Amazing peaks and valleys in new Orientale NAC oblique

Spectacular oblique view of the interior of the Orientale basin. LROC Narrow Angle Camera (NAC) mosaic M1124173129LR, LRO orbit 17842, May 26, 2013, centered at 24.23°S, 264.30°E. The scene cropped above shows a field of view approximately 16 km across, and the cliffs at center rise almost 2 km over the southwestern interior edge of the basin floor. Native resolution 2.59 meters per pixel [NASA/GSFC/Arizona State University].

Brett Denevi
LROC News System

With an estimated age of around 3.8 billion years, and a diameter of over 900 km, the Orientale basin is the youngest of the large lunar impact basins.

Its interior is relatively well preserved and its floor has not been completely buried under mare basalts, making it one of the most studied basins on the lunar surface in the hopes of unraveling the mechanics of multi-ring basin formation and the relationships of volcanic infilling to large basins.

Today's featured image highlights some of the more bizarre and complex features inside the southwestern portion of the basin, where primary features related to the basin itself meet the later-forming mare basalts in the basin floor.

Miniature (view the 1280x720 animation HERE) composition of five frames of HDTV captured by Japan's lunar orbiter Kaguya (SELENE-1) in November 2007.  In polar orbit more than 100 km over Mare Orientale Kaguya moves north. Beginning far to the south the slideshow begins with the inner mountain ring like a wide plateau looming on the horizon and minutes later the inner basin and lava-flooded basin floor comes prominently into view, including the area shown at high resolution in the LROC NAC oblique mosaic. Afterward, the final frames linger a moment over prominent Maunder crater and the high mountainous rings and valleys of north Orientale. Widespread terrain disruption by the basin-forming impact is uninterrupted throughout the entire sequence [JAXA/NHK/SELENE].

View the Kaguya Image Gallery HERE

A reduced-resolution version of the oblique NAC mosaic of the Orientale interior. The thumbnail above links to a 2470 by 740 reproduction HERE, while the zoomable, full-resolution view is viewable HERE [NASA/GSFC/Arizona State University].

The striking linear features seen in the top image are portions of a series of cracks that are near-radial to the basin and are unlike typical lunar graben. This portion of the interior is thought to have a high proportion of material that was melted by the extreme shock pressures of the impact event that crated the Orientale basin, and the cracks may have formed as the hot material, draped over underlying topography, cooled and shrank. It is hard to picture the effects of an impact so large it would have obliterated the state of Texas, but here you can almost see the molten and shifting terrain settling and cracking.

LROC Wide Angle Camera (WAC) context view of a portion of southwestern Orientale basin featuring the approximate area shown in NAC mosaic (white box) M1124173129LR [NASA/GSFC/Arizona State University].

You can also get a sense of how basaltic lavas, the lower-reflectance deposits seen at bottom right, poured in later, flooding low areas, lapping up against the higher-standing terrain, and leaving kipukas of original basin material exposed in some spots. These lavas are estimated to have erupted on the order of 100 million years after the formation of the Orientale basin, but were not as voluminous as the basalts that bury all but the rims of other lunar multi-ring basins, such as Serenitatis and Imbrium. The WAC image mosaic of the region, seen below, helps put these features into context. Here you can see the Orientale mare deposits lie largely within the innermost ring of the basin, the Inner Rook mountains. (The other rings are named the Outer Rook mountains, also seen below, and the Cordillera mountains, which lie outside of the context image.)

Why did these basalts fill regions largely contained within only the innermost ring of Orientale, whereas other basins were totally flooded? Orientale may have formed in a region of thicker crust, making it harder for basalts to erupt from the mantle to the surface anywhere but the center of the basin, where the crust was thinned the most. The composition of Orientale's basalts is also known to be different from the major nearside maria, with a lower concentration of radioactive heat-producing elements (known as KREEP), so there may have been less heat available to melt the mantle to produce basalts.

