Diversity of basaltic volcanism and the Marius Hills

A diverse range of volcanic morphologies: a sinuous rille, blocky lava flows and pyroclastic cones found in a portion of the controlled LROC NAC mosaic "NAC_ROI_MARIUS_ _LOC_E138N3037" centered on 13.476°N, 303.861°E, field of view approximately 21 kilometers wide [NASA/GFSC/Arizona State University].

J. Stopar
LROC News System

The Marius Hills region (located at 306.2°E 13.0°N) is known for its diverse and complex volcanic features: extensive mare basalt plains, small shield volcanoes, steep-sided cones possibly constructed from cinder or spatter, and sinuous rilles. One of these rilles has a small (approximately 50 m in diameter) pit in its floor, possibly a skylight opening into a lava tube. Today's featured image is a low-sun angle controlled mosaic of part of the Marius Hills region created by combining three LROC NAC left and right image pairs with similar lighting conditions (acquired on adjacent orbits). To create an accurate mosaic, a control network is used to tie the images together and correct for camera pointing and spacecraft position over the lunar surface (refer to the RDR description for more details). Today's featured mosaic illustrates all the major volcanic landforms found in the Marius Hills. 

The full mosaic can be downloaded HERE.

Thumbnail of complete controlled NAC mosaic "NAC_ROI_MARIUS_ _LOC_E138N3037" (down-sampled for web browsing) shows a larger area of the western Marius Hills region and a broad sample of its volcanic morphologies [NASA/GSFC/Arizona State University].

Two large sinuous rilles nicknamed "A" (larger, more southern rille in mosaic above) and "B" (smaller, more northern rille in mosaic above) by Greeley (1971) each have a source at a localized volcanic center within the Marius Hills and are embayed (overlapped) by younger mare basalts to the west. Rille "A" contains the Marius Hills skylight feature that was previously discussed in Featured Images Marius Hills Pit and Sublunarean-void!. This pit or skylight is intriguing because it suggests that many other lunar rilles may also have lava tubes or might be formed through lava tube collapse.

Section of today's featured mosaic that shows the Marius Hills pit crater skylight (center). Try to find this feature in the full mosaic. (Hint: the skylight appears darker and more shadowed than normal impact craters in this mosaic) [NASA/GSFC/Arizona State University].

Oblique view of the Marius Hills pit crater, originally discovered by Japan's Kaguya lunar orbiter in 2007, LROC NAC imagery soon revealed others, most notably in Mare Tranquillitatis and Mare Ingenii. LROC NAC observation M137929856R [NASA/GSFC/Arizona State University].

The volcanic shields, which are sometimes called domes due to their steep and abrupt margins, are constructed from short and blocky lava flows, but are probably not true volcanic domes. Observations from Clementine and LRO indicate that the blocky lava flows are generally basaltic in composition (e.g., Weitz and Head 1998; Lawrence et al. 2013) even though they appear thicker and shorter than the mare-style volcanism that dominates the lunar nearside. The shorter and less voluminous blocky flows likely result from slower eruption rates and more viscous lava.

Two examples of small shields that are primarily constructed from blocky lava flows. Try to find these features in today's featured mosaic [NASA/GSFC/Arizona State University].

The numerous volcanic cones found in the Marius Hills region are atypical for the Moon, though other examples do exist. The steep flank slopes (near 16°) and circular to elongate shapes in plan view (often C-shaped) suggest construction from pyroclastic materials, often called cinder and spatter, like similarly sized and shaped features on Earth. The Marius Hills cones range in size from a few kilometers to 500 meters in diameter. Many of the cones have blocky lava flows that appear to emanate from their central vent, resulting in a C-shaped cone.

Examples of C-shaped and elongate volcanic cones, these cones are often found in association with blocky lava flows. Try to find these (and more) pyroclastic cones in today's featured mosaic [NASA/GSFC/Arizona State University].

LROC NAC high-resolution view of "the love seat" among the Marius domes, from an assembly of 28 LROC NAC cropped images and slideshow, HERE [NASA/GSFC/Arizona State University].

The diversity and complex spatial and stratigraphic relationships of rilles, cones, and shields in the Marius Hills region makes it difficult to determine if eruption conditions changed over time. But, in general, the small shield volcanoes, blocky lava flows, and pyroclastic cones were probably formed nearly synchronously. Later, floods of mare lava partially overprinted the region, leaving only the uppermost peaks for us to observe today.

A gif animation showing the junction of the Reiner Gamma swirl and the southwest Marius Hills region under a variety of illumination angles, highlighting topography and albedo [NASA/GSFC/Arizona State University].

Today's featured image presented only one of the many controlled mosaics collected and produced by the LROC Team! Explore the more than 80 other controlled mosaics of various lunar terrains currently available on the RDR Products website.

