According to research presented at the 2017 European Planetary Science Congress (EPSC) in Riga, lava tunnels or underground caves created by volcanic activity could provide habitats sufficient to house streets on Mars or even cities on the Moon.
Lava tunnels can form in two ways: when lava flows fairly close to the surface, forming a hard crust that thickens to create a roof over the moving lava flow. When the eruptions end, the pipeline is cleared, leaving a tunnel several meters below the surface. The second option is when lava flows into existing cracks between rock layers or cavities of previous flows. At this time, it expands and leaves a huge network of connected galleries, after which it is pushed to the surface, and the tunnels remain empty.
Lava tunnels are found in many volcanic regions on Earth, including Lanzarote, Hawaii, Iceland, North Queensland in Australia, Sicily, and the Galapagos Islands. Such underground networks can be up to 65 kilometers long. Space missions have also observed chains of tunnels on the Moon and Mars. Recently, NASA's GRAIL mission provided detailed data on the Moon, suggesting the presence of huge subsurface voids associated with lava tunnels, formed during volcanic eruptions early in the Moon's history.
Currently, researchers at the University of Padua and the University of Bologna in Italy have conducted the first systematic comparison of lava tunnels on Earth, the Moon and Mars based on high-resolution digital elevation models (DTMs) generated from spacecraft data.
A comparison of terrestrial, lunar, and Martian examples shows that, as you'd expect, gravity has a large effect on the size of the lava tubes. On Earth, they can be up to 30 meters in diameter. In the low gravity environment of Mars, lava tunnels can be up to 250 meters wide. On the Moon, these tunnels can be a kilometer or more and can be many hundreds of kilometers in length. This was announced by Dr. Riccardo Pozzobon from the University of Padua. According to him, such results have important implications for studying the possibilities of human life on the Moon, as well as for the search for extraterrestrial life on Mars. Lava tunnels are places protected from cosmic radiation and protected from the flow of micrometeorites, potentially providing safe habitats for future human missions. They may alsolarge enough for fairly large settlements - they can accommodate most of the historical center of Riga.
