Researchers have verified that a depression in the Moon’s Sea of Tranquillity leads into an underground cave conduit, likely an ancient lava tube. Researchers from the University of Trento utilized radar data from the Lunar Reconnaissance Orbiter to verify the subsurface opening, offering a shielded site for potential future lunar exploration.
Decoding the Subsurface Radar Data
The lunar surface is pockmarked with more than 200 known pits where rocks and regolith collapsed into depths unknown. Recent investigations have revealed that a depression situated in Mare Tranquillitatis collapsed into a lava tube and made an underground cave conduit accessible from the lunar surface. According to Leonardo Carrer, a planetary scientist at the University of Trento, Italy, and first author on the research, the team found a sort of front door to enter the subsurface. This access to the otherwise shielded lunar subsurface makes the pit a tantalizing site for future human and robotic exploration while providing new insight into lunar volcanism.
Ancient magma oceans once blanketed the Moon before solidifying into the dark basaltic maria visible today. Experts in lunar science have long theorized that Earth’s satellite might harbor volcanic structures comparable to lava tubes found on our planet. These structures form when a lava stream cools and forms a hardened exterior shell while hot lava continues to flow through it like sludge in a pipe. Eventually, the molten rock drains out, leaving a hollow conduit that can connect to emptied magma chambers or caves.
From Terrestrial Analogs to Orbital SAR
To determine whether it was possible to map a hidden cave using orbital synthetic aperture radar (SAR) instruments, Carrer and fellow University of Trento planetary scientist Lorenzo Bruzzone tested their methodology at two terrestrial cave systems in Lanzarote, Spain, and the Well of Barhout in Yemen. By using SAR data to create three-dimensional reconstructions of those terrestrial caves near their entrances, the team verified that cave characteristics measured from space matched what speleologists measured on the ground. That ground-truthing gave them the confidence to deploy the technique on the Moon.
Researchers focused their attention on the Mare Tranquillitatis pit, a nearly circular sinkhole roughly 100 meters across and 105 meters deep. The Lunar Reconnaissance Orbiter collected the radar data employed for this evaluation back in 2010 by directing an angled signal downward into the pit and capturing the resulting reflection bouncing off the bottom. Bruzzone noted that it was quite easy to understand the signals seen on the Moon thanks to the Earth validation. Feeding those radar measurements into their computer model generated a 3D visualization of the lava tube along with estimates of its dimensions.
Dimensions and Environmental Protection
The resulting model suggests that the entrance is at least 45 meters wide. Depending on how sharply the conduit slopes downward, it extends 30 to 80 meters from the entrance and reaches 135 to 175 meters below the lunar surface. Robert Wagner, a planetary scientist at Arizona State University in Tempe who was not involved with the research, observed that the analysis certainly indicates a passage going deeper than visible-wavelength images can reveal. While he cautioned that the passage might not connect with a larger cave, he confirmed that the new study is consistent with present-day access to a lava tube.
The discovery carries significant implications for long-term habitation. Surface lunar temperatures fluctuate wildly, scaling up to 260 degrees Fahrenheit (about 127 degrees Celsius) during the day and dropping to minus 280 degrees Fahrenheit (minus 173 degrees Celsius) at night. In contrast, the temperature inside the permanently shadowed region of the pit fluctuates only slightly throughout the day, holding steady at around 63 degrees Fahrenheit or 17 degrees Celsius. Combined with the natural protection offered against meteorite impacts and solar radiation, these underground cave conduits present a highly promising environment to support sustained human exploration of the Moon.
