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.

 

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The Scientific Legacy of Apollo

By  Ian Crawford
(Department of Earth and Planetary Sciences,
Birkbeck College, London)

Fig. 1. One of the last two men on the Moon: Harrison Schmitt stands next to a large boulder at the Apollo 17 landing site in December 1972. (NASA).

This December marks 40 years since the last human beings to set foot on the Moon, Gene Cernan and Harrison “Jack” Schmitt of Apollo 17, left the lunar surface and returned safely to Earth. In the three and a half years between Neil Armstrong’s ‘first small step’ in July 1969 and the departure of Cernan and Schmitt from the Taurus-Littrow Valley in December 1972, a total of twelve astronauts explored the lunar surface in the immediate vicinity of six Apollo landing sites.

Fig 2. The Apollo landing sites. Note their restriction to the central part of the nearside – there is a lot more of the Moon to explore! (Image: NASA).

The total cumulative time spent on the lunar surface was 12.5 days, with just 3.4 days spent performing extravehicular activities (EVAs) outside the lunar modules. Yet during this all-too-brief a time samples were collected, measurements made, and instruments deployed which have revolutionised lunar and planetary science and which continue to have a major scientific impact today.

Fig. 3. A view across the Apollo 17 landing site in the Taurus-Littrow Valley. The Apollo 17 Lunar Roving Vehicle is in the foreground, and the Lunar Module is in the middle distance about 300 m away. The black box in the foreground is one of eight explosive packages deployed to provide data for the lunar seismic profiling experiment which measured the thickness of regolith and the underlying lava in the Taurus-Littrow Valley (NASA).

In their cumulative 12.5 days on the lunar surface, the twelve Apollo moonwalkers traversed a total distance of 95.5 km from their landing sites (heavily weighted to the last three missions that were equipped with the Lunar Roving Vehicle), collected and returned to Earth 382 kg of rock and soil samples, drilled three geological sample cores to depths greater than 2 m, obtained over 6000 surface images, and deployed over 2100 kg of scientific equipment.

Fig 4. Jim Irwin next to the Apollo 15 LRV with the 4.6 km high Mt Hadley in the background; note the sample bags attached to the rear of the LRV (NASA).

These surface experiments were supplemented by wide-ranging remote-sensing observations conducted from the orbiting Command/Service Modules.

Fig. 5.The Scientific Instrumentation Module (SIM) bay of the Apollo 15 Command/Service Module (CSM). On Apollo 15 the SIM included mapping cameras, a laser altimeter, and ultraviolet, X-ray and gamma-ray spectrometers (NASA).

 Probably the greatest scientific legacy of Apollo has resulted from analysis of the 382 kg of rock and soil samples returned to Earth. One of the key results has been the calibration of the lunar cratering rate. Only by comparing the density of impact craters on surfaces whose ages have been obtained independently by laboratory analyses of returned samples is it possible to determine the rate at which meteorite impacts have created craters on a planetary surface. Analysis of the Apollo samples (supplemented by those obtained by the Soviet Union’s three Luna robotic sample missions) has enabled this to be done for the Moon, which remains the only planetary body for which such a data-set exists. Not only has this facilitated the dating of lunar surfaces from which samples have yet to be obtained, but it is used, with various assumptions, to estimate the ages of cratered surfaces throughout the Solar System from Mercury to the moons of the outer planets.

Another important result of Apollo sample analysis by seo services uk has been the evidence provided for the origin of the Moon. In particular, the discovery that lunar materials have compositions broadly similar to those of Earth’s mantle, but that the Moon is highly depleted in volatiles compared to the Earth and has only a small iron core, led to the current view that the Moon formed from debris resulting from a giant impact of a Mars-sized planetesimal with the early Earth. It is very doubtful that we would have sufficient geochemical evidence usefully to constrain theories of lunar origins without the quantity and diversity of samples provided by Apollo, and indeed these samples are still being actively exploited for this purpose.