GRAIL MoonKAM video stills sequenced into nadir and off-nadir low-orbit HD views of Mare Orientale (2:00). The area of interest is visible between 1:05 and 1:15 [NASA/JPL-Caltech/Sally Ride Science].

This interplay of spectacular, complex features related to basin formation and later volcanic eruptions means Orientale is a high-priority target for exploration. Samples would pin down the exact age of the basin, important for answering questions about chronology across the Solar System, as well as answer a host of other questions about basin formation and evolution. And what a beautiful view you'd have, standing at the base of Orientale's cliffs!

View the full-resolution NAC mosaic of beautiful Orientale HERE.

Related LROC Featured Images:
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Dark halo crater in Orientale!

Numerov's Graben

Normal faults in regolith formed remarkably small graben in Nectarian age Numerov crater (70.7°S, 160.7°W). Only a handful of small craters superpose the faults, indicating a young age. LROC NAC M171619370RE, image width is 600 m [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Graben on the Moon come in a variety of sizes. Some of the larger rilles in the maria stretch for several tens of kilometers and can be a few kilometers in width. These linear rilles are thought to be the result of extensional stresses near the edges of the maria and are thus graben.

Since the mare basalts are dense, they weigh down the crust in the center of the deposit, pulling rock near the margins inward.

However, the Featured Image today shows much smaller graben that span only hundreds of meters in length and tens of meters in width. To complicate matters, these graben are not in mare basalts, they are inside a crater!

Context image for today's Featured Image. The graben are pointed to by the arrow. A nearby lobate scarp extends from A to A', its low relief enhanced by the low Sun mosaic. Image width is 100 km [NASA/GSFC/Arizona State University].

The LROC Wide Angle Camera (WAC) context image (above) helps us decipher the origin of these graben, as a nearby lobate scarp can be seen at this scale. Lobate scarps form in compressional stress environments as layers of rock or regolith fold and thrust upwards. The thrusting might cause nearby crust or regolith to uplift and bend.

The graben and scarp are only hundreds of meters apart which argues for a compressional interpretation.Thus the interplay between compressional and extensional stresses is reflected in the distribution of tectonic features within Numerov crater. The end result is that we see small graben situated very near to lunar lobate scarps!

Numerov show its great Nectarian age at minimal shadowing in this LROC QuickMap 125 meter resolution orthographic projection assembled from LROC WAC photography and the LROC WAC-based digital terrain model (DTM). By contrast, its larger neighbor shouldered against it's western edge is Antoniadi, an uncharacteristically youthful (Upper Imbrium) impact crater for this part of the lunar surface, deep within South Pole-Aitken basin, and home of the Moon's deepest elevation. The smaller stress affects discussed in the post by Drew Enns are not as apparent at this scale, though other stress affects, scarps in particular, are easier to pick out [NASA/GSFC/ASU/DLR].

Explore more of the lobate scarp and graben in the full LROC NAC, HERE.

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LROC WAC mosaic presented using the Virtual Moon Atlas 6 shows Numerov in context with Antoniadi and Minnaert, a triple astrobleme that is easy to spot on maps of the farside and South Pole-Aitken basin [NASA/GSFC/ASU/VMA6].

Up and Down, Back and Forth

Compression and extension in Mare Tranquillitatis. A northeast-trending wrinkle ridge has overridden a northwest-trending graben. LROC Narrow Angle Camera (NAC) image M192774961L, LRO spacecraft orbit 13437, May 27, 2012; angle of incidence 69.44° at 0.96 meters resolution from 115.91 km [NASA/GSFC/Arizona State University].

Jeffrey Plescia
LROC News System

Today's Featured Image shows the amazing intersection of a northeast-trending wrinkle ridge and a northwest-trending graben, both found in Mare Tranquillitatis (intersection is located at 9.2°N, 27.66°E).

This juxtaposition of structures indicates that the area was extended in a NE-SW direction pulling the surface apart and forming the graben. Later, it was compressed along a NW-SE direction pushing the surface together and forming the wrinkle ridge.

