LROC NAC August Close-Ups, Part 3

This reproduction of a roughly 188 meter wide segment (between lines 18602 and 19053) of LROC Narrow Angle Camera (NAC) observation M168000580R may not be the best view of the Apollo 17 lunar module descent stage or the rover tracks and foot prints left behind by Cernan & Schmitt in 1972, still it was collected from an altitude of only 22.41 kilometers on August 14, 2011; LRO orbit 9892, official resolution 0.41 meters per pixel with an incidence angle of 45.17° [NASA/GSFC/Arizona State University].

Joel Raupe
Lunar Pioneer

Since the latest release of Lunar Reconnaissance Orbiter Camera (LROC) images on December 15 we've been able to get a better idea of what flight directors were up to last August. As advertised, the record-breaking spacecraft's roughly 50 kilometer high circular polar orbit was briefly lowered to allow a narrow window for very low altitude photography.  The lowest perigee (or perilune) appears to have been engineered into orbits 9838 through 9973, between August 10 and August 21, 2011. At least that's the period where LRO Narrow Angle Camera (NAC) frames from last summer are available at resolutions higher than 40 centimeters per pixel.

The area covered, moving westward with the Moon's easterly rotation under the LRO orbit, begin near the nearside's east limb at the 85th meridian traveling short of the 60th meridian west (near 310° east). That period in LRO's August close-up maneuver featured perilunes as low as 22 kilometers over the nearside equator with apogee back up near the Nominal and Science Mission altitude higher than 40 kilometers while over the Moon's farside. Put another way, the very highest resolution LROC NAC frames were captured last August between Mare Marginis west to Aristarchus and the Marius Hills.

Our continued, now more extensive tour, of the LROC August low-altitude close-ups has uncovered many extensive fields of boulders and their trails. The largest boulder seen above on the floor of the Vera (26.32°N, 316.28°E) rille formation, directly adjacent to the Prinz ghost crater and head of a long and deep sinuous rille in Oceanus Procellarum, is roughly 22 by 22 meters in size. Many of these August perilune NAC observations appear fore-shortened in this raw first look. A description of the full-width NAC frame which included the detail above is reproduced below [NASA/GSFC/Arizona State University].

Vera-Prinz.  The full width of LROC NAC observation M168488930L, orbit 9964, August 20, 2011; 0.41 meters per pixel with an illumination incidence angle of 43.82° from 26.43 kilometers. The wider image does not provide the context of a Wide Angle Camera image but at least it shows where the boulders further above originated. Is "Vera" is not a crater but a caldera, the "cobra head" of a long and winding rille. The whole scene rests high above the Procellarum basin floor, on the still exposed northeastern ejecta blanket of the almost completely buried ghost crater Prinz. Still, Vera is a deep formation. The lowest elevation inside Vera above is about 550 meters below the surrounding terrain (which only looks flat) [NASA/GSFC/Arizona State University].

The primary purpose of the low altitude maneuvers last August was to allow a last, very close look at three of the six Apollo landing sites, but LROC's targeting team took advantage of the 11 day window to gather hundreds of observations. In addition, the period allowing for greater than 0.4 meter per pixel NAC resolutions was bracketed by a slow, probably energy conserving re-circularizing of the LRO orbit back to within 50 km. There are far more observations among those images between June 15 and September 14 released in December with resolutions higher than the mission average of half a meter per pixel.

There will undoubtedly be thousands more NAC observations captured through December (scheduled for release in mid-March). Though LRO will be placed at an extended mission altitude of greater than 100 km in January it's likely more than half of the Moon's surface will soon be mapped at high resolution, a very successful legacy indeed.

Not every feature on the lunar surface is billions of years old. On edge of the floor of the crater Milichius (9.86°N,329.75°E), seen above, a six meter boulder clearly rolled down the steep wall and came to rest before one of several subsequent dry flows covered the end of its trail, without moving or covering the boulder. The full width of the frame is detailed immediately below [NASA/GSFC/Arizona State University].

As context for the previous full resolution field of view (white rectangle), the full width of LROC NAC frame M168401046L, orbit 9951, August 19, 2011; resolution 0.395 meters per pixel with an illumination incidence angle of 37.62° from 23.08 kilometers. A 64 meter per pixel Wide Angle Camera LROC QuickMap mosaic of the vicinity is available HERE [NASA/GSFC/Arizona State University].

A full resolution frame from a very high resolution LROC NAC observation of a cross-section of landmark nearside crater Bessel (21.73°N, 17.92°E), prominent in the southeastern Mare Serenitatis. This dry flow is composed of material shed from the crater's southwestern wall that did not (or hasn't yet) reached the crater floor. The full width of the NAC frame is reproduced for context in the next image [NASA/GSFC/Arizona State University].

