"Boulder 668" at Descartes C

Fig. 1. A mere dimple in Apollo-era orbital surveys, this cracked, relatively large (around 53 meters on its long axis) boulder is nested on the north rim of Descartes C crater, lording over steep walls and an interesting strategraphy of the crater's frozen "over spray" of impact melt. It was tossed up from a depth and now sits high over the 4.3 km-wide, 900 meter deep hole from wince it came. LROC Narrow Angle Camera (NAC) observation M175172374R, LRO orbit 10,949, November 4, 2011; angle of incidence 42.42° from the east-northeast, at 40 centimeters resolution and from an altitude of only 23.9 kilometers [NASA/GSFC/Arizona State University].

Fig. 2. Earth's Moon, Waxing Full, April 1, 2012.
The area in yellow is shown at full resolution below,
in Fig. 3; and the full mosaic, by Yuri Goryachko,
Mikhail Abgarian & Konstantin Morozov of Belarus
can be viewed HERE. [Astronominsk].

Joel Raupe

Lunar Pioneer
 

A boulder that seems precariously balanced on the rim of 4.3 kilometer crater Descartes C (11.028°S, 16.273°E) was photographed by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC), last year, as LRO happened to be maneuvering through a cycle of exceptionally low orbital passes. It was a fortunate happenstance for Larry F. Scott and myself, because we once suggested a spot only a few meters away for a notional unmanned landing target. Though our area of interest in 2008, the Descartes Formation, so far, appears only sporadically in the released LROC NAC catalog, the team at Arizona State, headed by Mark Robinson, could not have picked a better target for us when LRO was "skimming the Moon," last year.

In 2011, after maintaining the LRO primarily in a low,  near-circular polar orbit, between 35 and 65 km high, for nearly three years flight directors began a change-over to their preferred method for extending the record-breaking mission into 2015, to raise LRO's orbit above 100 kilometers, which they accomplished very early in January.

Their plan reduces, but does not eliminate, the demands on LRO's limited supply of propellant needed to maintain a minimally useful near-circular polar orbit. (See, "Skimming the Moon," September 6 2011.)

The Moon is anisotropic, or "lumpy," as Dr. Robinson reminded us, and its notorious mascons and mass-voids put an uneven drag on LRO's baseline altitude, and this requires well-planned periodic maneuvers to prevent the vehicle from crashing after only seventy days or so. But before raising LRO's orbit, twice in 2011 the spacecraft was maneuvered through brief periods when the low point in its orbit brought the spacecraft down to within 25 kilometers from the surface, and this allowed for some really spectacular and detailed surveys, including even more extraordinarily detailed examinations of the Apollo landing sites.

These close passes also presented opportunities to gather other NAC observations to within 40 cm per pixel, and among the smaller NAC footprints delivered up in March was a nearly complete cross section of Descartes C, a typical small crater situated in a familiar, though unusual, location in the Southern Highlands.

Fig. 3. At full resolution, a 550 km field of view marked off with by a yellow rectangle in Fig. 1, up above, directly centered on the bright Descartes albedo 'swirl;' viewed through a better-than-average telescope. Slightly above and to the left (northwest) of the swirl both North and South Ray craters can be seen, making the Apollo 16 landing site one of the easiest to "pick out" with the mind's eye, even with a telescope back on Earth. Again, the really breathtaking full mosaic by Yuri Goryachko, Mikhail Abgarian & Konstantin Morozov of Belarus, can be viewed HERE. [Astronominsk].

Descartes C marks one indefinite extent of the Descartes formation, a small field of furrows and segmented hills stretching to the crater's northwest, topped with a distinct and bright, but small and amorphous "swirl" resembling a fresh snowfall. This 400 sq. km. patch of optically immature surface, nested inside a remarkably intense local magnetic field, rates higher than average scientific interest.

Our 2008 proposal included a teleoperated robotic ground survey beginning at Descartes C, lengthwise, through the heart of the unusual terrain and swirl, and its lunar magnetic anomaly, perhaps eventually emerging near the landing site of Apollo 16. Today we have an additional stop on that 'fantasy tour' a short distance from our originally proposed landing site. 

