Saturday, October 29, 2011
Dr. Ronald Greeley
(1939 - 2011)
(1939 - 2011)
Paul D. Spudis
The Once & Future Moon
Smithsonian Air & Space
Yet another lunar and planetary scientist has departed this world. My former teacher, dissertation advisor and mentor Professor Ronald Greeley passed away this week at the age of 72. The news of his death came as a true bolt from the blue – Ron was in apparent good shape, good humor and active in his scientific research. And so sadly, I say an untimely goodbye to another friend and colleague.
Ron became involved in planetary geology while fulfilling his military service requirement at NASA Ames Research Center. Ames needed a geologist, and though trained as a paleontologist, Ron was assigned the task of examining images of the Moon to study volcanic landforms. He quickly became interested in lava tubes (large horizontal conduits that transport lava from the eruptive vent outward as flows). After an eruption, lava tubes sometimes drain out, leaving behind an empty cave. Lava tubes can be many kilometers in length and tens of meters in cross section.
Sinuous channels wind their way across the relatively flat smooth surface of the lunar maria. Some workers noted the similarity of these features to terrestrial lava tubes and postulated that sinuous rilles were remnants of lava tubes and channels on the Moon. Ron examined this idea in detail by mapping and studying lava tubes on terrestrial volcanoes and by analyzing the images returned by orbiting lunar spacecraft. He wrote a key paper on Hadley Rille (a large sinuous rille at the base of the Apennine Mountains) the outer ring of Imbrium basin and the largest impact feature on the lunar near side. This area had been chosen as the landing site for the future Apollo 15 mission and understanding the origin of sinuous rilles was one of the mission objectives. Ron detailed the evidence that Hadley Rille is a collapsed lava tube. He noted the rille originated in an elongate, volcanic depression, had slightly raised edges and trended generally down slope to the north. Parts of the rille were still roofed, raising the possibility that caves could exist on the Moon. Years later, I had the honor to be a co-author with Ron and Gordon Swann (Principal Investigator of the Apollo 15 Field Geology Experiment) on another paper about Hadley Rille, modifying and extending the model Ron had developed in 1971.
While taking an undergraduate course at ASU in meteoritics, I wrote a term paper on the geology of Hadley Rille. I was just getting into lunar science and as a big fan of the Apollo 15 mission, I had read Greeley’s paper with interest. In a strange coincidence, Ron came to ASU that semester to give a talk on planetary geology and I arranged to meet with him after his seminar. We ended up talking for a couple of hours and he offered me a job for the summer at NASA-Ames.
For a starry-eyed space cadet, this offer was almost too good to be true. I worked the summer of 1976 on a Mars mapping project for an advanced mission study. That was the summer of the Viking landings on Mars, and I spent part of my time in Pasadena as a JPL intern. Ron was a team member of the Orbiter imaging team and arranged for me to work with him and John Guest on certifying the landing site for Viking 2. It was a memorable and exciting introduction to planetary exploration and I will always be in Ron’s debt for having given me that opportunity.
After studying for my Master’s degree at Brown, I returned to work with Ron at Ames. When he moved to ASU, I applied there to get my Ph.D. Ron agreed to take me on and I became one of his first doctoral students. Ron was a great mentor and a role model for a modern working scientist. Even as his academic group grew to where he needed to assign work and follow up later with discussion, I was always welcomed into his office to discuss science or other concerns. Besides showing his students how to do science, Ron also taught us how to survive scientifically. Science is a social activity. Navigating the treacherous political shoals of science is a learned and acquired skill and Ron generously passed those valuable lessons on to his students.
One of Ron’s best qualities as an academic mentor was assuming the role of what most graduate students desperately need, yet few ever get – a merciless and persistent editor. I never learned how to write until I worked for Ron. Hopefully I would turn in drafts of papers only to have them handed back to me in (almost literally) shreds. (This was before the days of word processing – we typed our papers and then literally cut-and-pasted the text into some kind of readable form.) Being told that your prose “stinks” is an infuriating rite of passage if you hope to become an acceptable writer. Working with Ron all those years convinced me of an uncomfortable truth – there is only one way to learn how to write and that is to write often and be edited heavily. Many do the first part, but few are fortunate enough to have a good editor for the second. Of course, I didn’t see it that way at the time; getting a copy of your work covered in red ink is annoying as hell. But an edit from Ron always improved the text, regardless of what it did to my blood pressure. Again, I am in his debt.