A full width view of LROC NAC frame M168088745L, a breathtaking north-south cross-section of the crater Bessel, August 15, 2011; orbit 9905, resolution 0.395 meters per pixel with an illumination incidence angle of 45.41° from 23.07 kilometers. In this slightly foreshortened view the most prominent feature is the 1325 meter plunge down the southwestern wall of Bessel, from rim to the wall's contact zone with the crater floor [NASA/GSFC/Arizona State University].

Lava Flows Exposed in Bessel Crater

Spectacular example of layering exposed just inside the rim of Bessel (21.8°N, 17.9°E), a familiar 17 kilometer-wide nearside crater in Mare Serenitatis. LROC Narrow Angle Camera (NAC) observation M135073175R, field of view above is 500 meters; LRO orbit 5029, July 29, 2010; solar incidence 13° See the Full-Size LROC Featured Image, HERE [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

The outcrops exposed on the interior wall of Bessel crater (~16 km in diameter) are remarkable since they are most likely preserved layering of mare basalt. Today's Featured Image shows a portion of the northern wall, which contains multiple layers that probably represent discrete lava flow deposits in Mare Serenitatis. Over time, large, but relatively thin, lava flows spread across the extent of Mare Serenitatis.

Lunar pits imaged by LROC also give us a good look at basalt flow layers. Boulders broken off of the mare layers tumble down the wall toward the floor of the crater.

Bessel crater is named after Friedrich Bessel, the developer of Bessel functions. By measuring the thickness of layering found in Bessel and other craters, scientists can put constraints on the thickness of individual lava flows. What else can Bessel crater tell us about Mare Serenitatis?

The original LROC Wide Angle Camera (WAC) 100 meter/pixel monochrome mosaic context image is seen here draped over the high-resolution Digital Terrain Model of the Apollo science mission corridor, available to users of Google Earth. Over that the LROC NAC frame was added (along with the Featured Image, barely visible inside the northwest rim. Bessel's interior shows slumping of material from the walls onto the floor [NASA/GSFC/Arizona State University].

Explore the entire NAC frame!

Related images:
Linne Crater
Dark streaks in Diophantus crater
Kepler's Rim

The view from the northwest floor (actually standing on slumped material) gazing up more than a kilometer along the longitudinal length of LROC NAC frame M135073175R and the location of the LROC Featured Image, almost to the crater rim (beyond line of sight). The Apollo Corridor was photographed in detail during the final Apollo "J" science missions, allowing for an assembly of a detailed terrain model, a method now being applied to the entire Moon by the LROC, LOLA and other instrument teams operating the Lunar Reconnaissance Orbiter.

Hopping digitally up to the rim of Bessel for a view south to the opposite rim, from a vantage near the location of the Featured Image. Similar lava layers are exposed at the same height 16 kilometers away. The southern Mare Serenitatis spreads out beyond. Examining LROC photography in this way demonstrates the global potential of the vast data being still being collected by LRO science teams, already many times over more information than all previous deep space missions combined.

LRO's Mini-RF Serenitatus Swath

LRO Mini-RF S-band zoom synthetic aperture radar (SAR) image strip through central Mare Serenitatis on the near side of the Moon (approximate longitude of strip ~ 18° E; center latitude ~ 20° N). (Full Image HERE.)

The radar strip runs through the crater Bessel (inset; 17 km (10 mile) diameter; center at 21.8° N, 17.9° E) and covers the highlands of the Haemus Mts. (rim of Serenitatis basin) in its southern (bottom) third. The full-resolution SAR data are 30 m (90 feet). The streaks of bright and dark material in the walls of Bessel probably reflect the blockiness of landslides within the crater, brighter streaks having more blocks of the 10-cm (4-inch) scale. The radar strip covers a major geological boundary in Mare Serenitatis; the darker, lower maria has higher titanium content than central Serenitatis. We see this geological boundary in the Mini-RF radar image, caused by higher absorption of RF energy by the high content of the iron-titanium oxide mineral ilmenite. Thus, Mini-RF SAR images can be used to map the titanium content of the lunar maria. The background image is part of the Clementine global mosaic [NASA/GSFC].

For no particular reason, a notional view of the interior of Bessel as seen within the Apollo corridor, the high-definition digital elevation model supported in all current versions of the Moon as seen in Google Earth application. Bessel stands out in minimal telescopes from Earth. It's easy to spot, just south of the basin's center, though there is some question whether the 17 km crater actually lies inside a prominent ray of Tycho, running north from Menelaus. That ray does seem to originate at Tycho, though it is slightly "off" when traced all the way back to the more famous ~109 year old crater 2100 km away in the southern highlands. But this "discontinuity" may just be a matter of perspective, an optical illusion, since the ray lines up well with Tycho when viewed from directly overhead.