Tuesday, July 29, 2014

Making a splash at King crater

Impact melt ponds adorn the lumpy terraces of King crater. If you look carefully, you can see small fractures in some of the these ponds. 10.9 km field of view from LROC NAC mosaic M1159315479LR, LROC orbit 22783, July 6, 2014; 53.77° incidence angle, resolution 1.16 meters from 114.15 km over 6.41°N, 120.37°E [NASA/GSFC/Arizona State University].
H. Meyer
LROC News System

In the image above, impact melt pooled among the terraces in the walls of King crater, a complex Copernican crater on the lunar farside.

Impact melt is generated when kinetic energy associated with an impact is transferred to the target rock. Shock waves cause the rock to melt nearly instantaneously. The impact melt is flung all over the interior of the crater and some even makes it out of the crater.

Melt splashed on the crater walls tends to drain back down pooling on ledges. This downward flow resulted in a coating of impact melt called a veneer, visible in this region, along with other features like fractures.

Upon closer inspection, these impact melt ponds display the fractures and oddly shaped craters typical of the transitory impact melt ponding at King crater. 1.34 km field of view from LROC NAC mosaic M1159315479LR [NASA/GSFC/Arizona State University].
Fractures in impact melt rocks can be the result of tectonic stresses and cooling and contracting of the impact melt. Sometimes craters also form before the impact melt has fully solidified, resulting in oddly shaped craters that resemble craters that form when pebbles are thrown into mud.

Aside from its rather peculiar central peak, King is rather unique in that the impact melt is not evenly distributed around crater, but rather accumulated in the north-northwest region outside the crater rim. This may be the result of an oblique impact. In the case of an oblique impact, impact melt is concentrated downrange of the incoming projectile.

Contextual view of farside Copernican Age crater King (76.2 km; 4.96°N, 120.49°E), with footprint of LROC NAC mosaic M1159315479LR (orbit 22783, July 6, 2014) and LROC Featured Image field of view. 50 km-plus field of view from LROC Wide Angle Camera (WAC) monochrome (643 nm) observation M176845114CE, LRO orbit 11197, November 25, 2011; 61.87° incidence angle, resolution 57.8 meters from 42.52 km [NASA/GSFC/Arizona State University].
Impact melt forms many exquisite features on the lunar surface such as flows, lobes, pits, fractures, channels, polygonal patterns, and more. Explore the impact melt features of this region in the full NAC mosaic HERE.

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Thursday, July 24, 2014

Banded Slump in Berzelius W

A complex interplay of slumping and slides in the northwest wall of Berzelius W result in banding patterns; downslope is toward the bottom right in this 230 meter-wide field of view from LROC NAC observation M174921824R, LRO orbit 10912, November 3, 2011; 53.59° incidence angle, resolution 40 cm from 23.87 km over 38.06°N, 53.02°E [NASA/GSFC/Arizona State University].
J. Stopar
LROC News System

Berzelius W (7.17 km; 38.137°N, 53.103°E), on the northeast limb of the Moon (as seen from the Earth), exhibits abundant evidence for mass wasting.

Materials of varying albedo create intricate patterns on the walls of the crater, including the banding patterns featured in the opening image. 

This particular portion of the wall includes a block of slumped material, as indicated by the characteristic arcuate faults near the crater's rim crest (see image below). The slumped material is overprinted by finger-like flows of finer particles that moved as slides of dry debris.

Arrow indicates the arcuate faults at the head of the slumped material in the wall of Berzelius W. This slumping may have triggered the associated narrower and finger-like landslides of lower albedo (darker) boulders and debris; 600 meter field of view [NASA/GSFC/Arizona State University].
When did these mass-wasting events occur? Did they occur during the impact event, shortly after while the landscape was still ringing from the shock of impact, or millennia later? The lack of high albedo (bright) ejecta around the crater and the subdued appearance of the rim crest indicate that this crater did not form particularly recently; perhaps it is between 1 and 2 billion years old. 