Related Posts:

Marius Hills Survey (Slideshow)

Rima Marius Layering (June 2, 2013)

A fascinating effusive dome in Mare Vaporum (February 19, 2013)

Shield volcanoes in Lacus Veris (February 2, 2012)

Magnetic Moon (October 19, 2011)
Dark Surface Materials Surrounding Rima Marius (May 16, 2011)

Discontinuous Rilles (May 15, 2011)

Volcanic Shields on the Moon (March 21, 2011)

Morphometry of lunar volcanic domes (February 22, 2011)

Sublunarean-void! (February 8, 2011)
The largest volcano on the Moon (October 19, 2010)
Marius Hills Constellation Region of Interest (June 2, 2010)

Hearts of Marius, Shadows of Yulu (May 29, 2010)
Craters on the Schrödinger Pyroclastic Cone (April 24, 2010)

LRO/LROC/LOLA: Marius Hills (March 20, 2010)
Marius Hills Pit - Lava Tube Skylight (October 22, 2009)

Rima Marius Layering

Basalt layering, a slice through the floor of Oceanus Procellarum, is visible along the wall of this section of Rima Marius. LROC Narrow Angle Camera (NAC) Extended Science Mission observation M1103881010R, LRO orbit 14991, October 3, 2012; 21.88° angle of incidence over a 1.3 km-wide field of view, resolution 0.99 meters from 121.1 km  [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

Mare basalt layering is visible in the walls of a number of impact craters such as Caroline Herschel Crater and Pytheas Crater. Layers were seen in the wall of Hadley Rille near the Apollo 15 landing site and Today's Featured Image shows a few layers of mare basalt along the top edge of the wall of Rima Marius.

Look closely at the Featured Image to see the individual layers.

Rima Marius is about 280 km long, sinuously slicing through large extents of mare basalt. The are seen in the Featured Image is centered at 14.986°N, 311.565°E.

LROC Wide Angle Camera context view of the southern leg of winding Rima Marius. The arrow marks the location of the field of view shown at high resolution in the LROC Featured Image. LROC WAC M166161047CE (604 nm) spacecraft orbit 9621, July 2, 2011, 63.53° angle of incidence, 58.9 meters resolution from 42.35 km [NASA/GSFC/Arizona State University].

Rilles form when large volumes of low viscosity magma erupt and flow turbulently. The erosive force of the turbulent flow carves a channel into the lunar surface and then drains away, leaving behind an empty groove in the Moon. Studying the thickness of mare basalt layers using areas like the Feature Image help scientists model the viscosity and eruption volume of single eruption events.

The 280 km length of Rima Marius and the LROC Featured Image field of view (arrow) as seen from Earth is more easily seen through telescopes from Earth with the lengthening shadows of local late afternoon illumination, a few days after a Full Moon. In this crop, from a high-resolution lunar mosaic captured by Yuri Goryachko and colleagues at Astronominsk in Belarus, September 25, 2008, shows vast context for Rima Marius within central Oceanus Procellarum, from the Aristarchus Plateau in the North to the Marius Hills, Marius crater and Reiner Gamma swirl albedo to the south [Astronominsk].

Explore the entire LROC NAC for more Rima Marius, HERE.

Related Images:
Dark surface materials surrounding Rima Marius
Discontinuous rilles
Hadley Rille and the Mountains of the Moon
Layers near Apollo 15 landing site

LRO teams deliver 12th quarterly release to PDS

The 8000 meter wide pyroclastic vent high on the second outer ring of Mare Orientale, at very high resolution, has nearly invariably been in shadow, or the LRO spacecraft has been at lower altitude and too close to catch this breathtaking view in one take. As it is, the full observation was repeated in sequential orbits, the makings of a spectacular stereo 3D anaglyph. From a mosaic including both the left and right frames of LROC Narrow Angle Camera (NAC) M1099502843, orbit 14378, August 13, 2012; resolution 0.75 meters from 72.12 km [NASA/GSFC/Arizona State University].

The Lunar Reconnaissance Orbiter Camera (LROC) team at Arizona State University, and investigation teams overseeing the other instruments on-board the robust LRO platform, are once again on time with their 12th quarterly release to the Planetary Data System (PDS). Its another impressive store, with more data gathered over three months than most deep space missions sweep up in an entire tour. All together, the LRO mission has again broken its own record, one unlikely to be surpassed for many years. LRO has returned more data than all present and past deep space missions combined.

 

To a widespread, devoted and grateful group unashamed to call themselves "lunatics," Christmas has arrived early once again this year.

Far to the northeast of the more familiar heart of the Reiner Gamma albedo swirl (and magnetic anomaly) in Oceanus Procellarum, the bright but thin layer of optically immature regolith meanders up into the Marius Hills. The higher altitude assumed by the LRO mission this year allowed the diffuse contact region to be photographed in one observation, under the same lighting conditions. The LROC Wide Angle Camera 100 meter global mosaic is used as context for that area, swept up in the NAC observation below [NASA/GSFC/Arizona State University]. 

Still at high resolution, the physical relationship at the surface between the Reiner Gamma swirl and the Marius Hills volcanism can be studied under similar lighting condition, and in one take, in this LROC NAC mosaic. LROC NAC M1099209032LR, orbit 14337, August 10, 2012; resolution in the original 0.98 meters from 118.85 km, angle of incidence 40.15° [NASA/GSFC/Arizona State University].

"The 12th LROC Planetary Data System release includes images acquired between June 16 to September 15," according to the announcement, posted by LROC team member Ernest Bowman-Cisneros.

This LROC release totals 16.54 TB, and includes ten more Narrow Angle Camera (NAC) Digital Terrain Models (DTM) and 6 NAC image mosaics of important Region of Interest (ROI).

"To date," Bowman-Cisneros writes, "the LROC Team has delivered 893,493 LROC images and over 8,653 derived (RDR) data products to the NASA Planetary Data System. The complete LROC PDS archive can be accessed via the URL http://lroc.sese.asu.edu/data or a search for specific images or mosaics can be made using the LROC WMS browser. Also be sure and try out QuickMap!"

