Wednesday, October 31, 2012

Ghosts of Fecunditatis

A gentle but distinctive topographic high designates the location of an ancient crater rim, nearly covered by basin flooding by volcanism, inside the boundaries of Mare Fecunditatis. LROC Narrow Angle Camera (NAC) frame M146662326L, illumination is from the east, angle of incidence 72.34° ; an approximately 1 kilometers-wide wide field of view at 0.98 meters resolution (in the original) from 47 kilometers altitude [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

Today's Featured Image focuses on ancient craters that predate mare basin flooding, and which are often recognized by subdued, sometimes discontinuous circular patterns best seen near local lunar sunrise and sunset (high solar incidence as measured from the surface normal). These circles mark the locations of once majestic excavations in the lunar crust. However the emplacement of volcanic deposits filling, surrounding, and overtopping the rims have buried these ancient craters in many instances. The presence of the near-surface rims produce local stresses in the deposits, which in turn deform the mare layers. The result is a wrinkle ridge-like topography with a circular pattern. They are thus often referred to as "ghost" craters, and can be found haunting many large, basin-filling mare deposits.

LROC Wide Angle Camera (WAC) mosaic centered on the Featured Image field of view. Note a second and more prominent ghost crater (Goclenius U) in the southeast corner of this approximately 120 km-wide frame [NASA/GSFC/Arizona State University.

At least two large ghost craters can be found here in the Mare Fecunditatis basin just south and southwest of the crater Ibn Battula. Some portions are simply unrecognizable as former crater rims without the large scale mosaic for context (see example below).

Another portion of the crater rim gives a muted appearance like that of a snow-covered park bench. Field of view width is ~700 meters.

Examine the full NAC frame (HERE) to see a greater length of ghost crater rim. Another example of a ghost crater is presented in the Ghost Crater in Southern Mare Crisium, a Tier 2 Constellation program Region of Interest. Contrast the appearance of a ghost crater to that of a flooded crater (e.g., as in Balcony Over Plato).

The Old and the Young in Tsiolkovskiy

A recent impact narrowly missed an ancient extensional crack in the mare-filled floor, the northeast interior of Tsiolkovskiy crater (18.97°S, 130.34°E). That recent impact's crater is at lower left. An 800 meter-wide field of view from LROC Narrow Angle Camera (NAC) frame M161455429R, spacecraft orbit 8927, May 31, 2011; illumination from the east by northeast at 67.26° incidence at 61 cm per pixel resolution (in the original) from an altitude of 59 kilometers [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

A common question from people examining the Moon through a small telescope for the first time is 'Why isn't the Earth covered with large craters like the Moon?' The first answer is that the Earth does have a number (more than 200) of known impact craters, but clearly nothing like the profusion of craters we see on the Moon. The more complete answer is that the Earth, during the first 600 million or so years of Solar System history, was indeed subject to the same relentless pounding that resulted in the Moon's population of large basins. However, early basins were subsequently obliterated by plate tectonics and surface weathering on the Earth. The Moon's features have been preserved because without wind, water, or plate tectonics, it takes a much, much longer time for the lunar surface to be reworked.

Today's Featured Image provides an example of this difference. We examine a length of shrinkage crack along the northern shore of Tsiolkovskiy's mare basalt volcanic deposits. Visible also in the WAC mosaic below, these features appear to result from contraction of the molten basaltic deposits during cooling. Also present (immediately adjacent to this portion of crack) is a relatively young impact crater. The two formation events are separated in time by several billion years, yet both present bouldery surfaces, and relatively sharp topographic expressions. Because of the slow rate of "space weathering," Lunar scientists must use more subtle indicators of freshness, or stratigraphic and other cross-cutting relationships, to determine the relative ages of landforms. The Apollo and Luna samples allow us to anchor portions of the lunar geologic time scale to radiometric age dates.

A12,922 line by 8,734 sample from a mosaic of both the left and right frames from LROC NAC observation M161455429, resampled to allow a medium resolution context for the field of view seen at higher resolution further above (white square). Again, that square represents approximately 800 meters and this re-sampling is about 4 kilometers across [NASA/GSFC/Arizona State University].
Compare the shadows within the linear feature to those of the recent impact in the southwest corner of the opening image. Because the crater is known to be an excavation, and the eastern walls of both features are in shadow, it is clear that the linear feature also shows negative relief. Contrast these shadows to those of this feature from within the same basin, which shows a similar outline, but represents a topographic high. Clearly several processes have been at work within these deposits. The crater is another example of a special type of impact feature called a bench crater (see October 17, 2012 LROC Featured Image).

LROC Wide Angle Camera (WAC) mosaic of the northeast quadrant of Tsiolkovskiy, field of view about 115 km across [NASA/GSFC/Arizona State University].
The full NAC image can be explored HERE. Other LROC Featured Images highlighting landforms related to mare tectonics include Tectonics in Mare Frigoris, and Not Your Average Scarp.

Since the very first grainy, misunderstood images of the Moon's far side were returned to Earth by the Soviet Union in 1959, Tsiolkovskiy immediately stood out, strongly underscoring the remarkable differences between the tidally locked hemispheres .
Weaving Boulder Trails on the Moon (July 12, 2012)
Bulging wrinkles at Tsiolkovskiy (January 11, 2012)

Saturday, October 27, 2012

Impact melt outside Wiener F

Fractures in impact melt pools outside complex farside lunar crater Wiener F, 470 meter-wide field of view from LROC Narrow Angle Camera (NAC) frame M141287909R, LRO orbit 5955, October 9, 2010. Resolution 52 centimeters per pixel from 47.29 kilometers  [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System

The impact melt fractures in the Featured Image are located in the impact melt pools outside of the crater Wiener F. Wiener F is a 47 km diameter crater on the farside, located at 40.89°N, 149.94°E. Impact melt is formed during the crater excavation process due to the intense heating of the target rocks. The fractures seen here most likely formed as the impact melt pool cooled. Exterior melt ponds are often seen with fresh craters of this size or larger. For example, the crater Anaxagoras has exterior melt pools, including many fractures. Wiener F is also interesting because it is superposed over a much older impact crater. In the LROC WAC context image below, you can see the remains of the northern rim of the older crater. The interior of Wiener F has been modified by slumping, which may be due to the irregularity of the terrain of the target material (the rim of the older crater).