Berzelius W (7.17 km; 38.137°N, 53.103°E) in 35.7 km-wide field of view from LROC WAC monochrome (604 nm) mosaic of M161965061C and M161971828C, LRO orbit 9003 and 9004, June 6, 2011; 73.5° incidence at 61.5 meters resolution from 44.9 km [NASA/GSFC/Arizona State University].
The crisp edges of the debris flows and arcuate scarps in the walls, however, suggest that they are much younger than the crater. So, while this landslide probably did not form yesterday, it is likely significantly younger than the crater itself, probably less than half its age. However, without more data, it is impossible to know precisely when these events occurred. Repeated imaging over many decades may provide more insight into how crater walls age with time. Alternatively, samples returned from crater walls may provide a method to age-date mass-wasting events. 

Try to find at least three other examples of mass-wasting features in the western half of this crater below. Pan and zoom to find examples of landslides, talus deposits, and boulder tracks:

Full 850 meter-wide field of view from remarkably high resolution LROC NAC observation M174921824R [NASA/GSFC/Arizona State University].
View full-window HERE.

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Monday, July 21, 2014

Ballistic boulder at Hecataeus N

A house-sized boulder left a clear impression, immediately beyond the east rim of a young 1.6-km crater (rim crest to the left), all in a full-sized reproduction, 988 meter-wide field of view from LROC NAC observation M182995612R, LRO orbit 12068, February 4, 2012; 48.59° incidence, resolution 85 cm from 83.56 km over 20.98°S, 80.74°E [NASA/GSFC/Arizona State University].
J. Stopar
LROC News System

The boulder above (21.085°S, 80.777°E) in the opening image is likely debris ejected during the violent excavation of the 1.6-km diameter crater immediately to the west (left).

The boulder was deposited ballistically; the distance it travelled and its time of flight are related to its ejection angle and velocity.

For the boulder, was this flight a "small step" or a "giant leap?"

Looking at the image above, we can deduce that the boulder was deposited with enough force to make a noticeable impression in the ground. However, a more forceful landing would have highly fragmented the boulder.

More examples of surface impressions formed by ballistic boulders from the fresh impact near Hecataeus N. Spotty trails mark where boulders rolled into a pre-existing crater (small yellow arrows). Another larger, boulder (25 meters in diameter, large white arrow) was thrown out of the crater to the southwest and carved a furrow in the ejecta blanket, coming to rest when it intersected a pre-existing crater rim (topographic high) [NASA/GSFC/Arizona State University].
The boulder is located about 500 meters east of the crater rim crest, which is only about a third of the crater diameter. Thus, this boulder did not travel very far or very fast.

Explore the entire crater and its ejecta below:

Fresh impact on the west flank of Hecataeus N 4.3 km field of view from LROC NAC mosaic M182995612LR, LRO orbit 12068, February 4, 2012; 48.59° incidence, resolution 85 cm from 83.56 km over 20.98°S, 80.74°E [NASA/GSFC/Arizona State University].
View full-window HERE.

The house-sized boulder (yellow arrow), which left its impression just beyond the east rim of the unnamed young 1.6-km crater that, in turn, sits on the west flank of Hecataeus N, shown in the context of a 7.86 km-wide field of view from LROC NAC mosaic M182995612LR, LRO orbit 12068, February 4, 2012; 48.59° incidence, resolution 85 cm from 83.56 km over 20.98°S, 80.74°E [NASA/GSFC/Arizona State University].
The fresh crater (center) on the southwest slope of Hecataeus N (10.82 km; 20.91°S, 80.944°E) excavates the deepest material originally turned up by "N" while both, in turn, sampled the very ancient Hecataeus interior (southwestern half of this 40-km wide field of view) and, even further, material turned out by Humboldt, to the south (see below), a powerful impact that significantly filled in and covered over the floor of Hecataeus. This is an example of something planners look when making good landing site choices, ones likely to efficiently utilize precious resources. LROC WAC observation M177109146C (604 nm), LRO orbit 11236, November 28, 2011; 68.1° incidence, resolution 58.91 meters from 43.76 km [NASA/GSFC/Arizona State University].
A general schematic map of geological types, representing the relative stratigraphy of the lunar surface affected by Hecataeus and Humboldt, in the south equatorial latitudes of the far eastern hemisphere, where the farside highlands begin, The fresh crater on the west flank of Hecataeus N is marked by an arrow.
The grooves carved by boulders ejected at relatively low velocities are in many ways similar to the spotty tracks etched by boulders sliding, rolling, and bouncing down steep slopes. Also, boulder tracks (like these) often resemble the astronauts' footprints on the lunar surface, since both have relatively recently disturbed the soil in narrow paths. 

