A lava tube with a view: The 60-meter wide "skylight" as seen by the Kaguya satellite.DCL

One of the key questions when we eventually return to the Moon will be: where are we going to live? Perhaps one solution has just presented itself in the form of a hole in the lunar surface, possibly leading to a hollow lava tube. This natural formation is known as a "skylight" and up until now little was known about where they are located and how many there are.

A Japanese team headed by Junichi Haruyama has analyzed data from the JAXA Kaguya mission to find these holes in the ground, and after two years of searching, they've found one in the Marius Hills region of the Earth-facing side of the Moon. The skylight is approximately 60 meters wide and it provides a tantalizing glimpse into what could be a future target for manned lunar exploration.

Carolyn van der Bogert, a co-investigator from University of Münster in Germany, has kindly taken the time to answer a few questions from Discovery News about this fascinating skylight and what it could mean to the future of lunar exploration…

Ian O'Neill: How long have lunar missions been searching for lava tube "skylights"?

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.

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 find?

Carolyn: The search for this first skylight covered one of the most likely regions where they might be found: Marius Hills. A 500 square kilometer (200 square mile) area was searched, and this one skylight was found. This indicates that skylights exist, but are probably rare. However, we would be very surprised if there are not skylights in other similar volcanic provinces. We are certainly looking for more skylights.

Ian: What are your plans for the future of the Marius Hills observations? Will your results be used by space agencies investigating potential habitats for future space settlers?

Carolyn: Both Harald Hiesinger (a co-author on the skylight discovery) and I are involved in the LRO Camera team. Hiesinger is also a co-investigator on the instrument. We are targeting the Marius Hills Hole (MHH) for imaging with the Narrow Angle Camera (NAC). The highest resolution images acquired by the Kaguya Terrain Camera were 6 meters/pixel, whereas the LROC-NAC images will have a resolution of 0.5 meters/pixel. It may also be possible to make some oblique observations as well.

One of the requirements for the LROC mission is to supply NASA with data for evaluation of Constellation Program landing sites. Information, like the locations of potentially useful lava tubes, will be included in this information and used to select Constellation landing and outpost sites.

Ian: Is there any indication to suggest the Marius Hills skylight could access an underground network of tunnels?

Carolyn: It is hard to say whether the MHH might access an underground network of tunnels. It is a possibility, because we know that lava tubes on Earth can be many kilometers long.

In the paper, using geometry, we calculated that the tube width might be on the order of 370 meters, but it could be anywhere from meters to kilometers long. Remote observations cannot answer this question for us — it's like we're trying to see an entire room by looking through a keyhole.

Ground-based exploration could answer this question. For example, Haruyama and Japanese colleagues, including Hideaki Miyamoto (also a co-author), have tested whether ground penetrating radar (GPR) might be useful in detecting intact sub-surface lava tubes. They tested their technique at Mt. Fuji, and were successful. It would certainly be interesting to do a GPR study in any region of the Moon where lava tubes might be present!

Ian: Personally, what would you expect the mission profile of a lunar mission would look like if we used natural shelter (such as lava tubes) to protect ourselves from radiation? Do you think this would be a cheaper alternative than building surface habitats?

Carolyn: Lava tubes would not only protect us from radiation, but also from meteoroid impacts and the extreme temperature changes over the lunar day. Near the equator, the lunar night is very cold (about -150 °C), and the lunar day, it is very hot (about 110 °C). A lava tube has a constant temperature, about -20 °C — just like caves on the Earth have very even temperatures.

Not being an engineer, I'm not sure that I can comment on the costs of surface versus subsurface habs. It does seem to make sense that using a natural, easily accessible shelter would help reduce costs.

However, it is important that the costs of accessing the geological shelter not exceed the costs of a surface shelter. There is a lot of discussion about whether it would be easy to access a lava tube from a skylight — there would need to be some sort of elevator to carry people and materials to the floor. It is also not known whether a sufficient open space might be available near collapsed sections of lava tubes, as proposed in Coombs and Hawke.

However, most importantly, the finding of the skylight confirms that there are intact lava tubes on the Moon, so we can keep our planning options open.