A few weeks ago, another Featured Image described the exterior impact melt of Wiener F, but for an area closer to the rim of the crater. During the excavation stage of the impact, ejecta scoured the rim and an impact melt veneer covered the scoured area. The impact melt veneer from the earlier Image and the exterior impact melt ponds in today's Image are different due to the volume of impact melt emplaced at each location.

WAC context image of Wiener F. The white asterisk shows the location of the impact melt fractures seen in the Featured Image [NASA/GSFC/Arizona State University].

Explore the entire NAC frame for more interesting impact melt features, HERE.

Related Images:

Thursday, October 25, 2012

Making the Moon: Two New Models

Simulation of a Moon-forming impact [Harvard University].
"A common origin for the Moon and Earth is required by their identical isotopic composition. However, simulations of the current giant impact hypothesis for Moon formation find that most lunar material originated from the impactor, which should have had a different isotopic signature. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to the present day; however, Earth-mass planets are expected to have higher spin rates at the end of accretion. Here, we show that typical last giant impacts onto a fast-spinning proto-Earth can produce a Moon-forming disk derived primarily from Earth's mantle. Furthermore, we find that a faster-spinning early Earth-Moon system can lose angular momentum and reach the present state through an orbital resonance between the Sun and Moon."

- Matija Ćuk & Sarah T. Stewart-Mukhopadhyay, "Making the Moon from a Fast-Spinning Earth: A Giant Impact Followed by Resonant Spinning," Science DOI: 10.1126/science.1225542 (Online October 17, 2012)

Scientists have long believed the Moon formed as a result of a collision between the early Earth and a smaller planet, but computer models of the giant impact have always predicted the wrong composition for the Moon. Now researchers at Harvard University and the SETI Institute are proposing a new spin on the giant impact model to match the observed composition of the Moon. Understanding how the Moon formed is important for astrobiologists who are studying how the Earth became habitable for life as we know it.

The previous giant impact models have held that the small planet, Theia, hit the Earth, sending a cloud of debris from Theia into orbit that formed the Moon. But the chemistry of the Moon matches the Earth. Now Sarah T. Stewart-Mukhopadhyay, a professor in Harvard's Department of Earth and Planetary Sciences, and her SETI colleague Matija Ćuk propose a new giant impact model that resulted in pieces of the Earth breaking off and forming the Moon.

The researchers present a dynamic model of their theory, motivated by the results of chemical analyzes of isotopes from the Earth and Moon, in a paper published online today in Science. The results were also presented at the 44th meeting of the AAS Division for Planetary Sciences in Reno, NV.

Additionally, Stewart and Ćuk propose that prior to the collision and creation of the Moon, the Earth was spinning much faster than it does now, and had a day that was only two to three hours long.

Many scientists believe that Earth itself emerged from a series of giant impacts. These impacts made the early Earth spin near its stability limit of about 2 hours per revolution. The last giant impact, they believe, formed a Moon that is a twin of the Earth. Stewart and Ćuk posit that when the giant impact occurred between Theia and the fast-spinning Earth, the high speed of the Earth's spin caused the ejection of material from Earth into orbit. The ejected material formed a Moon with chemical composition similar to Earth. After the impact, the rapidly rotating Earth was slowed down by the gravitational interaction between the Sun and the Moon.

Previous giant impact models could match the size of the Moon and the present angular momentum of the Earth and Moon but did not explain the similar chemistry of the Earth and Moon. But the new theory, with the discovery of a mechanism to slow the spin of the Earth after the impact, explains how a giant impact with a fast-spinning Earth could result in a Moon with a similar chemical composition

Almost a "double planet," the Earth-Moon system imaged by the ESA Mars Express in Mars orbit [ESA].
As part of their dynamic model, Ćuk and Stewart found that a resonance between Earth's orbit around the Sun and the Moon's orbit around Earth can pass angular momentum to the Sun. Furthermore, Ćuk and Stewart showed that if the Earth was fast-spinning before the impact then a giant impact would eject enough Earth material into orbit to make the Moon.

Today, tides between the Earth and Moon both slow Earth's rotation and push the Moon's orbit further away. But the total angular momentum of the system is conserved. The finding is significant because without a fast-spinning Earth preceding impact, "a giant impact could not make the Moon originate from the Earth's mantle with today's angular momentum," says Stewart.

The origin of the Moon had been called into question by isotope analyzes of material from both Earth and the Moon. The isotope signatures of celestial bodies differ greatly and often are used to 'fingerprint' different planets and meteorite groups. The data show that the Earth and Moon are 'isotopic twins,' a contradiction to the Moon origin story from the original giant impact model. If the original model were correct, then the Moon should have had a different isotopic fingerprint than the Earth.

Nineteenth century scientists speculated about a fast-spinning early Earth. George H. Darwin, son of Charles Darwin, studied the relation between tides and the Moon. In 1879, he suggested that the Moon formed by fission from the Earth, but he did not know how early Earth might have being spinning so quickly. A similar dynamic model for a great impact resulting in the formation of the Moon from Earth material is described in a second paper in the same issue of Science. This alternative dynamic model is presented by Dr. Robin Canup of the Southwest Research Institute (SwRI).

