Somewhere beneath the Moon’s near side there may be an enormous natural cave, and NASA wants to know for sure. As Aerospace America reported on October 21, the agency is moving forward with plans to send a rover carrying a suite of instruments to the Marius Hills pit, a hole in the lunar surface that scientists think opens into a lava tube.

The instrument package has a name Tolkien fans will love: GIMLI, short for Geophysical Instruments for Marius Lunar pit Investigation, a nod to the dwarf from The Lord of the Rings. Fittingly, its job is to look underground.

What is a lava tube?

On Earth, lava tubes form when the outer surface of a lava flow cools and hardens into a crust while molten rock keeps flowing underneath. When the eruption ends and the lava drains away, it can leave behind a long, hollow tunnel.

The Moon had plenty of volcanic activity billions of years ago, and scientists have long suspected that similar tubes, possibly much larger thanks to the Moon’s lower gravity, survive beneath its surface.

The Marius Hills pit

The Marius Hills pit sits in the Moon’s northern hemisphere on the near side. It was discovered in 2009 by Japan’s SELENE lunar orbiter, also known as Kaguya. According to the report, shadows in orbital images suggest the pit has an overhanging roof, a clue that it could be a skylight into a larger void below.

That’s a promising hint, but it isn’t proof. That’s where GIMLI comes in.

How GIMLI will look underground

The plan calls for launching a rover in 2031 carrying three main geophysical instruments, plus optical cameras:

  • Ground-penetrating radar to bounce signals off buried surfaces
  • A gravimeter to detect tiny changes in gravity, since an empty cavern means less mass below
  • A seismometer to listen to vibrations traveling through the ground

In a first since the Apollo era, GIMLI will also carry an active-source seismic “thumper” that strikes the surface to send seismic waves into the ground, which then reflect off underground structures.

A commercial lander, not yet chosen, would deliver GIMLI to the Marius Hills region. A rover, also yet to be selected, would drive to the pit’s rim to begin observations. Depending on its health and power, it could then drive a crisscross pattern over the area thought to hide the tube.

“The main goals are to detect whether there is in fact a void space down there, and then to try to get the dimensions of it,” Nathaniel Putzig, GIMLI’s principal investigator at the Planetary Science Institute (PSI) in Arizona, told the magazine. If the cave is real, the instruments should reveal how deep it is, where its floor and roof sit and how stable the roof is.

Using several methods at once is deliberate. As PSI senior scientist Gareth Morgan put it, if all the instruments show evidence of a void, “that increases your confidence that it’s real.”

Who is building it

NASA selected GIMLI for development on September 30, and PSI won the award with a bid under the $40 million project cap. Honeybee Robotics in Colorado, owned by Blue Origin, is set to build all of the instruments except the radar. The University of Oslo in Norway will build the radar, drawing on its experience with the RIMFAX ground-penetrating radar aboard NASA’s Perseverance Mars rover. Putzig said the team is reusing well-developed technology to meet its five-year timeline.

Why a Moon cave matters

Life on the lunar surface is harsh. There’s radiation from space and wild temperature swings between day and night. A large cave could offer natural shelter from both. Morgan pointed to earlier NASA estimates that the temperature inside such a cave could stay at a steady, comfortable 17 degrees Celsius (63 degrees Fahrenheit).

“If you think about how much expense goes into just regulating the temperature of a spacesuit or structure, a very large cave does that for you,” he said.

NASA is careful not to oversell it. In a statement to Aerospace America, the agency said it is currently focused on building a Moon Base in the lunar South Pole region, where no evidence of lava tubes has been found, and that lava tubes are not part of current habitat planning. Still, it said continued research “could help inform future exploration opportunities elsewhere on the Moon.”

The wrap-up

GIMLI won’t send astronauts into a lunar cave, and it may find that the Marius Hills pit is less dramatic than hoped. But if its radar, gravity and seismic readings all agree, it could confirm one of the most intriguing features on the Moon and give future explorers a natural shelter to consider. For now, the countdown to 2031 begins.

Image: Moon Surface by Matt Bango via StockSnap, CC0 1.0