In honor of the 45th anniversary of the Apollo 11 lunar landing (July 20, 1969), revisit some of our previous posts about large boulders visited by astronauts:

Finally, revisit some of the best LROC images of the Apollo 11 landing site and see if you can find any large boulders. You should find very few large boulders, as the mission planners sought a low-risk site for the first Moon landing:

Friday, July 18, 2014

Dorsum Nicol

This tectonic feature was formed as stresses built up in the lunar surface until it gave way. The energy released was immense, and the displaced rock is the north-south trending wrinkle ridge seen today in southeast Mare Serenitatis. LROC NAC-derived digital terrain model, slope angle and recent imagery. Field of view is 4765 meters across [NASA/GSFC/Arizona State University].
Aaron Boyd
LROC News System

Dorsum Nicol is a wrinkle ridge found in southern Mare Serenitatis.

The opening Featured Image is a LROC NAC image, juxtaposed with a slope map, NAC-derived DTM and recent LROC NAC observation of area.

Slope maps are useful to planetary scientists because topographic features like craters and small ridges really stand out. At the  location in the LROC Featured Image field of view, the mare on the west side of the ridge is about 100 meters higher in elevation than the mare on east side of the wrinkle ridge, and the peak elevation is around 50 meters above that (see profile below).

Elevation profile across Dorsum Nicol, at field of view for LROC  Featured Image released July 17, 2014. (Points A and B correspond to A and B in LROC WAC context image, below [NASA/GSFC/Arizona State University].
The difference in elevation between the eastern and western flanks of the ridge could be due to the lunar surface buckling and folding beneath the surface, or it could be from mare fill after the wrinkle ridge was formed.  Dorsum Nicol has a width of 10 km at its widest and 5 km at its narrowest. Take a look at the full feature in the context image below.

LROC WAC context image of Dorsum Nicol in Mare Serenitatis. The yellow box is the approximate location of today's Featured Image, the red box is the location of the full NAC DTM. A profile taken along the white line from A to B and is shown above. Context image spliced from LROC Wide Angle Camera (WAC) monochrome (604 nm) mosaic of three observations swept up over three sequential orbital passes (LRO orbits 9031-9033) June 8, 2011; incidence 75.86° resolution 58 meters from 42 km [NASA/GSFC/Arizona State University]. [NASA/GSFC/Arizona State University].
Wrinkle ridges are are surface expressions of compressional forces being released; they are seen in all large maria on the Moon. The loading from the massive flood basalts during mare volcanism could have caused the lithosphere to flex because the density of the flood basalts is higher than the anothositic highlands material. Buoyancy forces were at work here, causing viscoelastic relaxation and inducing forces in the mare rock.

Full-sized LROC WAC mosaic, from three sequential passes June 8, 2011. See full size mosaic HERE [NASA/GSFC/Arizona State University].
Southwestern Mare Serenitatis in HDTV. Dorsum Nichol (next to the ghost crater Brackett, butted up against Rimae Plinius, together with Dorsa Lister, and in the foreground, the belt of darker basalts encircling the entire impact basin, Rimae Plinius and Promontorium Archerusia are at lower left, a view Harrison Schmitt describes as close to what Apollo expeditions saw in orbit. From a HDTV still, captured from Japan's lunar orbiter Kaguya (SELENE-1) in 2008. See the original release image HERE [JAXA/NHK/SELENE].
These forces caused the once convex surface of the maria to become more planar as time went on. This is a problem because a plane has less area than a curved surface when they are bounded by the same radius. Where was the rock going to go? Well, the maria resisted this change in topology until something broke! Wrinkle ridges are the expression of that thrust fault behavior.

