The eroding fractured floor of Humboldt crater

An approximately 1 km-wide field of view showing boulders strewn across the wall and floor fracture in Humboldt crater. LROC Narrow Angle Camera (NAC) observation M146466012R, LRO orbit 6718, December 8, 2010; angle of incidence 71.68° at 94 cm per pixel resolution from 45.14 km over 24.87°S, 79.03°E.  [NASA/GSFC/Arizona State University].

H. Meyer
LROC News System

Many lunar craters contain floor fractures and evidence for volcanic activity. In the case of (the LROC) Featured Image (released December 23, 2013), the fracture seen eroding to boulders formed in the large impact crater Humboldt, whose floor has been partially covered by dark mantle material (possibly pyroclastic in origin) and lava flows.

There are a few possibilities for the formation of these floor fractures. The two most common are viscous relaxation and intrusive magmatic activity, though intrusive magmatic activity is currently the favored model.

Roughly 39 km-wide field of view as context for the more detailed view of erosion and floor fractures on the northwest floor of Humboldt crater (199 km, 27.02°S, 80.96°E). Red rectangle outlines field of view swept up in LROC NAC M146466012RE and the yellow box designates the area withing the LROC Featured Image released December 23, 2013, LROC monochrome (604 nm) Wide Angle Camera (WAC) observation M161787569CE, LRO orbit 8976, November 6, 2011; angle of incidence 70.87° at 67.16 meters per pixel resolution from 47.6 km [NASA/GSFC/Arizona State University].

Further context of western Humboldt crater, a 103 km-wide area, from a stitching together of three LROC WAC observations, including LROC WAC observations swept up in orbits immediately before and after on November 6, 2011. View the full-sized original mosaic HERE. [NASA/GSFC/Arizona State University].

LROC WAC mosaic, showing Humboldt crater and its immediate vicinity, with a false-color overlay representing the LROC photography-based digital terrain model (DTM) at 250 meter resolution through the LROC QuickMap web-based application [NASA/GSFC/Arizona State University].

The lava flows, some of which can be seen in the WAC context images above, are limited to only a few small areas within Humboldt and only partially fill some fractures, so they were probably not extruded during the formation of the fractures. If they had been emplaced during the formation of the fractures, then we would expect to see lava flows everywhere there are fractures, which is not the case. However, the intrusion of the magma into the subsurface of the crater floor (that eventually erupted onto the surface) could have contributed to the formation of the fractures by causing the floor to dome up.

View the full processed LROC NAC observation, HERE.

Related LROC Posts:
Karpinskiy Floor Fractures (August 22, 2013)
"Star light, star bright" (May 29, 2013)
Numerov's Graben (February 14, 2013)
A small crater's disappearing floor (November 4, 2011)
Last Portion of the Original Floor (November 3, 2011)
Pull Apart Grabens (December 16, 2009)

Revealed Surface, eastern Mare Insularum

Southern slope of unnamed fracture along the eastern mare/highland boundary of Mare Insularum. LROC Narrow Angle Camera (NAC) frame M1114199297R, LRO orbit 16439, January 30, 2013; 1147.2 meter field of view centered on 13.135°N, 355.638°E, 42.94° angle of incidence, resolution 0.96 meters per pixel from 114.18 km. (Downslope toward upper-right, north at top) [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

Today's Featured Image highlights a portion of an unnamed linear fissure located along the eastern edge of Mare Insularum, near the mare/highland boundary.

The width of this fissure varies from about 1.5 to 2 km, its  length is about 90 km, and it extends in the northwest-southeast direction.

The upper-right portion of the opening image, showing a shallow groove extending from up to middle right of the image, corresponds to the bottom of the fissure. Thus most of the image reveals the southern wall of the fissure.

The LROC Featured Image field of view rendered approximately in elevation data from the LROC WAC DTM. Local slopes in the vicinity of the fracture of interest can be difficult to otherwise see. LROC QuickMap [NASA/GSFC/DLR/Arizona State University].

On this slope, there is a high reflectance area with sinuous boundaries. This unit is hard to interpret in terms of what is on top and what is below, stratigraphically. The sunlight is from left side, highlighting what appears as a slightly raised boundary between the two units (arrows). Elsewhere it looks as if the high reflectance material overlies the lower reflectance material. Which unit is younger? Try counting craters between the two, but be careful, if the units have different hardnesses, then the more coherent unit may preserve craters better. 

