Archive | July 2010

Albedo, Color and the Interesting Case of Lunar Sample 12047,6

One thing that I have learned here at Moon Zoo as a full member-newbie is that when considering the lunar landscape what is dark may not be black and what is light may not be white. A case in point is this photograph that stumped me shortly upon arrival at Moon Zoo.

Moon Zoo Image

Multicolored Lunar Landscape?

Luckily, Forum moderator Thomas J introduced me to a term called albedo. He said:

“There are a number of reasons for this change in shade. The reflectivity of the surface material is known as its albedo; material that is highly reflective will have a higher albedo than that which is not as reflective. In some situations certain areas may display high albedo material due to geological activity such as impact effects, volcanic effects and even Moon quakes. It is, therefore, not uncommon to see two contrasting shades in adjacent regions. Also, the Moon’s surface is not flat, the topography rises and falls with slopes, hills and mountains. When the Sun is low in the sky a slight slope downward can leave an area in shade. So, in this image it may be that the material on the right has a higher albedo, or it may be a downward slope on the ground level to the left with a rise to that on the right.”

Why do the tracks of the astronauts on the lunar surface appear darker than the surrounding area? One answer from spacefellowship.com is:

” This effect is most likely due to compaction of a very loose surface powder by simply walking around. The more walking in a given area, the more compaction that takes place, and thus the lower the albedo.”

Moon Zoo Image AMZ1002u1l

Moon Zoo Apollo 16 landing site image showing where astronaut activity has lowered the albedo.

Lunar and Planetary Institute

Orange soil at Shorty Crater

We know certain areas of the lunar surface have coloration due to crater impacts and lava flows such as the famous orange soil of Apollo 17 near Shorty Crater but generally the lunar landscape was described by the Apollo astronauts as concrete, mouse grey and/or light brown in color.

This brings me to the Apollo 12 lunar sample 12047,6 that is now on loan from NASA to the Seattle Museum of Flight.

Apollo 12 astronauts Pete Conrad and Alan Bean collected this specimen while at the mare region called Ocean of Storms. It is an Ilmenite Basalt sample. From the black and white photograph you can tell that it is a concrete like color not unlike many of the photographs we view here at Moon Zoo. What is interesting is that there is more to this specimen than what you see here and read about in the paper linked to above and that is the albedo effect and color change.

I happen to be lucky and live about 15 miles from the Seattle Museum of Flight. However, it was not until I viewed an online photograph of NASA 12047,6 taken by an amateur photographer visiting the museum that I found out that the sample was on display there. On my first visit I was perplexed as the sample did not look quite like the beautiful one in the photograph but more like the concrete colored black and white photograph, though not quite as drab looking. For a month or so this mystery nagged on me and I went so far as to question the on site guides about the authenticity of the display item. Maybe it was another sample or a copy. However, I was told that it is a genuine lunar sample.

Searching the internet I found reference to color change on the lunar surface:

“The orbiting Apollo astronauts noticed a peculiar phenomenon when they observed the lunar surface at a small angle related to the sun’s light. At such small angle, the lunar surface appeared warm brown colored.”

Yes, the light bulb went on in my head and I returned to the museum several months later. When moving as close to the display case as possible and viewing the lunar rock at a sharp angle, the brown color showed up. So, here is a photograph of NASA 12047,6 taken at the Seattle Museum of Flight by amateur photographer Svacher. Just as the astronauts did, we can see the brown color of the lunar surface when the sun or in this case the viewer is correctly positioned.

cc svacher

12047,6 (cc svacher)

Here’s some more related reading:

Apollo Photography and the Color of the Moon
A Colorful Moon

And thanks to Moon Zoo Team member Katie Joy for providing the following additional information and links:

Lunar Sample Atlas
Katie says: “Thin or polished samples of lunar rock can be quite pretty. Some have hints of orange (glass), others bright green (the mineral olivine), some pink (the mineral spinel.) When you view them under the microscope using polarized light rather than direct light then all the minerals appear as a really bright range of colours and look like a stain glass window.”