Ron never let scientific grass grow under his feet. He developed an interest in the geological effects of wind and was the first to determine the wind speeds needed to start sandstorms on Mars. He made geological maps of every planet and was involved as an investigator on most of the robotic planetary missions of the last 30 years. He served the scientific community through numerous committee memberships and chairmanships. If Ron was asked to study an issue and write a report on it, you could be sure that his report would encompass the best thinking on a subject – lucidly and concisely presented. He was a superb speaker and presenter of scientific results, always fluent, interesting and engaging. Beyond science, Ron’s students learned how to write and speak, two critical skills for a working scientist.
In addition to being a good scientist, Ron was a fine man. He cared deeply for his family and spent as much time with them as he could, taking his lovely and gracious wife Cindy and children Randy and Vanessa with him on many of his national and international travels. He was a role model for his students both personally and professionally. If one wants to be remembered as living a productive and valuable life, emulate Ron Greeley.
Thursday, October 27, 2011
|The rim of a fracture inside Sarton Y. Left of the bright rim is outside the fracture and right of the rim is inside the fracture. Image field of view is 825 meters. See the full size LROC Featured Image HERE, a frame from LROC Narrow Angle Camera (NAC) observation M140697409L, LRO Orbit 5868, October 2, 2010. [NASA/GSFC/Arizona State University].|
LROC News System
In the farside highlands at 51.3°N, 238.6°E, Sarton Y and Z stand out among the other craters. Take a look at this area in the LROC WMS Image Map.
Other craters in the region have filled floors, however, Sarton Y and Z are the only two craters to also have floor fractures.
Why is this? What makes Sarton Y and Z special?
|Full 3.3 kilometer width frame from LROC NAC M140697409L, October 2, 2010 [NASA/GSFC/Arizona State University].|
Perhaps the subsequent impact of Sarton Y allowed the material in both craters to undergo a thermal evolution different from their surrounding counterparts, possibly causing the fractures as the floors cooled. Was there a difference in the surface material when Sarton Y and Z were formed? Both craters show evidence of slumping, but so do the other craters in the region. Age or the composition of the impactor may also play a role.
the NAC frame!
Wednesday, October 26, 2011
|Narrow fractures extend across a complex intersection of lunar surface types northeast of Mare Serenitatis, in Lacus Somniorum. LROC Narrow Angle Camera (NAC) observation M168007062L, from the period last August when the LRO spacecraft's orbit was altered to include very low passes over selected areas (resolution 25 centimeters per pixel); orbit 9893, August 15, 2011. Field of view a bit more than 550 meters wide [NASA/GSFC/Arizona State University]|
LROC News System
In this 0.25 m/pixel NAC frame we found narrow fractures that extend across the lunar surface, in the mare basalt of Lacus Somniorum. Narrow fractures like these are probably extensional features caused by tension stress that pulls the rocks apart in opposite directions. Similar fractures have been found before on top of wrinkle ridges.
The fractures in today's Featured Image are also associated with a small wrinkle ridge. In the LROC Wide Angel Camera (WAC) 60 meter per pixel context image below, you can see the ridge (yellow arrow) between two hills of non-mare material. The fractures are located west of the ridge (yellow box). There are a number of other wrinkle ridges in the area as well.
|LROC Wide Angel Camera (WAC) Observation M150321508C (604 nm), LRO Orbit 7288, January 22, 2011, local late afternoon illumination at incidence angle 67.201° The yellow box at center bounds the fractures seen in the LROC Featured Image, released October 26, 2011. The yellow area designates a ridge possibly related to those fractures. [NASA/GSFC/Arizona State University].|
Examine the rest of the narrow fractures in the NAC frame!