Fig. 6. The current theory of the Moon’s formation from debris produced by a giant impact on the early Earth is largely based on the geochemical analysis of samples collected by the Apollo missions (image: Wikipedia Commons).

Beyond this, the Apollo samples have been vital to our understanding of the Moon’s own geological history and evolution. While lunar geology may at first sight appear to be a relatively parochial area of planetary science, it is important to realise that the Moon’s surface and interior retain records of planetary processes which will have occurred in the early histories of all the terrestrial planets, such as the formation of cores and crusts. In all these respects the Moon acts as a keystone for understanding the geological evolution of all the rocky planets.

Fig. 7. Fragments of Apollo 12 soil sample 12023 at the Lunar Sample Laboratory at the Johnson Space Center, being selected for a study of lunar volcanism in 2009. Forty years after they were collected, Apollo samples like these are still being used for scientific investigations (photo: I.A. Crawford)

In addition, Apollo samples of the lunar regolith have demonstrated the importance of the lunar surface layers as an archive of material which has impacted the Moon throughout its history. These include records of solar wind and cosmic ray particles, and meteoritic fragments. Extracting meteoritic records from lunar regolith samples is especially important for planetary science as it potentially provides a means of determining how the flux and composition of asteroidal material in the inner Solar System has evolved with time.

Last, but not least, the Apollo samples have been used to calibrate remote sensing investigations of the lunar surface. The visible, infrared, X-ray and gamma-ray spectral mapping instruments carried by a host of recent orbital missions to the Moon have produced a wealth of information regarding the chemical and mineralogical nature of the lunar surface. Although these orbital missions post-date Apollo, the reliability of their results largely depends on their calibration against known compositions at the Apollo landing sites. Without the ‘ground truth’ provided by the Apollo samples, it would be difficult to have as much confidence in the results of these remote sensing measurements as we do.

 In addition to study of the Apollo samples, many other areas of scientific investigation were also performed by the Apollo missions, especially geophysical investigations of the Moon’s interior. Key results included the discovery of natural moonquakes and using them to probe the structure of the crust and mantle, geophysical constraints on the existence and physical state of the lunar core, and measurements of the flow of heat from the Moon’s interior. Although these data are over thirty years old, advances in interpretation means that they continue to give new insights into the interior structure of the Moon. For example, only last year an apparently definitive seismic detection of the Moon’s core, and strong evidence that, like the Earth’s, it consists of solid inner and liquid outer layers, was made by a re-examination of Apollo seismic data.

Fig. 8. Apollo 14 seismometer deployed on the lunar surface; the silvery skirt provided thermal stability. These instruments, also deployed at the Apollo 12, 15 and 16 landing sites, constituted the Apollo passive seismic network which remained active until 1978 and yielded valuable data about the interior of the Moon (NASA).

Looking over the totality of the Apollo legacy, I think one could reasonably make the case that Apollo laid the foundations for modern planetary science, certainly as it relates to the origin and evolution of the terrestrial planets. Arguably, the calibration of the lunar cratering rate, and its subsequent extrapolation to estimating surface ages throughout the Solar System, could alone justify this assertion. If one also considers the improvements to our knowledge of lunar origins and evolution, and the records of solar wind, cosmic rays and meteoritic debris extracted from lunar soils, it is clear that our knowledge of the Solar System would be greatly impoverished had the Apollo missions not taken place.

 However, it is also clear that Apollo did little more than scratch the surface, both literally and figuratively, of the lunar geological record. With only six landing sites, all at low latitudes on the nearside, it is clear that much remains to be explored. Therefore, as we pass the 40th anniversary of the last human expedition to the Moon, there are good scientific reasons to start planning for a return.

Fig. 9. Artist’s concept of astronauts supervising a drill on the Moon. Returning humans to the lunar surface later in the 21st Century would facilitate larger scale exploration activities than was possible with Apollo, and will further increase our knowledge of lunar and Solar System evolution (artwork: NASA).

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.