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Dark and light streaks on crater walls, Moon. (click to enlarge)

Although the Moon’s gravity is low, only about 0.165 of the Earth, rock and soil move down slope over time.  In geology, such processes are called mass wasting and is one of the principal sources of erosion on the Moon (the other being meteorite bombardment).  Mass wasting includes both gradual, infinitesimally slow soil creep on slopes and rapid, catastrophic mass movements, called landslides.  Long trains of rock debris can form scree slopes, loose fragments lying precariously at the critical angle beyond which they move, the angle of repose.  Because impact craters make steep walls and the larger ones bring up peaks in their centers, most mass wasting on the Moon is found in and around impact craters of all sizes.

As the number of high resolution images taken from the LRO mission continues to proliferate, several interesting and underappreciated lunar surface phenomena are becoming more apparent.  Among the fresh craters of the Moon, we find light and dark steaks on the walls of the ubiquitous craters of the Moon.  Although it is not surprising that material might move or flow down steep slopes on the Moon, the appearance of these flows can be startlingly similar to those seen on other planets, particularly Mars, where such streaks have been cited as evidence for the presence of subsurface water.

The new narrow angle LRO camera can see objects on the surface smaller than one meter (typically, 50 cm per pixel resolution).  These new views have shown us a wide diversity of new features within impact craters and have given us a new appreciation for mass wasting.  Larger crater walls are slumped, with stair step-like wall terraces, concentrically arranged around the crater between rim and floor.  In detail, these terraces show ponds of dark material that seem to collect in low areas.  Most of this material looks like it was once molten but now congealed; it is probably solidified impact melt.  Flows of melt may cascade down and over the walls of fresh craters.

However, many “flows” of both dark and light material on the Moon seem to consist of loose fragments of rock debris lying on steep slopes.  These debris flows show a variety of morphologies, including simple flow shapes, cascades, ponding, and fan-like termini.  Sometimes the dark and light flows intermingle within a single crater while others show only one type.  These debris flows can usually be traced back to outcrops of bedrock in the upper portions of the crater wall.  As the bedrock erodes (usually by meteorite erosion and disaggregation due to the intense fracturing induced by the original impact that formed the crater), it sheds small fragments that train down slope, forming flow-like landforms.

Because crater walls are uneven, undulating surfaces, the rates of down slope movement can vary widely over small distances.  This sometimes results in multiple, overlapping flows of debris.  Factors that control the albedo (reflectivity) of the debris flows are not well understood.  It could be related to composition (for example, dark, iron-rich mare basalt vs. white, anorthositic highland rocks).  Another factor might be particle size; small pebble-sized rock flows could be bright as new, fresh surfaces are constantly exposed.  Flows that contain mixed soil might be darker than normal, as this soil could cover the fragments and reduce its average reflectivity.  But while all these factors may be of significance to one degree or another, the brightness of a streak is not particularly indicative of origin.

Dark streaks on crater walls, Mars. (click to enlarge)

On Mars, many dark streaks are evident on crater walls and, as on the Moon, come in a wide variety of forms and occurrences.  Martian dark streaks have been variously interpreted as being caused by compositional and particle size differences, but the most popular idea is that the dark streaks are wet soil, i.e., they represent areas where liquid water is seeping out from the planet’s subsurface and moistening the surface.  One observation supporting this idea is an apparent correlation of some of the dark streaks with surface temperature, with warmer slopes showing more.  As liquid water is not stable on the martian surface, salt-rich brines (which would have much lower melting points than pure water) have been invoked as the possible liquid phase.

The dark streaks on the crater walls of the Moon call water-related interpretations of similar features on Mars into question.  The nature of down slope movement on Mars is likely to be controlled by even more diverse factors than the lunar case.  For example, large landslides partly cover the floor of the Valles Marineris, the large canyon system on Mars.  These landslides can extend tens of kilometers across the valley floor and the mass flow might have been lubricated by trapped atmospheric gas; this “cushioning” effect occurs within some landslides on the Earth.  Such a process would not occur on the Moon.  The diversity of geological processes on Mars suggests that explanations for dark wall streaks could encompass many more possibilities than simple wetting of the surface.