Fig. 4. LROC QuickMap (250 meter resolution) view, also centered on the 'anomalous' albedo and the Descartes Formation, seen here mixed with the false color of the LROC Wide Angle Camera (WAC)-derived topography. Again, a yellow rectangle marks off the field of view visible in Fig. 7, below. Note the "centipede" chain of half-kilometer long hills, training to the northeast from Descartes C. That feature is the most obvious distinction setting the formation apart from nearly every other spot on the Moon's surface. The age and wear of Descartes crater becomes more obvious as one closes in on the area [NASA/GSFC/DLR/Arizona State University].

Fig. 5. Simulated slightly oblique view over Descartes (29 km), from the Cayley Formation plains explored by Apollo 16 in the northwest, 80-plus kms southeast over the Descartes Formation and its swirl albedo to the highly-eroded main Descartes crater in the south. LROC WAC mosaic, from observations collected in three sequential orbital passes December 3, 2011, averaging 52 meters resolution, from 38 km, 70° angle of incidence [NASA/GSFC/Arizona State University].

Fig. 6. "Figure 2" from "Correlation of a strong lunar magnetic
anomaly with a high-albedo region of the Descartes mountains,"
by Richmond, Hood & Halekas, et al. (GFL, V. 30, # 7, 2003)
"Contour map of the two-dimensionally filtered magnetic field
magnitude (in nano-Teslas, or nT) at an altitude of 18.6 km in the
vicinity of the Apollo 16 landing site (boxed cross). The photo-
graph is a portion of Apollo 16 mapping camera frame 0161
(AS16-M-0161). Several exposures of the Cayley formation (CF)
and the adjacent Descartes mountains (DM) are indicated"
[Lunar Prospector Magnetometer data, 1999].

Our paper broadly outlined a very notional multipurpose robotic lander-rover mission in support of the proposed International Lunar Network (ILN). We advocated discovery of ground truth about one lunar magnetic anomaly in particular, and its well-known relationship with a bright surface swirl marking. Also, we wanted to add our small voices to the chorus recommending a cautious approach to the scientifically valuable (and remarkably fragile) artifacts of Apollo, "from the ground, and from a distance." Of course, since then, a growing chorus has out-grown much need for small voices. The NASA Human Exploration and Operations Directorate's recommendation "to space-faring entities," released in July 2011 explicitly spells out the agency's similar concern.

Interest in lunar swirl "patterns" appears as strong as ever, and may be growing. It's become difficult to remember that little more than a decade ago the anomalous albedo 'swirls' today associated with features near Descartes and nearby Airy craters (both easily visible from Earth) were still little recognized.

Because the more famous, more aesthetically pleasing swirl fields at Reiner Gamma, Mare Ingenii and Mare Marginis have been properly associated with local crustal magnetism the recognition of anomalous optically immature regolith elsewhere on the Moon was "reverse engineered."

In the case of smaller 'smudges' near both Airy and Descartes craters, for example, acknowledgement as true swirls has depended on the fleeting detection of magnetic fields at both locations late in the Lunar Prospector mission, not long prior to its eventual crash landing in Shoemaker crater, near the Moon's south pole, in 1999.

For many years after the demise of that small spacecraft researchers continued to tease more and more data from a telemetry stream that today seems remarkably sparse when compared with oceans of data continuously relayed back from LRO. Magnetometer readings from only two low altitude fly-over encounters by Lunar Prospector with the Descartes Formation delivered sufficient evidence to demonstrate a tightly wound mini-magnetosphere existed over the bright albedo swirl, upon on the unusual hills between Descartes crater and the landing site of Apollo 16. The relatively small magnetic anomaly may be the most intense crustal magnetism on the Moon (See Fig. 6).

Fig. 7. The swirl painted on the unique contours of the Descartes formation, just beyond the eroded northern rim of the main crater, is not as striking a in photographs taken from orbit, such as the picture taken from Apollo 14 and 16, or the LROC Wide Angle Camera images from close orbit. The estimated strength of the very localized magnetic field, as measured from Lunar Prospector from an altitude of 18.3 km in 1999, is indicated in nano-Teslas (nT).  LROC WAC observation M177535094C (604nm), LRO orbit 11299, December 3, 2011; angle of incidence 69.57° at 52.3 meters resolution from 38.27 km [NASA/GSFC/Arizona State University].

