Lava tubes that are large enough to contain entire cities are now believed to exist on the moon, and scientists believe that they could one day provide shelter from the cosmic radiation associated with space exploration.
Lava tubes form from the flow of lava from erupting volcanoes. The edges of the lava cool as the lava flows creating a large tunnel. When the eruption stops, most of the lava then drains out of the tube leaving a dry tunnel behind.
Large channels are actually visible on the lunar surface that are thought to have been created from ancient lava flows. Some like the sinuous rilles are up to 10 kilometers wide. This latest study looked at the possibility that other tubes exist that are as large or even larger.
"There has been some discussion of whether lava tubes might exist on the moon," said Jay Melosh, one of the researchers, in a news release. "Some evidence, like the sinuous rilles observed on the surface, suggest that if lunar lava tubes exist they might be really big."
The researchers looked at whether empty lava tubes more than 1 kilometer wide could exist. They found that if the lunar lava tubes existed with a strong arched shape, like those that can be found right here on Earth, then it is possible for them to be several miles wide.
"We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the moon," David Blair, a graduate student at Purdue's Department of Earth, Atmospheric and Planetary Sciences said. "This wouldn't be possible on Earth, but gravity is much lower on the moon and lunar rock doesn't have to withstand the same weathering and erosion. In theory, huge lava tubes -- big enough to easily house a city -- could be structurally sound on the moon."
This isn't normally possible here on Earth, but the gravity on the moon is much lower and there is, for all practical purposes, no atmosphere meaning that the moon and lunar rock doesn't have to withstand the same effects of weathering, erosion or the weight of gravity pressing down on it.
The team discovered that a lava tube's stability on the moon depended on the width, roof thickness and the stress state of the cooled lava. However, if they do exist, they could one day provide much needed shelter to humans as they use the moon for a launching point for further exploration into space.