Forming the Moon with an Earth-Like Composition via a Giant Impact (Canup, SwRI; Science)
Water from the Sun (October 17, 2012)
Hit-and-Run Science (September 30, 2012)
A Sawtooth-like timeline for the first billion years of lunar bombardment (August 28, 2012)
A new 'hit and run' Giant Impact scenario (July 28, 2012)
"Our view of the Moon has turned upside down" (April 26, 2012)
Ti paternity test fingers Earth as Moon's parent (March 28, 2012)
NLSI team sheds light on 'late heavy bombardment' (February 28, 2012)
Spudis: Cataclysmic Conundrum (February 14, 2012)
'Significant change' in bombardment timing (January 6, 2012)
LOLA reveals distinct populations in bombardment record,
Diviner finds "no pristine lunar mantle" even within SPA
(September 16, 2010)
'The Grand Lunar Cataclysm and how LRO can help test it' (September 7, 2009)

The Astrobiology Institute, Harvard Crimson, SETI Institute and Southwest Research Institute contributed to this digest.

Wednesday, October 24, 2012

Boulders bounce, roll and stop

Boulders bounced and rolled down the interior wall of the crater Shuckburgh E leaving a diagnostic trail. 655 meter-wide field of view  from LROC Narrow Angle Camera (NAC) frame M141885094R, spacecraft orbit 6043, October 16, 2010; angle of incidence 47.98° resolution 47 cm, from 39.84 kilometers [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System

Boulders of different sizes zipped down the interior slopes of Shuckburgh E, a 9.22 km diameter crater located at 44.042°N, 57.018°E. As the boulders bounced and rolled down the wall, they carved trails through the regolith. Click on the Featured Image above to take a closer look at more boulder trails. If the trail is discontinuous, like a dashed line, then the boulder was bouncing. If the trail is continuous, then the boulder was rolling. As they lose momentum the boulders stop bouncing and instead plow through the regolith until they come to a standstill. The largest boulder track in today's Featured Image is about 5-7 meters across! To put this into perspective, 5.5 meters is about the height of an adult giraffe.

Boulder trails are not only pretty, but in some cases they could help human or robotic explorers sample material that is otherwise hard to reach. For example, in Schrödinger basin, a rover could land on the relatively safe, smooth floor and sample boulders that fell from peaks and rims. Material at the top of peaks and rims is scientifically interesting since it is excavated from deeper in the lunar crust. Boulder trails help trace back to the original location of the boulder, making it possible to locate boulders that originated from different points on a peak. Boulder trails were used during Apollo 17 to determine the location of the Station 6 boulder!

LROC Wide Angle Camera (WAC) 100 meter resolution global monochrome mosaic draped over LOLA digital elevation model simulates the view 50 km over Shuckburgh E and Lacus Temporis to the northeast. NASA ILIADS application, Lunar Mapping and Modeling Project (LMMP) [NASA/GSFC/Arizona State University].

Explore the entire NAC frame, HERE.

Related Posts:
Sampling Schrödinger
Hole in One!
Rolling Rolling Rolling
Boulder trails in Menelaus crater

Tuesday, October 23, 2012

Beautiful Young Crater in Icarus

A beautiful, young crater inside of the complex crater Icarus. Field of view 550 meters from LROC Narrow Angle Camera (NAC) observation M156367058L, LRO orbit 8177, April 2, 2011; 0.6 meters resolution over an angle of incidence 10.53° from 58.61 kilometers [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System

Icarus is a large, complex crater (diameter 93.7 km) with a central peak, located at 5.584°S, 186.998°E. Icarus is named after the mythical Greek flyer. The Featured Image shows a young, fresh crater (located at 5.929°S, 187.696°E) superposed on the older terraces of Icarus. The ejecta of the impact is higher in reflectance compared to the surroundings since the newly excavated material has been exposed to space weathering for a relatively short time. Over time space weathering causes the reflectance of fresh regolith to decrease. The ejecta of the crater in the Featured Image will fade over hundreds of millions of years, until it can no longer be distinguished from the rest of Icarus crater.

While the rim and terraces of Icarus are heavily degraded by subsequent impacts, the crater's central peak is still quite tall. The central peak rises about 4475 meters above the crater floor! Compared to other craters of similar diameter, this is quite a tall central peak! Consider many of the other complex craters with central peaks featured in the LROC images: Moretus, Hayn, Aristarchus, Theophilus, Bullialdus, Langrenus, Copernicus, and Tsiolkovskiy. For example, Tycho (~82 km in diameter) crater's central peak is 2 km above the crater floor.

Topography of Icarus crater. The northeast edge of the rim is partially destroyed [NASA/GFSC/Arizona State University].
All of these complex craters have a few common characteristics. First, the impact has to be large enough to cause a complex crater. Relatively smaller impacts create simple craters, which are bowl-shaped and have no central peak or terraces. After the excavation phase of the impact, the transient cavity collapses. This collapse is driven by gravity, which causes the uplift of the central peak as well as the collapse of the rim inward, which forms terraces along the interior wall. Some times an impact is so large that it creates a ring of peaks instead of just one central peak.

LROC Wide Angle Camera (WAC) context image of Icarus, the white asterisk marks the location of the fresh crater in the Featured Image [NASA/GSFC/Arizona State University].
Explore more of the interior of Icarus with this LROC NAC, HERE.

