Local Topography and Reiner Gamma

Just what is "Reiner Gamma?" What characteristics does this well-known and most studied of the so-called 'swirl' markings on the Moon share with the countless other, similar bright albedo markings found on its surface? What puts Reiner Gamma in a class by itself? It's meandering and complex pattern seems finely printed like a photograph negative on the very surface along a 380 km-long path in the western Oceanus Procellarum, from among the Marius Domes southwest to the mountains of the ocean's edge. LRO laser altimetry (LOLA Image of the Week, released May 21) seems to confirm the position held by most observers that Reiner Gamma is without "topographic expression," invisible to high-resolution laser point elevation surveys from low orbit. However, LRO photographs centered on Reiner Gamma in the longest shadows of local sunrise appear to show, however briefly, a very shallow set of ridges and other features corresponding to the swirl phenomena, and LOLA investigators were hasty in concluding otherwise [NASA/GSFC/LOLA].

Since 1979 we've amassed three volumes filled with studies as part of an effort to understand so-called "swirl" phenomena on the Moon. Not surprisingly, a majority of that work centers on Reiner Gamma, the atypical stain of "ray-like" bright material stretching across the western Oceanus Procellarum and immediately obvious to any of those pioneering few who first traced out the features of the Moon's near side after the development of the telescope. For any feature there to stand out as being in a class by itself meant Reiner Gamma Anomaly (RGA) had to truly be unique.

In the modern era, or at least since the Moon's far side and more complete views its western and eastern hemispheres, and the context of the entire Moon's surface, became a part of our understanding of the Moon following the opening of the Space Age, scientists have come to see Reiner Gamma as merely the most grand example of so-called swirl phenomena. Other markings that share Reiner Gamma's signature characteristics were discovered all over the Moon, particularly in the middle latitudes south of the equator on the far side and in similar latitudes in the north along the eastern limb, as seen from Earth, particularly in around Mare Marginis and Goddard.

Remote spectrography, orbital photography and other instrumental surveys of the surface strongly infer a link between the bright bifurcated swatches of these relatively bright "anomalous albedo" markings with patches of crustal magnetism viewed as equally anomalous on a Moon where little or no global magnetic field is found.

Above is a modern global examination of lunar magnetic anomalies, showing the correspondence between some of the more intense clusters of local field lines to be veritable shadows, existing at the antipodes, of the most familiar impact basins on the near side. On the nearside, however, where locally intense magnetic clumps detected at low orbital altitudes exist over both Reiner Gamma in the west and over the Descartes mountains south and east of the central meridian, no such mirror relationship with a "basin-forming impact" has been found.

In addition, both of these near side magnetic anomalies appear to carve out "mini-magnetospheres" that are intense enough to cavitate the solar wind and interplanetary magnetic field, just as Earth does on a much larger scale. Among the far side's more numerous crustal magnetic field clusters, the clump "antipodal" to Mare Crisium also demonstrates intriguing evidence of a mini-magnetosphere.

Nearside equatorial swirls not linked with any shocking impact that occurred at polar opposite locations on the Moon's far side, unlike those opposite Mare Imbrium, for example, which dominates the top of this Full Moon as seen from Earth. Reiner Gamma has always stood out from the dark "optically mature" and smooth basin floor of Oceanus Procellarum. Though comparatively bright, the albedo swirls of the southern central highlands, over Descartes and northwest of Airy craters were unambiguously identified as swirl phenomena after attendant magnetic anomalies were detected in data collected from low, end-of-mission orbital altitude by Lunar Prospector (1998-1999).

Rare phenomena on the Moon's near side, equatorial albedo swirls are draped on the surface of the Moon apparently without regard to the nature of the topography upon where they lay, are clearly linked to locally intense magnetic fields. The lunar surface has been shown generally unable to resist space weathering and a roughly 2 million year cycle of micrometeorite gardening or, in particular, a dark reddening that should obliterate a ray system like that of Tycho, longer than about 900 million years.

Since the swirls of Mare Ingenii on the southern far side are linked to "basin-forming impacts" events nearly 4 billion years old, how do they stay stubbornly fresh? These magnetic fields have been shown, especially where intense enough to cavitate voids in the solar wind, to be sufficient to buffer against that most pervasive of weathering caused by solar particles and solar particle events. But even the strongest of these present no hedge against relativistic cosmic rays, for example, or micro-meteor bombardment.

The answer is not the influence of comets or cometary tails, as some have long suggested. Rather, there now appears to be evidence of a link with the deposition of peculiar kinds of the nanophase iron found everywhere on the Moon, perhaps made peculiar by the influence of the magnetic anomalies themselves, and daily transport of newly-gardened alternately ionized sub-micron-sized lunar dust. Still, the nature of the Moon's local magnetism is clearly not the same in every situation. Perhaps something a little different is happening at Reiner Gamma.

Oceanus Procellarum, 'the Ocean of Storms,' still poses mysteries. It's not round, for one thing, though it might be part of the remains of the hypothetical 'Gargantuan' impact, older and much larger even than four billion-year-old South Pole-Aitken (SPA) Basin. If confirmed, estimates gauge the Gargantuan 'event' to have created a basin larger than half the Moon centered on what today is northwest Mare Tranquillitatis. It's offered as explanation for, among other things, more obvious differences in character between the Moon's near and far sides.

