ASI W9700501r1.0

Moon Miners' Manifesto

#100 November 1996

Section the Artemis Data Book

12 Questions About Lunar Lava Tubes


The Lure of the Moon's Hidden Covered Valleys

In this Apollo 10 photo of Hyginus Rille in Sinus Medii (central nearside, 5xE, 8xN) are visible a number of "gaps" in the rille. The arrow points to the most prominent of these, about 10 miles long. The only geologically viable explanation is that this "interruption" is an uncollapsed segment of an original lava tube once well over a hundred miles long. Someday such ready-made sanctuaries from the cosmic elements may house the bulk of the Lunan urban population. Much more in six articles to follow.

Twelve Questions About Lunar Lava Tubes

What is a "lava tube"? How are they formed? A lava tube is a relic of a river of molten lava, self-crusted over on the top as the exposed surface cools, and then at least partially voided out as the lava spreads out eventually on the surface as a sheet.

EDITOR: scan in illustration from hardcopy

Where do we find them on Earth? in what kind of terrain? On Earth we find lava tubes in the flanks of shield volcanoes such as Mauna Loa/Kea in Hawaii and Medicine Lake in California. We also find them wherever we have had vast state-sized flood sheets of lava, as in Washington-Oregon, the Deccan flats of southern India, in northeast Siberia, and elsewhere.

How sure are we that similar features exist on the Moon? The lava tube-rich lava plains found on Earth are geologically analogous to the maria or Seas we find on the Moon. On those grounds alone, we would have a high expectation of finding lunar tubes. But for a second higher order of evidence we also have, in the same type of terrain, long sinuous valleys on the Moon called rilles (the Latin class name is rima]. We have found hundreds of these features in orbital photographs and have visited one (Apollo 15's visit to Hadley Rille). The consensus explanation of such features is that they represent collapsed lava tubes. For a third even more convincing order of evidence, some lava tubes are clearly segmented with interrupting stretches of valley-free surface [see the photo on page 1]. These can only be sections of the original lava tube that have not collapsed and remain still intact. Such sections should by themselves be enough to give future lunar developers ecstatic dreams. But if there are partially intact tubes, it is inconceivable that elsewhere, if not nearby, are to be found wholly intact tubes. Lava tubes are a natural concomitant of maria formation on the Moon, and will be common.

EDITOR: scan in illustration from hardcopy

Are they near surface objects only? Those we have direct or indirect evidence of (from rilles) are/ were near surface features. But keep in mind that the maria were filled with a series of lava floodings, and the formation of each successive sheet should have its own lava tubes. On the plus side, lava tubes in deeper layers have been more protected from collapse due to later meteorite bombardment. On the minus side, some, maybe most (a defensible guess for whatever your temperament), were filled up and plugged by later episodes of flooding. Deep tubes are unlikely to be discovered from orbit or from the surface. We could hope to find some of them serendi pitously (where tubes in successive levels just happen to cross) by radar soundings taken on the floors of near surface tubes by actual explorers.

How might typical lunar lava tubes differ from typical tubes found on Earth? (1) The formative episodes of lava sheet flooding on the Moon are all very ancient events on the order of 3.5-3.8 Billion years ago. Surviving lavatubes on Earth are all much much younger than that, some only thousands of years old. (2) In addition to being very ancient, lunar lava tubes differ in scale. Probably because of the lower gravity in which they formed (1/6th Earth's) tube-relic rille valleys already observed, photographed and visited run an order of magnitude (ten times) typical terrestrial dimensions in width, ceiling height, and total length. Lunar tubes are BIG. (3) lunar lava tubes have never been exposed to air or water (unless a comet happen to pierce the ceiling and vaporize inside with some of the volatiles freezing out on the tube's still intact inner surfaces - a real "lucky strike"!). Like tubes and caves on Earth, the temperature will be steady, but colder (Earth in general is 50xF warmer than the Moon because of the oceanic-atmospheric heat sink.)

How intact and stable would lunar lava tubes be? How prone to future collapse, total or partial? Any lava tubes that have survived to this day wholly or partially intact are likely to continue to do so for the rest of time. The vast bulk of major asteroidal bombardment which has pocked the Moon took place in the first billion years of the Moon's history. Lunar lava tubes, not subject to any sort of active geological forces or to any kind or weathering are perhaps the safest, most stable, protected volumes to be found anywhere in the solar system. They are veritable vaults, sanctums, sanctuaries we can bank on - no bet-hedging needed.

