Vertical hole and lava tube on the Moon
First, let us review what was discovered by KAGUYA. We discovered a vertical hole located at Lat. 14 deg N. and Long. 57 deg W. on the far side of the Moon. The region is called the "Oceanus Procellarum (Ocean of Storms)" where lava spreads broadly. The hole was discovered in the middle of a rille (trace of lava flow) located in the Marius hills. It is a unique area in the Ocean because it has many rilles and volcanic terrains called dome.
During the 21-month period of KUGUYA's observation, we observed the vertical hole nine times using the onboard Terrain Camera and Multi-band Imager. From the observations, we estimate that the hole is almost circular with a diameter of 60 to 70 m.
The nine observations were conducted at various sun angles. Once the diameter of the hole and the angle of the sun are known, it is easy to estimate geometrically the length (i.e. depth) of solar lighting to the hole's vertical wall. The estimated depth of the vertical hole based on the imaging observation when the solar angle was low agreed with the depth estimated geometrically. From this result, we judged that the hole has an almost perpendicular wall. In addition, when the solar angle was high, the bottom of the hole was observed. Thus, we found that the depth is 80 to 90 m under the surface.
A vertical hole of such diameter and depth could not be formed by an ordinary meteorite impact. With meteorite impacts, the ratio of diameter and depth is about 5:1 in deeper case. A vertical hole with the ratio of 1:1 is extraordinary. This kind of structure can be created by fumarolic gas on the earth but there is no sign of any matter scattering around the discovered hole. First of all, it is proved from lunar material samples that the Moon is very dry, so it is hard to imagine that fumarolic gas occurred on the Moon. It is natural to think that a hollow lava tube existed originally under the surface and, then, the hole was formed by a meteorite impact or a collapse induced by a moonquake. In fact, it was verified by an impact experiment to the thin layer that a through-hole with a diameter-depth ratio of one could be formed. Thus, we reached the conclusion that this vertical hole is a possible skylight for a lava tube underground.
Assuming that a lava tube exists underground, we assessed its width. With a computation based on a simple beam theory used for lava tubes on earth, we found that the width could be a maximum of 370 m. If it were wider, its ceiling would collapse under its own weight. We should keep in mind, however, that this is only one possible maximum diameter. If the ceiling were arched, for example, the diameter could be bigger because of the increased strength. On the other hand, if the tensile strength of the ceiling regarded as a beam is weaker, the diameter could be smaller. Thus, in the above case, there is a high possibility that the tube diameter could be smaller than 370 m. Further study and analysis are required to obtain a real value of the tube diameter.
The hole is located at almost the same distance from the levees on either bank of the rille's middle area in the Marius hills. If a lava tube is present under the hole, it is no wonder that the tube could extend several tens of km from the rille's upper area to the lower area. However, there are many unknown factors about the length of the lava tube, and it may have ended close by after filling up with lava. Nonetheless, past studies on lava tubes indicate that those on earth extend to very long distances. There is high possibility that lunar lava tubes, just like those on earth, stretch very far underground.