Edward Williams

University of Maryland

Locating And Characterizing Lava Tubes Using Surface Observations

Lava tubes are long sinuous caves that form inside lava flows. Several lines of evidence indicate that they also exist on other planetary bodies, especially the Moon and Mars, where they present numerous scientific and exploration opportunities. Therefore, it is important to develop methods to locate and characterize the shape and size of lava tubes in advance of in situ exploration by astronaut or robotic crews. I describe here two kinds of surface features, parallel fracture lines and inflation ridges, that can be observed at the surface above a lava tube and through a combination of modeling and fieldwork on terrestrial analogs link their characteristics to those of the underlying lava tube. 

The roofs of lava tubes bend slightly downward into the cave under their own weight. If of sufficient amplitude, this bending leads to fracturing both inside the tube and at the surface above it. My simulations show that no more than 500 m of the floor of a large lunar lava tube should be devoid of cracks or debris falling from the roof and, therefore, may be usable by exploration teams with minimal preparation or precautions. At the lunar surface above the tubes, linear cracks are expected to develop on either side of the tube axis at distances proportional to tube width. I identified possible examples of this kind of crack surrounding three lunar rilles: Rima Sharp, Rima Mairan (Fig. 1a), and possibly Rima Marius. This discovery implies that these rilles are associated with tubes with a width of ~ 1 km, smaller than the ones found from gravity inversion.

Inflation ridges develop when the interior of a lava flow, which may become a tube, becomes pressurized while the flow is active. I developed theoretical models that link the tube's size, shape, and the thickness and elastic strength of its roof to the height of an inflation ridge. This model shows that the ridge above Valentine Cave, in Lava Beds National Monument (CA), is consistent with an inflation ridge that formed within a week of flow emplacement (Fig. 1c). A simple analytical model based on the flexure of a uniform plate links ridge height to an apparent roof thickness, which can be linked to the actual height, roof thickness, and width of a half-elliptical tube. This relationship makes it possible to put limits on tube shape and depth using the height of an inflation ridge. These estimates can be refined if additional constraints are available, such as roof thickness deduced from oblique imagery of collapse pits, or tube width from parallel surface cracks. 

Figure 1: Summary of observable surface features above lava tubes. a.) Apparent linear surface cracks at the end of Rima Mairan, suggesting the tube extends into a rille. b.) Schematic diagram illustrating the development of surface cracking (left) and an inflation ridge (right). c.) Range of possible inflation conditions for two terrestrial cases, constrained using observed inflation ridge heights.