A Final Thought:

Being one of many amateur scientists here at MZ, the notion of albedo and color change as it applies to larger natural occurring and unaltered geology samples comes to mind such as the Apollo 12 lunar sample above. Here the surface of the sample is in its natural state showing the color change in the visible light spectrum similar to what the Apollo astronauts would have seen in similar lighting conditions.

Analyzing existing samples at the Johnson Space Center in their natural-unaltered state (if not already performed) while replicating the effect of a lunar sun at all possible angles may provide valuable information about the lunar landscape – a more holistic perspective. It may also give clues to color change of lunar formations helping to explain some anomalies associated with transient lunar phenomena.

Mapping color change of the lunar landscape at specific areas may be a helpful tool to acclimate astronauts as they once again walk on the Moon.


Tom128 is a Moon Zoo participant and a regular contributor to the Forum.

The Moon Has Its Faults

At Moon Zoo, they have asked for your help in identifying linear features on the Moon, some of which are likely to be faults. So, what are faults? Faults are fractures in a planet or moon’s crust that accommodate some movement along them. If the movement along a fault is great enough, a quake can be felt.

On Earth, the majority of faults occurs at plate boundaries, like the San Andreas fault which separates the North American and Pacific plates (Figure 1) or along the boundary between the Indian and Eurasian plates which forms the Himalaya Mountains.

Fault lines

Figure 1. San Andreas fault in California. A) Aerial view of the Carizzo Plain. Note the offset drainages. B) Annotated image showing the fault trace (black line) and the arrows denote the direction each side of this strike-slip fault moved. Image A: IPGP, Paris.

However, a planet or moon does not need to have plate tectonics (as on the Earth) in order to have faults. Faults can form anywhere where there is enough stress (the amount of force distributed over an area) in the crust to make it break – or fault.

There are several ways in which faults form. On the Moon, the source of stress is most commonly associated with impact basins, so most of the faults are found in and around the large impact basins on the lunar nearside.

There are two types of faults found on the Moon. Normal faults occur where there is tension (stretching) of the crust. When this occurs, the crust in that area gets thinner and longer, which means that one side of the fault must move down relative to the other (Figure. 2). This happens when the weight of the mare in the basins pulls the crust apart as the middle of the basin subsides.

Graben

Figure 2. Cartoon showing cross-sectional and map views of a pair of inward dipping normal faults, resulting in the formation of a graben. The red arrows show that the crust is being pulled apart and white arrows show the movement directions along the faults. Image: Sue Selkirk/ASU.

Normal Faults: on the Moon, there are several types of normal faults: straight rilles normal faults and thrust faults.

1. Straight rilles are long, narrow troughs and they exhibit two fault-related surface morphologies: linear (Figure 3) and arcuate. These are formed by faults and are found in the highlands near mare basins as well as near their margins. They have steep sides, which are interpreted to be the normal faults, and flat floors. These structures with two inward sloping walls are called graben (meaning trench in German) (Figure 3).

Rima Ariadaeus

Figure 3. Linear rilles or graben of Rima Ariadaeus is about 300 km in length and is located between Mare Tranquillitatis and Mare Vaporum. Image: WAC M116222025MC NASA/GSFC/Arizona State University.

2. Isolated normal faults also exist, but are rare compared to rilles (graben). Like some normal faults on Earth, these tend to be very linear as shown by the Rupes Recta (“Straight Wall or Cliff”) in Mare Nubium (Figure 4).

Rupes Recta

Figure 4. The Rupes Recta normal fault (shown by black arrows) is ~110 km long and is located in the southeastern part of Mare Nubium on the nearside of the Moon. The fault scarp is a few km high and is in shadow. Image: JAXA/NHK.

3. Thrust Faults: When stress acts in an opposite sense, compression of the crust occurs and thrust faults are formed. In this case, the crust gets thicker and one side of the fault moves upward over the other (Figure 5).

Fault Cartoon

Figure 5. Cartoon showing a thrust fault. U: up, D: down. Large black arrows show compression direction and small half arrows show movement directions along the faults. The surface trace of the fault is what geologists would map. Image: American River College.