Structural stresses in the crust are not always simple! Another large scale example of this type of intersecting compression and extension environments exists in eastern Mare Tranquillitatis at Cauchy rupes and rimae.

Larger view of the area in the Featured Image. The wrinkle ridge extends to the northeast and southwest changing morphology from narrow to wide. Sun is coming from the east (right). LROC NAC image M192774961L [NASA/GSFC/Arizona State University].

The width of the ridge varies from about 700 m to more than 2500 m, it is about 120-150 m high adjacent to the graben. Large variation in the width of wrinkle ridges is common.  In this case, the thrust fault that formed the ridge is interpreted to dip to the southeast, based on the asymmetry of the topography (the northwest side of the ridge has a steep slope and the southeast side has a gentle slope).

The lunar surface exhibits a variety of tectonic features that are produced when the stresses in the crust exceed the strength of the rock and the rocks break.  Depending upon the magnitude of the stress, the strength of the rock, and the orientation of the stress different types of tectonic features are produced. The two most common on the Moon are graben and wrinkle ridges. Graben are narrow blocks of the crust that have been down-dropped between two normal faults. Wrinkle ridges are thrust faults in which a part of the shallow crust has been thrust over the adjacent area; the upper part of the crust has been folded like an anticline into the wrinkle.

LROC Wide Angle Camera context, the graben -wrinkle ridge intersection below left center in a composite of three sequential orbital WAC observations collected December 12, 2010 (orbits 6767-6769), averaging 73° angle of incidence, 62 meters per pixel resolution from 45 km [NASA/GSFC/Arizona State University].

In this particular location the type of stress reversed over time. First there was an northeast-southwest directed extension that produced the graben, which is about 500-700 m wide and extends for more than 50 km to the northwest. Later, the crust was compressed along a northwest-southeast direction that resulted in the wrinkle ridge.  This ridge is part of a system of wrinkle ridges that extend southwest from a ridge of exposed older highlands material ~30 km to the northeast.

The annotated image below shows the stress directions associated with the graben and the wrinkle ridge.  Red arrows denote the tensional stresses that produced the graben; blue arrows denote the compressional stresses that resulted in the formation of the wrinkle ridge. What happened to cause this reversal, and when did it happen?

Blue arrows denote direction of compression producing the wrinkle ridge; red arrows denote direction of extension producing the graben [NASA/GSFC/Arizona State University].

Explore the entire tectonic feature in the full LROC NAC image, HERE.

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LROC: Fresh crater in Komarov's fractured floor

A fresh crater splashing ejecta across the edge of a fracture in Komarov crater. Field of view is 2.5 kilometers, from LROC Narrow Angle Camera (NAC) observation M191967463R, LRO orbit 13324, May 18, 2012; native resolution 1.52 meters. View the 1650 x 1650 LROC Featured Image, HERE [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

The wispy, bright rays of this small crater (~475 meters in diameter, 24.801°N, 151.687°E) extend down into the fracture (graben).

You can see that this small crater is younger than the fracture because the bright rays of the crater are not visibly deformed by the edges of the fracture.

Gradually, cratering events like this contribute to the erosion and infilling of fractures and other craters on the lunar surface.

Komarov crater is on the southeastern edge of Mare Moscoviense and is located at 24.59°N, 152.25°E (diameter 80.43 km). The floor was long ago filled with mare basalt, and then cut with a spectacular set of intersecting fractures, or graben. Graben form when a section of the crust sinks as two parallel faults pull the crust apart. Note that the northwestern section of Komarov's rim has an irregular shape. the irregular shape is likely due to a preexisting impact crater. The older crater influenced the formation of Komarov's rim, and may have been partially flooded with molten mare material when Komarov's floor was filled in.