The anatomical mix of dark and relatively bright featured tossed up by simple craters impacting the Marius Hills can also be examined from the unique perspective presented in the LROC NAC oblique images, this one being the first we stumbled across, a brief study of the shield volcano's interior. LROC NAC M1096851065LR, LRO orbit 14007, July 13, 2012 [NASA/GSFC/Arizona State University].

LROC releases 57 narrow angle elevation models

False color model of the highest vents of the many Marius Hills now believed to be a single shield volcano in central Oceanus Procellarum, a region which may have remained active until 1.1 billion years ago. Nearby (bottom, north) is the tadpole head of the unofficially named "Sinuous Rille A." This area was a prime potential landing site in the Apollo era, more recently in the second tier of 50 Constellation Regions of Interest. The low profile of the Marius Hills, surrounded by relatively flat Procellarum mare, has made appreciating their anatomy difficult except after local sunrise and before local sunset when even low features cast long shadows. LROC Narrow Angle Camera photography taken from overhead and from a slewed angle in orbits before and after such opportunities has made it possible to build very high resolution models when the rare opportunity presents itself [NASA/GSFC/Arizona State University.

On January 15, the LROC team at Arizona State released 57 new high resolution digital terrain models (DTM). According to principal investigator Dr. Mark Robinson, "the new DTMs total about 170 Gbytes of data, and cover a variety of high science value targets."

"Start exploring today," Robinson urged. HERE.

The Marius Hills ROI (color strip closer to the horizon) in the context of LROC Wide Angle and Narrow Angle Camera photography showing a handful of the surrounding hills to the southwest and including the grayscale DTM of the Reiner Gamma albedo swirl over a 600 kilometer stretch of the Procellarum mare, a trail that seems to begin in those hills meandering to the famous central "eye" of Reiner Gamma, also a Tier 2 Constellation program Region of Interest and location of the Moon's most familiar crustal magnetic field. Since its discovery early in the history of nearside telescopic study observers have speculated whether the distinctive bright swirl of anomalously low optical maturity was accompanied by a topographic component. The grayscale DTM sliced through the dense "eye" of the formation appears to show nothing like a different crater count or change in elevation that explains the highly visible swirl, seemingly painted over the wide space of Procellarum mare.

The search for the once-elusive highest elevation on the lunar surface came to an end in the 21st century, first with the arrival of Japan's SELENE-1 (Kaguya) and soon confirmed by two digital elevation models under development using data from LRO laser altimetry (LOLA) and offset orbital photography from the LROC NAC and WAC instruments. Somewhat isolated from hills nearly as high along the northern outer rim of the ancient South Pole Aitken basin, north of the Korolev impact basin, this high promontory adjacent to the eastern wall of Engel'hardt crater on the Moon's farside tops out at 10,786 meters above global mean, almost 2 kilometers higher than Mt. Everest [NASA/GSFC/Arizona State University].

Explore the LROC Narrow Angle Camera Digital Terrain Models HERE.

New view of the Sinas pit crater

Latest publicly available close-up of the Tranquillitatis pit crater (8.337°N, 33.219°E), LROC Narrow Angle Camera (NAC) observation M155016845R, LRO orbit 7979, March 17, 2011; whose interior is finally seen under a high Sun (incidence angle 10.58°) at a resolution of 47.4 centimeters per pixel from 39.67 kilometers. LROC QuickMap link [NASA/GSFC/Arizona State University].

Joel Raupe
Lunar Pioneer

Since its arrival in lunar orbit in June 2009 every three months LRO investigators have made available data collected by their respective instrument teams during the 90-day period between six and three months prior.

On September 15, for example, the LROC team, led by Dr. Mark Robinson at Arizona State University (and by far the most consciencious of these admittedly very busy science teams), released to the PDS (Planetary Data System) photography collected by their Wide and Narrow Angle cameras (WAC and NAC) between mid-March through  the middle of June 2011. The LROC team has also continued to make improvements to their already impressive set of web-based tools, created for sorting through these vast stores of data, most recently marked improvements to the ACT-REACT QuickMap interface.

So, every ninety days "lunatic" investigators, even rogue investigators like ourselves, everywhere on Earth jump on these data and begin scrambling through an ever-growing list of favorite targets, anxious for any new views or improved resolutions, even a different degree of illumination. Though it's rare when a new high resolution NAC view of a previously imaged location on our list becomes available it's difficult to complain when remembering more than a third of the Moon's surface has now been mapped at a half-meter per pixel resolution or better, or that the entire Moon has, by now, been photographed at 50 to 60 meters resolution several times over, at a respectable variety of illuminations.

After September 15, the date that a seventh batch of LROC observations were released, nearly two months passed before we finally reached the Moon's now-famous "skylights" on our list, in particular the now-famous pit craters at Marius Hills, Mare Ingenii and Mare Tranquillitatis.

No new LROC NAC views appears to have yet been processed of the "Haruyama pit," in the upper stretches of the Sinuous Rille A in the Marius Hills (14.094°N, 303.224°E). A low angle, very detailed image of the huge pit in the Mare Ingenii basin (35.95°S, 166.06°E), captured in 2010, will be hard to improve upon, though perhaps a view revealing more of that formation's boulder-strewn floor under a high sun would help expand our understanding of its extent.

