GRAIL twins put a new face on the Moon

This graphic depicting the crustal thickness of the moon was generated using gravity data from NASA's GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter [NASA/JPL-Caltech/IPGP].

Scientists using data from the lunar-orbiting twins of NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission are gaining new insight into how the face of the moon received its rugged good looks. A report on the asymmetric distribution of lunar impact basins is published in this week's edition of the journal Science.

"Since time immemorial, humanity has looked up and wondered what made the man in the moon," said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology in Cambridge. "We know the dark splotches are large, lava-filled, impact basins that were created by asteroid impacts about four billion years ago. GRAIL data indicate that both the near side and the far side of the moon were bombarded by similarly large impactors, but they reacted to them much differently."

Understanding lunar impact basins has been hampered by the simple fact that there is a lack of consensus on their size. Most of the largest impact basins on the near side of the moon (the moon's face) have been filled with lava flows, which hide important clues about the shape of the land that could be used for determining their dimensions. The GRAIL mission measured the internal structure of the moon in unprecedented detail for nine months in 2012. With the data, GRAIL scientists have redefined the sizes of massive impact basins on the moon.

Maps of crustal thickness generated by GRAIL revealed more large impact basins on the near-side hemisphere of the moon than on the far side. How could this be if both hemispheres were, as widely believed, on the receiving end of the same number of impacts?

Maps of the thickness of the Moon's crust from GRAIL data reveal the sizes of the Moon's great basins with unprecedented detail. The differences between the Moon's near and far sides run deep [NASA/JPL/MIT].

Scientists have long known that the temperatures of the near-side hemisphere of the moon were higher than those on the far side: the abundances of the heat producing elements uranium and thorium are higher on the near side than the far side, and as a consequence, the vast majority of volcanic eruptions occurred on the moon's near-side hemisphere.

"Impact simulations indicate that impacts into a hot, thin crust representative of the early moon's near-side hemisphere would have produced basins with as much as twice the diameter as similar impacts into cooler crust, which is indicative of early conditions on the moon's far-side hemisphere," notes lead author Katarina Miljkovic of the Institut de Physique du Globe de Paris.

The new GRAIL research is also helping redefine the concept of the late heavy bombardment, a proposed spike in the rate of crater creation by impacts about 4 billion years ago. The late heavy bombardment is based largely on the ages of large near-side impact basins that are either within, or adjacent to the dark, lava-filled basins, or lunar maria, named Oceanus Procellarum and Mare Imbrium. However, the special composition of the material on and below the surface of the near side implies that the temperatures beneath this region were not representative of the moon as a whole at the time of the late heavy bombardment. The difference in the temperature profiles would have caused scientists to overestimate the magnitude of the basin-forming impact bombardment.

Work by GRAIL scientists supports the hypothesis that the size distribution of impact basins on the far-side hemisphere of the moon is a more accurate indicator of the impact history of the inner solar system than those on the near side.

Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow by schoolchildren in Montana, operated in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometers) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface.

NASA/JPL-CalTech news release by D.C. Agle of JPL, Dwayne Brown at NASA HQ and Sarah McDonnell of MIT.

Related Posts:

The Moon's antipodal magnetism mystery (June 19, 2013)

Origin of lunar MASCONS found in GRAIL data (May 30, 2013)

March of time paces changes on the lunar surface (May 21, 2013)

Thin crust Moon (April 24, 2013)

GRAIL - Lunar Reconnaissance Orbiter Science Update (March 24, 2013)
LAMP detects hydrogen and Hg in GRAIL impact plumes (March 19, 2013)
Graves of the GRAIL twins (March 19, 2013)
Parting shots from "Ebb" and MoonKAM prior to impact (January 10, 2013)
Impact on Mons Sally Ride, Ebb & Flow finale (December 17, 2012)
The surface of the Moon: What lies beneath? (December 11, 2012)
GRAIL twins coax out Moon, Solar System's deeper history (December 8, 2012)
JPL releases most detailed map of anomalous lunar gravity (December 5, 2012)
GRAIL twins uncover unexpectedly thin lunar crust (September 20, 2012)
GRAIL extended science mission begins (September 1, 2012)
Ebb & Flow complete primary mission (May 30, 2012)
Zuber shares lunar morphology insights at Harvard (April 20, 2012)
Flying Formation - Around the Moon at 5,800 kph (March 27, 2012)
GRAIL twins begin science mission (March 7, 2012)
The thinking behind the GRAIL twins (September 11, 2011)
GRAIL twins on their way (September 10, 2011)

Thin Crust Moon

Map of the thickness of the Moon's crust from GRAIL mission gravity data. Mean thickness are estimated to be 34-43 km.