Although the existence of dark lunar streaks does not negate water-related interpretations of similar features on Mars, they do call attention to the need to keep alternative hypotheses in mind.  For many years (and with some success), planetary geologists have extrapolated landforms and processes (thought to be understood) on Earth, to similar appearing features on the planets.  In the case of the dark streaks, terrestrial water seepages in the desert can be darker than surrounding desiccated terrain.  A wide variety of evidence indicates that water is present in the subsurface on Mars but sometimes other effects such as rock composition or particle size are responsible for the streaks and alternatives to seepage should always be kept in mind.

Professor Ronald Greeley, 1939-2011

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.

The north pole of the Moon: Real or facsimile?

Of all the wonders depicted in science fiction books and movies, one of the most intriguing is the machine that makes anything that you need or desire.  Merely enter a detailed plan, or push the button for items programmed into the machine – dials twirl, the machine hums and out pops what you requested.  Technology gives us Aladdin’s Lamp.  A handy device that will find many uses.

We’re not quite there yet but crude versions of such imagined machines already exist.  These machines are called “rapid prototype” generators or three-dimensional printers.  They take digitized information about the dimensions and shape of an object and use that data to control a fabricator that re-creates the object using a variety of different materials.  Typically, these machines use easy to mold plastics and epoxy resins but in principle, any material could be used to create virtually any object.

What’s the relevance of this technology to spaceflight and to the Moon?  One of the key objects of lunar return is to learn how to use the material and energy resources of the Moon to create new capabilities.  To date, we have focused our attention on simple raw materials like bulk regolith (soil) and the water found at the poles.  It makes sense to initially limit our resource utilization ambitions to simple materials that are both useful and relatively massive, which currently have those killer transportation costs when delivered from Earth.  Bulk regolith has many different uses, such as shielding (e.g., rocket exhaust blast berms) as well as raw material for simple surface structures.

However, once we are on the Moon and have met the basic necessities of life, we can begin to experiment with making and using more complex products.  In effect, the inhabitants of the Moon will begin to create more complicated parts and items from what they find around them, just outside their door.  The techniques of three-dimensional printing will allow us to discover what makes life off-planet easier and more productive.  We will experiment by using the local materials to maintain and repair equipment, build new structures, and finally begin off-planet manufacturing.

During the early stages of lunar habitation, material and equipment will be brought from Earth.  With continued use, particularly in the harsh lunar surface environment, breakdowns will occur.  Although initially we will use spare parts from Earth, for simple uncomplicated structures that are needed quickly, a three-dimensional printer can make substitute parts using local resource materials found near the outpost.  Most existing 3-D printers on Earth use plastics and related materials (which are complex carbon-based compounds, mostly derived from petroleum) but some processing has used concrete, which can be made on the Moon from sieved regolith and water.  In addition, we also know that regolith can be fused into ceramic using microwaves, so rapid prototyping activities on the Moon may eventually find that partially melting particulate matter into glass is another way to create useful objects.

The lunar surface is a good source of material and energy useful in creating a wide variety of objects.  I mentioned simple ceramics and aggregates, but additionally, a variety of metals (including iron, aluminum and titanium) are available on the Moon.  Silicon for making electronic components and solar cells is abundant on the Moon.  Designs for robotic rovers that literally fuse the in-place upper surface of the lunar regolith into electricity-producing solar cells have already been imagined and prototyped.  We can outsource solar energy jobs to the Moon!

These technical developments lead to mind-boggling possibilities.  Back in the 1940s, the mathematician John von Neumann imagined what he called “self-replicating automata,” small machines that could process information to reproduce themselves at exponential rates.  Interestingly, von Neumann himself thought of the idea of using such automata in space, where both energy and materials are (quite literally) unlimited.  A machine that contains the information and the ability to reproduce itself may ultimately be the tool humanity needs to “conquer” space.  Hordes of reproducing robots could prepare a planet for colonization as well as providing safe havens and habitats.