Locating Dorsum Nicol in southeast Mare Serenitatis is easier than actually seeing these features, through a modest telescope. The stacked photograph above, assembled by Astronominsk in Minsk, Belarus, was captured at the best illumination incidence for such a purpose, on June 28, 2009, right after local sunrise, before First Quarter (in an early evening sky, here on Earth). Locating Taurus Littrow valley, the landing site of Apollo 17, and Promontorium Archerusia, along with other features in the contact area between Mare Serenitatis and Mare Tranquillitatis are relatively easy, however, throughout the lunar day [Astronominsk].
Check out the full NAC frame on the the LROC DTM product page, HERE.

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Tuesday, July 15, 2014

Birt E and the fiery rift valleys of Nubium

LROC Narrow Angle Camera-derived Digital Terrain Model (DTM, color gradations superimposed) on a close-up view along the northeastern long axis of Birt E (5.34 km; 20.72°S, 350.337°E), the vent structure of Rima Birt, a sinuous rille in east Mare Nubium immediately west Rupes Recta. The vent is thought to be a source region for pyroclastic flows that traced out Rima Birt. The mare basalt, visible as slightly darker material in a fan spreading north from the vent, is older than 3.4 billion years. A roughly 3 km-wide field of view from LROC NAC mosaic M1144849711LR, LRO orbit 20750, January 20, 2014; 46.21° incidence angle, 84 cm resolution from 78.77 km over 20.69°S, 351.11°E [NASA/GSFC/Arizona State University].
Aaron Boyd
LROC News System

Birt E (20.72°S, 350.337°E) was not created like most craters on the Moon; there was no meteorite impact. Lava sputtered out of this pyroclastic vent in Mare Nubium over 3.4 billion years ago, dispersing lava onto the surface and leaving the crater we see today.

How can we tell it is a volcanic vent and not an impact crater?

Impact craters and volcanic vents can be differentiated because vents often have an irregular or elongated shape (as with Birt E). Impact craters are usually circular in shape, created by the shockwave during an impact event.

Pyroclastic vent structure at Rima Birt, officially "Birt E," though it is not a crater; west of Rupes Recta in east Mare Nubium. 4.66 km-wide field of view from LROC NAC mosaic M1144849711LR [NASA/GSFC/Arizona State University].
Also, the vee-shape of this crater is likely a product of the formation mechanism. Vee-shaped vents are thought to be formed from a pyroclastic eruption. Gasses fractionating out of the liquid rock create violent events during eruptions. Explosive eruptions created the shape that we see today, but Birt E could have had a complex history with effusive eruptions forming Rima Birt, the short sinuous rille flowing from Birt E to the south-southeast.

Birt crater group and Rupes Recta, the eye-drawing structure in telescopic views of Mare Nubium. (See a larger, unnoted reproduction HERE.) LROC Wide Angle Camera (WAC) monochrome (604 nm) mosaic stitched from observations swept up over three sequential orbital passes June 10, 2011; incidence roughly 78° and 58 meters per pixel resolution, from 43 km [NASA/GSFC/Arizona State University].
East Mare Nubium, an orbital view north from 100 km altitude, an HDTV still from Japan's lunar orbiter Kaguya (SELENE-1) in 2008. (See the wallpaper-sized release HERE.) [JAXA/NHK/SELENE].
Over long enough time scales Birt E will be filled in with ejecta from newly formed craters around Mare Nubium or by mass wasting of the walls into the crater. Let’s enjoy this ancient crater today while we still can!