Stress and pull
Fractures in the mare of Tsiolkovskiy crater
|A boulder perched on the north rim of a Copernican Age crater situated directly atop the Dorsa Smirnov wrinkle ridge in Mare Serenitatis. LROC Narrow Angle Camera (NAC) observation M168041107L, LRO Orbit 9898, August 15, 2011; resolution 0.25 meters per pixel, width of the field of view is only 145 meters [NASA/GSFC/Arizona State University].|
LROC News System
Posidonius Y is a 2 km diameter Copernican Age crater located at 30.04°N, 29.4°E, in the upper northeast section of Mare Serenitatis. The crater is remarkably bouldery, which is a indication of a relatively young age. The boulder seen on the rim in today's Feature Image is about 25 meters long from stem to stern (about half the length of an Olympic sized swimming pool).
Since Posidonius Y is superposed on the wrinkle ridge system known as Dorsa Smirnov, the crater is also relatively younger than these tectonic features. The WAC context image below shows the beautiful combination of mare basalt volcanism, tectonics, and impact cratering that shape the lunar surface.
Explore the entire NAC frame!
Layers in Lucian Crater
Recent Impact in Oceanus Procellarum
Monday, October 24, 2011
The Lunar and Planetary Institute (LPI) is hosting a special summer intern program to evaluate possible landing sites for robotic and human exploration missions. The LPI invites applications from graduate students in geology, planetary science, and related programs. The program is also open to undergraduate students in geology, astronomy, chemistry, and physics with at least 50 semester hours of credit so that they, too, can participate in lunar exploration activities. The goal of this program is to integrate NASA's lunar science priorities with the exploration components of the new exploration program that takes us beyond low-Earth orbit. This will be a unique team activity that should foster extensive discussions among students and senior science team members.
The 10-week program runs from May 29, 2012, through August 3, 2012. Selected interns will receive a $5000.00 stipend to cover the costs associated with being in Houston for the duration of the program. Additionally, U.S. citizens will receive up to $1000.00 in travel expense reimbursement, and foreign nationals will receive up to $1500.00 in travel expense reimbursement.
Please pass this information on to any students who might be interested.
APPLICATION DEADLINE: January 20, 2012
Applications are only accepted using the electronic application form found at the LPI’s Lunar Exploration Summer Intern website: www.lpi.usra.edu/lunar_intern/
For more information:
Lunar Exploration Summer Intern Program
Friday, October 21, 2011
|Ejecta with a subtle chevron texture drapes the Anaximander crater group region in the highlands of the nearside north. The ejecta is pockmarked by few small craters, suggesting it is from a recent impact. LROC Narrow Angle Camera (NAC) M170937919R, LRO orbit 10325, September 17, 2011, image field of view is 500 meters. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
LROC News System
What created the chevrons in today's featured image? Chevron textures like these are secondary results of impacts. As ejecta is thrown out of its parent crater, the ejecta crashes back into the surface creating secondary craters and small linear streams of locally derived immature material.
Even better, the chevrons created during this process point back towards their parent crater! These features are not scale dependent, as we observe chevrons on both small and large craters.
So if these chevrons were formed from a recent impact, where is the parent crater?
The (original) context image (accompanying the LROC Featured Image) doesn't reveal any obvious choices, but they could be outside of this image. Try looking for potential candidates using the LROC QuickMap (don't forget to change the projection for the north pole).
Ed. Note: Drew Enns is correct about the original WAC mosaic showing no obvious crater of origin for the ejecta field and pattern shown in detail in the Featured Image. It "could be outside" the area covered in that same image, and thus also in the WAC mosaic, shown at 60 meters and 240 meters per pixel resolution, respectively, immediately above. But it isn't. Comparing the narrow and wide angle fields of view above, the subtle ejecta patterns and especially differences in crater ages in the area appears to point toward the unnamed ~3 km crater about 12 kilometers up over the crest of the slope from the area highlighted in the Featured Image. But, if the welcome exercise draws interest to the dramatic improvements in the LROC QuickMap, it was well worth the investigation!
Are there more ejecta deposits within the full NAC frame?
Ejecta from Van de Graaff Crater
Thursday, October 20, 2011
|Layers of material are exposed forming small cliffs just within the south rim of Lucian, perhaps exposing the volcanic strategraphy and geologic history of Mare Tranquillitatis. LROC Narrow Angle Camera (NAC) observation M170321251R, LRO orbit 10234, September 10, 2011; field of view above is 290 meters. View the full resolution field of view in the LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
LROC News System
Lucian crater, located in Mare Tranquillitatis at 14.3°N, 36.7° E, is a small (~7 km in diameter), relatively fresh crater. Because Lucian is still young, rock is freshly exposed in its wall, and the rocks are layered! But how do these layers relate to Mare Tranquillitatis? Scientists think that the maria were formed as the product of large scale flood volcanism.
