In the camera data retrieved from the lunar orbiting satellite SELENE (KAGUYA) (Fig. 1), we finally discovered a vertical hole, evidence of the existence of an underground lava tube on the Moon. This article discusses this new discovery.
What is a lava tube?
A lava tube is a hollow tube formed after lava flow. Most of the caves commonly seen on the Hawaiian Islands or at the foot of Mt. Fuji are lava tubes. Sometimes they are also called air holes or ice caves. Mt. Fuji, the islands of Hawaii and Maria (seas) of the Moon are composed of a rock called basalt and its composition is very similar. Accordingly, it is no surprise that lava tubes are also found on the Moon. Naturally, for lava tubes to be formed, lava would run through the tubes. If so, lava flowing in the tubes could reach far greater distance than that spreading two-dimensionally on the lunar surface. Thus, there is a possibility that lava tubefromation could play an important role in creating the vast lunar seas. By close investigation of tube's interior, we should be able to identify the age, volume, rate, etc., of the lava eruption. Therefore, lunar lava tubes are very important scientific objects.
The lava tube is also very suitable for a lunar base. The lava tube has a roof, meaning the tube interior is protected from radiation exposure and micrometeorite collisions, which occur on the Moon with no atmosphere. The temperature of the lunar surface varies drastically in the range of -200 deg. C to +100 deg. C. Since lava tubes are underground, the temperature in the tubes is kept almost constant. The in-situ exploration in the Apollo program showed that the temperature a few meters under the surface around the landing area is constant at about -20 deg. C. On the other hand, final-flow lava coagulates at the bottom of the lava tubes making it flat frequently. In other words, the bottom is just like a naturally paved surface. In addition, when tubes were formed, their inner walls were often rapidly cooled, which made them very airtight. By sealing the front and back tube exits, it is easy to create a pressurized space. Furthermore, inside lava tubes, there is no need to care about the finely crushed-ash-like sand covering the lunar surface, which severely hampers activities on the lunar surface.
Since lava tubes are present underground, it is difficult to discover them by usual observation by cameras from the space. But we should note that, when seeing aerial photos of lava tubes on Earth, they sometimes appear like a series of subsidence structures. This is because some sections of the tube collapse. To check whether there is such terrain on the lunar surface, researchers scrutinized images taken by the Lunar Orbiter and Apollo programs 40 years ago. In fact, a line of subsidence structures or craters was discovered. Many researchers have worked hard to discover an entrance to the lava tubes, closely examining images to confirm whether there is a lateral cave near the subsidence areas. However, until now nobody had discovered an exposed lateral cave that was probably formed by partial tube collapse.
The possible existence of lava tubes is not limited to the Earth and Moon. In the images of the Martian surface shots by the U.S.'s Mars explorer Mars Global Surveyor, series of subsidence craters were found. It is almost accepted that lava tubes exist there too.
In 2006, in an image taken by the European Mars Express, researchers discovered a place where traces of lava flow overlapped. Lava tubes extend underground and their structures are sometimes very complex. Since lava moves three-dimensionally underground, lava tubes naturally can become three-dimensional structures. The traces identified by the Mars Express have a structure that cannot be created by flows of water or lava on the surface of Mars. It is the aftermath of a lava tube's collapse.
In Aug 2007, one month before the launch of KAGUYA, it was announced that the U.S.'s Mars Odyssey discovered seven vertical holes distinctly exposed on the Martian surface. They are very similar to that on the lunar surface discovered by us. As flows of water and volatile substances exist under the surface of Mars, some researchers insisted that it is hard to say that lava tubes were present under the vertical holes. In this dispute, our discovery of a vertical hole on the lunar surface with no water and volatile substances suggests that the existence of lava tubes is more probable not only on the Moon but also on other planets.