As the GRAIL gravity probes mapped the Moon's basic anisotropy in 2012 detecting the densities of "deep fiery rift valleys," a probable source of a half billion year period of pyroclastic volcanism surrounding the Moon's Procellarum terrain, added important pieces to the tossed puzzle of the Moon's morphology. In the newest maps of the Moon's own squarish "ring of fire," there is still a missing link under the Southern Highlands, but a source for the multiple inundations of the nearside's lowlands, weight that may have created the Straight Wall fault in east Mare Nubium, may have been found [NASA/GSFC/SVS].
LOLA laser altimetry color-coded over LROC WAC 100 meter global mosaic demonstrates where Rupes Recta marks a neat break in the continuity of the floor of an otherwise almost entirely erased 190 km crater. The weight of repeated inundations gradually sloped that crater's floor 1500 meters in elevation deeper on the west side, gradually, aside from the 100 to 300 meter "snap" represented by Rupes Recta [NASA/GSFC/Arizona State University].
Check out the topography in the full NAC DTM HERE.

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Friday, July 11, 2014

"Donut Hole" structures on the floor of Harriot B

Some among kilometer-sized mounds on the floor of Harriot B (38.1 km; 33.356°N, 114.41°E), in the farside highlands. LROC NAC observation M180430508R, LRO orbit 11710, January 5, 2012; 78.76° incidence angle, resolution 1.57 meters from 159.56 km over 33.77°N, 114.38°E [NASA/GSFC/Arizona State University].
Raquel Nuno
LROC News System

The planets in the Solar System are continuously bombarded by space rocks. This violent process early on formed the planets by accretion, and impacts still shape the surface of terrestrial and icy bodies today.

Since the Moon lacks an atmosphere it preserves the impact history record of the inner Solar System. However, this record is less than complete; volcanic resurfacing and crater saturation of the surface erase part of the record. Ancient volcanic activity, resulted in massive outpourings of lava over parts of the Moon, filling in and covering old craters beneath layers of basalt. Crater saturation is a term used by scientists to describe a point after the planetary body has been completely covered with craters of a certain size such that every new crater of that size overlaps an older one, obliterating the evidence of the older craters' existence. 

A full-width view of LROC NAC observation M180430508R [NASA/GSFC/Arizona State University].
Today’s featured image is a portion of the central mounds, ~1 km in diameter, found on the floor of crater Harriot B, a highland crater located at 33.356°N, 114.410°E.

These mounds are likely weathered central peak formations. Central peaks form by the gravitational collapse of the crater walls which pushes material into the center of the crater and from the rebound of the floor; both of these events occur at the time of crater formation during the modification stage of the crater.

A more recent, lower incidence angle LROC NAC mosaic of the floor of Harriot B. LROC NAC mosaic M1159343420LR, LRO orbit 22787, July 7, 2014; 59.56° incidence angle, 1.4 meters resolution from 114.29 km over 33.39°N, 114.56°E [NASA/GSFC/Arizona State University].See the larger reproduction HERE. [NASA/GSFC/Arizona State University].
Harriot B crater formed, slightly off center, over the much older and degraded Harriot crater (see the WAC context image below). The older crater's walls have slumped down, resulting in a rounded rim, in contrast to Harriot B’s sharp rim. Since Harriot has a greater diameter than Harriot B, 53 km and 38 km respectively, we can assume the impactor that formed Harriot B had less energy (less mass, less velocity, or both) than the impactor that formed Harriot.

Superposition of Harriot by Harriot B. The inexorable erasure of the Moon's surface very often strikes more suddenly than the 3 cm every 2 million years "gardening" rate of micro-meteorite and superluminal bombardment, established in the Apollo era. The concentric and near concentric "hole in one" of "macro-bombardments" occurs more often than seems obvious at first glance. 97 km-wide field of view from LROC WAC-derived imagery shows how the formation of Harriot B fit nicely in the bounds of older Harriot, and the coincidence may have had some, as yet undetermined role in the morphology of mound "clusters" on the floor of the latter [NASA/GSFC/Arizona State University].
Succeeding impacts will ensure that the visible evidence of Harriot crater will cease to exist.  This becomes a problem when using crater counts to date planetary surfaces.  Once a surface reaches crater saturation, scientists can only estimate a lower bound for how young the surface is.