Related Images:
Lunar Topography - As Never Seen Before!
Copernicus Central Peak From The West
View From The Other Side

Faculty Position: University of Arizona

Department of Planetary Sciences
Lunar and Planetary Laboratory
University of Arizona

The Department of Planetary Sciences/Lunar and Planetary Laboratory at the University of Arizona has available a tenured or tenure-track faculty position. Candidates in all areas of planetary science are encouraged to apply. Current faculty and research staff are engaged in many facets of planetary science, including planetary surfaces, interiors, atmospheres, the Sun and heliosphere, exoplanetary systems, comparative planetary studies, origins of planetary systems, and orbital dynamics. They employ tools such as theoretical studies and data analysis, laboratory and field investigations, telescopic observations, remote sensing, and spacecraft development, operations, and instrumentation. The faculty, research staff, and graduate student body are drawn from the diverse backgrounds of planetary science, astronomy, chemistry, geology, physics, and engineering. Additional information concerning the Department/Laboratory is available at

Successful candidates will teach at all levels, from freshman through advanced graduate classes. They will establish and maintain a distinguished research program in the field of planetary sciences and will supervise graduate students. Salary is dependent on qualifications. To be considered for an appointment above the rank of Assistant Professor, candidates must have an internationally recognized record of distinguished scientific achievement, leadership, and teaching ability in the planetary sciences. To be considered for appointment at the rank of Assistant Professor, candidates must demonstrate clear promise of such achievement.

Review of applications will begin on November 15, 2012, and will continue until the position is filled. The starting date for the appointment is anticipated to be August 12, 2013. Applicants must complete the online application at (search for Job Number 51233). Inquiries and supporting application materials, including a CV with publication list, statement of research interests, statement of teaching philosophy, and the names and addresses of at least four references may be submitted to:

Professor Timothy D. Swindle
Head, Department of Planetary Sciences
Director, Lunar and Planetary Laboratory
The University of Arizona
1629 E. University Blvd.
Tucson, Arizona 85721-0092
(520) 621-4128

The University of Arizona is an EEO/AA employer - M/W/D/V.

Thursday, October 18, 2012

Debris flow at Clavius E: How Recent?

Granular debris flows cascade down the wall of a young crater. Geologically these are young features - but how young? Approximately a 696 meter-wide field of view from LROC Narrow Angle Camera (NAC) frame M185961505L (downslope to lower right, north is down), spacecraft orbit 12483, March 9, 2012; resolution 60 centimeters per pixel from 57.29 kilometers, angle of incidence 53.52° [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System

Granular debris flows are found in many young impact craters and are the products of mass-wasting. Over time, material from the crater rim and walls erodes into successively finer particles, and when the influence of gravity becomes too much this material moves downhill. Observations of mass-wasted material are prevalent in the LROC NAC images, and the detailed morphology of the flows are striking. Examples are composed of low-reflectance material when compared to the crater walls, fine-grained fingers superposing coarser-grained flows, as well as meandering flows interweaved with one another. Although debris flows on the Moon exhibit many spectacular morphologies and features, the presence of the flows represents relatively recent activity of a geologic process that is still active today.

Two kilometer-wide field of view, resampled to 12 percent of original NAC frame, with north at top, shows the granular flow in context with the southeastern rim, wall and floor of Clavius E. Boulder trails, shown at much greater visibility through the LROC Image Browser, abound [NASA/GSFC/Arizona State University].

Some debris flows show evidence of multiple formation events in the form of superposed lobes of material or braided channels, and today's Featured Image is no exception. The debris flow descending across the center of the image (51.725°S, 347.058°E) has braided, meandering channels at the upper left (uphill) that gradually disappear downslope into larger lobes of material. The lobes are most easily distinguished at the lower right corner of the image (downhill) where the flow terminates, and there are at least three individual flow units (and thus separate depositional events) that can be distinguished by a faint outline of higher-reflectance material. However, what makes today's flow special is the ~13 m diameter impact crater superposed on the flow because both of these features are geologically young. The presence of an impact crater on a debris flow suggests that the flow may not be geologically active at present and may not have been geologically active for some time (although constraining that time period is difficult).

Simulated view north over Clavius E (51.509°S, 347.278°E, ~15 km diameter), from a perspective originating 130 km over Clavius proper, south of Tycho. LOLA elevation model under LROC 100 meter Global WAC mosaic, ILIADS application from NASA Lunar Mapping and Modeling Project (LMMP).
Because the debris flows form in the young, least-degraded craters and do not often exhibit degradational features (such as superposed impact craters), determining the absolute age of the flow is impossible because the flow may have formed 100 million years ago or yesterday. However, the presence of a superposed impact crater on a debris flows constrains the relative age of the flow because the crater must have formed after the flow was deposited. Crater counting techniques are frequently used by lunar scientists to estimate an absolute age-date for a surface, but there have to be craters to count! For the moment, we must be satisfied knowing that enough geologic time has passed for the flow pictured above to accumulate one crater. Alternatively, LROC could target this debris flow (or others like it) and acquire additional observations over time in an attempt to understand the geologic history of these spectacular features. Because who knows - maybe a seismic event (moonquake, impact) will dislodge material upslope to flow downhill that erases the crater!

Can you find additional impact craters superposed on the debris flows in the full LROC NAC, HERE?

Hint: there is at least one other crater superposed on a debris flow in the opening image.

Related Posts:
Debris Channels
Lunar landslides!
Granular Flow

Wednesday, October 17, 2012

Water from the Sun

The Sun exudes a constant stream of hydrogen, called the "solar wind."
Paul D. Spudis
Smithsonian Air & Space

New data returned from a fleet of orbiting satellites changes our perceptions of the history and processes of the Moon.  Concentrated at both lunar poles, and to date the most striking discovery, is the documentation of the presence of large amounts of water.  Though this water has been confirmed by several differing techniques (from multiple missions), we remain uncertain about its source.  Two principal origins have been proposed: 1) water added by the in-fall of water-bearing meteorites and comets during the impact bombardment of the Moon; and 2) the manufacture of water from hydrogen implanted in the lunar soil by the wind from the Sun.