LROC Wide Angle Camera context image of Komarov Crater; the red box marks the total area imaged in the LROC NAC frame containing the field of view in the LROC Featured  Image. View the original LROC context image HERE [NASA/GSFC/Arizona State University].

Explore the rest of Komarov's fractures in the full resolution LROC NAC frame, HERE.

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Virtual view from an imaginary point 93 km over the lunar farside, south of Komarov. The LROC 302 ppd WAC mosaic draped over LOLA 128 laser ppd topography shows how pyroclastic flow overran the mare-filled Moscoviense floor. Both the famous long floor and Komarov are each well inside the larger, circular and less obvious Moscoviense basin [NASA/GSFC/LMMP/Arizona State University].

LROC: Rock silde in Rima Hyginus

A rock slide along a section of the northern wall of Rima Hyginus. LROC Narrow Angle Camera (NAC) observation M111545012R, LRO orbit 1572, October 30, 2009; angle of incidence 27.62° at a native resolution of 0.48 meters from 47.28 kilometers. See the 576 meter-wide field of view of the area in the LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Sarah Braden

LROC News System

Rima Hyginus is a linear rille which branches to the northwest and east of Hyginus crater.

The rock slide shown in the Featured Image is located on the northern wall of the eastern branch of Rima Hyginus at 7.393°N, 7.954°E. Bright boulder-rich material from the edge of the rille slid down the wall, possibly during a period of tectonic shaking due to a moonquake or forces associated with a nearby impact.

A trio of large boulders also left trails as they tumbled down the rille's wall.

LROC NAC and WAC mosaic overlay showing a cross-section of Rima Hyginus at the point of the rock slide of interest, LROC QuickMap at 4 meters per pixel resolution [NASA/GSFC/Arizona State University].

Rima Hyginus formed through faulting, and is actually a graben. A graben is a section of the crust that sunk as two parallel faults pulled apart. Remember, the term linear rille is just a fancy way of saying a graben. After the graben formed Rima Hyginus, the landscape changed again due to volcanic activity, specifically the collapse craters easily seen in the the WAC context image here. The craters follow the slight curve of the rille, which indicates that they are not simply a chain of secondary craters that happened to land on top of the existing graben. These craters also do not have raised rims, and they probably formed when the volcanic structures underlying the graben collapsed.

Branch of Rima Hyginus trailing away east from the Hyginus crater, with the subject rock slide designated with the yellow arrow. Cropped at its full 52.5 meter resolution from LROC Wide Angle Camera monochrome (604nm) observation M177582468C, LRO orbit 11306, December 3, 2011, from 38.58 kilometers [NASA/GSFC/Arizona State University].

Examine more of Rima Hyginus in the full LROC NAC frame HERE.

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Inside Hyginus Crater

Read more about the Hyginus region in the Icarus paper, "An igneous origin for Rima Hyginus and Hyginus crater on the Moon."

It's the Moon's fault

Linear rille in Mare Tranquillitatis, the result of extensional stresses. What caused the offset in the rille on the east wall? LROC Narrow Angle Camera (NAC) observation M146858595LE, LRO orbit 6776, December 13, 2010, field of view 700 meters. See the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Linear rilles are so named because of their nearly-straight morphology and surface expression. Unlike sinuous rilles, which are volcanic, linear rilles are tectonic in nature. Similar features on Earth are termed graben, and are created when two normal faults border a block of rock which has been depressed, producing a valley.

Since normal faults are understood to be the products of extensional stresses (see yesterday's Featured Image post), we can assume this region of the Moon was "pulled apart" - creating these normal faults, dropping the middle blocks, and producing the linear rilles. So a linear rille is the lunar analog of a graben on Earth!