On the Moon, this happens most often in impact basin interiors where the weight of the mare causes the basin to subside, or bend downwards, causing compression in the mare. Thrust faults on the Moon have two morphologies: wrinkle ridges and lobate scarps. Wrinkle ridges (Figure 6) are found in the mare in the basins and are very complicated structures. They consist of a broad arch (the ‘ridge’ in wrinkle ridges) with narrow, often sinuous ridges superimposed (the ‘wrinkle’). Lobate scarps, which are much simpler in their morphology (Figure 7), are almost exclusively found in the lunar farside highlands far from the influence of basin subsidence. These structures are instead thought to have formed from the slow cooling of the lunar interior that led to a small amount of global contraction.

Mare Serenitatis

Figure 6. Wrinkle ridges in Mare Serenitatis. Note also that normal faults are present at the basin margin in the lower corners. Both wrinkle ridges and normal faults are primarily concentric to the basin. Image width is ~280 km. Image: mosaic of Apollo Metric camera frames AS17-450 and AS17-454. NASA.

M104318871LC_pyr

Figure 7. Lee-Lincoln scarp in the Taurus-Littrow valley (Apollo 17 landing site). Image is 1.6 km across. Image: NAC frame M104318871LC. NASA/GSFC/Arizona State University

Why are faults important to lunar and planetary scientists? On the Moon, since most faults form in response to stresses from the lunar basins, they can tell us about the geologic and tectonic evolution of these basins, particularly the mare-filled ones on the lunar nearside. Wrinkle ridges and rilles can be used to determine the structure of the basin and in some cases how thick the mare are. The fact that lobate scarps are found primarily in the lunar highlands and wrinkle ridges are found only in the lunar mare suggests that these materials behave differently under stress, which is important when trying to puzzle out the geologic history.

The tectonic evolution of a planet or moon can also give us important information about what the interior is like since directly accessing the deep interior is nearly impossible. For example, the lobate scarps (along with the presence of mare) tell us that the Moon was initially hot and that it cooled some finite amount over its lifetime. This has been determined from the offsets along the faults to be about 70 m.

This post was written for Moon Zoo by by Amanda Nahm at the Lunar and Planetary Institute.

Lunar Swirls

One of the great things about the Moon Zoo forum is the way you can learn new things from questions posed by users.

At the end of May 2010, Darrin Cardani started a thread called What causes erosion on the Moon?.
This is an interesting question! There is no weather on the Moon (as we Earthlings define weather) as there is no atmosphere and no liquid water.

When the Moon was young, erosion was mainly due to impacts (from comets, asteroids and meteorites) and volcanism. Today the main causes of erosion are micro-meteorites, the solar wind, moonquakes and degradation of rocks by the temperature change as the surface alternately heats up and cools down.

Recently, a new user to the Moon Zoo forum, astrodel, posted in the “What causes erosion on the Moon?” thread with a reference to an article in Nature about superficial weathering on the Moon. I couldn’t access the original article as it requires payment but dug around a bit and found a reference to “Lunar Swirls”!

The term Lunar Swirls describes unusual sinuously-shaped features on the lunar surface. They have been described as looking like the swirls on the top of a mug of coffee when cream is poured in and slightly stirred! The following image shows an example of a lunar swirl in Oceanus Procellarum and is the largest and longest swirl on the near side.

Reiner Gamma swirl (7.4 N, 300.9 E). [LROC WAC M114342152CE]

All the lunar swirls found so far appear to be associated with magnetic anomalies on the lunar surface but their formation is still a bit of a mystery. There are three different models for swirl formation according to “The Lunar Swirls” document (see References at the end of this posting).

  1. The solar wind deflection model.
  2. The cometary impact model.
  3. The meteoroid swarm model.

A short quote from this document may help explain what swirls actually are:

At the resolution of current data, the swirls appear to overprint the topography on which they lie, indicating that they are quite thin or a surface manifestation of an underlying phenomenon that is manipulating normal surface processes. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain may be less prominent and exhibit simpler shapes such as single loops or diffuse bright spots. – [The Lunar Swirls; A White Paper to the NASA Decadal Survey]

Two of the swirls on the far side of the Moon are directly opposite the centres of two large near side impact basins, Mare Imbrium and Mare Orientale, so there appears to be some connection with a large impact causing a swirl to appear on the opposite side of the Moon.

