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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