A recent discovery may shed some new light on the origin of lunar water.  Researchers conducting detailed examination of tiny fragments of glass in soil returned by the Apollo astronauts found the molecule hydroxyl (OH) present in the glass.  Interestingly, the isotopic composition of these OH molecules indicates the bulk of the hydrogen comes from the Sun, not from cometary and asteroidal impacts.

The Moon has no atmosphere and no global magnetic field.  As a result, the solar wind – the stream of atoms and molecules constantly emitted by the Sun – directly impinges upon the lunar surface.  Most of this solar wind consists of hydrogen, either in the form of neutral atoms or positively charged ions (i.e., protons).   After it encounters the Moon, this spray of hydrogen has a complex fate, with at least some of it being implanted into the lunar dust.  In a process called adsorption, many of the hydrogen atoms stick to the surfaces of the dust grains.  The amount of adsorbed hydrogen varies by position and chemical composition around the Moon, but it can be present in quantities ranging from less than 10 to over 100 parts per million (ppm).

Impact glass is a major component of lunar regolith – up to 60% by weight of the soil at some landing sites.  The constant bombardment of the lunar surface by microscopic meteorites crushes and grinds up the surface rock, continually mixing the outer layer of the Moon.  When a micrometeorite strikes a rock, it forms a micro-crater (wholly melting the surface beneath this pit) and creates a clear, chemically homogeneous glass particle.  However, when a micrometeorite strikes lunar soil instead of rock, its energy is converted mostly into heat.  This flash heating creates a mixture of melt and mineral debris called agglutinate glass.

The new work details results of analyses of agglutinates returned from several lunar landing sites.  Their study measured both the amounts of hydroxyl present and its isotopic composition.  A normal atom of hydrogen is a single proton and an electron.  But in a rare form of hydrogen, called deuterium, the nucleus contains both a proton and a neutron.  The ratio of this form of “heavy hydrogen” to “normal” hydrogen is unique for different materials throughout the Solar System.  By tracking the D/H ratio in the sample, one can assign a source origin to the measured hydrogen.

When the lunar agglutinate glasses were studied, it was found that their D/H ratios indicated that most of the hydrogen in the hydroxyl molecules came from the Sun and not from cometary or meteoritic sources.  However, the source of the hydrogen is not completely solar, as the D/H ratios suggest some mixing with a subordinate component of either lunar or cometary origin.  The authors of this study suggest that the hydroxyl found on the Moon was created when a small impact flash heated the soil, releasing the adsorbed hydrogen and chemically reducing the metallic oxides in the soil into native metal (found as extremely tiny grains on the surfaces of the agglutinates) and hydroxyl molecules.  Multiplied by billions, such a process could account for the generation of water on the lunar surface.  Subsequent migration of these molecules toward cooler-than-average areas of the Moon (i.e., the higher latitudes, up to and including the poles) may have created the polar ice deposits found by numerous techniques.  In the view of the authors of this study, lunar water comes mostly (but not entirely) from the Sun.  This constant process, occurring on the sunlit hemisphere of the Moon, could create an enormous reservoir of hydroxyl molecules (in motion due to their thermal instability), slowly but constantly moving toward the poles.

If such a process occurs on the Moon, one might expect the accumulation of water in every location where water is stable (i.e., within every permanently dark and cold region near both poles).  But it appears that ice at the poles is not uniformly distributed, occurring in high concentration in some areas while absent in others.  This pattern suggests that the source of polar water might be controlled by a non-equillibrium process, such as episodic bombardment by asteroids and comets.  In fact, both solar wind-produced and cometary water may be present at the poles, but until the ice there is actually analyzed for its D/H content, we cannot be certain of its origin.  Such a measurement does not require the return of a polar ice sample to the Earth.  It could be made remotely in situ on the Moon with a properly instrumented robotic spacecraft.

It is important to emphasize that although the quantities of water generated by this process are potentially very large, the hydroxyl in agglutinate glass should not be considered an economic resource.  These molecules occur globally but at very low levels of concentration (tens of ppm).  Even if this water is the primary and ultimate source reservoir of lunar water, the migration of the molecules and their subsequent collection by the cold traps near the poles serve as a concentrating mechanism, where ice accumulates in large quantities, confined within small areas — the classic definition of an ore body.

What a change has occured in the mindset the lunar science community in the past few years!  From a bone-dry lump of rock in space to a complex, still mysterious body with a dynamic hydrological cycle.  It’s clear that many more discoveries about our Moon and its resources have yet to be revealed.  The more we learn about the Moon, the greater the range of processes we must account for and the more subtle and complex its history becomes.

Originally published at his Smithsonian Air & Space blog The Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.

A Beautiful Bench Crater

A beautiful bench crater, formed in melt trapped on a western wall terrace of Rutherfurd crater, south of Clavius on the lunar near side. A 300 meter-wide field of view from LROC Narrow Angle Camera (NAC) observation M185961318R, spacecraft orbit 12483, March 9, 2012; resolution 0.52 meters from 51.29 km [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System

Regolith covers the lunar surface, and the thickness of regolith on the surface is related to the age of the surface. Older surfaces have thicker regolith layers than younger surfaces, and observations of crater morphologies are used to learn about the regolith for a specific area. Bench craters form in layered targets when there are variations in strength between the layers because different strength targets require different amounts of energy during the excavation phase of impact cratering. On the Moon, bench crater formation is usually interpreted to result when a bolide punches through an unconsolidated regolith layer to excavate a more cohesive layer such as mare basalt bedrock. The 75 m diameter bench crater in the opening image (61.504°S, 346.728°E) is a prime example of a bench crater that formed in an impact melt pond that is covered by a thin layer of regolith. However, observations of LROC NAC images show some bench craters like the one above to be self secondary craters, formed during the last stages of the impact process. It may be that the bench crater above was one of the last secondary craters formed during the Rutherfurd impact event, soon after the melt was emplaced, but without further study, we cannot be certain.