Paul D. Spudis
The Once and Future Moon
Smithsonian Air & Space

Imagine a system of molten silicate material, where low-density minerals float and higher density minerals sink.  Minerals rich in iron and magnesium (such as olivine and pyroxene) will settle toward the bottom of the magma body while those rich in the elements aluminum and calcium (such as plagioclase feldspar) will float.  Just such a scenario – on a global basis – is thought to have created the crust of the Moon.

Before Apollo, many believed that the Moon was a primitive, undifferentiated lump of cosmic debris.  By studying the samples returned by Apollo 11, scientists identified small fragments of white, plagioclase-rich rocks (anorthosite).  There are no known magma compositions corresponding to this rock type – anorthosite is created by removing low-density plagioclase from a crystallizing system and concentrating it by floatation.  From the evidence of fragments in the lunar soil, large amounts of anorthosite were inferred to be present in the nearby highlands of the Moon.  As the highlands make up more than 85% of the surface of the Moon, it was postulated that the crust of the Moon formed early in its history by global melting, an episode termed the “magma ocean.”

Expecting only minor volcanic activity and perhaps a local igneous intrusion, the concept of a global ocean of magma was surprising to most scientists.  Given its small size and consequent paucity of radioactive heat-producing elements, the idea that most of the Moon might have melted and differentiated was astounding.  The existence of an early magma ocean, which implied high-energy processes, provided us with clues to lunar origin.  Once it was recognized that the Moon had a crust, it was important to gain an understanding of its composition and physical nature.

On subsequent missions, Apollo astronauts were tasked with laying out a series of seismic stations across the near side.  These stations allowed us to measure “moonquakes” – both natural events as well as those created artificially by slamming spent rocket stages and satellites into the Moon.  Seismic recording allowed us to infer the speed at which seismic waves traveled through the lunar interior.  These estimated speeds indicated densities that implied composition, allowing us to deduce the probable chemical and mineral composition of the lunar interior.

The Apollo seismic network indicated that the crust of the Moon was about 50-60 km thick in the central near side, a surprisingly large value, especially compared to the thickness of the crust of the Earth (which varies from as thin as 5-10 km under the ocean basins to over 30 km in continental areas).  Such a thick crust for the Moon led to the postulation of a global magma ocean, as so much anorthosite could only be produced under the conditions of near global melting.  Subsequent studies incorporating gravity data from Lunar Orbiter and other missions suggested that the lunar crust is variable in thickness, with values exceeding 100 km in some regions of the far side highlands.

Re-analysis of the Apollo seismic data gave the first indication that those values might be overestimated.  Using modern techniques on these old data, new analysis revealed that the crust might be thinner than we had originally thought, on the order of 40-50 km thick.  This lower value of crustal thickness had some implications for estimating the bulk chemical composition of the Moon, but because it was considered to be a relatively minor adjustment, it caused no major difficulties for the rest of lunar science.

However, the recent GRAIL mission to the Moon (using high precision gravity mapping) ascertained the thickness of its crust to be 34-43 km.  Why should this new value worry some scientists?  Because we are now entering realms in which the new estimates of crustal thickness create consistency problems for other aspects of lunar science.  A crust as thin as 35 km on the near side of the Moon implies that the largest impacts – the multi-ring basins – should have excavated considerable amounts of material from the layer below the crust, the mantle of the Moon.  One might object that, as this region of the interior is inaccessible, we don’t know what the mantle would look like.  But in fact, the density constraints imposed by the seismic and gravity data dictate that it must be a rock type rich in iron and magnesium, made up mostly of the minerals olivine and pyroxene.  Such rocks are not unknown in the lunar collections, but they possess chemical and mineralogical characteristics indicating their origins at much shallower (crustal) depths.  In other words, there does not appear to be any material from the lunar mantle in the Apollo collections.  Given our obviously incomplete sampling of the Moon, should this be a problem?

