About Those “Polar Lava Tubes”

For decades, the idea of useable caves on the Moon has been studied and discussed at various venues and science gatherings. Our fascination with the availability of underground planetary structures stems from the possible benefits such features may afford humans trying to live in an off-world, hostile environment. Humans are vulnerable on the Moon because it lacks the protective atmosphere and magnetosphere that we enjoy here on Earth. A thick layer of solid rock provides protection for people and equipment from galactic cosmic rays and solar particle events, and thus, a lunar lava tube cave becomes a very attractive place. In addition, as rock is a good thermal insulator, an underground void space most likely would be thermally stable, lacking the enormous temperature swings (more than 250° C) of the ordinary lunar surface.

In addition to providing shielding from radiation, the interiors of lunar lava caves would be very cold (probably around -100° C), possibly preserving trapped volatiles. If such water deposition occurs, it would be an extremely slow process. There is no groundwater on the Moon; water is added by impacting objects. Thus, any addition of water comes in the form of vapor, most of it lost to space and the remainder finding its way into lava tubes or permanently dark craters at the poles.

The poles are attractive destinations because of the water ice residing in the permanently dark craters close (~100-200 km) to the poles. Due to the obliquity of the spin axis of the Moon, the sun doesn’t rise and set here, but circles around just above the horizon, maintaining a near constant surface temperature of about -50° C. The interiors of polar craters act as cold traps, accumulating water ice for billions of years. Quasi-permanent sunlight is available on the nearby peaks—places where electrical power could be generated nearly continuously by solar arrays. These unique features make the Moon’s poles the most valuable piece of real estate in the Solar System. To be able to live on the Moon, having the availability of power is everything, and finding energy near deposits of water ice is priceless.

So, naturally, there’s been a flurry of coverage in the media about last week’s press release from the SETI Institute, claiming the discovery of lava tube skylights (surface entrances to underground caves formed by past running lava) located “near the North Pole of the Moon.” Unquestionably we would make use of lava tubes near the Moon’s poles—if they existed. However, the problem with the discovery of polar lava tubes is two-fold—they are not polar, and they are not lava tubes.

The SETI Institute press release describes features found on the floor of Philolaus, an impact crater about 70 km in diameter. At 72° N, 33° W, this crater is over 500 km (more than 300 miles) from the Moon’s north pole (the Moon’s surface area is roughly the size of the continent of Africa). Unlike areas that are near the Moon’s north pole, the location of Philolaus crater is like most of the lunar surface, which experiences the usual lunar 28-day diurnal cycle of 14 days of blazing sunlight (100° C), followed by 14 days of bone-chilling (-150° C) darkness. It is very difficult to survive the 14-day lunar night without a reliable source of electrical power. The best way to address this problem is to deploy a nuclear reactor, which can generate power continuously, but such a reactor does not now exist and we are unlikely to develop one of sufficient size any time within the next couple of decades.

Philolaus is an extremely fresh crater, with rough, blocky surfaces and rayed ejecta. It is very young in lunar terms (probably much less than one billion years old) and thus, has had little time to accumulate much water. In contrast, the permanently shadowed areas near the poles—over 500 km distant from Philolaus—have been exposed to space for at least the last 2-3 billion years—capturing water continuously, as molecules migrate poleward from the lower latitudes. These molecules can be trapped by the extreme cold inside the permanently dark craters (as cold as -250° C, colder than the surface of Pluto), and as the water (of any variety) cannot sublime or be lost to space at such temperatures, it is permanently preserved there.

The features described as “lava tubes” in the press release do not occur in lava, as there is no lava in the crater Philolaus. The floor of this crater is lined with impact melt, rock produced by the intense shock of the collision of an asteroid with the Moon. Impact melt rock is formed from crystallized molten silicate liquids, as is internally generated lava. However, in the latter case, lava is erupted from a source vent as a continuous stream of liquid rock. The lava spreads out onto the surface and cools from the outside inward. As the lava continues to erupt, the gradual constriction of flow inward results in a subsurface conduit of molten lava encased within a crust of solidified rock. When the eruption ceases, this lava may drain out from the tube, leaving a void space, or cave, behind it. In some cases, this feature is ephemeral and immediately collapses due to the overlying weight of the solidified lava crust. However, on occasion, the crust is thick enough to maintain its structural integrity, resulting in the creation of a cave.

Impact melt is not lava. The shock melt is produced in a small zone around the point of impact, and coats the growing cavity during formation of the crater. The impact process is dynamic, with many particles in motion; these unmelted fragments (clasts) intimately mix with the shock melt, quenching and solidifying the impact melt sheet very quickly. Such a situation is not analogous to the continuous supply of magma and flow of uniformly melted lava that characterizes a volcanic eruption. Although we often see flows of impact melt around fresh craters, they are lobate, tabular layers of rock, sometimes with levees that outline a channel, but never a tube. They do not flow very far (usually less than a kilometer or two), and seem to be have been molten for a very short period of time. Neither condition is conducive to the formation of lava tubes.

The floor of Philolaus is covered with fractures, some of which are made up of aligned rows of pits. These features can resemble lava tubes in that we often see aligned pits as parts of lunar sinuous rilles. However, in the case of fresh crater floors, the pits are not produced by collapsed roof segments of drained lava tubes but by the degassing and settling of the molten impact melt sheet after crater formation. This melt sheet contains many inclusions of non-melted material picked up during the impact, ranging in size from a few millimeters to several tens of meters across. As the melt sheet cools and settles, the liquid portion drains away from the unmelted clastic material, leaving mounds and cracks that resemble (but in reality are not) lava tube skylights. They are unlikely to lead to large caves, as the draining liquid leaves behind loose crater debris that may collapse into the void. That said, it is possible that small void areas might exist in melt sheets, but they would be on the order of a few meters across, and at least partly filled with rock debris.

The poles are the desirable locales on the Moon because: 1) the peaks of near-permanent sunlight allow us to reside continuously on the Moon without the requirement of developing a nuclear power source; and 2) they are the locations on the Moon known to contain significant deposits of water ice. I am puzzled by this obsession with finding lunar lava tubes to inhabit. It’s true that living underground on the Moon is highly desirable, but this can be easily achieved by placing a habitat module in a small crater, then backfilling it with regolith (local dirt). No lava tube is required—we simply bury our habitat beneath the lunar surface.

In contrast to the claims in the SETI Institute press release, Philolaus crater is not anywhere near the polar ice deposits. Philolaus is an impact crater; it is not a lava bed with lava tubes. And the postulated volatiles within its “caves” have not been proven to exist.

Journalists must remain vigilant when circulating science and engineering achievements announced through press releases. At a minimum, they should dig into the story and learn if the work has been submitted to reputable scientific journals that follow a peer review process. The failure to adhere to even such a minimal investigative posture does not engender excellence, nor does it bode well for science or journalism.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.