It's difficult to recall any controversy over the origin (and natural sustaining) of optically immature regolith, at Reiner Gamma, for example. Nevertheless, some very respectable researchers still insist the swirl albedo patterns are the result of scant, recent encounters with comets. The most detailed crater counting methods have all but ruled out any swarming impact origin to the Reiner Gamma swirl. The magnetic field strengths associated with many of these fields are, in some cases (e.g., Descartes and Gerasimovich), sufficiently intense to refract solar wind, but these fields are too small in scale to refract their less frequent but cumulative encounters with the most energetic and heaviest cosmic radiation.

Remote sensing of the Descartes swirl indicates the presence, in abundance, of nanophase iron in the surface grains, thought to be at least one of the ingredients of optical maturity, and a strong indicator of the transparency of the local magnetic field to iron nucleons, a big part of the cosmic ray mix. (Unless, of course, lunar micro-grains implanted with nanophase iron arrived at the site by another mechanism.)

Swirl fields and their associated magnetic fields along the north rim of 4 billion year-old South Pole-Aitken basin are each individually, very closely associated with the antipodes of the most easily-recognized nearside impact basins. The lovely swirls of Mare Ingenii, for example, are nearly on the direct opposite side of the Moon from Mare Imbrium, and the jumble of swirls in and around Goddard crater and Mare Marginis are similarly on the opposite side of the Moon from Mare Orientale.

Because these basins are still believed to be between 3.85 and 3.1 billion years old, respectively, the ages of the magnetic fields clustered at their antipodal foci are thought to be at least as old as those impacts. (But, it should be noted here that no basin-forming impact has yet been associated with the antipodes of smaller nearside swirl patches near Airy or Descartes craters, nor, for that matter, with the unique and much more widespread Reiner Gamma swirl within Oceanus Procellarum.)

The persistent mystery of lunar swirl patterns is still the longevity of "optical immaturity," brightness at the lunar surface that constitutes the swirls themselves. As soon as these "patterns" were associated with local crustal magnetism it was quickly suggested that some refraction, even reversal, of the relentless solar wind kept the surface under their influence from being "darkened." Experiments with high-energy radiation bombardment under laboratory conditions, the energetic variety of cosmic rays that can't be steered away by these fields, appears to indicate that, eventually, any lunar regolith will mature, after only 900 million years or so. As the upper few centimeters of the lunar surface is eventually pulverized into abrasive powder, the process of maturation by hard radiation gets underway.

So how is the optically immature regolith of these swirls kept fresh?

It was our suggestion in 2008 that the answer rests in the very slow migration of lunar dust. The supply of fresh, optically immature dust is continuously supplied by the gardening of impacts, both large and very, very small. And then, at the beginning and end of a daily cycle of charging and discharging of these smallest grains, these nested magnetic fields (which are likely to have more than one kind of origin) preferentially lose and accumulate both mature and immature dust, dividing up both the levitation and fallout along opposing polarities, in a very slow process that still manages to out pace the relentless process of "reddening" or "darkening" by hard radiation, admittedly a less frequent kind of radiation than the bulk of solar wind, but just as relentless.

Which brings us to "Boulder 668," on the north rim of Descartes C, a crater that is itself nested on the north rim of a far more ancient crater, Descartes. The number "668," by the way, marks the boulder's elevation, according to a rough reading of the LROC QuickMap website and its WAC-derived digital elevation model. Obviously there's nothing official about the name.

The boulder reminds us of "House Rock," as well, its smaller cousin ejected out from the North Ray crater impact, and at one time closely examined and sampled directly by John Young and Charles Duke in 1972 (and only about 80 km away from Boulder 668 and Descartes C).

Charlie Duke samples a shatter cone formation in Outhouse Rock, a large fragment shed off the southern end of House Rock, at North Ray crater during the third and final EVA of Apollo 16.  AS16-116-18649 [John Young/NASA/JSC/ALSJ].