Close view of a section of the central peak of Hausen crater (65.11°S, 271.5°E), among the few craters thought by some to have excavated more than 27 km and candidate for sampling below one of the thinner areas of the lunar crust. If the Moon's crust is as thin as GRAIL data indicate, however, why did basin-forming-impacts like Imbrium not appear to have excavated material from far deeper? LROC Narrow Angle Camera (NAC) mosaic M105100555LR, LRO orbit 643, August 16, 2009; angle of incidence 72.47° and 48 cm per pixel resolution, from 41.38 km [NASA/GSFC/Arizona State University].

Several Apollo landing sites (e.g., Apollo 14 and 15) were specifically chosen to maximize the chances of sampling ejecta from the enormous 1100 km diameter Imbrium basin (one of the biggest impact features on the Moon).  Virtually any reconstruction of the dimensions of the excavation cavity of this basin indicates that it should have dug up material from tens of kilometers depth, much deeper than the new value of crustal thickness implied by the GRAIL data.  So where is this debris from the mantle of the Moon?  True enough, it is possible that it may have been missed during the limited exploration time available to the Apollo crews, but the astronauts were trained to recognize such rocks and none were found.  Additionally, because we can map rock types by remote sensing (both from spacecraft and from Earth), we have an understanding of the regional distribution of rocks around these large impact features.  Despite a 30-year, exhaustively detailed search of the Imbrium impact basin (an area larger than Texas), we have found no convincing evidence for mantle material on the surface of the Moon.

So where does this leave us? In science, new data can solve some problems but at the same time, it may also create new ones.  Modern analyses of the old seismic data and new information on the Moon’s gravity field both suggest a relatively thin crust, with mantle material being very close to the surface (a few km) in some areas.  On the other hand, none of the ubiquitous impact basins and large craters of the Moon show evidence for mantle material in their ejecta, either in the Apollo collections or in remote sensing data.  Could our understanding of impact mechanics be completely wrong?  How could an event that formed an impact crater thousands of kilometers across excavate only a few kilometers deep?  Or are we misunderstanding the new gravity data?

Originally published April 24, 2013 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 but are better informed than average.

GRAIL twins uncover unexpectedly thin lunar crust

GRAIL gravity measurement schematic

Eugenie Samuel Reich
Nature

A sneak peek at the first results from a NASA mission to measure the Moon’s gravitational field hints at a lunar crust that is only half as thick as once thought.

There were a few gasps among scientists in the audience at a 13 September seminar at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, as they took in the data revealed by Maria Zuber, principal investigator for NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission. Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, showed a crisp, high-resolution gravitational map made with data collected by GRAIL’s twin spacecraft between March and June of this year.

“We are three to four times better in resolution compared to Kaguya and Lunar Prospector,” said Zuber, referring to two previous missions that mapped the Moon's gravitational field. GRAIL’s results have not yet been published or released publicly by NASA, and Zuber was not at liberty to give an interview.

Yet her talk, and the thrilled reactions from those present at the seminar and others interviewed by Nature, suggest that GRAIL is poised to have a profound effect on scientists’ understanding of the origins and early evolution of the Moon when its results are released in the coming weeks.

Read the full article HERE.

Related Posts:

GRAIL extended science mission begins (September 1, 2012)

Ebb and Flo complete primary science mission (May 30, 2012)

Zuber shares lunar morphology insights at Harvard (April 12, 2012)

LROC: Bedrock atop central peaks of Theophilus

The sun casts long shadows on the central peak of Theophilus crater. Oblique from LROC Featured Image, October 18, 2011; Narrow Angle Camera (NAC) observation M135019514R, LRO orbit 5031, July 29, 2010; field of view about 280 meters. See the full 700 meter field of view in the LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Theophilus is a large 102 km diameter crater located at 11.4°S, 26.4°E.

Like many other complex craters, it has terraced walls, a flat floor, and a large central peak. The Theophilus central peak even has abundant exposed rocks at its summit! 

The impact process excavates material from depth, and the deepest material forms the central peak. This exposed rock must then be lunar crustal rock weathering out of the central peak!

