Thank you Moon Zoo!
The MoonZoo science team would like to extend a gigantic thank you to all 20,627 users who contributed in counting craters (and more!) relating to the Apollo 17 landing site (Taurus-Littrow)!
Let’s ponder on some astonishing numbers: to date, around 8.5 million craters in total have been marked by MoonZoo citizen scientists, with around 670,000 (~8%) relating to the A17 region (from 21 selected NAC images, Figure 1); further, 3.3% (22,063) of these craters have been classified as containing boulders and 6.9% (45,893) were found to be non-circular.
Figure 1. On the left we see the MoonZoo users crater input. Different colours relate to different NAC images basemaps. On the right we see the A17 landing site (red dot) and the astronauts exploration paths and stations.
Our next step is to compare your input with the ‘expert’ count looking to validate and quantify your contributions. The ‘expert’ in question is a professional lunar scientist who has published research including the statistical occurrence of impact craters on planetary surfaces. The logical assumption is that given a more or less constant collision rate of interplanetary bodies (asteroids and comets), a surface will carry the record of impact products (craters and pits) as a function of time, i.e., from the time of resurface (maybe a lava mare flow) the scarring would be proportional to the length of exposure.
As most things in geology, this scenario is true but with caveats… : first, the resurfacing by lava flow or ejecta mantling might have only partially buried ancient craters, or, more probably, only the smaller ones, thus skewing the crater-size statistical record; crater rims erode with time, even on an airless body like the Moon, at a rate of around 0.06-1 cm per million year. This might not seem much, but in the lunar chronology scale, measured in billions of years, this factor becomes significant; in reality, the biggest source of uncertainty is represented by secondary craters: most impacts generate coherent distal ejecta that, when landed, produce smaller craters virtually indistinguishable from space-born ones. And this is fractal, i.e. scaled: big impacts will generate hundreds of smaller craters that will overlap with similar ones from nearby big impacts…
The hard reality is that there are no cast-iron methods to establish the origin of each excavation (although it has been advocated that a secondary crater might be somewhat shallower in comparison to a similarly-sized primary one). So, an ‘expert’ becomes so by developing a ‘sense’ or instinct on what ‘feels’ a statistically significant crater against one that is not. This approach is more akin to ‘artistic interpretation’ than ‘hard’ science, but qualitative investigation of certain geological features is an acceptable compromise when a physical method is either not yet available or even impossible to develop.
These considerations do not stop the development of alternative methodologies though; indeed, we are working closely with a research group at Manchester University which is building an automated pattern recognition software of circular features (and others) based on theoretical models, and actual data: ‘expert’ counts, AND MoonZoo users’ data.
Now, whatever approach brings us closer to a reliable crater counting method this cannot be easily accomplished by even a troupe of crater-counting planetary scientists: the 8.5 million craters noted by the MoonZoo community would have taken years to harvest otherwise!
So, what is going to happen now? Well, the ‘expert’ and pattern recognition software data will be compared with the MoonZoo output, uncertainties and limitations of all approaches established and, hopefully, develop a method that will represent the basis for ‘trusting blind’ the MoonZoo craters stats. In practice this will translate into something like “MoonZoo crater data are consistent with other methods for crater of sizes ‘x’ to ‘y’, in images with resolution higher than ‘z’ meters, and illumination of ‘n’ degrees or higher”.
Ultimately, the crater statistics (Cumulative Crater Frequency) plotted against known crater accumulation functions (i.e. Neukum, 1983, 2010) give us an estimate of the age of the lunar region. Using these data from landing sites allows for comparison with returned samples whose age has been established in the laboratory.
Figure 2. Age estimates based on estimated crater frequency distribution against crater size (diameter)
Our next journey will focus around the Apollo 12 landing site, in Mare Cognitum. The geology of this region is radically different from the Apollo 17 and it should serve as a perfect complement to our work so far. Elsewhere my colleagues will discuss and introduce the region in more detail, including ulterior scientific reasons behind the choice of this landing site.
We shall keep you informed of all further developments and new projects, and, once again, thanks for your patient and enthusiastic contribution to planetary science!
References:
Michael G.G., Neukum G., Planetary surface dating from crater size-frequency distribution measurements: Partial resurfacing events and statistical age uncertainty, Earth and Planetary Science Letters, 2010, DOI: 10.1016/j.epsl.2009.12.041.
