Ever since Neil Armstrong first set foot on the moon, scientists have been toying with the idea of forming potential colonies there (and more recently, uncovering potential signs of life on Mars). But extreme fluctuating temperatures, cosmic radiation and micrometeorite showers (colloquially known as space dust) pose challenges to human excavation of both the moon and Mars, writes Dr. Francesco Sauro from the European Space Agency's (ESA) Pangaea and Caves training program. However, astrobiologists are in the process of exploring nifty geological structures that could serve as natural shelter from these harsh elements: lava tubes.
What Are Lava Tubes?
"Lava tubes are caves that are carved by flowing lava that is eventually drained out, leaving a subsurface void," Dr. Riccardo Pozzobon of the Department of Geosciences at the University of Padova, said via email. Pozzobon has been at the forefront of European research on lava tubes.
Although there are different types of lava tube formations, these caves often form out of a type of fluid, basaltic lava, that flows down a slope like the side of a volcano. As the outermost portion of the hot lava flow comes into contact with the cold air, it cools rapidly, forming a hardened crust, explains Dr. Richard Léveillé. Léveillé is an adjunct professor at McGill University's Department of Earth and Planetary Sciences and a member of the McGill Space Institute. But liquid lava continues to flow like water in a channel underneath this newly hardened surface. At some point, that liquid lava runs out and cools underneath the surface, forming a curvy, tube-shaped structure. Boom — a lava tube is born.
"And these kinds of lavas we know have erupted on the moon and on Mars. So...we would expect to find lava tubes on the moon and on Mars," says Léveillé.
Extraterrestrial Lava Tubes
Geologists know lava tubes from volcanic areas in Hawaii or Iceland, but they've also become a hot commodity within the astrobiology community due to high-resolution images indicating that lava tubes may exist on the moon and Mars as well. Take, for example, these photos taken by the Japanese Aerospace Exploration Agency (JAXA)'s SELENE/Kaguya spacecraft of a potentially collapsed lava tube in a feature of the moon known as Mare Ingenii. In 2009, JAXA also spotted a vertical pit about 262 to 295 feet (80 to 90 meters) deep — a likely lunar lava tube — in the volcanic Marius Hills region.
Many sites thought to be lava tubes, like the Marius Hills Hole, are detected by the presence of "sinuous rilles" or curvy channels. And more recently, the SETI Institute announced the discovery of possible "skylights" or lava tube openings in the Philolaus Crater near the North Pole of the moon using images obtained from NASA's Lunar Reconnaissance Orbiter (LRO).
But lava tubes are a tricky business. Scientific technology is still playing catch up in identifying these underground habitats. "The main difficulty comes from the fact that lava tubes are essentially subsurface structures. Very few instruments are capable of performing direct measurements of underground structures," says Leonardo Carrer of the Remote Sensing Laboratory (RSLab) at the University of Trento in an email. But Carrer's team is working to modernize the technology and thus aid future human settlement of these lunar caves. The technology involves using radar, which can detect lava tubes from orbit based on their "unique electromagnetic signatures" to probe "below the lunar surface with low frequency electromagnetic waves and [measure] the reflected signals." The reflections offer insight into a lava tube's characteristics like shape, size and composition.
But one thing is clear. Lava tubes on the moon and Mars are invaluable as natural potential habitats — or at the very least, serve as convenient storage units between space missions. "These voids, if accessible, could be exploited not only for human settlement but also for material storage," says Pozzobon.
Using Earth's Lava Tubes to Explore Life on Other Planets
Meanwhile, back on Earth, scientists are preparing for future missions to the moon and Mars through a little cave diving. Pangaea is a program developed by the European Space Agency that prepares European astronauts to explore other planets. One of its projects concerns the 4.9-mile (8-kilometer) long Corona lava tube in Lanzarote, Spain. According to Sauro, who is a course designer for Pangaea, the team has undertaken advanced mapping of the tube, which has produced "the most complete 3D model of a lava tube on Earth ... with a millimetric precision." They've also been testing out new robots or rovers to identify how best to navigate these tubes, developing a greater understanding of the challenges associated with incursions into lava tubes on other planets in the process.
Other researchers have also taken an interest in exploring the microbiology of lava tubes by focusing their efforts on the Lava Beds National Monument in California. Léveillé, who heads this project funded by the Canadian Space Agency, says that his team is looking to explore lava tubes as habitats of microorganisms, which may leave traces or "biosignatures" behind through certain minerals, and thus indicate the presence of life once upon a time on the red planet. "And of course, the big question is "'How would we ever get into one of these lava tube caves [on Mars], which are quite irregular here on Earth?'" Léveillé says.
So what's the difference between lava tubes on Earth and their lunar and Martian counterparts? Well, gravity, for one. Pozzobon cites a NASA Gravity Recovery and Interior Laboratory (GRAIL) mission that he says detected "enormous subsurface voids" or potential lava tubes beneath the lunar surface. He describes how the lower gravity on the moon and Mars impacts the size of lava tubes significantly. Tubes on Mars can stretch for 820 feet (250 meters) in width, and tubes on the moon can reach more than a whopping 3,281 feet (1 kilometer) across. Pozzobon notes another important effect of lower gravity, which stabilizes the roofs of these tubes and causes fewer collapses — especially on the moon — thereby creating a potentially safer dwelling for human habitation. But otherwise, lava tubes on Earth are fairly similar in composition and structure to those on the moon and Mars and serve as excellent reference points for researchers.
The potential for lunar caves — and possible human settlements — has many people excited. Even the White House is making a bid for moon colonies in the near future. And the possibility of answering whether life has existed — or may still flourish — in caves on Mars is a tantalizing one for space explorers. But if you're wondering whether or not Martians will be found hanging around these lava tubes on the red planet, the answer is likely no — unless you count microbial critters. Radiation, a dry environment and frigid temperatures make the planet inhospitable to most forms of life. "There's no obvious indication of life, so most scientists would agree that if there was life in the subsurface, it would be microbial or bacterial in form," says Léveillé.