Unnamed fracture running northwest to southeast on the eastern side of Mare Insularum and surrounding vicinity in LROC WAC monochrome mosaic (100 meters per pixel), centered is 13.12°N, 355.66°E. The LROC NAC footprint (blue box) and location of the field of view in the Featured Image (yellow arrow) are marked [NASA/GSFC/Arizona State University].

Since this whole area is on a slope, slope failure may have revealed an underlying immature surface. Indeed multiple higher reflectance boulders are sitting at the downslope side of this high reflectance unit. But the upper complicated shapes are difficult to explain by this simple story. Or perhaps low reflectance materials could have slumped and covered portions of the high reflectance material? A high resolution NAC DTM would help scientist unravel this complicated morphology.

Explore this enigmatic patterned surface in full NAC frame yourself, HERE.

Related Posts:
Inside Hyginus Crater
Bright ridge near Mons Hansteen
Wrinkle Ridge vs. Impact Crater
Really Wrinkled
Boulders In The Sea Of Serenity
Ghost crater in Mare Imbrium
Zebra Stripes
Aitken Central Peak, Seen Obliquely
Constellation Region of Interest at Mare Tranquillitatis

LROC: Dense Fractures

Fractures at the edge of an impact melt pool within the crater Larmor Q. Field of view 312 meters across from the LROC Featured Image, February 29, 2012 (540 meters), LROC Narrow Angle Camera (NAC) observation M151726155R, orbit 7494, February 7, 2011; 0.57 meters per pixel from 54.8 kilometers altitude [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

The crater Larmor Q, located at 28.630°N, 176.240°E, is 18.3 km in diameter, has slumped walls, and impact melt in its floor. The Featured Image show fractures within this impact melt at the boundary of boulder-rich slumped material and the impact melt pool itself. The fractures may have formed two ways: from post-impact modification of the crater floor, or from a volume change associated with the cooling of the impact melt. Post-impact modification means that the shape of the crater changed, due to slumping walls, or changes in the crater floor topography caused by strain due to the redistribution of material during the impact itself.

The fractures are closer together (denser) near the edge of the impact melt. This may be where the impact melt is the thinnest, depending on the topography of the crater floor. Imagine an empty bowl (the crater) and then fill the bottom 5% of the bowl with ketchup (impact melt). The ketchup will be shallowest closest to the sides of the bowl. Often impact melt pools are not of uniform thickness due to variations in the shape of the crater's floor. Also, within the same crater there may be multiple impact melt pools of different overall depths since the emplacement of melt is not symmetric with respect to the crater.

When a lava (or impact melt) cools, it reduces in volume, which may have formed these cracks. Or, the thinnest part of the impact melt might be expected fracture more in the case of changes in the floor topography. Lunar scientists will have to study fractures in impact melt pools to determine which of these answers is correct. Perhaps it is a combination of the two causes of fractures, or something not yet considered.

LROC Wide Angle Camera (WAC) image of the 18.3 km diameter crater Larmor Q. Slumping of the crater walls has not yet covered all the impact melt on its floor. From LROC WAC observation M136389155 (604nm), orbit 5233, August 14, 2010; angle of incidence 54.83° with a resolution 82.1 meters per pixel from 58.4 kilometers  [NASA/GSFC/Arizona State University].

Find other areas in Larmor Q where fractures are denser towards the edge of the impact melt in the full NAC frame!

Related Posts:
More Impact Melt!
Fractured Impact Melt
Melt Fractures in Jackson Crater
Fractures in Ohm's Melt

LROC Melt fractures in Jackson crater

Fractures can be seen in profuse abundance on the Jackson crater melt pond surface. Illumination from west, a field of view roughly 700 meters across swept up at an incidence angle of 71.13° LROC Narrow Angle Camera (NAC) observation M118560367L, LRO orbit 2606, January 19, 2010; resolution 0.84 meters from 52.97 kilometers altitude. View the full-size Featured Image HERE [NASA/GSFC /Arizona State University].

James Ashley
LROC News System

As molten rock cools, it shrinks and often cracks. In this case of impact melt ponded within the Jackson crater floor (22.18°N, 197.24°E), the cracking rate was so high that unfractured melt is almost more of an exception than a rule!

Radial and divergent patterns can be seen among the fracture sets that tell a story of the cooling history. The context image below shows a portion of their wider distribution.