We can experiment on the Moon with self-replicating machines because it contains the necessary material and energy resources.  Of course, in the near-term, we will simply use this new technology to create spare parts and perhaps simple objects that we find serve our immediate and utilitarian needs.  But things like this have a habit of evolving far beyond their initial envisioned use, and often in directions that we do not expect; we are not smart enough to imagine what we don’t know.  The technology of three-dimensional printing will make the habitation of the Moon – our nearest neighbor in space – easier and more productive.  Even now, creative former NASA workers have found a way to make this technology pay off.  In the future, perhaps their talents could be applied to making the Moon a second home to humanity.

Note:  The image at the beginning of this post is a model of the lunar north pole, made using a three-dimensional printer and LRO laser altimetry data by Howard Fink of New York University.  The scale of the model is about 30 cm across.

The lunar feature Ina, an extremely young, unusual depression that may represent a gas eruption site on the Moon. LROC narrow angle camera images.

There are times when seemingly unrelated discoveries about other planets come forward to enlighten us about the history and processes of the Moon. A recent paper, using data from the orbiting MESSENGER mission mapping Mercury, describes a number of newly discovered rimless pits and depressions.  These pits (called hollows by the mission team) are difficult to explain by impact processes and are hypothesized to be the products of outgassing from the planet’s interior.  They are often associated with color anomalies (which implies compositional differences from the surrounding terrain) and frequently found on the floors of impact craters and basins.

Impact craters come in a wide variety of sizes, but within selected size ranges, they all appear more or less similar.  Small craters are nearly perfectly round and bowl-shaped with smooth rims that are raised above the surrounding terrain.  Craters with irregular shapes and no raised rims suggest that processes other than impact might be at work.  It has been suggested that on Mercury, these “hollows” were created by the violent release of volatile substances.  Such a release of gas under pressure accompanies volcanic eruptions called pyroclastic, meaning “fire-broken” (fine liquid rock (magma) fragments spewed into space and cooled during flight).

We’ve known about pyroclastic eruptions on the Moon for many years, evidenced by the green glass of the Apollo 15 site and the orange-black glass from Apollo 17.  Careful search of the images taken from lunar orbit reveal the rimless pits that served as vents for the pyroclastic eruptions that produced these Apollo glasses.  They are distinct from impact craters and often are found on the floors of craters and basins along fractures, the conduit by which volcanic magma travels to the lunar surface.

Sometimes pit craters or “hollows,” found across the surface of the Moon, take unusual form.  The kidney-shaped feature shown above is named Ina; after its discovery in one of the Apollo orbital images, it was informally named the “D-caldera” after its shape and the interpretation that it represented a volcanic collapse feature.  Ina is about 3 km across and consists of a series of small platforms, mounds and holes within a larger irregular depression.  Other similar pits and hollows occur elsewhere on the Moon (e.g., on the floor of Rima Hyginis).  And while not major features, they have been found often enough to bother many lunar scientists, who had no good explanation for their origin.

About five years ago, we got a clue as to the possible origins of these features.  Pete Schultz and associates from Brown University published a paper showing Ina displayed unusual spectral reflectance characteristics.  The slow micrometeorite bombardment of the Moon adds craters to the surface and also makes small iron-rich glass particles that darken and redden the surface.  As these glass particles build up in the soil, a soil is said to “mature.”  Fresh surfaces are more “blue” in color (actually, less red) and become redder with time as the soil matures.  Most lunar features show age or “become mature” on timescales of millions of years.  Ina shows very few impact craters on top of it, meaning that geologically, it is very young.  Moreover, the soils associated with Ina are much bluer than surrounding areas.  Both of these observations suggest that Ina is young with immature surfaces.