And we've still not had the time to look for any better views of the natural bridge north of King crater, nor what appear to be small pits on the floor of Messier A.  There does not seem to be anything new to add to the little we know about the other, much smaller and far less dramatic "pit" north of the skylight pictured above (and below).

The LROC Science Operations Center at Arizona State University has taken advantage of at least five separate opportunities to capture the Tranquillitatis pit crater, surrounded by the Sinas crater group . All five close-ups are reproduced here in an animated gif, including one "double exposure," positioning the second observation with the third to obtain a better look at the interior before a fifth observation swept up on St. Patrick's Day. A fourth oblique view offered a breathtaking view of subsurface layers, and thus the long and apparently very eventful early history of Mare Tranquillitatis [NASA/GSFC/Arizona State University].

The Sinas group pit crater captures our imaginations perhaps primarily because, like all great discoveries, it raises so many more questions than it answers. These initial historic surveys will be among those things 100 years from now LRO investigators will be credited for as paying the treasure and devotion paid to the entire LRO mission all by themselves.

LROC Quickmap improvements dazzle

Roof top of the Moon (10,786 meters (35,387 feet) above global mean elevation), as determined by LRO investigators a year ago, is high on the lopsided eastern rim of Engel'gardt crater (5.7°N, 159.0°E), in the farside highlands; 44 km-wide and seen here immediately left of center in a field of view roughly 325 km wide and includes the northern Korolev basin (below). All these features are difficult to spot in cameras, not least of the reasons being in an area criss-crossed with superimposed bright rays. After this past weekend, however LROC premiered an overlay with a variable opacity showing their Global Wide Angle Camera (WAC) digital terrain model (DTM) in false color (here seen at the default 30% over the hybrid LROC NAC and WAC Global mosaic) is now an integral part of the ACT-REACT LROC Quickmap feature on their popular website, improving the map's usefulness when searching through LROC's vast data contribution to the Planetary Data System immeasurably [NASA/GSFC/Arizona State University].

An hour "playing" with the 'new and improved' LROC Quickmap enabled us to assemble this exploration of the tenuous connection between the Marius Hills and the Reiner Gamma albedo feature (with it's attendant crustal magnetic anomaly), both familiar features in Oceanus Procellarum. It's a place to begin digging deeper into the three dimensions of data from LROC already available to the public, especially with the new addition of the LROC DTM. Is Reiner Gamma's long swirl and it's intense local magnetism a result of a sub-surface flash flood of volcanic material? With the new LROC DTM overlay, it's much easier to demonstrate those features with little to no corresponding topographic expression and others nearly invisible except at very high sun angles [NASA/GSFC/Arizona State University].

Not very far from the Moon's highest point is what appears to be it's lowest, within the South Pole-Aitken basin, at the bottom of the large crater on the southern floor of Antoniadi (or, near 70.38°S, 187.2°E, over 9,000 meters below global mean elevation). This mix of LROC WAC imagery overlaid with the false-color WAC DTM adds more than just a feeling of depth of field. Most camera views of the floor of Antoniadi, and mare-filled features everywhere else on the Moon, the surface looks misleadingly flat. Even at 500 meter per pixel resolution, the wide deep flat floor of Antoniadi shows an uneven, almost "dune-like" roughness, lost in surveys based on albedo alone [NASA/GSFC/Arizona State University].

Another instant study increases the opacity of the LROC WAC DTM overlay from the base WAC optical mosaic of a nearside portion of the lunar south pole environs, offering an informative look at one of the Moon's last terra incognitas [NASA/GSFC/Arizona State University].

Related Post: LROC Quickmap

Volcanic Shields of the Moon

Shield Volcanoes of the Solar System: Marius Hills on the Moon (above) and Marion Island, Indian Ocean, Earth (below) -- Twins?

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

Come home with your shield, or on it – Spartan women to their husbands, marching off to war.

From the giant Olympus Mons shield on Mars (600 kilometers across and 27 km high) to the large volcanoes of Venus, shield-building was thought to be a common expression of volcanism on all rocky Solar System bodies; the Moon appeared to be a conspicuous exception. In geology, a shield volcano is a volcanic construct with a broad, low profile made up primarily of thin lava flows with little ash deposits. Earth’s shield volcanoes range in size from a few to more than 200 km for the Big Island of Hawaii, the extent of its base on the sea floor beneath the surface of the Pacific Ocean.

Our understanding of lunar volcanism has been informed and shaped both by images and samples. The large-scale shield volcanoes so prominent on Mars, Venus and Earth were believed to be absent on the Moon. Before the Apollo 11 astronauts visited Mare Tranquillitatis in 1969, we understood that the dark maria of the Moon were volcanic lava plains. Orbital images showed us a landscape of domes, small cones, sinuous lava channels (rilles) and collapse pits – surface features created by volcanic activity. Many of these small volcanic features tend to be clustered in provinces concentrated on the western near side.

Rocks from the maria are basalts, the most common type of igneous rock in the Solar System. They are rich in iron and magnesium and poor in silica. On Earth, when such rocks are molten, the resulting magma has a very low viscosity (i.e., they are very fluid, spreading onto flat surfaces in thin sheets). We understand lunar lavas to be similarly fluid, having erupted in thin sheet-like flows onto the airless surface of the Moon. The maria formed as this geologic process of massive high-volume eruptions built up stacks from the thin, fluid flows which extend for hundreds of kilometers. Scattered within the ancient maria are numerous small volcanic constructs, previously believed to be the only manifestation of central-vent volcanism on the Moon.