What aspects of lunar lava tube internal environ-ments are most attractive for human purposes and to what uses might we put them? (1) "lee" vacuum protected from the micrometeorite rain, from cosmic rays, from solar ultraviolet, and from solar flares, and unlimited volumes of it, is a priceless and odds favoring handicap toward lunar outpost and settlement establishment, expansion, and maintenance. In these conditions, only simple unhardened lighter weight pressure suits need be worn, for much greater safety, comfort, and convenience. Lee vacuum is ideal as well for storage and warehousing and in-vacuum manufacturing. (2) steady temperatures at all times (-4xF), protected both from dayspan heat (+250xF) and nightspan cold (-200 some xF), the "body-heat" of the subsurface Moon being much higher than the "skin" heat of the exposed surface (3) Lunar lava tubes are dust free. The regolith Moondust blanket is the result of eons of micro-meteorite bombardment or gardening of the lunar surface. The unexposed surfaces of lunar lava tubes have been protected from all that and, good house-keeping measures adopted and religiously followed, will remain dust-free sanctuaries. Given the insi-dious invasiveness and machinery- and lung-fouling character of moondust, this asset is a clincher!

For construction purposes, shielding now provided as a transcendental given and dust-control vastly easier, lava tube sites will be much simpler and easier places in which to build. We have only pressurization to provide and maintain within these natural macro-structures.

As a package, lava tube assets effectively remove (squelch, eradicate, nuke) most of the common objections to the Moon as a development and settlement site, reducing worries to lack of around-the-clock sunshine (an engineering energy-storage and usage/scheduling question) and gravity one-sixth Earth normal (as if life hasn't always been able to adapt to anything!).

Are there any more special resources we might find in lunar lava tubes here and there as extras? - Mineralogically, lavatube surfaces and their host terrain will be boring, fairly homogeneous basalt. Other elements, not present in local basalt, can be mined and processed elsewhere and the products made from them brought to the site. But not to be overlooked is the possibility that we have hit the cosmic jackpot with a volatile-rich comet strike of just the right size to puncture, but not collapse, a lava tube. Frozen volatiles would be the prize. These would not be subject to most of the loss mechanisms that will surely operate in polar permashade ice fields (micro-meteorite bombardment, solar flares and solar wind, cosmic rays, splashout from other impacts). To date, the only (and it's inconclusive) teasing evidence we have is an anomalous reading over western Mare Crisium that on first interpretation would seem to indicate subsurface water-ice. This reading has been (but should not be) routinely dismissed as spurious.

What lava tube uses are near term, what uses are more challenging and likely to be realized only in the far future? Warehousing and storage; industrial parks; settlement as opposed to outpost; archiving. All of these can benefit from the use of lava tubes much as we find them, without wholesale modification. The idea of pressurizing tubes for more "terraform" settlement presents a number of enormous hurdles (sealing methods, sealant composition, pressurization stress, importation from Earth of astronomical volumes of nitrogen, etc.) and while in toto vastly easier than wholesale terraforming of a whole surface (e.g. Mars) is still something we will not tackle for some generations perhaps.

How much total ready to go protected volume are we talking about? For political purposes internal to the pro-space movement, let's express our back-of-envelope guesstimate range of the total available volume of intact lunar lava tubes in terms of O'Neill Island III Sunflower space settlement units. That's ready-to-occuppy-and-use-NOW (for those without 1-G and 24-hour sunshine hangups - they can wait the generations it will take to build Sunflower units from scratch !)

The surface area of the host terrain, the lunar maria, comprise some 17% of lunar surface = 2.5 million square miles - compare with 3 million square miles for continental U.S. Now if (we have to start the argument somewhere!) we assume that available floor and wall terrace surface of intact lava tubes compares to 1/1000th the taking 1/1000th of this aggregate lunar maria surface area, we get 2,500 square miles. This is in our estimate, a very conservative fraction. Counting supposed lava tubes in lower level lava sheets, 1/100th is a fraction that could be closer to reality. That would yield 25,000 square miles, an area comparable to West Virginia.

Subtracting for window strips (as we have for lava tube upper walls and ceilings), an O'Neill cylinder, if ever realized in full ambitious scale, might have 100 square miles of habitable inner surface. Argue about the figures, it won't change the overall picture. We are talking about ready to occupy network of lunar lava tubes that compares to 25 to 250 Island III units. If you are going to hold your breath until these free space oases are built, I can only hope your life expectancy is much more Methuselahn than mine {P. Kokh].

Can we expect to find other similar hidden covered valleys elsewhere in solar system? Yes, as they seem to be a standard concomitant of lava sheet flooding and of shield volcano formation, we might expect to find lava tubes on Mars, Mercury (the temperature swing refuge would make them hot property), Venus (they would be too hot, and share Venus' over-pressurization), Io (protection from Jupiter's radiation belts), and even on little Vesta.

By what Latin class name are such features likely to be referred? (e.g. rima = rille) Cava, tubus, and ductus are available Latin words. The latter better indicates the mode of formation. MMM

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