a bit of a mystery to this day. Some other swirls are located at the diametrically opposite point on the lunar surface from a major impact zone. However, there seems no such feature which could have resulted in seismically focused energy to lift surface material on the other side of the Moon to form Reiner Gamma.
The feature gets its name from the 30km crater Reiner which lies 120km to the east.
Reiner is foreshortened into an oval as we see it from Earth, but is well defined with a sharp rim which leads steeply down to a level floor. The floor has a bump in the centre and appears malformed in its southern half, with a curious, almost folded cleft line running north–south.