When the Moon’s topography was mapped with laser altimetry (first by Clementine in 1994, then at greater resolution by the Japanese Kaguya spacecraft and NASA’s Lunar Reconnaissance Orbiter mission), it showed clusters of many small volcanoes occurring on topographic highs that are quasi-circular, with low relief and shield-shaped. Pat McGovern, Walter Kiefer (colleagues at the Lunar and Planetary Institute) and I were intrigued by this correspondence. We studied these areas by mapping volcanic features, integrating the new topographic data, and examining their gravity signatures (the amount the local gravitational attraction is enhanced or depleted from normal).

We found that these large shield-shaped topographic swells are made of basaltic lava and display concentrations of volcanic features. Such a structure found on Venus or Mars would be classified as a shield volcano; therefore, we interpret these features on the Moon as shield volcanoes. We have found seven of these large structures on the Moon, ranging in size from 66 to almost 400 kilometers in diameter and from 600 to over 3200 meters in height. Such sizes and shapes are very similar to large shields on Earth, Venus and Mars. The average slopes on these volcanoes are very low, typically less than a few degrees, as would be expected for structures made from very fluid lava. These lunar shields display abundant volcanic features, including domes and cones, sinuous rilles (lava channels and tubes) and collapse features – all common morphologies in terrestrial shield volcanoes.

Topographic map of the Marius Hills shield on the Moon from LOLA laser altimetry. A broad topographic swell with many small cones and domes on it [NASA/GSFC].

Although we believe these features are shield volcanoes, this new interpretation is not without some difficulties. Unlike most shield volcanoes on the other planets, none of the lunar shields has a central collapse pit (caldera). However, many shields – especially those on Venus – likewise do not show central calderas. Additionally, while evidence for some lunar shields such as the Marius Hills is pretty convincing (e.g., shield shape, high gravity signature indicating dense stacks of lava), the evidence for others is not as clear. The largest feature we identified, the Cauchy shield, possesses the correct topographic shape and has numerous small cones, rilles, and vents on it, but remote sensing data suggest that the lava thickness in eastern Mare Tranquillitatis is relatively thin, which might mean that Cauchy is not a thick stack of lava as Marius appears to be. We still think that Cauchy is a shield volcano, but acknowledge that our interpretation is tentative and we will continue studying these enigmatic features to better understand their history.

But the real story here is not whether these features are true shield volcanoes or not, but rather, how the advent of new, high-precision data (high resolution topography) can cause scientists to reexamine areas and processes long thought understood and perhaps come to surprisingly different interpretations. We are currently in the midst of a revolution in lunar science. The 42^nd Lunar and Planetary Science Conference held this month in Houston highlighted new scientific findings about the history and processes of the Moon. New, high-quality data coming from an international flotilla of lunar orbital mappers – Chandrayaan, Kaguya, Chang’E and LRO – has scientists seriously reconsidering our current understanding of the processes, history, resources and potential of the Moon.

Related Reading:
LROC: Morphometry of lunar volcanic domes
February 22, 2011

The largest volcano on the Moon
October 19, 2010

LROC: Marius Hills ROI
June 2, 1010

Hearts of Marius, Shadows of Yutu
May 29, 1010

Local Topography and Reiner Gamma
May 22, 2010

LRO/LROC/LOLA: Marius Hills
March 20, 2010

LROC: Haruyama Cavern in the Marius Hills
March 2, 2010

Sublunarean Void

The LROC Narrow Angle Camera acquired an oblique view of the Marius Hills "Haruyama Skylight" pit at just the right angle to reveal an overhang. The pit is about 65 meters in diameter (LROC NAC observation M137929856R, LRO orbit 5460, August 31, 2010) [NASA/GSFC/Arizona State University].

Marc Robinson
Principal Investigator
Lunar Reconnaissance Orbiter Camera
Arizona State University

Since LRO completes a full cycle of lunar imaging each month, it is possible to follow up previous discoveries and re-image targets under different lighting conditions. The LROC team waited patiently until the Sun and orbit position in the Marius Hills region was such that the bottom of the previously imaged pit wall was illuminated at just the right angle so that if there was an open lava tube extending horizontally its floor would be illuminated. The spacecraft slewed 43° to the east and the solar incidence angle was 34° from vertical.

Schematic of the imaging geometry in cross section, allowing a view of the lava tube floor [Arizona State University].

In this geometry, the NAC was able to image a few meters under the overhang discovering a sublunarean void! Will astronauts someday explore under the mare? What scientific riches wait to be discovered within the unseen reaches of sublunarean voids?

LPSC XLII (2011) #2771, Figure 2b. "The Mare Tranquilitatis pit, imaged at LRO nadir (0.00°) (2a) and -51° (2b) slew angles; images; M126710873R and M144395745L, respectively. Note layering complexity, differentially modified pit wall profile, and funnel-shaped rim in 2b (red scale bars are ~ 100 meters along each length" [NASA/GSFC/Arizona State University].

Also note how the oblique angle really brings out the layered nature of the mare bedrock in the pit walls. These exposed layers give scientists important clues as to how the vast mare were deposited.

Explore the entire oblique image! Read the 2011 Lunar and Planetary Science Conference abstract describing details of this fascinating discovery.