Billions of years ago large volumes of lava were erupted from fissures in the lunar crust to cover 17% of the Moon's surface. But this process occurred over a period of time with a very low viscosity lava. This model of mare formation predicts that the mare are made up of many distinct layers. Prior to LROC some scientists argued that the layers were thin, and some scientists argued that the layers were thick. LROC data shows Lucian and other craters have thin layers that are a few meters thick. LROC has again helped scientists understand more about the nature of the Moon!
|Full width 2 km-wide view of the layering under the surface of the Sea of Tranquility exposed by the explosive impact that formed Lucian crater probably late in the Eratosthenian era (1.1 - 3.1 billion years ago). The flat floor of the crater formed from pooled impact melt is just out of view above, though it is visible in the full NAC frame HERE, and in the Wide Angle Camera (WAC) view immediately below [NASA/GSFC/Arizona State University].|
|A quick view from the new and improved LROC QuickMap centered on Lucian composed of a base LROC WAC mosaic of northeast Mare Tranquillitatis with the WAC digital terrain model (DTM) overlay (500 meter resolution) at its default opacity of 30 percent [NASA/GSFC/Arizona State University].|
How many layers can you count in the full NAC frame?
Layers near Apollo 15 landing site
Layering in Messier A
Wednesday, October 19, 2011
|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].|
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].|
|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?
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].|
Monday, October 17, 2011
|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].|
|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].|
Saturday, October 15, 2011
|Low-reflectance granular material flowed down the northeastern wall of an unnamed crater and formed interweaved tendrils. LROC Narrow Angle Camera (NAC) observation M169398317R, LRO orbit 10,098, August 31, 2011; downslope is to the lower left, image field of view is 290 meters. (View the full 500 meter field of view in the LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
LROC News System
Although LROC collects images of craters on the lunar surface at only one moment in time, impact craters are not as static and unchanging as these images may lead you to believe. The majority of material movement occurs during the impact event over a very short time, sometimes lasting only a few seconds, but post-impact modification plays a large role in crater erosion over time. In fact, post-impact modification begins immediately after the crater is formed! Wall slumping that forms terraces or debris piles on the crater floor and solidification of impact melt ponds and flows are just two examples of modifications that begin soon after a crater is formed. Over historical time (one year, ten years, 100 years) as well as geologic time (tens to hundreds of millions of years), crater modification proceeds to degrade the pristine crater into a shallower, less-distinct crater (you can see some of these types of craters in the WAC context image below).
|Crop from LROC Wide Angle Camera (WAC) monochrome (643 nm) observation M118695906ME, LRO orbit 2626, January 26, 2010 of the 44 km-wide crater Virtanen (below, near 15.80°N, 177.39°E) and the unnamed crater that impacted into its eastern wall, a scene from the middle latitudes of the lunar farside. From 54 kilometers overhead the scene does not capture a fell for the slope angles of the topography in this terrain as well as the false-color images built up from LOLA laser altimetry further below. The location of the scenes in the unnamed crater's inner walls are noted by the two blue arrows; the lower arrow notes the location of an additional explanatory close-up further down in this posting (the crater at the top of the image above is Virtanen Z). [NASA/GSFC/Arizona State University].|
Today's Featured Image highlights a granular debris flow that originated near the crater rim and flowed downhill from the northeastern wall of an unnamed crater within Virtanen crater (15.80°N, 177.39°E). Along the way, the dry particles were disturbed by boulders that deflected the material. In a previous post, a boulder acted as a dam to stop the debris from flowing and created a "flow shadow" where the low-reflectance material did not reach - similar to what is visible here. However, in some cases, there is space between the boulder and the location at which the debris forks for its detour. Why might this be? Here's a hint: take a look at these boulders - do any of them have boulder trails or do they look like they are eroding out of the crater wall itself? There are no visible boulder trails, and the boulders of variable sizes are not sitting on the crater wall surface. In fact, most of the boulders look partially buried. So, it is likely that the low-reflectance granular material deflected around these boulders because the boulders are eroding out of the wall material and represent a small topographical high compared to the smoother, unbouldered portion of the crater wall. What do you think?
