TL;DR: NASA selected GIMLI to make the first on-the-ground measurements around the Marius Hills Pit to test whether the 65-by-90-meter opening leads to a lunar lava tube.

NASA picks a cave hunt for the Moon

NASA has selected a mission proposal that will examine the Marius Hills Pit to determine whether it opens into a hidden lava tube, according to Astronomy.

The selection was announced Sept. 30, according to Astronomy. The investigation is called GIMLI, for Geophysical Instruments for Marius Lunar pit Investigation, according to Astronomy.

The target is the Marius Hills Pit, a large hole approximately 213 by 295 feet (65 by 90 meters), according to Astronomy. The pit is located in Oceanus Procellarum, one of the Moon's most volcanically varied regions, according to Astronomy.

GIMLI will map and measure the ground beneath and around the pit, according to Astronomy. The work could provide the first on-the-ground evidence for a lunar lava tube, according to Astronomy.

NASA selected GIMLI alongside two other proposals for funding through the Payloads and Research Investigations on the Surface of the Moon (PRISM) program, according to Astronomy. The agency says PRISM will help lay the groundwork for a sustained presence on the Moon, according to Astronomy.

"NASA Science is building the ultimate interplanetary survival guide to ensure that science goes first to the lunar surface to provide our future astronaut crews with the vital information, resources, and safety precautions needed ahead of time to survive the night on the Moon," said Nicky Fox, associate administrator of NASA's Science Mission Directorate, in a press release, as reported by Astronomy.

Why Marius Hills Pit matters

The Marius Hills Pit is in a region of the Moon that previously saw a large amount of volcanic activity, according to Astronomy. Scientists believed the pit to be a skylight, an opening where part of a lava tube's roof collapsed, according to Astronomy.

Japan's Kaguya mission photographed the Marius Hills Pit in 2008, according to Astronomy. Since then, nearly 300 lunar pits have been identified, according to Astronomy.

A 2017 study of the Marius Hills Pit paired radar data from the Kaguya orbiter with gravity measurements from NASA's GRAIL mission, according to Astronomy. That study found strong evidence for a void beneath the surrounding surface, according to Astronomy. That result marked the pit as an area of interest for future exploration, according to Astronomy.

A 2024 analysis examined a pit in Mare Tranquillitatis using radar data from NASA's Lunar Reconnaissance Orbiter, according to Astronomy. That analysis found that the pit might lead to an underground cavern around a hundred feet long, according to Astronomy. That finding was described as the first evidence of an accessible lava tube on the Moon, according to Astronomy.

Scientists have proposed lunar lava tubes since the Apollo era, according to Astronomy. No one knows for sure whether such tubes exist on the Moon, but recent evidence leans in their favor, according to Astronomy.

What a lava tube could mean for astronauts

The open lunar surface offers astronauts no cover from the harsh environment, according to Astronomy. A cave stretching past the pit's mouth could protect explorers, according to Astronomy.

GIMLI's findings will inform future crewed missions and uncover new information about the Moon's geology, according to Astronomy.

Researchers think the Marius Hills Pit may connect to a vast, underground cave likely created by lava flowing in the ancient past, according to Space.com. If true, confirmation could shed light on the history of volcanic activity on the Moon, according to Space.com.

"Confirming a substantial lava tube would give us an insight into how volcanism operated on the moon," said GIMLI deputy principal investigator Gareth Morgan, as reported by Space.com.

Morgan said lava tubes are a common feature of basaltic volcanism on Earth, according to Space.com. He said identifying them on the Moon means researchers could use knowledge of such terrestrial caves to better understand lunar history, according to Space.com.

If researchers confirm the lava tube, astronauts could potentially use it as a natural shelter on the lunar surface, according to Space.com. The caves would provide general shelter as well as protection from extreme temperatures and radiation exposure on the Moon's surface, according to Space.com.

Lava tubes could also serve as a shielded location where future astronauts could be protected from micrometeorites, according to Space.com. A future Moon base will probably include human-made shelters, but natural caves could be an existing resource that astronauts could tap into, according to Space.com.

NASA aims to build a future Moon base near the south pole, according to Space.com. Confirming the presence and size of underground spaces could influence where NASA considers placing future habitats and how those habitats are designed, according to ScienceDaily.

How lava tubes form

On Earth, lava tubes form when the outer layer of a lava flow cools while molten rock keeps moving beneath it, according to Astronomy. Once the flow stops and the lava drains away, the hollow channel remains as an underground tunnel, according to Astronomy.

Scientists have long theorized that this process likely unfolded over the course of lunar geological history as well, according to Astronomy.

Terrestrial lava tubes top out at about 100 feet (30 meters) wide, according to Astronomy. Studies suggest that under the Moon's weaker gravity, tubes could reach 1,260 feet (385 meters) across, according to Astronomy.