Also check out previous Featured Images of the Mare Tranquillitatis and Mare Ingenii pits.

The largest volcano on the Moon

From LRO WAC Album 1 -

The Heart of the Marius Domes (27.2 km field of view), from LROC WAC M116683214ME (December 29, 2009), much as they would appear as the morning terminator begins its sweep across the central Oceanus Procellarum, today, October 19, 2010 [NASA/GSFC/Arizona State University].

Sunrise at Marius Hills is a significant time for Moon watchers, even for those equipped with modest telescopes. The myriad domes there, only a bit higher in profile than the rolling elevation of the surrounding, vast basaltic plains of Oceanus Procellarum are briefly very starkly highlighted by long shadows. Very soon after, the terminator continues it endless westward, march and the domes (near 12.0°N, 306°E) quickly become difficult to see, overwhelmed by brighter albedo contrasts as a chief marker of topography under a high sun.

But the Sun was less than two degrees over the east when LRO swept up the dark scene above, much as it should be for the next few hours today, Tuesday, October 19, 2010.

In the long shadows above is the "Heart" of the Marius Domes, where a sinuous rille winds between the two largest domes of an enormous volcano China's Chang'E-1 investigators have labeled "Yutu." LRO Laser altimetry (LOLA) shows the many hundreds of domes west of Marius crater are clustered atop a larger bulge near the center of the Procellarum expanse.

No one has yet definitively identified Procellarum as a typical, fully formed round basin, though there are many theories, including the Gargantuan impact theory, where Procellarum originates as part of an impact, centered in northeastern Mare Tranquillitatis, whose outer circumference is larger than the entire Near side as the original source of the Moon's Near and Far side discontinuity.

Whatever the source of Procellarum, we know it's huge expanse was covered by molten flows in many places at many times. Thankfully, the occasional asteroid, comet or large meteor has pre-excavated the Procellarum floor, uncovering the history for future definitive dating.

Perspective view of the Constellation Region of Interest landing zone at the Sinuous Rille A "cobra head" formation in the Marius Hills, derived from LROC Narrow Angle Camera Digital Terrain Models [NASA/GSFC/Arizona State University].

LRO and the others building the huge new legacy of an international fleet of 21st century orbiters, are allowing investigators a closer look at the Marius Hills, and in a wider context - tied into neighbors like the Reiner Gamma swirl, the surrounding plains typified by minerals found nowhere else on the Moon, and perhaps even the Aristarchus Plateau and Mons Rümker. A casual glance at Oceanus Procellarum appears to show a huge semi-circle lunar "sea" devoid of anything as dynamic as "storms," but there is much more to be seen here for the patient observer.

The full LRO Wide Angle Camera (WAC) monochrome (689nm) observation M116696805ME was swept up from an altitude of 45.4 kilometers over the course of 2 minutes, 51 seconds during LRO orbit 2331, December 29, 2009.

Additional Reading:
Reiner Gamma in color
October 15, 2010

NASA@Science: "Down the rabbit-hole"
July 13, 2010

LROC: Marius Hills ROI
June 2, 1010

Hearts of Marius, Shadows of Yutu
May 29, 1010

Local Topography and Reiner Gamma
May 22, 2010

Lunar Swirl phenomena from LRO
May 17, 2010

LRO/LROC/LOLA: Marius Hills
March 20, 2010

LROC: Haruyama Cavern in the Marius Hills
March 2, 2010

From Lunar & Planetary Science Conference Album -

Marius Hills is the largest volcanic dome field on the Moon. The region is an area of high interest because it contains approximately half of the Moon's known volcanic domes. These domes range from 200-500 meters in height. In comparison, the Hawaiian volcano Mauna Loa, which is the largest shield volcano on Earth, is 17,170 meters high. This LOLA image covers the area of the Moon from 9.5 - 17N°, and 303.5 - 311°E [NASA/GSFC].

Fresh views inside the mare crater pits

Updated September 16, 2010 1958 UT

Spectacular high Sun view of the Mare Tranquillitatis pit crater reveals boulders on an otherwise smooth floor. Image is 200 meters wide, north is up, NAC M126710873R (400 meter-wide original LROC Featured Image, HERE.) [NASA/GSFC/Arizona State University].

Mark Robinson
Principal Investigator
Lunar Reconnaissance Orbiter Camera
Arizona State University

When the Sun is well overhead, the floor of the Mare Tranquillitatis pit is illuminated. With an incidence angle of 26.5° and a shadow of 55 meters, scientists can estimate the depth to be a bit over 100 meters. That estimate is from the edge of the shadow, which begins a slightly downslope from the gradual margin of the pits. When measured from the level of the surrounding mare plain, the depth of the pit is even greater. Compare this depth to the width, which ranges from 105 to 115 meters across the sharp precipice.

Two views of Mare Ingenii pit [NASA/GSFC/Arizona State University].

A pair of Mare Ingenii pit images (each panel is 150 meters wide) reveals different portions of the floor as the Sun crosses from West to East (Left M123485893RE, Right M128202846LE). Shadow measurements indicate that the Ingenii pit is about 70 meters deep and its width is about 120 meters. The Sun angle, direction, and elevation perfectly illuminate the layered nature of the mare basalts. Each shelf corresponds to a local lava flow event. By climbing down this "staircase" a geologist astronaut can sample increasingly further back in time.

