for National Geographic News
A "skylight" found on the moon's surface could provide access to a cozy underground shelter for future humans on the moon, scientists say.
Japan's Kaguya spacecraft recently captured pictures of the curious dark hole, which may open onto a large underground lava tube.
Scientists have long searched for easy access to lava tubes on the moon, since the lunar caves hold promise as natural shelters, noted Junichi Haruyama, of the Japan Aerospace Exploration Agency's Institute of Space and Astronautical Science.
"Lava tubes provide ready-made protection from the harsh lunar environment: meteorite bombardment, radiation from space, and the large changes in temperature through the lunar day," Haruyama said.
On Earth, lava tubes form after volcanic eruptions, when underground "rivers" of flowing lava run out and leave behind empty channels in the rock.
When a section of the tube's roof erodes or otherwise collapses, a "skylight" hole may be created.
Researchers believe the moon's volcanoes were active until about three billion years ago, although recent data from Kaguya indicate there might have been volcanic activity as recently as 2.5 million years ago.
Due to the moon's volcanic past, scientists have long expected that lava tubes exist in the lunar underground.
But even with decades worth of pictures from various lunar orbiters, no skylights had ever been spotted. That's because the holes can be difficult to distinguish from craters when seen from orbit.
Kaguya took high-resolution pictures of the entire moon from December 2007 to June 2009. Pictures taken in May 2008 finally revealed a skylight in the moon's highly volcanic Marius Hills region.
The JAXA team analyzed several images of the same dark spot taken at different times of day and used the changes in shadows to calculate the newfound spot's depth.
The hole appears to be as much as 289 feet (88 meters) deep—too deep to be one of the moon's many impact craters, Haruyama and colleagues report in a study set to appear in the journal Geophysical Research Letters.
Provocatively, the 213-foot-wide (65-meter-wide) hole is in the middle of a rill, a type of sinuous, line-like feature that meanders across the moon.
Rills are thought to be the surface evidence of underground channels that once carried ancient lava flows and may now house empty tubes.
If the skylight does provide entry to an interior cavern, the study suggests, the cavern should be a minimum of 1,214 feet (370 meters) wide.
Andrew Daga, of consulting firm Andrew Daga & Associates, has been researching the feasibility of using lava tubes for lunar bases.
Any standalone surface shelter, whether rigid or inflatable, would be more complicated, heavier, more expensive, and necessarily smaller than a structure designed to sit inside a protective lava tube, Daga said.
"Nothing that we can build on the surface using reasonably available technologies could provide the same protection as the interior of a lava tube," added Daga, who was not involved with the new study.
In addition, such tubes should be safe in the long run, since they've stood the test of time, he said.
Based on a count of nearby craters, Haruyama's team estimates that the skylight's tube was created more than 3.5 billion years ago.
That means any areas of the tube still in good condition are unlikely to collapse anytime soon.
Better Moon Landing Target?
Next steps could include sending a robotic rover armed with ground-penetrating radar to take critical measurements, such as the thickness of the moon cave's basalt "roof."
If the idea of underground shelter gains traction, lava-tube locations could join potential water sources and other factors in the debate over just where people should put down roots on the moon.
In fact, the Marius Hills region was proposed as a landing site during the Apollo era and is now under consideration for NASA's Constellation program, which aims to return humans to the moon by 2020.
"Volcanic regions like the Marius Hills may be good places to find resources on the moon," Haruyama said. That's because the moon's volcanic eruptions created basaltic rock, which could be mined for rare-Earth metals, silicon, and oxygen.
And now, Haruyama said, "we think this cavern could be useful as a lunar base."
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