How are these features created?  Significant volcanism on the Moon largely stopped at least a couple of billion years ago.  The Brown team thought that the combination of young age, low maturity and unusual morphology suggested a relatively uncommon pit-forming process.  They proposed that the explosive release of volatile substances from the lunar interior would have disrupted the surface, created a chaotic mixture of rock and soil, exposed fresh surfaces (creating the immature spectral signature), and formed a collapse depression caused by the instantaneous removal of mass from below.

Now we can see that the new Mercurian hollows have morphologies displaying spectral anomalies similar to the lunar collapse pits such as Ina.  The new data suggest that Mercury contains significant volatile substances.  These volatiles must be present at some depth, accumulated under high pressure until crustal failure ensues and a massive gas release results in an “eruption.”  This explosive event leaves behind a chaotic, disrupted surface (“immature,” with fresh bedrock and deep regolith “newly” exposed to space).

In the case of Ina on the Moon, its extreme youth is suggested both by the lack of overlying impact craters of almost any size, as well as the sharp preservation of topography in its cliff and pit interior morphology.  This extreme youth may be on the order of thousands to hundreds of thousands of years, not the millions and billions of years that typify most lunar landforms.  Such youth and the widespread distribution of Ina-like collapse pits across the lunar surface implies that outgassing events are occurring on the Moon now; it is highly unlikely that we were just lucky enough to find a singular or unique occurrence.

What might these volatile substances be?  Before the recent lunar missions flew, it was common to declare that water was not a possibility.  However, we recently discovered from study of the lunar samples that water was present in the deep interior of the Moon during the epoch of mare volcanism three billion years ago; water could still be present in the subsurface.  There are many other volatile substances that could be responsible as well, including carbon monoxide, hydrogen sulfide, gaseous sulfur, as well as other more exotic gases.  Because the compositions on Mercury are poorly known, the possibilities for exotic materials there are even more extensive.

The explosive release of gas from the deep interior (without the eruption of magma) appears to be an ongoing lunar process.  This gas release could provide at least a partial answer to two vexing lunar problems: the accumulation of volatiles at the poles of the Moon (discussed in my blogging many times, most recently HERE) and the infamous phenomena of Lunar Transient Phenomena (LTP), described as glowing reddish “clouds” hovering over the lunar surface that mysteriously appear and disappear.  Telescopic observers have reported seeing LTP for many years.  Unfortunately, we have not been able to verify and document these events, largely because they are transient.  Now we have direct morphological evidence for the venting of gas from both planets, making it possible that at least some LTP might be related to gas release from inside the Moon.  Stay tuned – the book of the Moon continues to be rewritten and expanded with new and interesting discoveries.

NOTE: The latest version of the paper Tony Lavoie and I wrote on using lunar resources to create a cislunar space faring system has been published in the Proceedings of the AIAA Space 2011 Conference.  A copy is available for download HERE.

The Great White Fleet of the United States Navy, 1907 -- We need a fleet of spacecraft to open "This New Ocean" of space

We seem to be in one of those periods in which basic reasons for doing what we do as a nation are called into question.  This includes our national civil space program, which for the last few years has engaged in an extended period of back-biting and navel-gazing.  Much of this “debate” has focused on either or both of two points: what rocket to build and where to go, and not on sustainability.

In an era of limited resources, our challenge is to create a worthwhile space program with an expenditure rate that falls at or below a level perceived as affordable.  Given this reality (regardless of prevailing agency direction or assertions about projected deep space destinations) it is highly likely that cislunar space will be the sphere of space operations for the coming decade or two. Thus the questions should be:  What are we doing in space and why are we doing it?  If the answer is a series of space exploration “firsts” (flags-and-footprints forever), that model will require specific activities and missions.  If the answer is that an incrementally developed transportation infrastructure is desired, one that creates an expanding sphere of human operations, then such a model requires a different set of specific activities and missions.