Wide angle view of Harriot B (33.356°N, 114.409°E), nested inside only slightly larger Harriot crater, in a stark demonstration of the principle of superposition in stratigraphy. LROC WAC mosaic from sequential 55.3 km passes over the area of interest in LRO orbits 11201 and 11202, November 25, 2011; incidence 67.6° at 55 meters resolution [NASA/GSFC/Arizona State University].
One day, Harriot B will suffer the same fate as Harriot.  A new impact will erase it, along with its central mounds that deliciously look like donut holes. The Moon’s surface is a dynamic place, because it seems static in human-appreciable timescales this is easily forgotten.  So, let’s enjoy Harriot B’s central formations while they’re still around, check out the full resolution NAC below:

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Tuesday, July 8, 2014

A rille to rover over near Rimae Hevelius

A rille found on the southwestern edge of Oceanus Procellarum, most likely formed from stress added to the surface as mare deposits were emplaced and cooled.  1200 meter-wide field of view from LROC NAC observation M1145219838R, LRO orbit 20802, January 24, 2014; 36.35° incidence, resolution 1.05 meters from 103.39 km over 0.5°N, 295.29°E [NASA/GSFC/Arizona State University].
Raquel Nuno
LROC News System

Remotely sensed data acquired by spacecraft allow scientists to study the geology of other worlds without ever setting foot there.

The investigation of image data is usually the first step in unraveling the origin of a planet’s observed landforms and their evolution -- this scientific study is called planetary geomorphology.

Characterizing geomorphologic features is of the utmost importance for planning both a safely landed mission as well as a rover's path – it would be unfortunate if a rover sent to the Moon got stuck because it was sent to a location it could not traverse. The curved depression in today's Feature Image, called an arcuate rille, is lined with boulders.  This is an example of a landing site that would be hazardous for a rover to land in without a very detailed look at the surroundings.

Location of the area viewed at high-resolution in the LROC NAC Featured Image (arrow) on the unnamed arcuate compression rille that runs almost parallel against Rima Hevelius I, longest of the Rimae Hevelius system and extending outward and beyond Hevelius crater, which is outside this 38 km-wide field of view; north of the pyroclastic vent structures near Lohrmann D. From LROC WAC observation M129716592C, LRO orbit 4249, May 28, 2010; 59.3° incidence angle, resolution 58.61 meters from 42.07 km [NASA/GSFC/Arizona State University].
This unnamed rille (0.141°N, 295.362°E), roughly 1 km across, is located on the edge of the southwestern region of Oceanus Procellarum.  As the mare deposits that formed Oceanus Procellarum cooled and contracted, fracture systems developed along the mare-highlands boundary. Near this boundary, loading of denser basalts (mare) over less dense crustal materials (highlands) results in tectonic stresses that can cause rock to pull apart along fractures, forming arcuate rilles.

The footprints of the left and right LROC NAC frames, from which the Featured Image was derived, installed on a digital elevation model and LROC Wide Angle Camera mosaic to offer a simulated 'real world' perspective, putting the the arcuate rille area of interest (arrow) in relation to Rimae Hevelius, inside and outside its namesake crater, and the other complexities of the Lohrmann and Hevelius pyroclastic area of the western equatorial border of Oceanus Procellarum [NASA/GSFC/Arizona State University].
The pattern of some of the complexities at the surface of west and southwest Oceanus Procellarum (and a lot of other areas on the Moon) were at least partially laid bare by the sensitive GRAIL A and GRAIL B gravity probes in 2012. Though the exact boundaries and morphology of the vast nearside basalt flood plain are still poorly understood, it's now clearer that the Moon once had its own "Ring of Fire," less dynamic than on Earth, but still a 'squarish' border in the form of deep volcanic rift structures partially surrounding an enormous plate. The inset shows how this system of rifts explains at least some of the surface faulting, the unique mountains and lateral pressures under the Hevelius, Rimae Hevelius, and the pyroclastic vents near Lohrmann D, and, indeed, much of the the western boundary of Oceanus Procellarum [NASA/GSFC/SVS].
The boulders on the floor of this rille are most likely material that has been eroding away from the walls. Some of the boulders reach diameters up to 12 meters. While an autonomous rover would likely require a lot of time to maneuver amongst the boulders, a human driver on the lunar surface might quickly and easily navigate a safe path around the boulders.