As context for the January 18, 2012 LROC Featured Image (field of view near where the impact melt inundating the crater floor emerges from eastern wall slump; the white box) a long view north and up the steep northeastern wall, nearly to the rim, courtesy of the digital elevation model combined in Google Earth [NASA/USGS/ASU/JAXA/Google].

Overhead context for Featured Image, a field of view roughly 2.5 kilometers across from the wider LROC frame.View the full-size LROC context image HERE [NASA/GSFC/Arizona State University].

Solid objects in the melt, together with the 'shore' of the pond, appear to have influenced the way the cracks organized themselves as the melt cooled. Note how the fractures bend around or radiate from some of the positive relief features in the images above. These could be ejecta blocks or portions of the slumped crater walls in the melt that served to locally accelerate cooling. Their influence might thus be to 'seed' the stress field within the shrinking melt volume, helping some of the cracking to grow from these points, and ultimately resulting in the patterns we see today. Sagging along the shore can cause the cracking to parallel the shoreline. Any motion within the volume of melt, possibly influenced by late-stage additions of molten material, may also have contributed to the patterns observed here.

Further context, from 100 kilometers altitude, this square crop from a highly detailed HDTV still frame was captured by Japan's lunar orbiter SELENE-1 (Kaguya) in 2009 [JAXA/NHK/SELENE].

The extent and complexity of the melt pond features can be explored in the full NAC frame HERE. Additional examples of impact melt cracking include Polygonal fractures on Tycho ejecta deposits, fractured impact melt in Thales crater, and Moore F.

Ed Note: In a way opposite and contributing to the low optical visibility of the vast majority of similarly sized craters in the farside Highlands, Jackson is easier for the eye to see than most. Like Tycho on the nearside, there are a lot of craters of similar size and origin everywhere on the Moon. The difference is age. Like Tycho, the ray system of Jackson (and the materials its progenitor impact threw out) shows Jackson's "optical immaturity." To illustrate, below are two representations of the farside quadrant with the highest of the Highlands scoured by the Jackson impact, likely less than a half billion years ago.

Jackson stands out in this global montage of Clementine (1994) Ultra-Violet/Visible (UVVIS) wavelength photography designed to better map the Moon's albedo, more than a decade ago. Similar craters, basins and the Moon's highest elevations are nearly invisible [NASA/USGS/DOD].

A white arrow is needed to designate Jackson out from the pocked highlands and several otherwise invisible basins stand out with exceptional clarity in this view of nearly the same terrain as a representation of differences in elevation from the LROC Global Digital Terrain Model, developed using LROC Wide Angle Camera survey photography [NASA/GSFC/Arizona State University].

Oblong Roche V

The boundary between the flooded crater floor and crater wall of Roche V  wall is subtle, except for locations where a distinct pinch represents the contact between units. LROC Narrow Angle Camera (NAC) observation M159100166R, orbit 8580, May 3, 2011; incidence angle 53.76° and reduced from a resolution of 0.59 meters per pixel from 57.3 kilometers. View the original, full size 590 meter wide field of view LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Lillian Ostrach
LROC News System

Roche V (38.86°S, 129.62°E) is one of five satellite craters associated with Roche crater. Compared to its siblings, Roche V is the ugly duckling of the bunch: it has an oblong, irregular shape, floor fractures, and mare fill. But, similar to the ugly duckling in the story, the geologic features that stand out and catch our attention make this 29 km diameter crater quite special indeed. The irregular shape tells us something about the crater formation, probably because the impactor hit the lunar surface at a highly oblique angle. But maybe more exciting is the flooded mare material in the crater, along with both linear and arcuate fractures. There are many floor fractured craters on the Moon, but most of them are much larger than Roche V (for example, Gassendi is 110 km in diameter). The cause of fractures in these larger craters remains poorly understood, but scientists do know that the floors of these craters are uplifted and many contain smooth deposits of basalt. Given the size difference between Roche V and these larger craters, can we explain the geology of the Roche V crater floor?

LROC Wide Angel Camera (WAC) observation M115462620CE, (604 nm) image of satellite crater Roche V. Roche, the parent crater, is located to the southeast of this view. Arrow marks location of pinch contact discussed above. LRO orbit 2149, incidence angle 77.07° with a resolution of 85.7 meters per pixel from 61 kilometers [NASA/GSFC/Arizona State University].

