Sometimes, a hole in the ground is just a hole. Other times, it’s a portal to an underground cave spanning miles in width and length. You don’t really know what’s inside unless you dive into the void.
Lava caves, formed from cooled lava flows underground, are found near shield, basalt volcanoes across Earth. These lava tubes should ideally be found anywhere in the universe near basalt volcanoes, like on the moon or Mars, but that notion has long been hypothetical – until now.
One such lava tube has been confirmed on the moon for the first time, according to a recent study. These caves could shelter future human visitors and even harbor signs of microbial life, scientists say. But to learn more about what these features may look like on the moon, they’ve been studying similar lava tubes on Earth.
“Everything that we learned and what we’re theorizing about the moon’s lava tube comes from what we’ve seen here on Earth,” said Jekan Thanga, an engineer at the University of Arizona who was not involved in the study.
‘Not just a hole in the ground’
From a bird’s-eye view, evidence of a lava tube just appears as a hole on the surface. The hole, known as a skylight, is an area where the tunnel collapsed from above and provides access to the rest of the cave. Inside, the tunnel is jet black because basaltic lava flows are dark, whereas more familiar karst caves, for example, are made of lighter limestone. The sides of the tunnel are smooth like glass, except where breakage has created shards. The floor is like cobblestone because of rock that is half melted.
“The darkness is really, really dense. It’s basically like being submerged by tar,” said Riccardo Pozzobon, a planetary geologist at the University of Padova and an author of the recent research published in the journal Nature Astronomy. “You could really see that you are in somewhere really special.”
Earth’s lava tubes are located on nearly every continent. They are found from the Canary Islands to Iceland, from the United States to China. Tourists can even walk inside the Lava Beds National Monument in Northern California.
The caves come in all shapes, sizes and ages. Recently crystallized ones are found in Hawaii. Caves more than a thousand years old can be found in places like Flagstaff, Arizona. Even older lava tubes can be found in New Mexico.
“It doesn’t feel like you’re on Earth to be in the cave. It feels like you’ve gone somewhere totally different,” said astrobiologist Magdalena Osburn, who has been to about ten lava caves and was not involved in the new study. “I find it very magical.”
In recent decades, more than 200 suspicious holes – called pits – have been detected on the moon by a camera on NASA’s Lunar Reconnaissance Orbiter (LRO). Using the spacecraft’s radar data that allows them to see below the surface, researchers confirmed one of these holes is in fact a lava tube. The team used laser scans and three-dimensional models of terrestrial lava caves to help learn what the lunar cave may look like.
“All we see are these holes in the surface,” said Noah Petro, NASA project scientist of LRO who was not involved in the study. “The radar data that we have actually measures the properties of the surface and meters below. Basically, it affirms that it’s not just a hole in the ground.”
Comparing lava tubes on Earth and the moon
The newly discovered lunar cave is in the Mare Tranquillitatis (Sea of Tranquility), the landing site of the Apollo 11 mission. It is measured to be at least 130 feet wide and 100 to 200 feet long, which is quite large by Earth standards. Lunar lava caves are often two to three orders of magnitude bigger than those on Earth – enough to host a small town – because lower gravity makes them larger and more spread out.
The cave, scientists say, could be a good place for astronauts or future humans to shelter for protection against radiation, micrometeoroids traveling 3 to 4 miles per second or even moonquakes. The temperature inside the caves – at least the ones on Earth – is also fairly consistent for people.
Pozzobon, who estimated he’s visited about 20 lava tubes around the world, said the newly identified lunar lava cave is most similar to the corona lava tube in the island of Lanzarote in the Canary Islands because of its size. That terrestrial lava tube is one of the largest lava tubes on Earth in terms of volume, although it’s still an order of magnitude smaller than the one on the moon.
Even so, the team was able to practice analyzing radar data on larger lava tubes with the corona lava cave and other well-known caves. These studies allowed them to come up with a new approach for characterizing potential ones on the moon. With data of previous visits and analysis of the corona lava cave, the new study stated it was “strikingly similar” in morphology and radar readings as the lunar cave.
“You can basically take the information that you have [on Earth] and then compare to what you observe on other planets and see what are the differences, what are the similarities?” said Pozzobon. Knowing similarities to the terrestrial caves, he said they could calculate the volume of a slice of the lunar lava tube, for instance.
The corona lava tube was also created similarly to those suspected on the moon, the team hypothesized. Lava tubes on Earth are most commonly formed when basaltic lava flows from a volcano and the outside cools faster than the inside, which results in a hardened shell and a cave. But the team thinks the corona lava tube – likely similar to the one on the moon – was created by a deep inflation process, which does not create any bulging or topographic effects on the surface.
What’s inside the caves
Inside lava tubes on Earth, scientists have found microbial life appearing like a sparkly, yellowish film on the walls. This microbial life, Osburn said, can live off carbon dioxide. Some are probably capable of eating trace gases, like methane and carbon monoxide in the atmosphere, which means they can live in oxygen-poor environments.
For that reason, Osburn said lava tubes on islands, like the Canary Islands or Azores, are often a good place to study caves that could be on the moon. The Canary Islands is more arid and perhaps more bare-bones. If you wanted a lava tube with a forest, you may go to Australia. If you want a cold environment with ice flows, go to Lava Beds National Monument.
“When you’re seeking an analog for a given extraterrestrial environment, it’s a game of sort of matching some features,” Osburn, an astrobiologist at Northwestern University, said. “You’re trying to limit aspects that make things more earthlike.”
It’s also not impossible to rule out potential evidence of life or biosignatures in these lava tubes on the moon, she said. The caves could also contain water ice, potentially shown to be in permanently shadowed areas on the moon.
Scientists say there are undoubtedly more lava tubes on the moon, but that’s not the next pressing question. Instead, they’re looking to explore them.
On Earth, scientists send drones and cameras in the caves, but some research groups are also designing and testing rovers. Thanga, who heads the Space and Terrestrial Robotic Exploration Laboratory at University of Arizona, is building a rover with his students that can hop or climb down the pit entrance.
As far as what’s down the pit, no one will know for sure without a trip into the abyss.