Swirls show up because they “weather” a lot slower than surrounding terrain. If the original impacts that formed the near side impact basins also somehow caused the magnetic domains on the antipodes to form with swirls above them then these swirls are very old. Mare Imbrium formed about ~3.8 billion years ago so the swirl on the opposite side (“butterfly” swirl) must have been formed then and is still protecting the surface from weathering. This is one of the many mysteries of lunar swirls.

In the early 1970s NASA put two small satellites in orbit around the Moon to measure Earth’s magnetic tail (the solar wind blowing against Earth’s magnetic field creates a “tail” that stretches more than a million miles away from Earth) and these satellites also measured the magnetic field of the Moon. To the scientists’ surprise they found that there were strange magnetic domains all over the Moon in no particular order. They also found that the strongest magnetic fields were above lunar swirls.
These magnetic domains may help to prevent the solar wind from weathering the surface so the albedo remains high.

If you happen to spot a lunar swirl please post it here: TLP Project – Lunar Swirls


“Butterfly swirl” in Mare Ingenii (directly opposite Mare Imbrium).
[LROC WAC M103439292MC]

References:

Reiner Gamma swirl: magnetic effect of a cometary impact?

The Lunar Swirls – PDF document

NASA Science News; Lunar Swirls

The still-mysterious Descartes formation

Dark Haloed Craters

Dark haloed craters are windows to the volcanic history of the Moon. This blog entry was inspired by Thomas’s choice of Image of the Week where he highlights impact craters that have dark material surrounding the crater hole itself.

Dark haloed craters provide us with key insights into what must have been a dramatic and violent volcanic period of lunar history. To probe a little further here is a little background about eruptive volcanism on the Moon:

Between about 4 billion years ago and 3 billion years ago the lunar mantle underwent a period of partial melting where magma was generated at depth and then propagated up through fracture networks and magma conduits towards the lunar surface. It has been proposed that some of this lava was very rich in gases like carbon monoxide that may have caused rapid upward movement (maybe on the scale of one to several days) and caused dramatic pyroclastic eruptions at the lunar surface. These fire-fountaining events are similar to, but on a much larger scale than, eruptions witnessed at volcanoes in Hawaii, with some plumes of lava being thrown up to 40 km in height above the surface of Moon!

hawaiilava

Fire fountaining: Volcanic eruption in Hawaii. Droplets of molten lava are thrown up into the air, where they rapidly cool to form glass beads called Pele’s tears. This is a good analogy to how volcanic dark haloed mantling deposits were formed on the Moon. Image: USGS.

Other lavas that were less gas-rich would have migrated to the lunar surface more slowly, and could have erupted more gently, forming long lava flows that travelled great distances from their volcanic vent site. These lava flows are thought to have the consistency of runny motor oil and easily flowed into topographic lows like impact craters. Sometimes large quantities of lava must have flowed in channel networks – forming rivers of fire across the lunar surface. It must have been a dramatic time, but as available heat sources were diminished in the lunar interior, less magma was generated and by about 1 billion years ago we believe the Moon’s eruptive volcanic history came to a close.

Lunar glass beads: Orange and black glass beads collected from a pyroclastic deposit at the Apollo 17 landing site. These types of beads form mantles around the volcanic vent site from which they were erupted, forming dark mantling deposits. Image: NASA.

Lunar glass beads: Orange and black glass beads collected from a pyroclastic deposit at the Apollo 17 landing site. These types of beads form mantles around the volcanic vent site from which they were erupted, forming dark mantling deposits. Image: NASA.

