NLSI Lunar Science Conference (2008)
LUNAR SURFACE PROPERTIES: WHAT DO WE KNOW, WHAT DON’T WE KNOW, AND WHAT DO WE NEED TO KNOW? J. B. Plescia, Applied Physics Laboratory, Johns Hopkins University, 11100 John Hopkins Road, Laurel, MD firstname.lastname@example.org Introduction: The surface of the Moon is covered with a layer of fragmental debris referred to as the regolith consisting of a range of particle sizes from micron to tens of meters with superposed impact craters. It is this surface layer with which landers, rovers, and outpost operations will interact. Thus, its characteristics are critical to understand. Of the relevant properties of the surface, there are site-dependent and siteindependent properties. It is the site-dependent properties that will control the location of the outpost. Rocks and craters of various sizes that occur on the surface are relevant to issue of hazard avoidance, safe landing and surface operations. A detailed understanding of geomechanical properties is relevant to excavation and transport activities. Based on data from Ranger, Surveyor, Lunar Orbiter, Apollo, Luna and Lunokhod a basic understanding of lunar surface processes exists that is sufficient to understand the range of characteristics that would be encountered, regardless of the outpost location (e.g., equatorial or polar). It is true, that since an outpost site has yet to be selected, a explicit understanding of that site is not yet possible. Regolith Properties: Unlike terrestrial soils, the lunar regolith is formed primarily by the process of impact-driven mechanical degradation with additional modification due to space weathering. Chemical and mineralogic alterations also occur due to the micrometeorite impact events. While a general understanding of the some of the geotechnical properties were obtained from Apollo and robotic missions, a great deal remains uncertain because direct geotechnical experiments were limited and analysis of returned material can not provide appropriate in situ values. Direct measurements of the regolith properties include only the Soviet Lunokhods and the Apollo 15 and 16 missions. Lunokhod carried a cone penetrometer and shear vane. Apollo 15 and 16 carried a penetrometer that used by the astronauts. These experiments, supplemented by analysis of boot prints, trenching and drilling, as well as the active and passive seismic experiments, provide the basic in situ data. Analysis of returned regolith samples provides information on the particle size-frequency distribution as well as their chemistry and mineralogy. A key problem however is that the bulk in situ regolith has a significantly higher density than can be achieved by recompacting returned samples, and the returned regolith cores lack particles larger than ~2-3 cm. Thus, Earthbased experiments conducted on the regolith provide only bounds on the range of in situ properties. However, it is the bulk in situ properties that are relevant to operations such as mining, site preparation and ISRU. Landing Hazards: The distribution of rocks and craters are the surface properties that are most relevant to landing safety. These are the features that will produce significant local topography and slopes which may be beyond the capabilities of the lander. In the case of the Apollo LMs, all of the vehicles were tilted due to the presence of crater or ejecta (Table I). Table I. LM Landing Tilts A11 4.5° A12 4° A14 8° A15 11° A16 2.5° A17 4-5° Slopes: At scales of kms to tens of km the highlands are much rougher than the mare because of the presence of numerous large-diameter craters. As the highlands are older, they are more cratered and thus rougher compared with mare. As the base-length decreases the roughness of the highlands and the mare converge because of the presence of small-diameter craters whose frequency is similar on both surfaces.
Figure 1. Cumulative size-frequency distribution for impact craters at various lunar locations plotted as a function of the number of craters >D per 106 km2. The dashed lines are suggested production functions; the red line is the steady state or saturation levels.
NLSI Lunar Science Conference (2008)
Crater Distribution: Craters in the size range of tens of cm to a few meters are the class relevant to landing as these are the craters that control short-scale topography critical to landing safety. At these diameters the lunar surface is in equilibrium – for each new crater formed, one is destroyed and thus the frequency of impact craters does not increase with time once this level is reached. Figure 1 illustrates the crater sizefrequency distribution. The relevant aspect of an impact crater is its depth as this will control the tilt of a vehicle which places one landing pad in an impact crater. For a fresh crater, the depth/diameter ratio is about 0.23 to 0.25, whereas mature (degraded) craters, which are more common, have depth/diameter ratios of 0.11-0.125. It is the fresh craters that are deepest and thus of the most concern. The frequency of craters in the 10 to 1000 cm range per 100 and 1 m2 are listed in Table II. Table II. Crater Frequencies Diameter Areal Frequency (cm) m2 100 m-2 >10 1000 10 >100 10 0.1 >1000 0.1 0.001 Rock Distribution: The distribution of rocks on the lunar surface is largely controlled by the proximity to fresh impact craters and the regolith thickness (which is a function of the age of the unit). For relatively young units such as the mare, the regolith is only a few meters thick; for the highlands, the regolith may be up to 15-20 m. Thus, on the mare a much smaller impact crater will excavate bedrock and distribute rock fragments compared to the highlands. Figure 2 illustrates examples of the size-frequency distributions of surface rocks at the Apollo 16 site. Table III lists the frequency of rocks >30 cm at several sites. The highest rock abundance occur with the ejecta of a fresh crater. At the Apollo 16 site, a highlands site, the most common fragments are 2-40 cm, rocks 25 cm cover about 25-90% of the surface. The frequency of large rocks (>15 cm) varies from <1% on North Ray ejecta to 16% near WC crater. It should be noted that these distributions are somewhat biased by the fact that the crew did not take panoramas in highly rocky areas, and thus do not reflect the most rocky locations. In addition, data were compiled out to distances of only 10 m. Thus, the largest rocks (e.g., boulders meters to tens of meters) are not included.
Figure 2. Cumulative size frequency distribution for rock fragments at the Apollo 16 site, stations 4, 5, and 6. Table III. Rock Frequencies (> 30 cm) Site 100 m-2 Surveyor I-III (Lunar Orbiter) 2-10 Surveyor (Surface imaging) 1-30 Apollo 16 3-11 Apollo 12 3 Discussion: Understanding the basic geology of a site, whether it occurs on highlands or mare, the distribution of fresh (i.e., rayed) craters, and the maturity of the regolith will allow an assessment of the likely frequency of rocks and craters which are of most concern. The distribution of fresh craters is the primary morphologic feature. The properties of the bulk regolith (including the frequency and distribution of subsurface rocks) are not well characterized and thus should be investigated prior to active mining and site modification. Regolith in areas of permanent shadow which contain significant volumes of ice, should they occur, would have bulk properties very different from that of illuminated regolith. Conclusions: Sufficient data exists on the properties of the lunar surface to allow for spacecraft design and most operational planning. The data necessary to facilitate extensive (deep) regolith mining, such as regolith thickness variability, size-frequency distribution of rocks and boulders in the regolith, and the properties of regolith with included frozen volatiles remain unknown.