The Marius Hills pit was discovered in images from the Japanese SELENE/Kaguya Terrain Camera and Multiband Imager, and reported in Geophysical Research Letters. The hole is nearly circular, 65 m in diameter, and located in a sinuous rille at the Marius Hills region, a volcanic province on the lunar nearside. The hole was observed at various solar illumination conditions and its depth estimated to be 80 to 88 m. The depth/diameter ratio is much larger than for typical impact craters. There are neither conspicuous deposits indicating volcanic eruptions from the hole, nor are there pit craters adjacent to the hole that could be related to an underlying fault or dike. The area around the hole is covered by a thin (20 to 25 m) lava sheet, which may help protect the lava tube from collapse due to meteorite bombardment.
Enlarged image of the Marius Hills pit showing a small crater on the northwestern edge and small boulders on the southern lip of the hole. The pit is about 65 meters in diameter. NAC M114328462R [NASA/GSFC/Arizona State University].
The LROC image presented here (above), at 0.5 meters/pixel, is the highest resolution image of the Marius Hills pit to date! (The SELENE/Kaguya Terrain Camera team also made a fly-over movie of the hole, which is available here.
The Japanese team, led by Junichi Haruyama, made multiple observations of the pit using both the Terrain Camera and the Multiband Imager at resolutions as high as 6 meters/pixel (see below).
How did the Marius Hills pit form? The Marius Hills region was quite volcanically active in the past, and contains numerous volcanic features, including sinuous rilles. Sinuous rilles are long meandering channel features, like those labeled Rilles A and B in the above figure. Before the Apollo missions, sinuous rilles were thought to be formed by running water on the surface of the Moon! However, today we know that sinuous rilles form in two different ways: as open lava channels and/or as lava tubes, many of which subsequently collapse. Because the Marius Hills pit is in the middle of a sinuous rille, it likely represents a collapse in the roof of a lava tube. The pit itself may have been caused by an impact that punched through the lava tube roof.
Lava tubes might be useful as locations for lunar bases (see a report by Fred Hörz of JSC here). The interiors of lava tubes could protect human explorers from different aspects of the lunar environment, including cosmic rays, meteorite impacts, and the extreme temperature differences between the lunar day and night. Just like caves on the Earth, lunar caves, including lava tubes, have temperatures that are constant.
Posted by: Soderman/NLSI Staff