Full two kilometer width segment of LROC NAC frame M146858595LE, showing the approximate location of the LROC Featured Image, September 15, 2011 [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) context images of the Rimae Sosigenes extensional linear rille system in the northeast Mare Tranquillitatis, between the Arago domes (out of view, to the south and east) and the craters Sosigenes and its smaller namesake Sosigenes A. one rille is cross-cut with a close-grouped and prominent secondary crater chain, well-known to well-equipped telescopic observers when the morning terminator passes over five days following a New Moon. WAC monochrome (566 nm) mosaic from orbits 4515-4517, June 18, 2010. See the original LROC WAC context image HERE [NASA/GSFC/Arizona State University].

In today's featured image, two normal faults appear to be offset.

What are we seeing here?

Is Mare Tranquillitatis really an impact basin? Looks can be deceiving, when comparing two familiar and neighboring basins, each flooded multiple times with volcanic flows. Dark and optically-mature regolith covers both Mare Serenitatis (top center) and Tranquillitatis (below - the area of interest is indicated with the yellow area), though the differences in color of each are obvious even in black and white photographs. But In this false-color LOLA elevation map, the nature of Mare Tranquillitatis is less obvious, until one examines more closely and sees how the weight of material infilling the Tranquillitatis plain may have led to finer features like wrinkle ridges and extensional rilles [NASA/GSFC/LOLA/MSFC/LMMP].

It is probably an en echelon step between the two normal faults making up the east wall of the rille. When two faults are near to each other, they can interact and create an en echelon step that helps to even out the displacement and forces that created the faults. En echelon steps are common, and are seen in other tectonic features on the Moon.

Can you find any more faults in the full NAC frame?

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LROC: Atlas

The interior of a crater-floor fracture within landmark nearside crater Atlas. LROC Narrow Angle Camera (NAC) observation M157303976L, LRO orbit 8316, April 13, 2011; incidence angle 47°, resolution 0.5 meters per pixel. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

Floor-fractured Atlas crater (46.7°N, 44.4°E) is 87 km in diameter. The cause of the fractures that cut the crater's floor is not well understood. It is thought that the fractures have wide, flat floors, like a trough (or graben) and that they record a period of uplift of the crater floor. The question is, what caused the uplift? Floor-fractured craters have been a known lunar feature since the days of the Lunar Orbiters, but with LROC images, geologists are working to better understand how they formed. LROC NAC frames allow for a look at the interiors of the fractures, and with stereo images we can measure their shapes.

A roughly 2 by 4 kilometer section of LROC NAC frame M157303976L, from which the field of view (red box) within the LROC Featured Image released August 26, 2011 can be found [NASA/GSFC/Arizona State University].

And, in turn, the field of view within the image above is seen in this small section of a much larger LROC Wide Angle Camera (WAC) 643 nm band mosaic gathered during LRO orbits 2750 through 2757, January 31, 2010. The field of view is roughly 50 x 100 kilometers [NASA/GSFC/Arizona State University].

After the impact that created Atlas, the floor of the crater was molten. As it cooled, the solid floor formed. In the case of Atlas, eventually uplift caused the floor to break and pull apart, forming the graben, or fractures. There are two theories for the cause of the uplift. One possibility is the slow readjustment of the crust after the crater-forming impact. During an impact, the energy released compresses the crust. However, over time the crust can rebound to its original, pre-impact position. This rebound would supply the uplift that forms the fractures on the floor of Atlas crater. A second possibility is that the fractures may be due to an intrusion of magma into the crust below the crater, which uplifted and disrupted the crater floor as it rose. When investigating floor-fractured craters, geologists often look for signs of volcanic activity related to an intrusion of magma. Unraveling the origin of lunar features like this one is a primary focus of LROC science.

One hundred meter per pixel WAC context view of Atlas, showing the field of view of the entire LROC NAC frame M157303976L View the full size LROC WAC context image HERE[NASA/GSFC/Arizona State University].

Explore the entire NAC frame!

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From an Earth-bound perspective Atlas (upper right) is the constant companion of it's neighbor to its west, 71 km-wide Hercules, seen in this oblique view captured when the Moon was Full, January 10, 2009, by Mario Weigand [LPOD/SkyTrip.de/VMA].