LROC WAC monochrome 64 meter resolution mosaic of Rutherfurd crater (61.186°S, 347.683°E, ~47 km diameter), from LROC QuickMap. Featured Image field of view noted by plot point on the southwestern crater wall [NASA/GSFC/Arizona State University].

The smoothed, softened texture of the pond surface, absence of cracks and fractures in the melt, and presence of superposed impact craters of various sizes and degradational states provide evidence of a layer of regolith in this area. If the 75 m diameter bench crater is not a self secondary crater, the projectile that formed the crater likely excavated roughly 7-8 m into the melt rock. Meter-sized boulders distributed within and around the eastern portion of the bench crater support an impact into a consolidated target and the formation of these boulders during excavation of the crater. Besides confirming the results of experiments conducted in the 1960s with layered targets, today's bench crater might be used to help constrain the depth of the impact melt pond. If there are other craters of similar degradational state in the pond, the morphology of these craters could be studied to help constrain not only the regolith thickness but also perhaps the thickness of the melt pond in this region. Unfortunately, it looks like the ~40 m diameter crater to the right of the bench crater may too degraded or affected by the boulders outcropping toward the upper right of the image. Additionally, finding these craters may prove difficult because the Featured Image may be the location of the only small melt pond with a bench crater in this portion of the Rutherfurd crater wall and any bench craters occurring elsewhere may reflect the strength contrast between the impact melt veneer on Rutherfurd's wall and the crater wall material.

How many bench craters can you find in the full LROC NAC frame? Are the bench craters located in small melt ponds or in the impact melt veneer on Rutherfurd's wall? If you find bench craters in the melt veneer, what two layers do you think might be responsible for forming the bench (hint: think about what the melt veneer covered) if the craters are not self secondaries?

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John Moore: Exaggerating the elevation

Exaggerating the elevation values in some LROC DEMs (and accompanying colour topographic overlays) can sometimes reveal more about features seen at normal elevation level. Here are just five, randomly-chosen DEMs, but many more available in the LROC website (see Credits) can be downloaded for further research.

Tuesday, October 16, 2012

Winding Channel of Melt

Impact melt channel winding its way down Anaxagoras crater wall, on its descent to the floor. LROC Narrow Angle Camera (NAC) frame M185956855L, LRO orbit 12484, March 9, 2012' image field of view 2.2 km (downslope to upper left), angle of incidence 73.06° at 1.76 meters resolution, from 179.79 kilometers [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System

Anaxagoras crater (73.458°N, 349.934°E, ~52 km diameter) is a complex crater with terraced walls, central peak, and substantial impact melt deposits. After impact melt is created during the impact event, significant portions are often ejected from the crater in much the same way as the unconsolidated excavated rock that forms the typical ejecta blanket. However, most melt does not contain sufficient energy to escape the crater interior and is instead splashed onto the crater walls. If the melt remains hot enough, the splashes and globs of melt on the walls may coalesce and descend toward the crater floor under the influence of gravity.

Sometimes, the flowing impact melt creates channels by eroding the substrate or building up levees. In today's Featured Image, an impact melt channel formed in the lower section of crater wall terraces, near the last major break in slope close to the crater floor (73.286°N, 351.500°E). Many small, channel-like features occur in this area of the crater wall, and the channel origin is difficult to discern and may actually be located upslope to the lower left, outside the view of the Featured Image. Often, channels form in a path of least resistance, that is, where preexisting fractures and weakness occur - and crater wall terraces are prime locations for heavily fractured target rock. This channel ranges from ~95 m to 160 m across and has clearly defined walls until the upper right of the image, when part of the channel disappears. Why the disappearing act?

LROC WAC monochrome (604nm) mosaic of Anaxagoras stitched from 10 passes March 21, 2011, or a bit less than a full year prior to the opening NAC observation. The arrow indicates the location of NAC Featured Image field of view [NASA/GSFC/Arizona State University].
Flowing impact melt has a specific lifetime and when the melt cools enough, it stops flowing. There are a couple of plausible explanations for the disappearance of one of the channel walls. First, some melt may have traveled down the channel until it cooled enough solidify near the tail end of the channel, thus clogging the channel pipeline. Alternatively, a relatively thick veneer of melt may have been splashed onto the wall at a late stage, thus obscuring and erasing the channel wall. Another possibility is that there is a change in slope that influenced the melt to breach the channel wall and instead flow toward the top of the image, an explanation perhaps supported by the change in concavity in the channel from concave up (somewhat U-shape) to concave down (upside-down U-shape). However, additional observations, including the use of a NAC derived topographic maps, are crucial to determine which hypothesis (or another!) is the best explanation.

How many impact melt channels can you find in the full LROC NAC image, HERE.

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Friday, October 12, 2012

'China's grand plan for lunar exploration'

China's second lunar orbiter, the ultimately successful Chang'e-2, is mated to it's launch vehicle in preparation for launch in October 2010. According to official state-owned news sources, the determined and methodical China Lunar Exploration Program (CLEP) marches on.
Wang Xiaodong
China Daily

A lunar probe scheduled to be sent to the moon in 2017 by China will bring back enough samples from the surface of the moon for research by various institutes, according to Ouyang Ziyuan, chief scientist for China’s lunar exploration project.

“The sample acquired by Chang’e-5 will be distributed to qualified institutes of many different sectors in China for research,” Ouyang, a member of the Chinese Academy of Sciences, said after an event organized by the Chinese Society of Astronautics to celebrate World Space Week.

A UN Generally Assembly resolution adopted in 1999 declared Oct 4-10 each year as World Space Week, to celebrate the contributions of space science and technology to the betterment of the human condition.