Lava tubes form when the outer surface of flowing lava cools and hardens while molten rock continues moving beneath it, according to ScienceDaily. Once the lava drains away, it can leave behind hollow tunnels, according to ScienceDaily. Scientists believe similar formations may exist beneath parts of the Moon's volcanic terrain, according to ScienceDaily.

GIMLI will use geophysical measurements to investigate what lies beneath the pit and determine whether it connects to a larger underground passage, according to ScienceDaily. The findings will also provide new insights into the volcanic processes that shaped the lunar landscape, according to ScienceDaily.

What GIMLI will do without entering the pit

GIMLI will not actually enter Marius Hills Pit, according to Astronomy. It will combine instruments to search below the surface around the opening and characterize what is hidden there, according to Astronomy.

The investigation will collect the first-ever on-the-ground measurements of a lunar pit, according to Astronomy. Pulses from ground-penetrating radar will map subsurface objects, according to Astronomy.

A gravimeter will look for missing mass where an empty chamber might sit, according to Astronomy. A gravimeter is a device for measuring the strength of gravity at a specific location, according to Astronomy.

Seismic sensors will measure the movement of sound waves through the ground, according to Astronomy. Principal investigator Than Putzig said that approach "has essentially not been done since the Apollo astronauts" ran the Moon's first seismic surveys, according to Astronomy.

Cameras will document what they can see of the surface and the pit's walls, according to Astronomy. If a void exists, the combined measurements are intended to reveal its size and extent, according to coverage of the investigation in Astronomy and Space.com.

GIMLI will work directly on the lunar surface, scanning beneath to look for signs of the lava tube, according to Space.com. From the surface, GIMLI will gather data using ground-penetrating radar, seismic sensors and a gravimeter to determine if there is a subsurface lava tube and how big it is, according to Space.com.

Who leads GIMLI and how it reaches the Moon

The Planetary Science Institute (PSI) will lead the mission, according to Astronomy. PSI is an independent nonprofit headquartered in Tucson, Arizona, according to Astronomy. Its work in solar system exploration spans more than 30 states and several other countries, according to Astronomy.

GIMLI's principal investigator is Than Putzig, a PSI senior scientist and the institute's associate director, according to Astronomy. The data collected will be studied by a team led by Putzig, according to Space.com.

Honeybee Robotics, a subsidiary of Blue Origin, will manage the project, according to Astronomy. Honeybee Robotics will build the mission's seismic sensors and gravimeter and mount everything on a CLPS lander and rover, according to Astronomy.

The Norwegian Space Agency will provide the mission's ground-penetrating radar, according to Astronomy. The mission will also include co-investigators from Boise State University, Johns Hopkins University, the Lunar and Planetary Institute, and the University of Oslo, according to Astronomy.

Funding will come through PRISM, according to Astronomy. The mission will be launched through NASA's Commercial Lunar Payload Services (CLPS), according to Astronomy. CLPS is a public-private partnership through which NASA awards lunar delivery contracts through a pool of approved commercial firms, according to Astronomy.

GIMLI does not yet have a confirmed launch date or launch provider, according to Astronomy. The gear will get to the Moon aboard a lander and rover via NASA's Commercial Lunar Payload Services program, according to Space.com.

Two companion PRISM selections for a Moon base

NASA is preparing for a Moon base with three investigations that will hunt for underground shelters, uncover hidden ice, and track conditions on the lunar surface, according to ScienceDaily.

The instruments were selected through PRISM, according to ScienceDaily. They will be delivered to the lunar surface through the CLPS initiative as part of the Moon Base Program, according to ScienceDaily.

Each investigation will address a different challenge of establishing a lasting human presence on the Moon, according to ScienceDaily. Together, the findings will support NASA's Artemis and Moon Base programs as the agency works toward long-term lunar exploration, according to ScienceDaily.

One companion investigation is the Lunar Environment Monitoring Station — South Pole (LEMS-SP), according to ScienceDaily. It will establish an autonomous station to track environmental conditions and potential threats over an extended period, according to ScienceDaily.

LEMS-SP will detect micrometeoroids, tiny pieces of space debris that strike the lunar surface, according to ScienceDaily. Unlike Earth, the Moon lacks a substantial atmosphere capable of burning up most incoming objects, according to ScienceDaily.

The station will also measure volatiles, substances that readily turn into gas, within the Moon's extremely thin outer layer of gases, according to ScienceDaily. It will carry a short-period seismometer designed to detect vibrations and seismic activity, including moonquakes, according to ScienceDaily.

The investigation will be led by Dr. Mehdi Benna of the University of Maryland, Baltimore County, according to ScienceDaily.

The third investigation is the Depth Imager with Spectral and Color Optics (DISCO), according to ScienceDaily. DISCO is designed to make the first direct measurements from the lunar surface of ice preserved inside small, exceptionally cold areas, based on the payload description reported by ScienceDaily.

Researchers will examine potential natural shelters, identify resources astronauts could use, and study conditions that future habitats and equipment will need to withstand, according to ScienceDaily.

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