So how do dark haloed craters fit into this story? Well they actually help address two separate lunar science questions as there are two types of dark haloed craters to keep an eye out for in Moon Zoo images (although we would please like you to classify them using the same button!)…

1. Volcanic eruption sites – these are rare places where pyroclastic beads, the ‘airfall’ deposits of lunar volcanoes, are concentrated on the lunar surface and form mantles around their source vents. These beads are a little bit like the volcanic ash or the Pele’s tears glass that gets erupted from volcanoes on Earth and you can read more about these types of dark halo mantles at:

2. Crater excavation sites – you are far more likely to spot these types of dark haloed craters in Moon Zoo images. The dark haloed craters provide us with a neat view down through a series of geological layers. These are formed when an asteroid or comets smashes into the Moon, punches through an overlying light coloured layer (probably an ejecta blanket material from a nearby highland impact crater) and excavates material from below that is darker in colour. This darker material is likely to be a lava flow that was buried at depth and that is now revealed by the impact cratering process.

Here’s a nice diagram of this process can be seen at where the lower image shows a schematic of what two dark haloed impact craters look like from side-on. You can also view a 3D perspective of this process.

Impact revealing buried lava flows: This LROC NAC image (taken from M112183669LE) is a good example of a dark haloed impact crater that has punched through a light surface deposit and has excavated darker material from an underlying lava flow. Image: LROC/NASA.

Impact revealing buried lava flows: This LROC NAC image (taken from M112183669LE) is a good example of a dark haloed impact crater that has punched through a light surface deposit and has excavated darker material from an underlying lava flow. Image: LROC/NASA.

We call these types of buried lava flows cryptomaria as they would otherwise be hidden from view if we had not have spotted the tell tale signs of dark haloed craters. By mapping the location and extent of dark haloed craters we can therefore map out the distribution of buried lava flows at depth across the Moon and get a much better idea of the amount of ancient volcanism on the Moon. This in turn helps to shed new light on the Moon’s thermal and magmatic history, helping us to understand geological processes on small rocky planetary bodies.

Good examples of these types of impact formed dark-haloed craters spotted by Moon Zoo users include:

So please do keep an eye our dark haloed craters on your Moon Zoo lunar exploring! Thanks to Irene Antonenko for providing helpful guidance about this topic.

Fresh White Craters, the brightest things on the Moon

Fresh white impact craters are the most recent impacts on the Moon. Anything less than a billion years old (which means it is from the current Copernican era), is considered young in lunar terms. Some may be very young indeed. There are very few large craters with the bright ray systems associated with fresh craters. Tycho is thought to be the youngest large crater unless a group of 12th century astronomers were right and this accolade should go to the far side crater Giordano Bruno.

They are important because their ejecta blankets are as fresh as they were on the day of the impact and have not been disturbed by micrometeorites. Many of these craters have extensive ray systems and in some cases ejecta was flung out for hundreds of kilometres (in Tycho’s case 1500 km stretching to the Apollo 17 landing site in the Taurus-Littrow region.) Landing sites for robotic or manned missions can, therefore, be chosen to take advantage of this to maximise the different types of rock available to analyse. You don’t have to go to Tycho to see what Tycho is made of. Analysis of the Tycho ray samples brought back by the Apollo 17 crew show Tycho to be around 100 million years old.

The rays of a fresh crater can be spectacular to view through binoculars or a telescope when the sun is overhead with respect to the crater so full Moon is the best time to observe them. The rays look white not because the rocks excavated are bright white in colour but because their newly exposed and broken surfaces are clean and shiny and have a relatively high albedo in comparison to the mature, darker mare material they lie on top of which has been battered and dulled by micrometeorite impacts.

So why is Moon Zoo interested in them? There are several reasons.

  • By counting the number of fresh impact craters the team can calculate the current impact rate of the Earth-Moon system which is of interest for assessing the risk of asteroid and meteoroid impacts.
  • Also small fresh, impact craters of of just a few kilometres in diameter are the most likely locations from which lunar meteorites found on Earth have been ejected and pinpointing the source of these meteorites is the subject of much research.
  • And because fresh craters are undisturbed their crater walls, interior features and secondary craters can be studied in detail.