“We will gather the strength of the whole nation for achievement of the highest level. And we will do what others haven’t done yet,” Ouyang said, referring to the research into samples to be brought back by Chang’e-5.

China’s lunar probe project consists of unmanned moon exploration, a manned moon landing and building of a moon base. Currently, China is in the first stage.

China launched its Chang’e-1 orbiter in 2007 and Chang’e-2 in 2010, and got a great deal of scientific data and a full high-resolution map of the moon.

China is scheduled to send its third probe, Chang’e –3, to the moon next year. After the sampling of the moon’s surface is done around 2020, China will start a manned lunar mission. But there is no clear timetable for that, Ouyang said.

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Thursday, October 11, 2012

Lunar accretion from a Roche-interior fluid disk

Julien Salmon, Robin M. Canup
arXiv 1210.0932 v.1

We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of the inner disk, resulting in a total lunar accretion timescale of ~10^2 years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing < 2.5 lunar masses (ML), we find that a final Moon with mass > 0.8ML contains < 60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.

Download, read and review the paper, HERE.

Hole on A Melt Sheet

A portion of the impact melt sheet on the floor of crater Korolev X. Image centered on 0.699°N, 200.594°E, field of view is 638 meters, illumination from the right, or east. From LROC Narrow Angle Camera (NAC) observation M145664820R, LRO orbit 6600, November 29, 2010; angle of incidence 61.4° at 0.64 meters resolution, from 61.62 kilometers  [NASA/GSFC/Arizona State University].
Hiroyuki Sato
LROC News System

Korolev X is a 25-km crater located at 0.54°N, 200.59°E. As seen in the bottom image, the northern rim of this crater was destroyed by a younger crater about 16 km in diameter. The heat from the impact that formed this younger crater melted a large volume of rock, which flowed down onto the floor of Korolev X, creating a sheet of solidified melt 14 x 5 km across. The opening image highlights a dent in the surface of this melt sheet.

This dent is about 105 meters in diameter. Considering the existence of multiple, similarly-sized (around 100 m in diameter) craters on this melt sheet, this dent is most likely an impact crater even though neither an ejecta blanket nor a raised rim can be clearly recognized. Along the top of the crater wall there appears to be a thin layer of the melt sheet that is exposed and highlighted by the angle of the sun. Below this surface layer no clear layering is observed, implying a rather homogeneous structure. Small craters like this are often observed in impact melt sheets, and why they lack typical features of impact craters (a well-defined raised rim, a thick ejecta blanket) is still not well known. Could these craters have formed when the impact melt was still partially molten?

Korolev X and surroundings from an LROC WAC monochrome mosaic (100 m/pixel) centered near 0.52°N, 200.57°E. The blue box indicates the footprint of LROC NAC observation M145664820R with their Featured Image field of view designated by the yellow arrow [NASA/GSFC/Arizona State University].

Explore various strangely shaped craters on this melt sheet in full NAC frame yourself, HERE.

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More detail, at small scale, shows the elevation range north and south of Korolev X, near the rim of mighty Korolev basin. Less than 200 km to the north of the area of interest is the Moon's highest elevation. Though all of Korolev range as high as 10.2 km lower than the east rim of Engel'gardt (and unlike the familiar nearside basins), nowhere in Korolev falls below the Moon's global mean. The farside of the Moon is very different than the nearside [NASA/GSFC/Arizona State University/DLR].

Wednesday, October 10, 2012

Branched Impact Melts

Impact melt on the northern rim of an unnamed crater. Image center is 17.378°S, 85.205°E, image width is 500 m, downslope is towards the top. LROC Narrow Angle Camera (NAC) observation M139348293R, LRO orbit 6559, September 17, 2010; angle of incidence 20.25° at 0.5 meters resolution from 44.7 km  [NASA/GSFC/Arizona State University].
Hiroyuki Sato
LROC News System

Today's Featured Image highlights the viscous impact melt flow features found near an unnamed 4 km-wide crater centered near 17.247°S, 85.2°E, on the rim of crater Gibbs (78.8 km in diameter). You can see the low-reflectance viscous flow features forking into smaller terminal fingers.

These distinctive flows are found on the top of the older impact flows (older by seconds or minutes), and you can also see small channels formed as the molten rock flowed around blocks on the rim. These relations show us that this distinctive fingered flow was emplaced after most of the other ejecta. So how did this melt get to this location? One hypothesis is that this deposit of melted rock, along with all of the other ejecta produced by the impact that formed this crater, was expelled almost vertically. In this scenario, the melt that formed this deposit was then simply among the last to fall back to the surface, where it flowed around and over some of the ejecta that had fallen previously. Alternatively, this melt deposit could also simply be some of the last impact melt to be ejected from the crater during the last part of the excavation stage. No matter its exact origin, it is inspiring to see the tremendous forces of nature creating artistic patterns on our nearest neighbor.

The unnamed crater and surrounding area in LROC WAC monochrome mosaic (100 m/pix). The blue box and yellow allow indicate the location of full NAC frame and today's Featured Image respectively. Image center is 17.25ºS, 85.20ºE [NASA/GSFC/Arizona State University].

Explore this fork-shaped impact melt deposit in full NAC frame yourself, HERE.

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Outside of Giordano Bruno
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'Once in a Blue Moon,' Our Satellite's True Color

"True" color (left) and "false" color (right) images of the near side of the Moon from Clementine. "Blue" units in Mare Tranquillitatis (right middle of false color image) are ilmenite-rich lavas.
Paul Spudis
The Once & Future Moon
Smithsonian Air & Space

The color of the Moon has been studied for years.  Lunar color is a subtle, yet fascinating phenomenon.  Just when it seemed that we had an explanation, complications would arise.  We now think we have a reasonable explanation for it.  So, why is the Moon gray?  Or to ask the question “scientifically”— What factors account for the range of spectral reflectance seen on the Moon?