Full width of the LROC NAC frame (M135019514R) shows the deep lunar crust that was almost instantly upthrust to the 1400 meter height of the four central peaks of Theophilus by a catastrophic Eratosthenian Age impact between around 3.2 to 1.1 billion years ago [NASA/GSFC/Arizona State University].

A 25 km-wide LROC Wide Angle Camera (WAC) monochrome (643 nm) mosaic of observations swept up during three orbital passes showing the dramatic central peaks of Theophilus in a local mid-afternoon illumination, February 1, 2010; average resolution 56 meters per pixel. The area near the summit of one of the four central peak spotlighted by the LROC Narrow Angle Camera is indicated in the reduced mosaic below. The software used to stitch together the panorama mosaic from three sessions, an hour and a half apart, left behind a few artifacts. [NASA/GSFC/Arizona State University].

Future explorers may sample these rocks from talus fields at the base of the peak. Since they are from the deepest material now exposed in the crater, valuable information about the three dimensional nature of the crust would be readily at hand.

Context image showing the location of the area spotlighted as the LROC Featured Image, October 18, 2010, a hefty portion of the Moon's original crust elevated to 1400 meters higher than the floor of Theophilus crater by rebounding forces at the time of the original impact (located in the yellow box. Image field of view is 140 kilometers. LROC WAC monochrome (643 nm) mosaic from three consecutive orbital passes, February 1, 2010. View the LROC WAC context image that originally accompanied their Featured Image HERE  [NASA/GSFC/Arizona State University].

Are there more exposed rocks in the full NAC frame?

Related Posts:
Central Peak of Bullialdus Crater
At the Top of the Avalanche
Tycho Central Peak Spectacular!

Deep lunar nearside crater Theophilus (11.4°S, 26.4°E) as seen in a still frame from NHK television's HDTV camera on-board Japan's groundbreaking first lunar orbiter SELENE-1 (Kaguya) in 2008. See the full width (at lower resolution) HERE [JAXA/NHK/SELENE].

The thinking behind the GRAIL twins

A useful view of our heterogeneous Moon. A practical illustration of the thinking behind the GRAIL project. From several thousand kilometers above the southern hemisphere and just below the equator of the lunar Farside it’s easier to see our Moon is “lumpy;” perhaps like the asteroids, it's own mass isn't high enough to crush it into a unified solid. From the standpoint of gravity the Moon retains the the memory of the smaller solid and semi-solid bodies from both before and after it's original formation. So nothing stays in close orbit around the Moon for very long without getting a frequent boost, and such boosts need fuel and fuel eventually runs out. This false color map of the lunar surface shows, in low resolution, differences from average elevation, or datum. Mare Orientale is on the right, and just beyond, so a crescent of the Nearside’s is visible. The expanse of the Farside here is defined, by the ancient South Pole Aitken basin, with the Moon's thinnest crusts, below center left, and by the Moon’s highest elevations and thickest crusts in the Farside highlands spread above the SPA rim (yellow box shows field of view in the next illustration [NASA/GSFC/MSFC/LOLA/LMMP/LP].

The Lunar Reconnaissance Orbiter (LRO) has orbited the Moon over 10,000 times since June 2009, mostly in a low and circular polar orbit. It requires a monthly boost to keep its record-breaking mission going. A common demonstration of the Moon's mass concentration (MASCON) problem is a thought experiment. A future astronaut stands on the rim of the Nearside impact basin Mare Imbrium holding a weight suspended a meter below a gloved hand sees that it doesn't hang straight down. Instead it hangs angling slightly toward the center of the basin hundreds of kilometers away. Anything in orbit is alternately tugged or gains slack changing its speed, causing it to eventually crash. This inconvenience, when carefully recorded and studied, is also a good way of mapping the Moon's interior in 3D.

The elevation map above shows how radically different the Moon’s Farside is from the familiar Nearside. In a photographs the extent of the 4 billion year-old SPA basin and the higher ground and its rim don’t stand out nearly as well. The map is plotted from millions of laser points measured from LRO's orbit to and from the lunar surface by the LOLA instrument, shown here using the ILIADS program available from NASA Marshall Space Flight Center. The yellow rectangle shows the field of view shown in an August 2011 release of LOLA science from the Goddard Space Flight Center.