Taking a look at the WAC image, there is a striking difference between the fractures in Roche V and those observed in other floor fractured craters, such as Atlas (87 km diameter). The fractures in Atlas and other floor fractured craters are large, somewhat resembling linear rilles in appearance. However, the fractures in Roche V are not as sharply defined and appear visually shallow compared to those observed in Atlas. The fractured and rougher mare fill (including the location of today's Featured Image) has a higher albedo than the smooth mare. Maybe the Roche V mare fill is partially covered by high albedo ejecta from a nearby recent impact; can you find any craters with high albedo ejecta blankets using the LROC WMS viewer?

The surface features of the floor material in Roche V are visually similar to the surface of a cornstarch and water mixture in a bowl as it dries. The material that lapped up on the sides of the crater stuck there as the lava cooled, but as the lava cooled it experienced a small volume change and shrank. The fractures observed may be the result of cooling within the lava pond. Much of the contact between crater wall and floor-fill is currently blurred and smoothed due to post-impact modification and regolith formation and at the NAC scale are very difficult to discern, but there are still a few distinct contacts visible in LROC NAC images.

What do you think - what formed these fractures? Take a look in the full LROC NAC image!

Related Posts:
The fractured floor of Compton
Alphonsus crater mantled floor fracture

"Two of these things are not like the others"

The rim of a fracture inside Sarton Y. Left of the bright rim is outside the fracture and right of the rim is inside the fracture. Image field of view is 825 meters. See the full size LROC Featured Image HERE, a frame from LROC Narrow Angle Camera (NAC) observation M140697409L, LRO Orbit 5868, October 2, 2010. [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

In the farside highlands at 51.3°N, 238.6°E, Sarton Y and Z stand out among the other craters. Take a look at this area in the LROC WMS Image Map.

Other craters in the region have filled floors, however, Sarton Y and Z are the only two craters to also have floor fractures. 

Why is this?  What makes Sarton Y and Z special? 

Full 3.3 kilometer width frame from LROC NAC M140697409L, October 2, 2010 [NASA/GSFC/Arizona State University].

Perhaps the subsequent impact of Sarton Y allowed the material in both craters to undergo a thermal evolution different from their surrounding counterparts, possibly causing the fractures as the floors cooled. Was there a difference in the surface material when Sarton Y and Z were formed? Both craters show evidence of slumping, but so do the other craters in the region. Age or the composition of the impactor may also play a role.

Sarton Y (l) superpositioned upon the slightly larger Sarton Z, in the farside northern highlands, as swept up April 26 and 27, 2011 in this approximately 44 km-wide frame from a LROC Wide Angle Camera 60 meter per pixel 604 nm mosaic, LOR Orbits 8471 - 8481. The yellow arrow designates the location of the rim of the fracture discussed above [NASA/GSFC/Arizona State University].

Explore more of the fractures in the NAC frame!

Related Posts:
Atlas
Gassendi's Fractures
Perched boulders

LROC: Extensional Fractures

Narrow fractures extend across a complex intersection of lunar surface types northeast of Mare Serenitatis, in Lacus Somniorum. LROC Narrow Angle Camera (NAC) observation M168007062L, from the period last August when the LRO spacecraft's orbit was altered to include very low passes over selected areas (resolution 25 centimeters per pixel); orbit 9893, August 15, 2011. Field of view a bit more than 550 meters wide [NASA/GSFC/Arizona State University]

Sarah Braden
LROC News System

In this 0.25 m/pixel NAC frame we found narrow fractures that extend across the lunar surface, in the mare basalt of Lacus Somniorum. Narrow fractures like these are probably extensional features caused by tension stress that pulls the rocks apart in opposite directions. Similar fractures have been found before on top of wrinkle ridges. 

The fractures in today's Featured Image are also associated with a small wrinkle ridge. In the LROC Wide Angel Camera (WAC) 60 meter per pixel context image below, you can see the ridge (yellow arrow) between two hills of non-mare material. The fractures are located west of the ridge (yellow box). There are a number of other wrinkle ridges in the area as well.

LROC Wide Angel Camera (WAC) Observation M150321508C (604 nm), LRO Orbit 7288, January 22, 2011, local late afternoon illumination at incidence angle 67.201° The yellow box at center bounds the fractures seen in the LROC Featured Image, released October 26, 2011. The yellow area designates a ridge possibly related to those fractures. [NASA/GSFC/Arizona State University].

Examine the rest of the narrow fractures in the NAC frame!

Related Posts:
Stress and pull
Fractures in the mare of Tsiolkovskiy crater