Variations in lighting reveal the structure of the fascinating lunar pit craters. The center panel, with the Sun high above, gives scientists a great view of the Marius Hills ("Haruyama") pit interior. Each panel is a 300 meter-wide section of LROC NAC observation; top M133207316LE, center M122584310LE, bottom M114328462RE See original three-panel view, HERE [NASA/GSFC/Arizona State University].

LROC has now imaged the Marius Hills pit three times, each time with very different lighting. The center view has an incidence angle of 25° that illuminates about three-quarters of the floor. The Marius pit is about 34 meters deep and 65 by 90 meters wide. Read more about the Ingenii, Tranquillitatis, and Marius pits.

Do any of these pits provide access to still-open/uncollapsed lava tubes? What could be learned by visiting and exploring one or all of these fascinating features? Imagine entering a preserved lava tube, unchanged for more than 3 billion years; such an opportunity is a geologist's paradise - a chance to travel back in time to see what brand-new lava flows look like! What types of rare minerals might exist on these hidden surfaces (if they exist)? Do you think we should send a robot into one of the pits? How about astronauts; is it worth sending humans in to explore? How would you like to explore this amazing feature?

Search the nearby area for clues in the full LROC NAC frame that may help determine if an extended lava tube system still exists beneath the surface.

How common are mare pit craters?

One of three large pit craters so far found on the Moon -- do these pits provide access to open lava tubes? From LROC NAC M106662246R, LRO orbit 861, Sept. 4, 2009; Alt. 133.78 km, resolution = 1.35, phase angle = 29.34° [NASA/GSFC/Arizona State University].

Mark Robinson
Principal Investigator
Lunar Reconnaissance Orbiter Camera (LROC)

The Kaguya team discovered three mare pit craters, all about 100 meters in diameter. Since the Kaguya Terrain Camera had a pixel scale of about 10 meters they could not definitively identify pit craters much smaller than 100 meters. This raises a key question - are there smaller pit craters to be found? The answer is most likely yes.

The LROC team is searching images that are already on the ground and is targeting areas around pits for more coverage. To date, we have about ten candidate pits awaiting confirmation.

Possible small (25 meter diameter) pit crater in Mare Tranquillitatis observed under high Sun (3° incidence angle). This feature is about 200 km NW of the large pit shown in the opening image. From LROC NAC M124382509L [NASA/GSFC/Arizona State University].

How and when did pit craters form? On the Earth volcanic pit craters are formed as the roof of a lava tube collapses, often while magma is still flowing underground. The resulting opening is often termed a skylight. Can we determine if the lunar pits formed during or after the mare lavas flowed? Perhaps the best place to start looking for evidence is on the pit crater floor. If the crater formed long after eruptions had ceased and the subsurface lava tubes were cold, you might find a chaotic pile of rubble on the floor. If the pit collapsed into an active lava tube you might find the smooth, frozen surface of the last lava to flow through the tube.

View down a skylight revealing magma racing through a lava tube in Hawaii. When lava stops flowing in tubes it cools and forms a smooth surface [photo by Mark Robinson].

The floor of the larger pit does look smooth, but the image of the smaller pit does not have high enough resolution to give a clear picture of the interior. One might infer that the larger pit was formed as an underlying tube was active, though at this stage such a conclusion would amount to healthy speculation. The LROC team is planning to acquire stereo images of the large pits and smaller ones that are now being discovered. The detailed topographic data will allow scientists to confirm the origin of these fascinating pits. Additionally, the LROC team will acquire images at a slant attempting to look for overhangs that might indicate the lava tubes are still open and accessible. Keep checking in as this exciting story unfolds.

Another possible pit crater in Mare Tranquillitatis. LROC NAC observation M109100697RE; LRO orbit 1212, October 2, 1009, Alt. 49.11 km, phase angle 2.54° [NASA/GSFC/Arizona State University].

Examine previous images of lunar pits at Marius Hills and Mare Ingenii.

Explore on your own.

Low-resolution context for two of the three Lunar Reconnaissance Orbiter (Narrow Angle) Camera observations of verified and unverified mare crater pits within Mare Tranquillitatis. (One of the three highlighted by Dr. Robinson up above is not yet released to the Planetary Data System.) Format note: A near perfect "National Geographic-like" font stylizes the upper loop in the '8' of Ranger 8, and it has unfortunately made it appear Ranger 6 impacted the Moon at two widely different locations. Ranger 6 impacted the Moon at or near the northern designation February 2, 1964, though it's television camera had already failed after inadvertently switching on during a booster separation. Ranger 8 was successful, however, in returning televised images of the lunar surface up until the moment of its intentional impact, February 20, 1965; not far from the eventual landing sites of Surveyor 5 and Apollo 11 [Clementine 750nm- NASA/DOD/USGS].

NASA@Science: "Down the lunar rabbit-hole"

The 'skylight' in the middle of Mare Ingenii, on the Moon's Far Side, show a tantalizing view of house-sized boulders on part of it's barely illuminated floor in this LRO Narrow Angle Camera (NAC) observation, LROC News System Featured Image released June 16, 2010. Two kilometers wide, the Ingenni pit is twice the size of the skylight previously unveiled by JAXA SELENE-1 (Kaguya) investigators in 2009 - [M128020284LE, LRO orbit 4026, May 11, 2010 - NASA/GSFC/Arizona State University].