Reiner Gamma is seen here in context, it's distinctive "eye" pooling just left, or west, of bottom center and rope-like streamers running from the southwest and northwest into the very middle of the Marius Hills., Those volcanic domes, seen here below center as hair-like structures on the north side of the ring of lava-inundated Marius, south of Aristarchus Plateau, have been proposed as vents of a single shield volcano that Chinese scientists have named Yutu. [Astronominsk/LPOD].

From it's distinctive central 'eye," Reiner Gamma's long uninterrupted northeastward streamer reaches into the heart of 'Yutu," directly among the low Marius Hills. That seems to link RG with underground phenomena, with sub-surface morphology and perhaps even with the network that includes the Harayuma Skylight 'cave' entrance. Unlike most lunar magnetic anomalies, the low optical maturity of materials on the surface within its influence at Reiner Gamma is not yet linked with any massive impact on the polar opposite side of the Moon. Unlike most lunar swirls, RG is lengthy, and even indirectly linked with the very widespread and sustained volcanism centered at Yutu, Aristarchus Plateau, Rümker, the Gruithuisen domes, etc., there's enough evidence for a link just shy of scientific certainty. [LO-IV-157-H2/NASA/JPL/USGS].

So how did we get sidetracked by Reiner Gamma? Well, originally we set out to complete a discussion about the unique swirl on the Descartes mountains, begun on May 11 in "The still-mysterious Descartes formation" and continued in Lunar Swirl Phenomena and the LRO on May 17. The latter and this copiously illustrated post include a lot of the background for that work which we have put off again for one reason only.

On Friday, May 21, Goddard Space Flight Center released the excellent LRO laser altimetry (LOLA) elevation survey of Reiner Gamma shown at the top of this article., accompanied by a Clementine (1994) albedo photography of precisely the same area. In the text of their release was the hasty assertion that "there appears to be no correlation between the Reiner Gamma swirl and its local topography."

This was not quite what we expected, though taken as a conclusion about most lunar swirls, including most of Reiner Gamma, the statement make perfect sense. In most cases there appears to be little to no connection between swirls and local topography. Dark bands regularly bifurcate anomalous bright albedo swirls that drape over hills and in and out of craters all over the Moon, like ghosts, or as though they fell like salt shaken from overhead, and most of Reiner Gamma is no exception.

V.V. Shevchenko of Sternberg State Astronomical Institute in Moscow, long a proponent of the "recent cometary influence" theory concerning the origin of Reiner Gamma, has pointed to a faint structure of increased crater count that overlaps a seemingly uninterrupted background noise of craters that is apparently coincident with some of the eastern side of Reiner Gamma's huge eye. This is definitely seen in earlier laser altimetry of the area and again in the LOLA image release on Friday.

Of course, This only very slight change in cratering character could be coincidental. Ordinarily, we would be ready to agree that the huge Reiner Gamma swirl has no surface component. The conclusion is actually news worthy, believe it or not, because the "extremely thin" swirl characteristic is said to hold up even in the unprecedented resolution made possible by the LRO laser altimeter. But during our review of data for the Descartes discussion, we encountered a wide angle camera image also swept up by the Lunar Reconnaissance Orbiter.

The portion of LROC Wide-Angle Camera image M116696805MC showing the area of Oceanus Procellarum corresponding to the eye (7.4°N, 301°E) and northeastern "streamer" of the Reiner Gamma formation, running south-southwest of the Marius Hills region, December 29, 2009 (LRO Orbit 2331). At local sunrise, LRO was at an altitude of 45.42 km and the resolution is 64.73 meters per pixel (LRO-lunar surface-Sun phase angle = 90.1°) [NASA/GSFC/Arizona State University].

The portion of LROC WAC M114342152CE showing a slightly slewed view of the 'central eye' of Reiner Gamma, December 1, 2009 (LRO Orbit 1984). It is early mid-morning, LRO altitude = 45.61 km, resolution = 64.82 meters per pixel; LRO-Subject-Sun phase angle = 60.51°

These two images really have to be seen in full and studied with some care, comparing crater with crater, to appreciate the magnitude of relief, allowing some very shallow features to be visible only in the long shadow available in the above image.

Both of these should be available at their maximum available size simply by clicking on the images themselves.

In the image below is the heart of Reiner Gamma at a level of clarity already equal to the previously unprecedented assembly of data put together from Japan's SELENE-1 (Kaguya) Terrain Camera. It's difficult, perhaps impossible at a quick glance to pick out any crater count above background corresponding to the surface albedo. A similar preliminary conclusion seems likely after examining cross-sections of the area available in LROC narrow-angle camera images at widely varying angles of solar incidence.

In the image above, a prominent set of ridges trail across the area that clearly follow, perhaps even channel, the original flow of molten material which may have induced a strong magnetism that may be at its strongest, today, on the eastern side of the eye. These features and others are clearly local topography that corresponds to the Reiner Gamma swirl.