|A small white arrow notes the location of Virtanen, well inside a 650 km-wide basin as mapped by LRO's LOLA, over the course of the past two years [NASA/LOLA/LMMP].|
The southeastern crater wall near the crater rim (below) is markedly different than the northeastern part of the crater wall nearing the crater floor (opening image). Instead of well-developed low-reflectance debris flows tendrils located downhill from the crater rim because of gravity, there is a mix of both high- and low-reflectance material on the crater wall slope. There are also large erosional troughs or alcoves from which the material forming debris flows originates. The contrast between reflectance of the crater wall and the lower-reflectance material traveling downslope (to the upper left) illustrates that the crater wall is not a smooth, flat surface. Small slope breaks in the crater wall acted as a dam to halt debris on their downward descent to the crater floor. Maybe these troughs will erode to the point where, in several millions of years, substantial material from the upper part of the southeastern wall will have mobilized downhill to form debris flows similar to those on the northeastern slope.
|The southeastern crater wall has deep alcoves from which material erodes. LROC NAC M169398317R, illumination is from the bottom/bottom left, image width is 290 meters [NASA/GSFC/Arizona State University].|
Explore these debris flows from the comfort of your computer seat in the full LROC NAC image!
Tendrils in Reiner Crater
Erosional trough on crater wall
Rock avalanche in Robinson crater
Thursday, October 13, 2011
|A mare wrinkle ridge transitions to a highland lobate scarp at the edge of Oceanus Procellarum. Illumination is from the lower-left in this 2.9 km wide mosaic of LROC Narrow Angle Camera (NAC) frames M107069913LE and M107069913RE, LRO orbit 918, September 8, 2009. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
LROC News System
A mare wrinkle ridge transitions to a highland lobate scarp at the edge of Oceanus Procellarum. Illumination is from the lower-left in this 2.9 km wide mosaic of LROC NAC images M107069913LE and M107069913RE [NASA/GSFC/Arizona State University].
Two types of compressional tectonic landforms are commonly observed on the Moon: wrinkle ridges and lobate scarps. Wrinkle ridges are long, often sinuous hills in mare basalts and are thought to be folded rock layers overlying deeper faults. Lobate scarps usually occur in the highlands and are interpreted as rocks lifted up by faults very near to or even breaking the surface. There are a few locations where a wrinkle ridge transitions into a lobate scarp or vice-versa, such as here at the northern edge of Oceanus Procellarum (60.5°N, 331.4°E). In this LROC NAC mosaic, the lobate scarp in the highlands massif to the northeast meets a wrinkle ridge to the southwest when it reaches the otherwise flat-lying Procellarum basalts. You can also see lots of boulders eroding out of the wrinkle ridge.
|Reduced resolution NAC mosaic of images M107069913L and M107069913R showing the wrinkle ridge - lobate scarp transition. Illumination is from the bottom-left in this 16 km wide mosaic. [NASA/GSFC/Arizona State University].|
Wrinkle ridges are thought to have formed after the basaltic lavas erupted, filling in the basins on the nearside, and weighing down the crust. All that extra weight probably made the ground sag and bend, causing the basalt to buckle and fold in some areas. On the other hand, lobate scarps are thought to form from radial contraction or shrinking of the entire Moon. The global radial contraction built up compressional stresses in the crust until the stress was great enough to fracture all the way to the surface. The transition from wrinkle ridge to lobate scarp may be due to the contrast in materials, especially if the basaltic lavas are layered and the highland massif lacks layering. However, the relationships between wrinkle ridges and lobate scarps at transitions like this are still being studied.
the full NAC mosaic - can you find any other scarps or ridges?