Neukum G., Meteoritenbombardement und Datierung planetarer Oberfl�chen. Habilitation Dissertation for Faculty Membership, Univ. of Munich, 186pp, 1983.
Flying over Taurus-Littrow
The LRO took many images of the Apollo 17 landing site at Taurus-Littrow. Here is a glorious oblique “spaceship-eye” view of the Sculptured Hills and massifs surrounding the landing site taken from from M1096343661R and L. The position of the Lunar Module is marked on the second image.
Two NAC images have been stitched together and the aspect ratio tweaked to around 4:1. (click image for larger version.)
December 19: Splashdown
After 13 days in space Eugene Cernan, Ronald Evans, and Harrison (Jack) Schmitt aboard the Apollo 17 command module Challenger parachuted to a safe splashdown at 19:20 GMT on 19 December 1972, 648 km southeast of American Samoa. The last humans to have walked on the Moon.
 Challenger makes a perfect splashdown |
 The crew arrive by helicopter aboard the rescue ship Ticonderoga |
images NASA
There is still much to learn from the Apollo 17 mission. Moon Zoo needs your help to explore the Apollo 17 landing site. Celebrate the anniversary with us. Go to http://www.moonzoo.org/ and start clicking! Follow “live” mission tweets from @moonzoo
December 18: 10 more facts about Apollo 17
1. Jack Schmitt discovered some unusual orange coloured “soil” later found to contain volcanic glass.
http://www.lpi.usra.edu/captem/slide_1.html
2. The Apollo 17 mission patch.
http://airandspace.si.edu/collections/imagery/apollo/PATCHES/Apollo17patch.jpg
3. Apollo 17 was the eleventh and final mission to carry astronauts in the Apollo space program.
4. The command module was named America and the the ascent stage of the lunar module was Challenger.
The descent stage was left on the Moon at coordinates 20.19080°N 30.77168°E.
LROC Video of the site here.
5. Lunar rovers were used on Apollo missions 15, 16. and 17.
Apollo 17’s Lunar rover Apollo Lunar Surface Journal
6. Apollo 17 was the only Apollo mission to carry the Traverse Gravimeter Experiment (TGE.) The TGE made measurements of the lunar gravity and its variation over time. It also investigated tidal distortions of the shape of the Moon.
7. Jack Schmitt turned to from space exploration to politics and in January 1977 he began a six-year term as one of New Mexico’s Senators in Washington. His was on the Commerce, Science and Transportation Committee; the Banking, Housing and Urban Affairs Committee and the Select Committee on Ethics.
8. The name of the Apollo 17 recovery ship was USS Ticonderoga.
9. There were no seats in the Lunar Module.
10. The Apollo programme defined 10 mission types from A (unmanned test flights) to J (extended lunar scientific missions). Apollo 17 was a J-type mission.
December 17: Eugene Cernan’s regret
Hindsight is a wonderful thing. Just when space travel seemed almost normal, Gene Cernan left his Hasselblad camera on the Moon as an experiment in how solar radiation would affect the lens. He assumed that another mission would be able to retrieve and study it firmly believing that the Apollo programme was just the beginning rather than the end of sending humanity to the Moon.
He said: “I left my Hasselblad camera there with the lens pointing up at the zenith, the idea being someday someone would come back and find out how much deterioration solar cosmic radiation had on the glass. So, going up the ladder, I never took a photo of my last footstep. How dumb! Wouldn’t it have been better to take the camera with me, get the shot, take the film pack off and then (for weight restrictions) throw the camera away?”
A Hasselblad camera like the one Gene Cernan left behind
NASA via Daily Mail
December 16: Moon Rover
Lunar rovers (or Moon buggies) were used on the last 3 Apollo missions. A rover allowed astronauts to explore further and carry more equipment. It had a 90 inch wheelbase and a top speed of 22 kph.