The compression rille is also visible, to the right of the upper center, east of Hevelius, north of the channel linking Grimaldi basin (lower left) with Procellarum (upper right), in this HDTV still captured by Japan's lunar orbiter Kaguya (SELENE-1) in 2007. See the full-size original image, HERE [JAXA/NHK/SELENE].
If you were an astronaut on the surface of the Moon, do you think you could find a safe path through this rille?  Trace your path on the full resolution LROC NAC below.

View full-window, HERE.

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Thursday, July 3, 2014

Five years after First Light

Close up on the rim of Shackleton crater, a portion of the first LROC image in lunar orbit, released as LROC Featured Image on the Fifth Anniversary of  "First Light," July 3, 2014. This region of the rim of Shackleton, near the lunar South Pole, is illuminated over 70% of the time, while the floor of the crater is permanently shadowed. LROC NAC mosaic M101013931LR, LRO orbit 72, June 30, 2009; 90.01° incidence angle, resolution 1 meter from 42.96 km over 89.38°S, 53.48°E [NASA/GSFC/Arizona State University].
Brett Denevi
LROC News System

The first image captured by LROC from lunar orbit was acquired on 30 June 2009.

When the LROC team first glimpsed that image, some twelve days after LRO’s launch from Cape Canaveral, there was a mix of pride that all of the hard work to build and calibrate LROC was coming to fruition, relief that the cameras were in perfect working order, and anticipation for the adventures to come.

Now, on the fifth anniversary of LROC’s first image, we examine it again, with the insights that come with five years of knowledge gained.

An Orbital Earthset, from Japan's lunar orbiter Kaguya, November 2007, behind Malapert massif, and 10 km-wide Shackleton crater, both features of the Moon's far south visible from Earth at favorable librations. Between them, in permanent shadow, like the interior of Shackleton, is invisible Shoemaker crater, where a signature of a kind of volatile frost, perhaps composed of water, has been detected [JAXA/NHK/SELENE].
As it happens, this first image shows off one of the most interesting lunar science and exploration sites, Shackleton crater. Shackleton is located at the Moon’s South Pole, with a portion of its rim just grazing 90° south latitude. One of LROC’s primary objectives, defined as the mission was being conceived, is to map regions of permanent shadow or permanent illumination.

Because the Moon is just barely tilted on its spin axis (1.5 degrees compared to the 28.5 degrees that give us our seasons on Earth), any low point near the pole has the potential to be forever shaded from the Sun and peaks could forever poke above the horizon to catch grazing rays from the Sun.

Quasi-permanently illuminated (and, thus, the better known areas in the vicinity of Shackleton crater and the Moon's south pole) are tinted purple on a topographic map built up from LRO's LOLA laser altimetry. The remainder of the map, including Shoemaker crater (51.8 km; 88.137°S, 45.91°E) at upper right and inside Shackleton are parts of the wide areas of the Moon now known to be permanently shadowed. The footprint of the first LROC NAC observation on June 30, 2009 is outlined in yellow [NASA/GSFC/Arizona State University].
Why do we care about lighting conditions at the poles? For any future long-duration missions to the lunar surface, it’s easy to see the benefit of finding a spot with near-continuous illumination to provide solar power. With that also comes a reprieve from the cold lunar night, which lasts a full two weeks at lower latitudes.

Because LRO has been in orbit for 5 years, we can now map with great precision how much sunlight each portion of the surface sees by examining repeat imaging as the illumination from the Sun changes in the sky.  In the image above, a four-km2 region on Shackleton’s rim (near the boulders) is illuminated over 70% of the time.

LROC has shown there are no true regions of perpetual sunlight, but within small areas the surface is in sunlight nearly 94% of the year, with the longest periods of darkness lasting just 43 hours.

"Still-mysterious Shackleton"..."twice as deep as the Grand Canyon," from "Tour of the Moon," a 2:30 video prepared by the Science Visualization Studio (SVS) at Goddard Space Flight Center in 2012 [NASA/GSFC/SVS].
Over the last five years, an even bigger revolution in our understanding concerns what happens in those areas that never see the Sun – regions of true permanent shadow like the floor of Shackleton crater.

