Ridges lava tubes and rilles

Arcuate ridges, also called wrinkle ridges, are found parallel and interior to the margins of many of the mare basins. They are irregular and extend brokenly for great distances. Their origin is uncertain, but they may represent compression features that result from the differential compaction of layers of rock over subsurface ring systems. They might also represent doming of the crust over shallow intrusions, or

9 Flood basalts are seen in a few locations on Earth, such as the Columbia River and Snake River plateaus of the western United States, and the Deccan basalts of India.

10 Viscosity: a property of fluids that causes them to resist flowing as a result of internal friction from the fluid's molecules moving against each other.

11 Mantle: the zone of a planet below the crust and above the core.

Figure 1.10. Lunar Orbiter 3 oblique view of Murchison Crater (foreground), 58 km across and Ukert Crater (back right-center) on the Moon. The southern edge of Mare Vaporum is visible in the background. Note the irregular features and rilles on the floor of Murchison. North is at 10 : 30 (Lunar Orbiter 3, frame M-85).

Figure 1.10. Lunar Orbiter 3 oblique view of Murchison Crater (foreground), 58 km across and Ukert Crater (back right-center) on the Moon. The southern edge of Mare Vaporum is visible in the background. Note the irregular features and rilles on the floor of Murchison. North is at 10 : 30 (Lunar Orbiter 3, frame M-85).

accumulation of extrusive lava flows along a linear volcanic fissure system. Ground-penetrating radar data of the southern Mare Serenitatis Basin suggests that differential compaction is the most likely origin for that particular ridge.

Lava tubes are among the most interesting features of the Moon, especially from the standpoint of human habitation. Lava tubes are natural caverns that are the drained conduits of underground lava rivers. Their existence is inferred from the inspection of sinuous rilles that appear to be discontinuous on the surface. Parts of these rilles may be "roofed over'' to create a structure that is similar to the lava tubes found on Earth.

The inside dimensions of lava tubes may be tens to hundreds of meters, and their roofs are expected to be greater than 10 meters thick (Horz, 1985), thus offering an environment that is naturally protected from the hazards of radiation, meteorite impact, and temperature extremes. Figure 1.11 shows a lava tube with segments of collapsed roof. According to Oberbeck et al. (1969), basalt "bridges" spanning a few hundred meters are possible on the Moon provided they are at least 40 to 60 meters thick. This is in good agreement with the fact that uncollapsed segments of lava tubes

Figure 1.11. A lava tube with segments of collapsed roof (Lunar Orbiter 5, frame 182).

display impact craters a few tens of meters across, occasionally as large as 100 m. These diameters correspond to crater depths of up to 20 meters (Gehrig, 1970). Roof thickness would have to be twice that value (40 meters) in order to withstand the impact (Horz, 1985).

The upper five to ten meters of any lava roof has probably been eroded by micrometeorite impact into a fine-grained lunar soil. Cracks associated with this regolith are probably a factor of three to five deeper (Pohl et al., 1977). It is difficult to predict the structural integrity and exact thickness of a lava tube roof with great precision (Horz, 1985). However, an obvious strategy would be to select roofs or roof segments that have only suffered relatively small cratering events. Additionally, future robotic rovers could use ground-penetrating radar to determine the structural integrity of the roof section.

Figure 1.12. View of Hadley Rille from the surface.

Horz (1985) concluded that natural caverns of suitable size to house an entire lunar base could exist on the Moon. Roof thickness in excess of ten meters would provide safe, long-term shelters as receptacles for modular habitats. It would be very unlikely that the lava tube itself could be modified to create a habitat, however. Site preparation inside the lava tube would consist of leveling the floor with lunar soil.

Horz (1985) also pointed out that the strongest advantage to using lava tubes is that the sheltered environment allows the use of extremely lightweight construction materials (inflatable habitats, etc.). None of the components would require shielding. Use of thin foil materials would be possible not only for the habitat modules, but also for ducts, storage tanks, and other structures. Not having to shield these mechanical systems from meteorite impact would be a great advantage when it becomes necessary to inspect, repair, or replace them.

Another strong advantage to housing lunar bases inside lava tubes is the relatively constant temperature (—20°C), due to the fact that one is underground and some tens of meters removed from the lunar surface (Horz, 1985). This would be much easier to work with than the constantly fluctuating temperatures (—180° to + 100°C) on the surface, where complex thermal insulation and control systems are unavoidable. In contrast, the relatively constant, benign temperature within a lava tube, coupled with the freedom from ultraviolet and infrared radiation, would allow the use of common materials such as plastics.

A drawback to using lava tubes is that steep slopes may limit their accessibility. However, relatively shallow rilles do exist. When the Apollo 15 crew visited the edge of Hadley Rille (Figures 1.12 and 1.13), they thought that their Lunar Rover could have negotiated its slopes (Horz, 1985). Horz also notes, "Location of a lunar base [in a lava tube may not seem] very economical from an energy point of view, because the mass will have to be lowered and especially raised when needed on the lunar surface and when being readied for export to LEO or GEO.

These energy considerations are, however, a matter of degree, because most large-scale industrial operations rely heavily on gravity for material transport. Some modest elevation difference between the source of raw materials and the processing plant is desirable even for such simple operations as sieving and magnetic separation. For this reason, a lunar base may be more functional if located at the base of some slope such as a sinuous rille/lava tube. Chutes or pipes may be laid out such that they terminate inside the lava tube at exactly that station where the high-graded raw materials are needed.

A great deal of work remains to be done to determine the feasibility of using lava tubes for lunar habitats. Detailed study of data from the Clementine and Lunar Prospector missions is needed to improve our knowledge of the inventory of lunar lava tubes and to determine their spatial distribution. These studies will also give us improved information on roof thickness and dimensions. The in-situ examination of lava tubes by tele-operated robots will be among the most interesting missions of the next stages of lunar exploration.

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