REINER GAMMA: LUNAR ENIGMA
 

By Daniel DelValle, Raffaello Lena, Massimo Giuntoli, Guido Santacana

There are strange and unique landforms on the Moon that defy description. They can be found in both lunar maria and highlands. The usual descriptions and understandings of lunar features do not apply to them. Exogenic and endogenic processes cannot explain their origin. Their unusual morphology may be the result of an interplay of processes. The mystery of these unusual features will remain until these areas are  mapped and sampled by future lunar missions.
 There is one enigmatic landform which is of easy access for a telescope. Near the crater Reiner, in Oceanus Procellarum, is Reiner Gamma. It is apparently an entirely flat feature formed by bright material. This bright material appears as a swirl-like deposit. Relatively young in appearance, Reiner Gamma has very few craters that overlie its area. Its swirls are related to a very strong magnetic field which was detected by orbiting spacecraft. Similar deposits of bright swirls can be found north of Mare Marginis and on the far side near the crater Van de Graaff.
 The formation of Reiner Gamma is uncertain. One theory is that volcanic gases vented from the surface discoloring the surface materials and causing the swirls. Another theory is that intense surface magnetic fields have shielded the surrounding regolith from darkening by the solar wind. So far these theories are concerned with endogenic processes. A third theory proposes an exogenic process: Reiner Gamma is the surface remnant of a comet impact. In this scenario hot plasmas and gases from the comet's nucleus scoured the surface creating the swirls. The intense magnetic field of the nucleus induced the local magnetic anomaly. Each of these theories is however far from conclusive. Reiner Gamma remains an enigma.
 An observer's challenge has been made by Harold Hill in his book, A Portfolio of Lunar Drawings. The challenge is to attempt to detect any discernable relief under very low lighting. This would mean to observe Reiner Gamma when on the terminator.

Lena and Giuntoli observed Reiner Gamma close to the terminator. It appears like an albedo feature.

Giuntoli on 27 october 1993 reported that the terminator is approximately bisecting reiner gamma but all thet one can see is a series of not very elevated ridges. Nothing is suggesting a mountain or a plateau, so it seems certain that reiner gamma is merely an albedo feature and not a topographical feature.

R. Lena. Observation carried out on 21 august 1997 at 23:30 UT. SCT 25 cm f/10 250X.

Reiner Gamma -Lunar orbiter image

Reiner Gamma- image processed using false color

 Another useful program would be to ascertain if there are any domes in the immediate area. Hill has reported a dome near the crater Reiner (East direction). Del Valle reported a suspected dome at longitude 55.09 W and latitude 8.05 N. It is of very low relief and elliptical in  shape. No detail cold be seen, such as a craterlets, on it.
 

D. Del Valle. Observation carried out on 3 june 2001 at 23:54 UT. SCT 20 cms f/10 339x.
 

 Santacana reported the presence of two small 3 km domes near reiner at about longitude 52.0 W and latitude 8.0 N.

G. Santacana. Observation carried out on 28 february 1999 at 00:38 UT, SCT 20 cm f/10 444x.

R. Lena. Observation carried out on 25 june 1999 at 21:00 UT 300x using a refractor 10 cms f/15. Two dome to the E and W direction.

CONCLUSION
Until spacecraft arrive at this peculiar feature to gather meaningful data, it will be up to the lunar observer, sitting in the backyard separated from our sister world by a few pieces of polished glass, to try to unravel the mystery of Reiner Gamma.