The choice for the Apollo 16 landing site, the only manned visit to the lunar highlands, and a landing site referred to as "Descartes," was made in the sincere belief that the apparently unique topography of the Descartes Formation strongly indicated the area to be volcanic in origin. Mission planners were disappointed to discover, almost immediately after Apollo 16 landed, however, no obvious sign of volcanism.

On their second EVA, Young and Duke drove up the slopes of "Stone Mountain," the northwest extreme of the Formation, and looked high and low for a sample uncontaminated by ejecta from nearby South Ray crater.

On their departure from the Moon Captain Young remarked about "still mysterious Descartes," unaware then of the tantalizing evidence they had almost inadvertently uncovered. Their haul of samples proved every bit as valuable as any from the Apollo missions to the overall body of lunar research in the decades following Apollo.

Almost as a footnote, their magnetometer readings proved to be the strongest ever detected on the lunar surface.

In the years since Apollo it's become generally accepted that the unique topography of the Descartes Formation, completely apart from its swirl albedo and magnetic personality, "probably" originated with the impact that formed Mare Imbrium, whose influence is so clearly etched into the landscape of the region, so obviously radiant from the center of that basin. Others say the Nectaris impact, before Imbrium and closer by, tossed up what may turn out to have been a very large, semi-coagulated chuck of impact melt that quickly fell back to the Moon more or less intact, immediately settling in and around existing crater remnants.

Also generally recognized as perhaps the oldest, remarkably intact feature of area is old Descartes itself, an apparently very worn and "tortured" crater differs in many ways from worn craters of apparently the same age elsewhere on the Moon. As worn as it is, it's concentric rings have, for the most part, not been erased (again, remarkably) appear more like a sand castle after the first wave of an incoming tide, without the notched rims characteristic of many largely intact older craters. Descartes seems over washed, with an infill of material around it which consists of more than just the convoluted terrain of the Descartes Formation plateau to its north.

In any case, Descartes, by all appearances, hollowed out a place in the ancient Southern Highlands, perhaps prior the supposed late heavy bombardment.

Though Descartes is now mostly "back-filled," today, more likely from a steady bombardment that erased many of its nearby contemporaries, the impact that formed the old crater tossed up its deepest excavated material and deposited this around its smoothed rim. Some time after this, apparently not from volcanic vent, the half-kilometer-scale chains of hills and furrows of the Descartes Formation arrived on the crater's north exterior, forming or deforming a plateau. Under that material, or, more likely, within the material itself is a very intense, very local crustal magnetic field. That field has since interacted with the slow process of lunar dust charging, discharging, preferential accumulation and levitation, dust migration and the forces driving optical maturity, to form the bright swirl within the magnetism's exceptionally strong influence.

Eons pass, and along came the progenitor that excavated Descartes C crater at the crossroads of all this ancient history. In its formation there was tossed up along its rim the long-buried material once tossed up from an even greater depth by Descartes. "Boulder 668" may represent a bulk of older material less shock metamorphosed than the melt splashed over its rim and pooled on its small floor. The boulder poses questions more, perhaps, than it answers.

In 2008 we fancifully suggested approaching the artifacts of Apollo 16 from the ground, and from a distance of 80 kilometers, all starting with a landing less than 100 meters from the north edge of Descartes C and "Boulder 668." Showing great faith in the future of robust teleoperated robotics we suggested being driven to reconnoiter the Descartes Formation to closely examine "still mysterious Descartes," its magnetic field and albedo. Now, thanks to this remarkably close examination of our proposed landing site we have our fantasy rover's first stop picked out for us for us to examine, and a reason to linger around the perimeter of Descartes and Descartes C a little longer than originally planned.

Fig. 8. Descartes C (4.32 kilometers, centered near 11.028°S, 16.273°E), nested on the deeply eroded rim of Descartes proper, has gradually been seen in increasing detail following multiple LROC Narrow Angle Camera (NAC) observations over LRO's three years in lunar orbit. No longer just a bright crater in a bright region, having excavated an unusually complex area, Descartes C is itself richly complex, with impact melt on steep walls and debris flows into a small kilometer-wide melt-flooded floor. The boulder at 668 meters elevation, high on its rim, was excavated from below the melt pond 750 meters below [NASA/GSFC/Arizona State University]