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Caves of the Moon: How radar sensing is finding new locations for future astronaut bases

Tags: InSAR

Image of the lunar surface showing crater topography.
The lunar surface. (Credit: NASA.)

Ambitions to return to the Moon in the 21st century with a manned mission are accompanied by plans to establish a lunar base. Finding a suitable location for long-term habitation, however, requires careful exploration of the lunar surface. What is known about lunar geology has largely been the product of the first sample return missions in the 20th century, as well as remotely sensed observations of morphology and chemistry. Moreover, future ambitions of lunar and Solar System exploration are underpinned by the necessity to construct a lunar base that avoids the perils of extended interstellar exposure, such as cosmic radiation. These limitations play into a dismal feedback loop: without manned or robotic exploration of the surface to probe for such optimal locations, there is little to go off of to support future missions.

A recent finding published in Nature Astronomy from a team headed by Leonardo Carrer sheds some much-needed light on this obstacle to human space exploration through investigating whether or not the well-known Mare Tranquillitatis pit (MTP) may be host to a system of cave conduits. Detection of such a cave could be the key to a sheltered lunar base – and would be the first discovery of such a cave.

Pits on the lunar surface have been found in droves since 2009, but it is unclear from orbiters if any of these pits lead to subsurface caves. By comparing to Earth analogs, scientists think these caves could be similar to collapsed roofs of lava tubes, which happen when the ceiling of a tube deteriorates and eventually falls inward. On Earth, lava tubes form when more viscous lava cools and hardens over still-running lava, creating a tunnel shape. Scientists know that the Moon had a volcanic history, and this is readily available to the naked eye – the dark splotches on the surface of the Moon, called lunar mare, are actually the evidence of past lava flows. So, it makes sense that there could be lava tubes beneath the surface of the lunar mare.

Variable and inconclusive evidence of some cave entrances has been found through previous investigations using techniques such as orbital ground-penetrating radar and gravimeters, but these have not been entirely successful looking past the pit openings. Some of this is because the geometry of observation is not suited for looking past a pit opening. Additionally, orbiters may also be limited by poor resolution that prevents an image from picking up a pit opening at all. 

On Earth, success has been found using the side-looking geometry of synthetic aperture radar (SAR) to image pits. In this configuration, the satellite is able to aim the electromagnetic field into the pit opening and light up some portions of the interior to detect a conduit if present. Utilizing data from this technique may provide a higher resolution look at lunar pits.

Luckily, a satellite in orbit around the Moon called the Lunar Reconnaissance Orbiter (LRO) carries a SAR instrument called Mini-RF (the miniature radio-frequency instrument). Mini-RF took coarse-resolution images of the Moon’s surface between 2009 and 2011, and these images have now been analyzed to see if SAR is capable of detecting cave openings.

Four-paneled figure showing SAR images of Mare Tranquillitatis Pit. There is a distinct round pit in black, and in two images there is an elongated brightening on the left side indicating radar light being reflected back.
Panel a shows the SAR image of MTP from Mini-RF with the RGB m-chi polarimetric decomposition. The white arrow indicates the look direction. Panel b highlights the scattering of the radar from the cave overhang. Panel c is stereo observation of the pit walls. Panel d is a 3D radar simulation of the pit walls and overhang, with the pit edges outlined in red. (Credit: Carrer et al. 2024, Nature Astronomy.)

Since its discovery in 2009, the MTP is thought to be a cave roof collapse feature, meaning the opening might lead to a subsurface cave system. In the SAR images analyzed in this study, an “anomalous increase in radar echo power originating beyond the west side of the pit” was detected. This was a tantalizing suggestion of a conduit extending laterally beneath the surface. After determining that the detected brightening could not likely be caused by another source, including by running radar simulations, the team determined that the signal captured by SAR matches well with what would be expected if a cave conduit was present beyond the shadows. 

After carefully fitting a 3D radar model to the SAR images, Carrer and his team were able to construct a likely geometry and geological explanation for the MTP and why a portion of its interior was detectable by Mini-RF. MTP is likely a collapse feature, as evidenced by the rock pile at the floor of the opening. The brightening feature detected in the SAR images is due to the angle of observation of Mini-RF, which is able to observe down into the pit and slightly into the mouth of the conduit. The team observed a west-east profile of a conduit, possibly extending along the other side of the MTP. 

It’s not known how extensive this conduit cave system might be, and that won’t be detectable without follow-up imaging or exploration. However, the geography of the MTP suggests that future astronauts using the Moon as a waypoint may seek to house their base within the pit or just within the conduit opening to minimize debilitating cosmic radiation exposure. 

Four-paneled figure showing two best-fitting models to explain the cave geometry from the SAR data. Left-side images are the schematics of what the cave looks like, while the right-side images are 3D models.
Two best-fitting scenarios from radar simulations that could explain the Mini-RF radar observations. Panel a shows a scenario in which the conduit has a shallow rock pile, while panel b shows a scenario in which the roof and rock pile are both more sloped. The surface and overhang model is developed using a 3D model by Wagner & Robinson (2022), while the conduit is visualized with an inversion of the Mini-RF data. (Credit: Carrer et al. 2024, Nature Astronomy.)

Considering that Mini-RF was not designed for this type of surface analysis, Carrer and team’s work is a remarkable finding and encouraging for future studies analyzing subsurface structures with SAR imaging. Building a lunar base in a cave on the Moon sounds like a sci-fi plotline, but may actually come true one day.