A cave underground is considered a particularly good location for a future lunar station, because astronauts would be protected there from the harsh conditions on the surface. Planetary researchers have now discovered such a cave. It is located in the Mare Tranquillitatis in a pit around 100 meters wide with vertical or even overhanging walls. Radar data now show that a cave passage leads from this round crater in the lunar surface. This lava passage is around 45 meters wide and at least 30 to 80 meters long – probably even longer. The images also suggest that this lava cave under the “Mare Tranquillitatis Pit” could be accessible to astronauts.
The moon is not a very hospitable place for astronauts – quite the opposite. The Earth’s satellite has no atmosphere and no protective magnetic field. Anyone on the surface is therefore exposed to harsh cosmic radiation, the solar wind and bombardment by meteorites. At the same time, there are extreme temperature fluctuations: when the sun shines, it heats the surface to up to 120 degrees. In the shade and on a moonlit night, however, temperatures drop to a frosty minus 170 degrees. If people are to return to the moon in the near future and stay there for more than just a few days, they will need special protection. The thin walls of a space capsule or even an inflatable habitat are not sufficient for this. In addition to massive lunar structures made of bonded regolith, planetary researchers are therefore looking for lunar refuges in the form of caves or other geological formations that could offer astronauts natural protection.
Oblique view into the Tranquillitatis Pit
Attention has long been focused on conspicuous “holes” in the lunar surface: mostly circular openings up to 100 meters in size, which were discovered primarily in the once volcanic basalt regions of the lunar mare. In some places, entire chains of depressions, sunken trenches and such openings even seem to occur. On Earth, such holes, known as “skylights”, are often found where hollow lava passages run underground. The openings were created by the collapse of the cave ceiling. “Although more than 200 such holes have now been detected in various geological formations and latitudes of the moon, it is still unclear whether these openings lead to longer cave passages underground,” explain Leonardo Carrer from the University of Trento in South Tyrol and his colleagues. This is because the cameras and radar instruments of space probes in lunar orbit can usually only look into these openings vertically or at most slightly obliquely. Although they showed steep walls and in some cases overhangs of the rock faces, they did not show whether the bottom of the holes continued laterally.
That’s why Carrer and his team have now analyzed the data from a radar device that was able to look at the lunar surface at a more oblique angle than others. This mini radio frequency instrument (mini-RF) on board NASA’s Lunar Reconnaissance Orbiter is a sideways-looking polarimetric synthetic aperture radar (SAR). “Under the right conditions, the electromagnetic field emitted by this radar can penetrate the openings and in some cases make underground passages visible in the form of measurable radar signals,” the researchers explain. For their cave search, they analyzed images that this radar took of the largest and deepest lunar skylight, the Mare Tranquillitatis Pit in the Sea of Tranquility. This cylindrical hole has a diameter of around 100 meters and is around 75 to 80 meters deep. Its bottom is covered by a mixture of finer regolith and boulders up to ten meters in size – possibly debris from the former cave ceiling.
A lava vein at the bottom of the mine
“In 2010, the Mini-RF instrument collected data that also included this opening in the Mare Tranquillitatis,” reports senior author Lorenzo Bruzzone from the University of Trento. “Now, years later, we have re-analyzed this data using complex signal processing techniques.” One thing became apparent: “The images reveal an anomalous increase in radar echoes beyond the western side of the pit,” they report. These bright signals cannot be explained by surface structures at this location and must therefore come from a structure that lies beneath the lunar surface in the immediate vicinity of the Mare Tranquillitatis Pit. As the researchers explain, very similar radar signals on Earth usually come from rock overhangs in skylights, under which a passage extends into the subsurface. To check which geological formations in the skylight of Mare Tranquillitatis could have caused the observed radar signals, Carrier and his team conducted a complementary computer simulation in which they virtually recreated various shapes and structures and their representation in the radar.
The analyses showed that the radar signals are best explained by the existence of a cave passage underground. This passage could have a diameter of around 45 meters and extend 30 to 80 meters laterally into the underground – possibly even further. Because of the oblique angle of the radar beams, they do not extend further into the cavity, as the team explains. According to their data, the passage is probably only slightly inclined and could therefore be easily accessible from the bottom of the skylight. “This discovery provides the first direct evidence of an accessible lava cave beneath the surface of the moon,” says Bruzzone. At the same time, the find could indicate that such lava passages – as has long been suspected – also exist in other areas of the lunar mare. These structures could therefore possibly serve as bases for future lunar missions, but also provide valuable information about the geological past of the moon. “Future direct exploration of these structures could provide crucial insights into the formation of the lunar mare by giving us access to lava samples of different ages.”
Source: Leonardo Carrer (University of Trento) et al., Nature Astronomy, doi: 10.1038/s41550-024-02302-y