#25 May 1989
Section 184.108.40.206.025.of the Artemis Data Book
by Peter Kokh
In the event that citizen-funded* Lunar Prospector 1 finds no indications of ice deposits in permanently shaded ('permashade') craters near the Moon's north or south poles, there will still be some debatable pluses for siting a base near one of the lunar poles along with a litany of disadvantages. What then?
On many occasions, we have stated that a mare/highland "coastal" site makes the most sense because it allows access to both major soil types on the Moon, important if we want to make intelligent use of lunar resources. Such coastal sites frequently come endowed with topographical features of enormous potential advantage: lava tubes and sinuous rilles. Indeed, the most important site advantage for a base designed with settlement expansion potential uppermost, will be close proximity to accessible lava tubes.
Our evidence for lava tubes on the Moon is threefold, and though indirect, quite strong. The first evidence is the existence in many mare areas of sinuous rilles or valley channels such as Hadley which was investigated by the Apollo 15 mission. These are typically hundreds of meters across and deep and can be a hundred or more kilometers in length. Our best explanation for these features, one now generally accepted, is that they represent collapsed lava tubes. (Rilles bear none of the water-flow signatures so marked in Martian valleys). The second evidence is the existence of chains (catennae) of rimless craters, often oval in shape, in several mare areas. Our best explanation for them is that they are collapse pits following along the top of a lava tube whose ceiling is within 40 meters of the surface, and with intervening stretches still intact. Finally, we find at least one "interrupted" rille, Hyginus, in which the interruptions appear to be intact lavatube sections, "bridging" the rille here and there.
There are many terrestrial examples of lava tubes, admittedly on a far smaller size scale (the considerably higher gravity on Earth being the determinant here) for example in the lava flow sheets covering much of Oregon and wherever the lava upwelling has had an especially low viscosity such as the Panhoehoe flows that have built up Mauna Loa/Mauna Kea (the Island of Hawaii). Lava tubes on Earth are typically 10-40 meters wide and high and may run several kilometers in length, and as a rule with a very gentle gradient. Their floors are sometimes flat (often with mid-floor channels handy for utility emplacement), sometimes strewn with rubble from ceiling spallation. We are only beginning to realize the extent of the honeycomb network of such tubes on the Big Island.
Our evidence that the lunar maria were formed by very low viscosity lava flows is substantial and based both on compositional analysis of the mare basalt samples returned and the topography of the very flat flows themselves. Relatively high titanium content may be a factor in this fluidity.
While all those tubes of which we currently have evidence lie near the surface, it is totally warrantless to conclude, as most writers seem to have done (we know of no exceptions), that this is the extent of their domain. On the contrary the morphological evidence is quite conclusive that the various mare areas have been built up by a succession of flows, each typically hundreds of meters thick. Total mare fill thickness can be deduced from the size of subsequent crater impacts that have 'bottomed out.' In the case, for example, of western Mare Crisium (Pierce, Piccard) this thickness must be two kilometers or more. Another indication is the size and extent of ghost rim craters on the mare (e.g. Yerkes in western Crisium, Prinz on the Mare Imbrium/Oceanus border). Thus Mare Smythii which contains many such features, must be comparatively shallow. Lava tubes in all probability radiate out from the source(s) of lava upwellings in one successive sheet above the other. Accordingly, some, subsequently filled or not, must lie quite deep and present a considerable challenge for detection and an invaluable especially pristine resource if found.
Some writers have suggested emplacing lunar bases within lava tubes. While it will be some time before we can afford to seal and pressurize even the smallest of these voluminous features, there are less ambitious ways to make use of them for initial bases or settlements. The Society's Oregon chapter has taken the lead in illustrating the very real advantages of near-surface intact tubes both for original siting and for subsequent base/settlement expansion, going so far as to carry out dry-run exercises with area Young Astronauts in suitable (but much smaller scale) lava tubes in the Bend, Oregon area east of the Cascades.
Lava tubes provide constant temperature volumes (about -4 degrees F, -20 degrees C) free from the hazards of micrometeorite bombardment, cosmic rays, ultraviolet radiation and solar flares (allowing lightweight inexpensive 'pressure suits') and thus ideal for warehousing and volatile storage (water-ice and gasses), expansive garaging space, and siting automated or teleoperated manufacturing facilities and laboratories that do not need, or even work best without, pressurization. Lightweight inflatable structures, perhaps of Kevlar, that do not need their own shielding overburden can provide whatever pressurized control centers or habitat spaces that are needed. Access can be by a shaft through the ceiling for freight and personnel elevators, utility conduits, even entry for sunshine concentrated and funneled by heliostats on the surface. It is, moreover, hard to conceive of a safer and more secure environment in which to emplace a nuclear power facility than an isolated section of lava tube.
As these features have already lasted 3.5 to 4 billion years (limestone caves on Earth are likely to last a few million years at best), and will outlast all existent terrestrial features without exception, a lunar lava tube might well be recommended someday as the best site in the entire Solar System to house some future grand archives and museum of all humanity. By the same reasoning, if you will pardon a little fun speculation, there would have been no better site in all the Solar System for ancient visitors from elsewhere who happened to have arrived millions, even hundreds of millions of years prematurely (from our point of view), to have left a calling card of sorts that would survive for as long as need be to be found by some as then barely conceivable native intelligent species (us). As such, lunar lavatubes have been aptly dubbed "attractors of alien artifacts."
Given the way they were formed, lava tubes may provide the best hunting grounds for future lunar gem collectors. At any rate, there is a future for lunar spelunking, although it will be quite a bit different from limestone cave exploration in karst regions on Earth.
The cost of providing access to an intact lava tube pales in comparison with the cost of providing comparable volume by any other method of base construction. So while at least the first residential and agricultural areas will likely be excavated or built in covered trenches, Lunar Industrial Centers built in conventient lava tubes will have an enormous advantage over those that are not.
Our recommendation: the National Space Society should consider raising funds for further studies of the existing photographic records for evidence of near surface lava tubes. Research into the best non-photographic methods of ferreting out such features from orbit also should have very high priority and if task-appropriate instrumentation can be devised, strong advocacy of a so-equipped follow-up probe in the Lunar Prospector series is in order.
* [When Lunar Prospector finally flew, some eight years after this was written, it was NASA who picked up the tap. Lunar Prospector was the 2nd outside mission to be picked up by NASA as part of its Discovery Mission Opportunity program. All attempts at private funding had failed.]
HARVEST MOON by Andy Weber
Contents of this issue of Moon Miners' Manifesto