 Apollo Lunar Surface Journal |
There is still much to learn from the Apollo 17 mission. Moon Zoo needs your help to explore the Apollo 17 landing site. Celebrate the anniversary with us. Go to http://www.moonzoo.org/ and start clicking! Follow “live” mission tweets from @moonzoo
December 15: Measuring the regolith thickness at the Apollo 17 site
By Ian Crawford
(Department of Earth and Planetary Sciences, Birkbeck College)
Estimating the thickness of the unconsolidated lunar regolith is one of the major scientific objectives of Moon Zoo. This is because understanding the thickness of the regolith in different regions of the Moon will address a number of important scientific questions. For example, as regolith thickness increases with time, measuring the regolith thickness in areas which have not been dated by returned samples will help provide additional surface age estimates. Conversely, measuring the regolith thickness on surfaces with well-determined ages (such as the Apollo landing sites) will help us determine the regolith accumulation rate. Improved global regolith thickness maps will also provide important information for future exploration of the Moon, including the quest to identify future lunar resources.
There are three ways in which studies of small craters can be used to estimate regolith thickness. The first is to determine the minimum size of craters which have excavated blocks of bedrock (i.e. boulders) from below the regolith layer (Fig. 1). If the crater dimensions are known, then an estimate of a maximum depth of excavation can be estimated as about one-tenth of the diameter.
Figure 1. LROC image of a boulder-covered bench crater. The crater has formed in a basaltic regolith close to the Apollo 12 landing site. The impact has punched through the thin regolith cover and into the harder rock, excavating large blocks that have covered the surrounding surface. This example is 130m in diameter, so the regolith here must be less than about 13m deep. By determining the maximum size of craters in this area which have not excavated boulders the actual depth of the local regolith can be determined. (LROC image M114104917L/ASU/NASA).
The second method relies on identifying flat floors or benches within a crater, which also indicates that a crater has penetrated an overlying regolith layer to a stronger layer beneath. Figure 1 again provides an example. For features like this a simple expression has been derived which estimates the regolith thickness from the ratio of the bench diameter to the overall crater diameter. For the example shown in Figure 1 this indicates a regolith depth of about 6 m, consistent with the upper-limit of 13m estimated from the presence of boulders around the rim.
The third method is more subtle, and exploits the process of impact gardening, whereby rocky surfaces are disaggregated and overturned by meteorite impacts, thus destroying the record of previous impact cratering events. The equilibrium diameter is identified when the cumulative number of craters seen on the surface is less than the number actually produced, and can be recognized as a change in slope in a graph which plots number of craters in a given area as a function of their size. Because the number of craters buried under new regolith depends on the regolith thickness, measuring the equilibrium diameter gives a guide to the latter.
In order to test these different methods it is necessary to apply them to areas where the regolith thickness has been directly measured. However, this can only be done at the small number of Apollo landing sites where seismic measurements of regolith thickness were conducted. By far the best estimates have been provided by the Apollo 17 Lunar Seismic Profiling Experiment (LSPE). For this experiment the astronauts deployed eight small explosive packages during their traverses around the Taurus-Littrow Valley (Fig. 2) which, when detonated, provided seismic signals for detectors setup close to the Lunar Module.
Figure. 2. One of eight explosive packages deployed by the Apollo 17 astronauts to provide data for the lunar seismic profiling experiment which measured the thickness of regolith in the Taurus-Littrow Valley. The Apollo 17 LRV is in the foreground and the lunar module, where a geophone detector array was deployed to collect the signals, in the middle distance about 300 m away (NASA)
By measuring the time taken for the seismic signals to travel from the explosive packages to the detector, geophysicists were able to determine the thickness of both the regolith layer and the underlying lava flows at the Apollo 17 landing site. The results are shown in Fig. 3.
Figure. 3. Subsurface structure under the Taurus-Littrow Valley, as determined by the Apollo 17 seismic profiling experiment. The numbers indicate seismic wave speed in meters per second. Yellow represents the lunar crust, which outcrops locally as the South Massif (“LM impact” schematically indicates where the Apollo 17 Lunar Module ascent stage was crashed into the South Massif to provide an additional seismic data point). The green layers indicate the thickness of basaltic lava that has flooded the valley to a depth of about 1.4 km. The thick black line shows the regolith layers (inset). (Image adapted from a paper by M.R. Cooper et al., published in Reviews of Geophysics and Space Physics, Vol. 12, pp. 291 – 308, 1974).
Five separate layers were identified below the surface of the Taurus-Littrow valley:
(i) The topmost layer, 4 m deep with the very low seismic wave speed of 100 m/s, is interpreted as being due to the local regolith.