Before LRO’s launch, and really since the Apollo days, the canonical thinking was that the Moon was completely dry. There was some evidence in the form of neutron spectrometer and radar data that regions of permanent shadow may harbor water ice, but no definitive answer. Thus many of LRO’s instruments were designed to provide a fuller picture, so to speak, of these shadowed regions. LROC has undertaken long-exposure images to see inside the shadows, Diviner has measured temperatures below -238° C (-397°F) to show a host of ices are stable, LEND has found evidence for abundant hydrogen, Mini-RF has shown that radar properties within shadowed regions are consistent with the presence of water ice, LAMP and LOLA have demonstrated that there are variations in the reflectance of materials within craters at the poles, possibly indicating ice is present at the surface.

And other spacecraft, including LCROSS, Chandrayaan-I, Deep Impact, and Cassini, have shown that hydrated materials are not necessarily limited to the poles. But as this wealth of new data has been gathered, synthesizing it into a consistent story to explain how the volatiles were deposited, migrated, and evolved over time has proved more complicated. Seemingly conflicting results (for example, LROC does not see strong reflectance contrasts within permanent shadow, but LOLA and LAMP indicate frosts may be there; LEND sees abundant hydrogen outside of regions of permanent shadow, which is a bit mysterious) have led to more questions and the need for future missions to land in these enigmatic areas. Hopefully, in a second extended mission, LRO will have the time to gather the data to give us insight to find the best landing sites.

This first image from LROC provides just one example of how our view of the Moon has been altered by LRO’s five years in orbit. Long-outstanding questions are being answered, and new questions are being raised as we examine the abundant data from LRO. This vast trove of data and wealth of new knowledge will be LRO’s legacy for decades to come. 

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Sun angles, over fresh ejecta, east of Buisson V

Overlapping views of the same 1 km-wide area of the lunar surface, immediately southwest of a fresh impact crater near Buisson V; 68° morning shadow and 8° mid-day albedo, respectfully. LROC NAC observations M146255155R (LRO orbit 6687, December 6, 2010; incidence 67.86° at 62 cm resolution from 60.47 km) and M187520776L (orbit 12701, March 27, 2012; 8.35° incidence, resolution 97 cm from 116.95 km) [NASA/GSFC/Arizona State University].
Hiroyuki Sato
LROC News System

Today's Featured Image highlights differences in how the surface appears as the Sun angle changes. As seen in the opening image, the two images show the exact same area, but their appearances are remarkably different. Both images show the southern portion of the ejecta from an unnamed fresh 1 km crater (0.4933°S, 110.7156°E, 9.8 km west by northwest of Buisson V, an ancient equatorial crater just beyond the east limb.

The darker image, highlighting the granularity and elephant skin texture of the surface, was acquired when the Sun was low on the horizon (incidence angle = 68°), and the image characterized by bright swaths of ejecta was acquired when the Sun was nearly overhead (incidence angle = 8°).  Both were acquired with the cameras looking straight down. The small crater (about 27 meters in diameter) in the upper-middle each segment is a good landmark to confirm both images show the same location. 

Context view of ejecta blanket, source unnamed fresh crater (0.4933°S, 110.7156°E) 9.8 km west by northwest of Buisson V. LROC NAC footprint fields of view designated, area shown at high resolution indicated with an arrow. 46.1 km-wide field of view from LROC WAC monochrome (604 nm) observation M176905846C, LRO orbit 11206, November 26, 2011; 62.67° incidence, resolution 58.44 meters from 43.14 km [NASA/GSFC/Arizona State University].
When the Sun is high no resolvable shadows are cast, and surface brightness (albedo) variations stand out. The low Sun image has sharp shadows, highlighting the surface texture. Due to these drastic changes caused by the Sun angle scientists use NAC images at specific lighting conditions for different studies, of morphology, composition, optical and molecular maturity, topographic mapping, etc).

Granularity, at the expense of albedo, becomes visible at relatively low sun (high incidence angles). See the image at higher resolution HERE.
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