Carolyn van der Bogert: Let me answer the question this way: The possibility of using lava tubes as shelters was first proposed, to my knowledge, in 1985 by Fred Hörz in a NASA report about lunar bases. Besides outlining the advantages to using a natural shelter as a base, Hörz argues that there are probably a lot of lava tubes on the Moon, because they are thought to be related to sinuous rilles...."
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Sinuous rilles are thought to form in two ways: (1) an open lava channel that thermally erodes into the lunar surface, or (2) subsurface lava tubes that eventually collapse. There were images of collapsed lava tubes that were discussed in the mid to late 1970's as evidence for volcanic activity and sinuous rille formation on the Moon.
In 1992, Cassandra Coombs published work that was part of her PhD thesis with B Ray Hawke that involved a survey of lunar sinuous rilles and other volcanic features to locate intact lava tubes.
They identified 4 rilles that had really strong evidence for intact lava tube segments (including the Marius Hills). They rated 67 probable tube segments for their suitability as lunar bases. We've had to wait until now -- with the help of JAXA's Kaguya and NASA's Lunar Reconnaissance Orbiter (LRO) -- to have higher resolution images than the data Coombs and Hawke used.
Haruyama has long been interested in lava tubes -- even before the launch of the mission -- and his team has been looking at Kaguya data for the last two years.
Ian: The SELENE/Kaguya mission has discovered the first skylight. Will you and your JAXA colleagues be continuing to search for more skylights? How many more do you expect to ..."
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