Forum member Tom128 developed an interest in freshly formed craters and started a forum thread to collect “Great Fresh Whites.” Here are some of the early finds:

AMZ20004r5 (Tom128)

AMZ20004r5 (Tom128)

AMZ20003g7 (DJ_59)

AMZ20003g7 (DJ_59)

AMZ1000j38 (Aliko)

AMZ1000j38 (Aliko)

AMZ100dn8 (Geoff)

AMZ100dn8 (Geoff)

Read more about craters here and watch a cool animation here.

And there is more information in the Fresh White Crater Reference Resource here.


Jules is the volunteer Moderator of the Moon Zoo Forum

Looking for Change?

For centuries Earth-based astronomers have reported seeing transient changes on the Moon’s surface, known as Transient Lunar Phenomena, or TLP. These have taken the form of short term brightness changes, coloured and non-coloured glows, obscuration of detail, shadow anomalies and flashes of light. The duration of TLP can be from a fraction of a second up to several tens of minutes; then everything returns to normal as far as we can tell from Earth. Unfortunately many of these so called TLP are from inexperienced observers, or made under poor observing conditions, however some TLPs are more intriguing as they are confirmed by more than one observer.

Although the Moon is widely believed by planetary scientists to be effectively geologically dead, there are four currently accepted theories that could explain some of these mysterious TLP reports: (1) Flashes of light seen on the night side are simply due to energy released at optical wavelengths from meteorite impacts – these are being studied by project ALaMO at NASA’s Marshall Space Flight Center and also by amateur astronomers at the Association of Lunar and Planetary Observers (ALPO).  (2) Eruptive out-gassing from residual radiogenic gases such as Radon and Argon that accumulate in deep seated cracks and are released during Moon Quakes – we would effectively see the dust kicked up from this out-gassing. A robotic telescope at Cerro Tololo, Chile, is being used to look for white light changes on the Moon every few tens of seconds, and amateur astronomers at ALPO and the British Astronomical Association (BAA) are searching for coloured events, and TLP in Earthshine or more shaded areas not easily accessible by the Cerro Tololo scope. (3) Electrostatic levitation of dust particles forming dust clouds in electric fields that build up around sunrise in the form of Moon dust fountains. (4) Internal reflectance off volcanic glass beads – analogous to rainbows. However the only attempt to prove a TLP was caused in this way, in Torricelli B crater, was unsuccessful at reproducing this effect.

figure1

Figure 1. (Left) part of an LROC image (M116282876RC) of the Ina formation with an enlarged inset (right) showing craters on the chaotic terrain

Here at Abewrystwyth University we have been analysing some of the statistics of past TLP reports. A preliminary report was given at the Lunar and Planetary Science Conference in Houston in 2009 and we will provide an update in September 2010 at the European Planetary Science Congress in Rome. However we are also very interested to hear from Moon Zoo users, via the on-line forum (e.g. Picture of the Week) of any interesting surface features that they think might have been freshly disturbed or perhaps a vent from which out-gassing has come from. You may find such features as you go through “Boulder Wars” or the “Crater Survey”. However for those really keen to find additional interesting objects, then they can dip into the Apollo Metric and Panoramic Camera or higher resolution LROC image archives, though the latter has a bit of a learning curve. Things to look out for include small craters which are completely shadow filled, whereas their neighbours are not – these could be ceiling vents into underground lava tubes. Also unusual disturbed surfaces that do not look like anything elsewhere in the vicinity such as Figures 1-4. We do not propose that any of these are examples of where out-gassing may have occurred, but they are the kinds of unique features that we would like to know about, just in case.

figure2

Figure 2 (Left) M104476560LE LROC image of a small part of the floor of Hyginus Crater containing some geologically young sunken areas. (Right) M111972680LE LROC image of bright ray crater (east of Reiner Gamma) with a remarkably featureless central floor patch.

figure3

Figure 3(Left) Unusual dark shaded depressions, or dark textured material, to the north west of Vallis Alpes from LROC image M122218486RE. (Right) deep seated fractures on the floor of Tycho from LROC image M114031031LE.

So as Jules says in an earlier Moon Zoo blog, please get out there, looking at the Moon close up and report your findings to the Forum. Just maybe you will be the first to identify a true TLP site, and if not then I am sure that your findings will interest the Moon Zoo geologists anyway!