Astrobotic Technology, Inc. and Carnegie Mellon University have received NASA funding to begin work on Spelunker, a prototype mission concept to explore a lunar cave.

Although the Moon has never had running water, which is responsible for most caves on Earth, it does have volcanic caves called lava tubes. In some locations, these tubes have partially collapsed to form openings called skylights. The Spelunker mission calls for landing on the rim of a skylight, followed by tethered descent of a power/communications hub and multiple robots. The robots would explore the interior of the cave using a combination of driving and hopping.

Right now, Spelunker is funded by a $498,411 Phase II grant from the NASA Institute of Advanced Concepts. Phase II studies will address the feasibility of skylight access, robot configurations for in-cave mobility and subsurface sensing, terrain modeling in darkness from a lightweight mobile platform, and autonomous exploration with hopping robots.

The studies will advance robot configuration, subsurface terrain modeling, and in-cave autonomy to NASA Technology Readiness Level 3 (analytical and experimental critical function or characteristic proof of concept).

Lunar Skylights

The first lunar skylight was discovered in 2010 using images from the Japanese Kaguya space probe. The skylight is 200 feet across and at least 250 feet deep. It sits in the middle of a rille that suggests an underlying lava tube as wide as 1200 feet. It’s located in the Marius Hills region of the Moon.

The find was quickly followed by the discovery, using images from Kaguya and the Lunar Reconnaissance Orbiter, of an even larger skylight in the Mare Ingenii region. This new hole, seen at the top of this article, is over 400 feet across.

Geologists are interested in lava tubes because they afford a chance to study the Moon’s interior without drilling through hundreds of feet of rock and regolith. Without water, even the lunar regolith becomes highly compacted and difficult to drill through, as NASA discovered during the Apollo missions.

A recent whitepaper by Andrew Daga of the University of North Dakota and seven other planetary scientists described the scientific benefits of lava tubes:

“To the geologist, discovering and gaining entry to a section of uncollapsed lava tube would permit direct examination of pristine bedrock, and potentially, materials brought up from depths that are inaccessible from the surface. This could show, in context, undisturbed native mineral composition as it flowed up from the lunar interior. It would be isolated from solar wind deposition and gardening, transported ejecta, asteroidal and cometary deposition, and mixing with shock-modified materials. Lava tubes are also useful in understanding the history of volcanism as well as the thermal history (heat flow) of a planet or moon. The same bedrock would also be an ideal location to conduct seismic investigations.”

As interesting as lava tubes might be to scientists, however, they may be even more important for future lunar development and settlement. Lava tubes have long been proposed as sites for future lunar bases. The roof of a lava tube would provide nearly ideal protection from solar and cosmic radiation, as well as the extreme temperature variations that occur during the lunar day/night cycle. Just six feet below the lunar surface, the temperature remains relatively constant at around -20 to -40° F (similar to the North Pole on Earth and more benign than the South Pole where 250 people winter over every year at McMurdo Station).

The problem confronting planners who would like to use the lunar lava tubes is that no one knows the interiors are like. The tubes may be hollow, filled with debris from partial collapses, or completely blocked by frozen lava flows. The presence of skylights shows that some of the tubes are at least partially open, but beyond that, we know little. Some researchers have proposed using ground-penetrating radar to determine the extent to which lava tubes are still open. The lack of moisture in the lunar rock and regolith would make it hard for scientists to interpret the radar returns, however. So, robots are considered a better option.

[youtube=http://www.youtube.com/watch?v=D15rjcwsk78&w=700]

Astrobotic hopes to explore a lunar lava tube on one of its future commercial missions to the Moon. According to the company’s Lander & Rover Payload Users Guide, the first mission (dubbed Lunar Cruiser) would be an equatorial mission to a lava-tube skylight or an Apollo landing site.

On to Mars

Skylights have also been spotted on Mars. The following image, taken by the HiRISE instrument on the Mars Reconnaissance Orbiter, shows a hole located on the dusty slope of the Pavonis Mons volcano. The hole, about 115 feet across, is believed to be an entrance to an underground cavern. The origin of the crater surrounding the hole is a mystery to researchers.

Not to be outdone, the Mars Odyssey orbiter has discovered seven possible cave openings using its Thermal Emission Imaging System. The seven openings, located on the side of the Arsia Mons volcano, range from 330 to 740 feet in diameter.

From a scientific perspective, Martian caves are even more interesting than lunar caves. Both geologists and and astrobiologists would love to examine the interior of such structures and sample materials from deep beneath the Martian surface. Some biologists believe such caves may be the best place to find life on Mars. Those hopes were reinforced by the discovery in 2011 of bacteria living in the ice inside a lava tube on Earth.

Unlike the Moon, Mars has had running water in the past, so there’s even the possibility of caves carved out by water rather than lava. The existence of such caves is pure speculation at the moment; we aren’t ware of any evidence to support it. But if such caves were discovered, we’re sure that geologists and astrobiologists would be really, really interested. We could probably throw in a few more really‘s.