Thus, the real debate is not about launch vehicles or spacecraft or even destinations; it is about the long-term – the paradigm or template of space operations.  One model requires mega-rockets to distant targets for touch-and-go missions; for convenience, I’ll call it the “Apollo” template (no denigration intended).  The other model is an incremental, go-somewhere-to-stay-and-then-expand-onwards mindset – call it the “Shuttle” template (again, same disclaimer).  The one that you adopt and follow depends on what purpose you believe human spaceflight serves.

Because Mars may harbor former or existing life, NASA has presumed that it is our “ultimate destination” in space.  In effect, the entire focus of the human spaceflight effort has devolved into a huge science project – “The Quest for Life” (which means finding pond scum, not ET).  Thus, debate about what to build, where to go and how to do it must be formulated towards attainment of Mars.

This unspoken assumption has been at the root of most space objective studies for the past 20 years.  Mars was the end point of President George H.W. Bush’s Space Exploration Initiative, President George W. Bush’s Vision for Space Exploration, of former Lockheed-Martin President Norm Augustine’s two reports, and a myriad of space groups and societies.  From the 1990′s to the present, a multi-billion dollar robotic campaign has sent mission after mission to Mars, each discovering that the red planet once had liquid water.  This mania for Mars and preoccupation with possible life there, has blinkered our perceptions of the space program and distorted our reality of what is possible or attainable on reasonable time scales with available resources.

Long term, the goal for human spaceflight is to create the capability to go anywhere we choose, for as long as we need, and do whatever we want to in space.  For the sake of argument, if one accepts such a goal, which model is more amenable to implementing it: the Apollo template or the Shuttle template?

If our goal is to “sail on the ocean of space,” we need a navy.  Navies don’t operate with just one class of ship because one class isn’t capable of doing all that is necessary.  Not all ships will look or operate the same because they have different purposes and destinations.  We need transports, way stations, supply depots, and ports.  In space terms, these consist of one to get people to and from space (LEO), one to get them to and from points beyond LEO, way stations and outposts at GEO, L-1, low lunar orbit, and to the lunar surface.  To fuel and provision our space navy, we require supply (propellant) depots in LEO, L-1 and on the lunar surface.  Ports of call are all the places we may go to with this system.  Initially, those ports are satellites in various orbits which require service, maintenance and replacement with larger, distributed systems.  Later, our harbor will be the surface of the Moon, to harvest its resources, thereby creating more capability and provisions from space.  Reliable and frequent access to the entire Solar System, not one or two destinations, should be our ultimate goal.

By designing and building mission-specific vehicles and elements, the “Apollo” template forfeits going everywhere and doing everything.  However, adopting the “Shuttle” model does not preclude going to Mars.  In fact, I contend that to go to Mars in an affordable manner that sustains repeated trips, one needs the infrastructure provided by a space faring navy.  Building a series of one-off spacecraft – huge launch vehicles to dash to Mars for expensive, public relations extravaganzas will eventually put us right back in the box we’re in now.

We have been arguing about the wrong things.  It is the mindset of the space program that needs re-thinking – not the next destination, not the next launch vehicle, and not the next spacecraft.  How can we change the discussion?  First, we need to understand and articulate the true choices so that people can see and evaluate the different approaches and requirements.  Second, we need to develop sample architectures that fit the requirements for “affordable incrementalism.”  Finally, we need to get such plans in front of the decision makers.  There is no guarantee that they will accept it or even listen to the arguments for it.  But right now, they are completely ignorant about it.

A cost-effective, sustainable human spaceflight program must be incremental and cumulative.  Our space program must continually expand our reach, creating new capabilities over time.  Moreover, it should contribute to compelling national economic, scientific and security interests.  Building a lasting and reusable space transportation system does that, whereas a series of PR stunt missions will not.  The original vision of the Shuttle system was to incrementally move into the Solar System – first a Shuttle to-and-from LEO, then Station as a jumping off platform and then beyond LEO into cislunar space.  We have the parts from the now retired Shuttle system and an assembled and working International Space Station.  We can use these legacy pieces to build an affordable system to access the near regions and resources of cislunar space.  In this new age of austerity, perhaps we will finally acquire the means to build our pathway to the stars.