After decades of increasingly more detailed photoreconnaissance of the moon, we have yet to discover compelling evidence of alien objects or structures on the surface. Many features that at first attract our attention are at the limits of resolution or are viewed at unfamiliar sun angles or highly oblique look angles. Under different conditions or in higher resolution images, most turn out to be craters, rock outcrops, or other natural features. 

Carl Sagan was one of the first scientists to suggest the possibility of extraterrestrial activity on the moon. In the 1960s William Blair announced the discovery of “lunar spires” that turned out to be large boulders lit at a low sun angle casting long shadows that looked like towers. George Leonard and Fred Steckling believed they had found extensive evidence of alien activity on the moon, which they described in their books Somebody Else is on the Moon and We Discovered Alien Bases on the Moon. Richard Hoagland argued that there were enormous towers and glass structures visible in Lunar Orbiter and Apollo photography. Attempts to locate these features, in particular, one known as “the shard”, were not successful.

In a previous post, we searched for evidence of a “monolith” that the Soviet Union claimed their Lunokhod 2 rover had found, photographed, and analyzed in 1973. Other possible structures purported to have been found on the moon have been investigated and determined to be optical illusions. One was a tower on the edge of the crater Lobachevsky first spotted by Johannes Fiebag. Higher resolution imagery revealed that the impression of a tower was created by a pattern of dark crater ejecta material and a reflection off the rim of the crater. Another was what appeared to be a bridge at the edge of the Sea of Crisis that turned out to be an optical illusion caused by the interplay of shadows.

Lunar Archaeology

In 1990 a colleague of mine and I first proposed the idea of using anomaly detection algorithms to search for artificial objects on planetary surfaces. Alexey Arkhipov described several such algorithms and provided a listing of more than a hundred “finds” or anomalies detected on the moon in Clementine imagery. Arkhipov’s paper “Towards Lunar Archaeology” is a blueprint for lunar SETI:

“Our Moon is a potential indicator of a possible alien presence near the Earth at some time during the past 4 billion years. To ascertain the presence of alien artifacts, a survey for ruin- like formations on the Moon has been carried out as a precursor to lunar archaeology. Computer algorithms for semi-automatic, archaeological photo-reconnaissance are discussed. About 80,000 Clementine lunar orbital images have been processed, and a number of quasi-rectangular patterns found. Morphological analysis of these patterns leads to possible reconstructions of their evolution in terms of erosion. Two scenarios are considered: 1) the collapse of subsurface quasi-rectangular systems of caverns, and 2) the erosion of hills with quasi-rectangular lattices of lineaments. We also note the presence of embankment-like, quadrangular, hollow hills with rectangular depressions nearby. Tectonic (geologic) interpretations of these features are considered. The similarity of these patterns to terrestrial archaeological sites and proposed lunar base concepts suggest the need for further study and future in situ exploration.”

Some of the rectangular features Arkhipov discusses appear to be related to patterns on the lunar surface known as the lunar grid, which was explored by Fielder and others.

Lacking evidence of ET activity on the surface leads to the question of whether or not there might be activity underground. In 2017 Ananda Sirisena, Fran Ridge, and I showed that certain features first imaged by Apollo in the crater Paracelsus C on the far side of the moon are present in more recent Lunar Reconnaissance Orbiter (LRO) imagery and appear even more remarkable up close. 3-D analysis suggests that one of the features may be an opening leading underground.

Lava Tubes

The idea of underground lunar settlements or bases is gaining support after the discovery of lava tubes on the moon by the Japanese Kaguya spacecraft in 2008. It appears that many long sinuous features on the moon called rilles are collapsed lava tubes. The existence of a lava tube is sometimes revealed by the presence of a “skylight,” a place in which the roof of the tube has collapsed, leaving a circular hole.

LRO has found hundreds of skylights. The Chandrayaan-1 orbiter imaged a lunar rille with an uncollapsed segment suggesting the presence of a lava tube near the lunar equator. Gravitometric observations by the GRAIL spacecraft suggest the presence of lunar lava tubes with widths of over 1 km. 

Large underground spaces the size of cities are possible. Assuming a width-to-height ratio of 3:1, a structure can remain stable with a ceiling that is 2 m thick. Lava tubes at least 500 m underground can theoretically remain stable with widths of up to 5 km.

Transient Lunar Phenomena

For hundreds of years, astronomers have noticed strange features on the lunar surface. The first recorded sighting was a star-like point of light on the dark side in 1540. Other features that have been reported include small whitish clouds by Cassini in 1671, ‘lightning’ by Louville and Halley in 1715, ‘vapors’ by Schroter and Olber in 1797, brilliant flashing spots on the dark side by Gruithuisen in 1821, dots and streaks of light by Slack and Ingall in 1869, a glow of light in the crater Plato by Fauth in 1907, a moving luminous speck near Gassendi by Haas in 1941, a pulsating spot on dark side by Emanuel and others in 1965, and hundreds more. In 1968, NASA published a compendium of these so-called lunar transient phenomena (LTP) sightings in a report entitled, Chronological Catalog of Reported Lunar Events.

A majority of TLP events occur near the maria, which is where many lava tubes are located. Could there be a spatial correlation between TLP events and skylights? 

If there were underground bases on the moon that were accessed through skylights one possible alternative SETI strategy might be to image skylights continuously, or at least as frequently as possible, to attempt to detect such activities.

Thermal Anomaly Detection

One of the instruments aboard LRO is the Diviner Lunar Radiometer that has mapped the global thermal state of the Moon and its diurnal and seasonal variability. If there is an active presence below the surface of the moon it is likely generating heat. Another alternative SETI strategy finds underground activity on the moon using thermal imagery. By correlating optical and thermal imagery it is possible to detect thermal anomalies – areas in an image that are different from the natural background that could be indicative of underground heat sources.

Summary

NASA’s recent technosignatures report proposes a new approach to SETI based on advanced technologies, future sensor capabilities, and crowdsourcing. Unfortunately, the report reads much the same way as previous SETI strategies that can be characterized as a search for ETs either a long time ago, or in a galaxy far, far away.

That reports of TLP and related events have persisted for hundreds of years suggests a more exciting and potentially more relevant strategy to look for signs of an extant intelligence underground. The benefit of this strategy is that even if we don’t find alien bases we might, in the process, find some good places to build our own.

Feature image courtesy V101 Science.