Free Astronomy Magazine May-June 2018

8 MAY-JUNE 2018 PLANETOLOGY Among the various hypotheses advanced to explain the origin of those channels, called “rilles,” one proposed that these channels were originally petrified lava tubes that had their roofs collapse. Examples of rilles are also present in Earth’s volcanic areas (Hawaii, Iceland, Australian North Queensland, Galapagos Islands, Lan- zarote and Sicily) and the mechanism through which they formed is rather sim- ple: a stream of lava emerged onto the sur- face and, following the surface features of the territory on which it flowed, this lava channeled along a slight slope. Due to the lower temperature of the external environ- ment, the outer surface of the flow cooled and solidified, creating a real lava rock pipe. When the eruption ceased, the re- maining lava in the tube exited, leaving the hollow, cooler shell behind. At this point, many external factors can act on the structure. If the tube roof is not thick S kylights found on the Moon, some of which may be the entrances to lunar caves. De- tailed exploration is required to verify cave presence. [LROC images, ASU] T he city of Philadelphia is shown inside a theoretical lunar lava tube. A Purdue Uni- versity team of researchers explored whether lava tubes more than 1 kilometer wide could remain structurally stable on the Moon. [Purdue University/courtesy of David Blair]