The Potential of Lunar Lavatubes
© 1996 Copernicus Construction Company
Peter Kokh and Doug Armstrong
Introduction
Twelve Questions about Lunar Lavatubes
? What is a "lavatube"? How are they formed? A lavatube 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.
? Where do we find them on Earth? in what kind of terrain? On Earth we find lavatubes 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 lavatube-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 such sinuous rilles represent collapsed lavatubes.
[log of Apollo 15 crew at Hadley Rille with photo links]
For a third even more convincing order of evidence, some lavatubes are clearly segmented with interrupting stretches of valley-free surface [location and photo link below]. These can only be sections of the original lavatube 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. Lavatubes are a natural concomitant of maria formation on the Moon, and will be common.
location of Hyginus Rille
Click on picture to get 1087x851 pixel photo (613K)
Hyginus Rille is located in Mare Vaprum (Sea of Vapors) in central nearside
? 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 lavatubes. On the plus side, lavatubes 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 serendipitously (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 lavatubes 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 lavatubes 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 lavatubes 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 50°F warmer than the Moon because of the oceanic-atmospheric heat sink.)
? How intact and stable would lunar lavatubes be? How prone to future collapse, total or partial? Any lavatubes 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 lavatubes, 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 lavatube internal environments 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 (-4°F), protected both from dayspan heat (+250°F) and nightspan cold (-200 some °F), the "body-heat" of the subsurface Moon being much higher than the "skin" heat of the exposed surface (3) Lunar lavatubes 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 lavatubes have been protected from all that and, good housekeeping measures adopted and religiously followed, will remain dust-free sanctuaries. Given the insidious 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, lavatube 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, lavatube 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 lavatubes 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 lavatube. 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 lavatube 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 lavatubes much as we find them, without wholesale modification. The idea of pressurizing tubes for more "terra-form" 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 lavatubes 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 lavatubes 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 lavatubes 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 lavatube 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 lavatubes 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 lavatubes 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.
Remote Mapping of Lunar Lavatubes
Teleo-Spelunking on the Moon
[Reprint of MMM #44, April '91, page 6]
EARTH-BASED SEARCHES FOR LUNAR LAVATUBES
Writing in Starseed, the newsletter of Oregon L5 Society, Oregon Moonbase researcher Thomas L. Billings discusses ways to search out lunar lava-tubes. Tube openings are hard to spot by camera unless you are right on top of them [but see note below]. While intelligent lunar base siting will require better orbital mapping than provided for the Apollo landings, the best method may be to look "through" the rock. The severe dryness of the lunar surface should make this possible for orbiting radar. (Airborne radar has been used successfully to find lava tubes on the big island of Hawaii.)
To provide deep radar imaging, the antenna diameter must be four times the radar wavelength being used. To penetrate deeply enough we'd need a wavelength of 5-20 meters, meaning an antenna 20-80 meters across! That's a lot of mass to put into orbit along with the ancillary equipment.
Billings suggests a way out. Readings from a number of smaller antennas in an interferometer array can substitute, synthesizing an image. It will be tricky to do this in orbit, and an intercontinental interfero-meter is an option Using a 7 meter wave-length, you'd have a 250 meter resolution and a penetration of 70 meters, good enough to detect a convincing sample, given that many tubes are likely to be larger than this.
However, a considerable amount of power will be needed if the signal returning to Earth is to be detectable. Computer algorithms needed to sift signal from noise are getting better. Nor need the search extend beyond a few months, so maybe the expense wouldn't be out of line with the rewards. TB
[Ed.: 1) Would it be practical to intercept that signal in lunar orbit where it would be stronger? 2) Would Earth-based searches be limited to central nearside? 3) We could use the same instrumentation package to search for tubes on Mars, Mercury, Venus, Io, and Vesta, worlds with shield volcanoes and lava sheets.]
Using Orbiting Infrared Cameras to Find Cooborating Evidence.