Early Apollo astronauts were very impressed with the Moon’s lack of color.  During Apollo 8 (first mission to orbit the Moon in 1968) Jim Lovell remarked, “The Moon is basically gray – no color.”   The Apollo 10 crew was struck by the numerous brownish hues exhibited by the Moon – from a bright tan to a dark, chocolate brown.  When the first astronauts landed and walked on the Moon (Apollo 11), they had an even closer view.  Buzz Aldrin mentioned that although the surface color was basically gray, he could see interesting colors within some rocks outside the LM window.  During the EVA, Aldrin mentioned to Neil Armstrong that he had seen “some purple rocks.”  Purple? — perhaps so.

The Apollo 15 crew was surprised on their 1971 mission to catch a fleeting glimpse of green on the surface (in film shot earlier by crews on the lunar surface, color was too subtle to be seen). When they raised the sun visors of their helmets to again see that the soil was gray, the disappointment in their voices was palpable.  But then, at the very next station, they again saw a flash of green and this time, it was still green when the visors were raised.  Despite the predictable remarks about “green cheese,” this lunar material – consisting of volcanic glass erupted from deep (> 400 km depth) within the Moon under high pressure – was still green when brought back to Earth.

During their second lunar traverse in 1972, the crew of Apollo 17 found orange soil at Shorty crater.  Also volcanic glass, this soil is made up of tiny (~50 micron) beads of orange glass, again erupted from great depth.  It is orange (as opposed to the Apollo 15 green glass) because of its relatively high titanium content.  It is mixed with black glass beads, of identical composition, but in this case, partly crystallized.  Subsequent study of the Apollo samples have found volcanic glass fragments in almost every color in the spectrum, from red to yellow and brown in addition to the two described above.

True colors of some selected lunar samples. Top left - green glass pyroclastics from the Apollo 15 landing site. Top right - orange and black glass from Apollo 17. Bottom left -- troctolite showing yellow-brown olivine crystals. Bottom right - brownish crystals of orthopyroxene in Apollo 17 norite sample.
At this point, it is tempting to ascribe lunar color seen at a distance to the intimate mixing of a variety of colors present at fine scale.  But this is not quite correct.  Most returned lunar samples are also gray, ranging from a very dark charcoal to a light, almost white-gray shade.  Minor variations can be seen as a result of the presence of certain minerals.  In particular, the mineral olivine (an Mg- and Fe-rich silicate) is abundant in the lunar crust and is often green or a brownish yellow.  Ilmenite (and iron- and titanium oxide) is bluish-black and probably the source of the  “purple” Aldrin saw in some rocks during the Apollo 11 EVA.  Moreover, the astronauts could sometimes see significant color units from space.  After his surface visit, Apollo 17 astronaut Jack Schmitt (in orbit) saw orange material, excavated by small craters on the southwestern rim of the Serenitatis basin.  He suggested that this material might be related to the orange soil collected at the landing site a few days earlier.

Interestingly, one can detect subtle color differences on the Moon with telescopes and from spacecraft.  Although the Moon appears gray at first glance, one notices different hues of gray in certain places.  The dark Mare Tranquillitatis on the eastern near side is a noticeably darker and “bluish-gray” compared to the dark mare plains just to the north in Mare Serenitatis.  Part of the reason the Moon looks whitish-gray in the sky can be attributed to the fact that it is the brightest object in the night sky – dazzling the eye when first looked at (either with your naked eye or through a telescope).  Spacecraft views also reveal color differences.  It is common practice for lunar scientists to work with “false color” composite images, where color variations are “stretched” to extreme degrees to exaggerate differences in order to make them easier to work with.  The typical “false color” version of the near side of the Moon shows brilliantly colored “blue” and “red” maria; these color units do not coincide with mare-highland boundaries.  The received wisdom is that the different color units in the lunar maria represent lava flows of differing composition. That some lavas are enriched in titanium was a major finding from the Apollo sample studies.  Interestingly, these high-titanium lavas come from “blue” regions in the maria.  Initially, this was only an empirical correlation but we now know that it is the presence of ilmenite (the iron-, titanium-rich oxide) in these basalts that makes them “blue.”

It should be noted that color differences on the Moon are extremely subtle, requiring intensive image processing to display them clearly.  Typically, color differences on the Moon are less than about one percent or so.  We are able to see these differences with a careful look, but mapping the detailed boundaries of individual lava flows requires image processing to make the “false color” composites.

Lunar soil from the Apollo 11 landing site. Mostly gray, the fine material shows splashes of colors, including green, red and brown. Image by Randy Korotev, Washington Univ.

The “true” color of the Moon is a brownish (i.e., reddish) gray, but overall, the surface is fairly neutral in tone.  If the Earth had no atmosphere, hydrosphere or biosphere, it too would be largely a brownish-gray, as its crust is made up (more or less) of the same silicate and oxide minerals as the Moon (in slightly different proportions).  It is the weathering effects of air and water and biological activity at the Earth’s surface that makes it so colorful.  The Moon – having none of these processes – displays the “true color” of the rocky planets of the Solar System.  The dominant mineral in the lunar crust is plagioclase, a calcium/aluminum-rich silicate mineral.  Plagioclase is gray.  Thus, the dusty surface of the Moon, derived from plagioclase-rich rocks, is likewise gray.  When we talk about “red” and “blue” in lunar terms (as in “blue mare basalts”), we mean bluer, or less reddish, than comparable mare deposits elsewhere on the Moon.  So in reality, lunar color differences are really just varying degrees of reddish gray, some more so than others.

And what of the blue Moon?  As Conan the Barbarian might say, “But that is another story…..”

Originally published at his Smithsonian Air & Space blog The Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.