(ii) Beneath the regolith is a layer with a velocity of 327 m/s, which is still too low for solid rock. It may be due to more consolidated regolith, or possible highly fractured lava.
(iii) At a depth of 32 m the velocity rises to 495 m/s, and this is interpreted to be the fractured and/or vesicular top of the lava flow filling the valley.
(iv) At a depth of 390 m the velocity rises to 960 m/s. This is interpreted as being due to a more coherent basalt unit.
(v) Finally, at a depth of 1.4 km the velocity rises sharply to 4.7 km/s, and this is interpreted as being due to crustal bedrock underlying the lava layers.
The deeper layers are too deep to be probed by craters found in the MoonZoo images, although the presence of a lava layer at a depth of about 30m is consistent with the excavation of basaltic blocks from 300-400 m diameter craters in the valley floor. Where MoonZoo can really help is to confirm that the seismic boundary at a depth of 4m (which will be probed by craters about 40 m across), and to determine whether the underlying layer is more consistent with fractured basalt or compact regolith.
In order to address these issues, we need MoonZoo users to look carefully at craters in the images of the Apollo 17 area, determine their sizes accurately, and note the presence of boulders around the rims and/or interior benches or flat floors. Don’t worry that scales are not provided on the MoonZoo images (this is deliberate to avoid the possibility of biasing the results), but users may be sure that the sizes and morphologies of all thecraters in these images are relevant to the task in hand.
Ian Crawford is based in the Department of Earth and Planetary Sciences, Birkbeck College, London, and is a member of the MoonZoo science team. This blog article is based on a longer article published in the December 2012 issue of the Royal Astronomical Society journal Astronomy and Geophysics.
December 13: It’s Orange!
Coloured soil on the Moon is unusual. Apollo 15 found green glass deposits but mostly the Moon is a variety of greys and browns. Jack Schmitt, a geologist, was, therefore, understandably elated to discover something very different – and orange. This is how Schmitt told Cernan what he had found:
SCHMITT There is orange soil!
CERNAN Well, don’t move it ’till I see it.
SCHMITT It’s all over, orange!
CERNAN Don’t move it ’till I see it.
SCHMITT I stirred it up with my feet.
CERNAN Hey it is, I can see it from here.
SCHMITT It’ s orange!
CERNAN Wait a minute, let me put my visor up, it’s
still orange!
SCHMITT Sure is. Crazy! Orange!
(from the Apollo Mission Transcripts)
Listen to and watch the discovery here.
The patch of soil was the result of fire fountains of volcanic lava which cool before falling back to the surface as tiny orange glass beads.
There is still much to learn from the Apollo 17 mission. Moon Zoo needs your help to explore the Apollo 17 landing site. Celebrate the anniversary with us. Go to http://www.moonzoo.org/ and start clicking! Follow “live” mission tweets from @moonzoo
December 12: I was strolling on the Moon one day…
The song “The Fountain in the Park” as featured in Tom and Jerry, Bugs Bunny and Mickey Mouse cartoons and sung by Judy Garland in “Strike up the Band” was given the Schmitt and Cernan treatment while setting up experiments during an EVA. Watch the astronauts enjoying themselves in this video. (placed on You Tube by Jordaxe12.)
Transcript from the mission transcripts:
SCHMITT (Singing) I was strolling on the Moon one day, in the merry merry month of December-
CERNAN May – May’s the month.
SCHMITT – May – that’ s right.
CERNAN May is the (garble) month.
SCHMITT – when much to my surprise, a pair of funny eyes (singing)
CAPCOM (Capsule Communicator back in mission control) Sorry about that, guys, but today may be December.
December 11: Moon landing at Taurus-Littrow
Challenger landed on the lunar surface at 19:48 GMT on 11 December 1972.
Taurus-Littrow valley
Apollo Lunar Surface Journal
The view from Challenger’s window (click to enlarge)
Apollo Lunar Surface Journal
The Taurus–Littrow valley was chosen because of its geologically varied terrain. The crew were hoping to collect different soils, relatively young lavas which filled the valley floor and older crustal material from the North and South Massifs. Several large boulders had rolled down the massifs providing additional sampling opportunities.
Moon Zoo 