Scarps in Schrödinger
Lunar Lobate Scarp
Slipher Crater Fractured Moon in 3-D
Forked Wrinkle Ridge
Stress and Pull
Wednesday, October 12, 2011
|The crest of a linear floor fracture wall is intermittently covered by boulders and smoothed terrain. The largest boulders (lower part of image) are around 30 meters across. LROC Narrow Angle Camera (NAC) M159065590R, LRO orbit 8575, May 3, 2011; from top to bottom (north is to the right) field of view is 960 meters. See the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
LROC News System
Boulders are found nearly everywhere on the Moon, and LROC NAC images allow scientists to study boulder populations. Why would anyone want to spend their time looking for boulders? Boulders represent erosional products on the Moon and can be used to help interpret geologic features and derive a geologic history for a region. Presently, erosion on the Moon largely occurs as a result of micrometeorite bombardment (for a short discussion, check out the Relative Age Relationships Featured Image). For example, the presence of boulders surrounding a 100 meter diameter crater in the mare suggests that when the crater formed, the impactor punched through the layer of regolith and excavated bedrock. Similarly, the boulders perched on wrinkle ridge crests and on the walls of sinuous rilles represent bedrock that is eroding out of these features over time. Perhaps the density or frequency of boulders on wrinkle ridge crests may be used to determine relative ages between features within a region. However, because erosion can be controlled by rock type and how fractured or deformed the rocks are, scientists need to carefully interpret their observations of boulder populations.
Today's Featured Image highlights boulders perched on the crest of a linear floor fracture wall within the central peak ring of Schrödinger basin (73.22°S, 133.82°E).Many of the boulders are around 5 to 10 meters across (although some are smaller), but near the lower edge of the image above the boulders are much larger, around 20to- 30 meters across. There are also more large boulders exposed downslope of the 20 - 30 m boulders compared to other boulder clusters. Why is there an apparent discrepancy?
To answer, we should consider several things. First, we must determine whether the boulders originate from the crest (in-situ) or are impact-derived (that is, they deposited by a nearby impact). Taking a look in the full Narrow Angle Camera frame, there is an approximately 7 kilometer in diameter Eratosthenian-Age crater about 30 kilometers north of the fractures. However, while there are boulders around 30 meters in size located in clusters surrounding the crater the continuous ejecta blanket (for a crater generally) is confined to one crater diameter. Since the fractures are much farther than 7 km from the crater the boulders on the fracture wall crest are probably not derived from that crater.
What about other nearby craters? While there are several craters nearby (check out the LROC WAC context below), none of them are particularly fresh nor bouldery. Based on these observations, the boulders probably orignated in-situ as a result of erosion. Thus, the apparent discrepancy in boulder sizes along the crest may be related to the deformation of the rock during fracture formation and mare flooding of Schrödinger. Right now, though, we do not have a definitive answer; however, to examine this topic further, we would need to complete a survey of boulder sizes and distributions along the length of the fractures. Even then, we may not be able to explain why some boulder groupings contain larger sized boulders.
|LROC Wide Angle Camera (WAC) monochrome mosaic showing and area located slightly north of the center of Schrödinger basin with the northern portion of the central peak ring near the top of the image. (Asterisk notes the location of today's Featured Image image.) View the full size LROC WAC context mosaic showing the deep interior of Schrödinger basin HERE [NASA/GSFC/Arizona State University].|
Now that we found evidence to suggest that the boulders are probably related to the erosion of the crest of the fracture wall we need to try to explain the smoothed portions of the crest.
In the opening image, there are boulder clusters separated by smoothed areas. What is this smooth stuff? Regolith, the lunar soil, is generated by impacts - big and small - and over time accumulates on the lunar surface. When a surface is covered entirely by regolith, it looks smooth and sometimes has a textured appearance. So, these smooth regions are accumulations of regolith on the wall crest and also along the fracture walls. But then why isn't the crest all smooth, or all bouldery? The illumination and where the shadows lie across the ridge crest may be a clue; these smooth areas have shadows that are not in-line with the bouldery portions of the crest and look like they may be lower topographically than the bouldery parts or may be less steeply sloped than the parts of the crest where boulders are. Again, taking a look at the full NAC frame may help, too, because in the full image there are other smooth regions that are associated with obvious changes in topography and slope along the crest. If we then assume that the smoother regions have a shallower slope at the crest, we may have an answer: boulders are more likely to form along steep slopes of crests where the slope break is steepest (here along the fracture wall crest, or along wrinkle ridges). This is only one explanation based on visual explanation, but we really should take some time to measure slopes in a NAC-derived Digital Terrain Model (DTM) to be certain that we are on the right track.