Dauna Coulter
Science@NASA

A whole new world came to life for Alice when she followed the White Rabbit down the hole. There was a grinning cat, a Hookah-smoking caterpillar, a Mad Hatter, and much more. It makes you wonder... what's waiting down the rabbit-hole on the Moon?

NASA's Lunar Reconnaissance Orbiter (LRO) is beaming back images of caverns hundreds of feet deep -- beckoning scientists to follow.

"They could be entrances to a geologic wonderland," says Mark Robinson of Arizona State University, principal investigator for the LRO camera. "We believe the giant holes are skylights that formed when the ceilings of underground lava tubes collapsed."

Japan's Kaguya spacecraft first photographed the enormous caverns last year. Now the powerful Lunar Reconnaissance Orbiter Camera (LROC, the same camera that photographed Apollo landers and astronauts' tracks in the moondust) is giving us enticing high-resolution images of the caverns' entrances and their surroundings.

Situated a bend at the northwestern reaches of the unofficially named sinuous rille 'A,' near the heart of the widespread Marius Hills volcanic region in Oceanus Procellarum, the "Haruyama" skylight (unofficially named, for the lead investigator of the JAXA SELENE-1 (Kaguya) science team who first identified it) is wide enough to swallow The White House in Washington, DC (LROC Narrow Angle Camera observation M114328462RE; LRO orbit 1982, December 1, 2009; Alt. 45.38 km, Resolution = 50cm per pixel) [NASA/GSFC/Arizona State University].

Back in the 1960s, before humans set foot on the Moon, researchers proposed the existence of a network of tunnels, relics of molten lava rivers, beneath the lunar surface. They based their theory on early orbital photographs that revealed hundreds of long, narrow channels called rilles winding across the vast lunar plains, or maria. Scientists believed these rilles to be surface evidence of below-ground tunnels through which lava flowed billions of years ago.

"It's exciting that we've now confirmed this idea," says Robinson. "The Kaguya and LROC photos prove that these caverns are skylights to lava tubes, so we know such tunnels can exist intact at least in small segments after several billion years."

Context for the LROC NAC image of the Marius Hills 'skylight,' is LROC Wide Angle Camera (WAC) image M117867923ME, showing a 26 km-wide area in the heart of the Marius Hills. The white arrow designates the location of the pit, at the bend in "Sinuous Rill A." The location is almost directly situated between the two main volcanic vents of what China's Chang'E-1 orbiter team have labeled "Yutu," theorized to be an extinct single volcanic under the entire Marius dome region, a claim perhaps backed up by LRO (LOLA) laser altimetry data [NASA/GSFC/Arizona State University].

Lava tubes are formed when the upper layer of lava flowing from a volcano starts to cool while the lava underneath continues to flow in tubular channels. The hardened lava above insulates the molten lava below, allowing it to retain its liquid warmth and continue flowing. Lava tubes are found on Earth and can vary from a simple tube to a complex labyrinth that extends for miles.

If the tunnels leading off the skylights have stood the test of time and are still open, they could someday provide human visitors protection from incoming meteoroids and other perils.

"The tunnels offer a perfect radiation shield and a very benign thermal environment," says Robinson. "Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C."

Lunar Orbiter IV (1967) photographed the Marius Hills region from 2668 km (1967,) shown in this "no dash" noise-reduction of the original image by the USGS Lunar Orbiter Digitization Project (not to be confused with the Lunar Orbiter Image Restoration Project, or LOIRP, who re-creating Lunar Orbiter images directly from restored telemetry). The sinuous rilles shown above are barely visible in the upper left center. Much detail from the original image is lost in the reduction to merely 400 pixels. Nevertheless, the larger context for the region-at-large is seen here, and the neighbors, including many of the Marius domes (visible with the Sun illuminating from the right, 27° over the eastern horizon. At center left is Rima Galilaei and the bright surface feature in the south is the frilly northeastern extremes of the Reiner Gamma albedo swirl phenomena, suggesting it also may be related to an extinct regional "Yutu" volcano; a very large part of Oceanus Procellarum. (Image width is about 160km) [NASA/JPL/USGS].

That may sound cold, but it would be welcome news to explorers seeking to escape the temperature extremes of the lunar surface. At the Moon's equator, mid-day temperatures soar to 100 deg C and plunge to a frigid -150 deg C at night.

Paul Spudis of the Lunar and Planetary Institute agrees that lunar lava tubes and chambers hold potential advantages to future explorers but says, "Hold off on booking your next vacation at the Lunar Carlsbad Hilton. Many tunnels may have filled up with their own solidified lava."

However, like Alice's Queen of Hearts, who "believed as many as six impossible things before breakfast," Spudis is keeping an open mind.

"We just can't tell, with our remote instruments, what the skylights lead to. To find out for sure, we'd need to go to the Moon and do some spelunking. I've had my share of surprises in caving. Several years ago I was helping map a lava flow in Hawaii. We had a nice set of vents, sort of like these skylights. It turned out that there was a whole new cave system that was not evident from aerial photos."

As for something similar under the lunar skylights?

"Who knows?" says Spudis. "The Moon continually surprises me."

This could be a white rabbit worth following.

Further Reading:

Depths of Mare Ingenii, June 16, 2010

Hearts of Marius, Shadows of Yutu, May 29, 2010

Local Topography and Reiner Gamma, May 22, 2010

LRO/LROC/LOLA: Marius Hills, March 20, 2010