According To Bryce Walden and Cheryl Lynn York of Oregon Moonbase, orbiting side-looking infrared detectors may on occasion peer into the entrance of a fortuitously oriented lavatube, detec-ting its characteristic sub-surface temperature, clearly distinct from ambient surface readings, in sunshine or out. Illustration on revious page.
Robotic On-site Exploration & Surveying
We are back on the Moon, to stay it seems, and we've detected a number of lavatubes from orbit, some handy to our first beachhead outpost. The catch is that there are so many things needing priority attention that we cannot afford the manpower and equipment costs to outfit even a single lavatube exploration expedition. But if we don't "go in" and actually explore and survey, how can we plan intel-ligently to "move inside" in concrete particulars?
Here is a way we can survey in detail all the lavatubes we have detected remotely from photo-graphic evidence, from orbiting radar and infrared equipment. The costs, in comparison to a single limited human expedition, would be negligible.
A surface crawling drilling rig, using high resolution orbital radar lavatube location data, finds its initial drill point over an indicated tube site. This rig can be teleoperated or manned. Given the repetitive nature of the tasks involved, a highly automated remote monitored operation will be ideal.
(1) Its first task is to drill and stabilize (with a sleeve? with side-wall fusing or sintering lasers?) a hole through the surface and penetrating the lavatube ceiling some tens of meters down. The hole might be only a few inches in diameter.
(2) Next the rig winches down through the shaft hole a radar-mapping instrument and/or CCD optical camera down to a height midway between lavatube ceiling and floor (determining that position is the first task of the radar device). Then a flare attached to the bottom of the instrument package is released and dropped. The radar mapper and camera pan 360°, and from near vertical up (zenith) to near vertical down (nadir). The instrument package is retrieved. A latitude/longitude/altitude benchmark is then lowered to the tube floor directly below.
(3) The rig then winches down to the same point a length of fiber optic cable, securing the top end to the collar of the shaft hole. At the top end is a solar light concentrator which passively gathers available dayspan sunshine and channels it into the optic fiber cable. At the bottom end a light diffuser scatters this light in all directions. The idea is not to provide future human explorers within the tube with enough light, throughout the surface dayspan period, to find their way around with the naked eye, but only with enough light that they can find their way using off-the-shelf night-vision goggles. Of course they will carry battery-pack spotlights to light up areas needing closer inspection, as well as for emergencies e.g. they are forced to stay inside after local sunset on the surface above.
(4) Meanwhile, data from the radar/camera probe is being turned into a contour map of the lavatube's inner surfaces. From this map, it will be clear in which direction the lavatube runs and the location of the next drill hole can be determined, picked so that data from it (and the reach of the left behind "solar flash-light" overlap conveniently).
As the instrument package is removed from each successive shaft hole, another passive solar flash light chandelier is installed. On and on until the entire intact lavatube is surveyed from source to outflow. The rig then moves to one end of the next orbitally detected site to be investigated.
The result will be a set of tube surveys and maps from which preliminary rational use scenarios can be put together all prior to commitment of man-hours and man-rated equipment packages. Now , with all of these robotic surveys, safely made, when we do go in to explore or set up shop, we can be sure that the tube section picked is right for the purpose intended, including the offer of adequate expansion room for foreseen development options.
This is the basic idea. Possible embellish-ments are designing the solar flashlight chandeliers to serve as line-of-sight relays for radio communi-cations by exploring crews, and/or as direct radio antennas to the surface.
If the tube surveyed by the surface-crawling robot drilling rig has already been picked for future development, a "sleeve-bag" of sundry provisions and resupplies could be lowered to the tube floor beside the benchmark prior to sealing the shaft with the solar light fixture apparatus. These provisions would lighten the burden in-tube explorers need carry along. Alternately, the solar light fixtures could be removable if the shaft is needed for lowering provisions or other narrow diameter equip-ment to the area below it.
This exploration plan will only work, of course, for those near surface tubes that have been sniffed out by our orbiting probes. But that will be an important start!