 

NASA/GSFC, August 15, 2011 - Twenty-five years have passed since seven brave astronauts lost their lives in the Challenger accident. As the Shuttle program comes to an end, we are reminded of those who lost their lives in the pursuit of human exploration. Shortly after the accident, the Challenger astronauts were memorialized by having lunar craters named after them. These seven craters, located on the far side of the Moon in the Apollo Basin, expose deep portions of the lunar crust.

This LOLA image reveals that the depths of McNair and Jarvis craters, in particular, reach nearly 7 km below the lunar datum (the Moon's equivalent of 'sea level'). The depth of McNair and Jarvis is due to their placement within the large Apollo Basin (an existing topographic low) as well as the Apollo Basins location in the even larger South Pole-Aitken Basin. When combined with data from other LRO instruments such as LROC and Diviner, and instruments aboard other spacecraft such as the Moon Mineralogy Mapper (M3) aboard Chandrayaan-1, the complex nature of the Challenger craters is revealed. Data from the M3 instrument reveal that Jarvis crater's composition may represents a deep portion of the lunar crust.

References: 

Steigerwald, B. (2010) "Biggest, Deepest Crater Exposes Hidden, Ancient Moon," June 2, 2011.

Robinson, M. (2011) "Challenger Astronauts Memorialized on the Moon," January 28, 2011, LROC

Petro, N., et al. (2010) "Lower Crustal Materials Exposed in the Apollo Basin Revealed Using Moon Mineralogy Mapper (M3) Data," 41st Lunar and Planetary Science Conference, #1802, March 2010.

LOLA original map: small | large 

Japan’s lunar orbiter Kaguya (SELENE-1, 2007-2009) vastly added to our knowledge about the “hidden Moon” originally gathered through the Apollo era and afterward, stitched together by 2005. Along with the first HDTV from lunar orbit, Kaguya was a platform for a variety of instruments, including laser altimetry, like LRO. The Kaguya LALT system itself built up an elevation map that is only very recently being surpassed by LOLA during the past two years.

Using their links with Kaguya, with its sub-satellite R-SAT, and in a manner very much like the mission plan for GRAIL-A and B, JAXA investigators delicately measured Doppler shift and subtle light-speed changes between each orbiting spacecraft and with the ground to built-up a detailed map of the Moon’s "gravimetric anomalies."

Together with the unprecedented detail of the Moon’s crustal thicknesses, seen in maps like the one below, Kaguya presented scientists with new and very much more detailed faces of the Moon. Kaguya investigators also helped refine the elusive center of the Moon, from within 20 to 2 kilometers, much more.

The relative thickness of the lunar crust as teased out by Japan's Kaguya orbiter and its sub-satellite R-SAT. The Moon's MASCONS and 'negative gravity anomalies' don't necessarily manifest themselves in surface features, like the one associated with Mare Imbrium.[JAXA].

GRAIL-A and B will join LRO and the recommissioned ARTEMIS twins for a grand total five American unmanned lunar missions, all orbiting the Moon at the same time by the end of the year. The skies above the Moon will become nearly as crowded as those of Mars.

The GRAIL twins will pick up the task of mapping our lumpy Moon’s mass, ARTEMIS the intricacies of the Moon’s plasma wake and its interaction with Solar wind as the Moon orbits through Earth’s magneto-tail with LRO continuing to map the lunar surface from more lasting, slightly higher polar orbit.

All this latter-day renewed interest in the Moon began as preparation for an eventual return, inspired by the loss of Columbia in 2003. That original timeline for renewed, extended human activity on the Moon may seem much further away once again, for the moment, but these unmanned “precursor missions” set into motion through the vagaries of reaction to tragedy or short-term public policy shifts are well along in the pipeline, on time and under budget.

LROC Wide Angle Camera (WAC) monochrome (604nm) mosaic of northeastern Apollo basin, from observations in LRO orbits 2068 and 2069, December 8, 2009; field of view roughly 120 km, resolution 78 meters per pixel, incidence 70° The depth of the interior floor of Jarvis and McNair, the larger and smaller of the two co-joined craters, respectively, and the largest feature seen above, are roughly 7,000 meters below lunar mean elevation. [NASA/GSFC/Arizona State University].