Degradation of the Lunar Vacuum by a Moon Base
Geoffrey A. Landis


originally published in Acta Astronautica, Vol. 21, No. 3, 183-187 (1990); available here with the permission of the author. A non-technical version of this article appeared in the magazine Analog.

Geoffrey A. Landis
NASA Lewis Research Center, 302-1
21000 Brookpark Rd.
Cleveland, OH 44135 USA
GLANDIS@LERC.NASA.GOV

ABSTRACT

High vacuum is required for many industrial processes which might be accomplished on the moon, such as electronic component and solar cell manufacturing or a large particle accelerator. Ambient pressure on the moon is in the range of 1 E-12 torr (night) to 1 E-10 torr (day). The effects of a 20-person base and a 250 person industrial facility on this vacuum are discussed. Exhaust from the transport spacecraft and leakage from the habitat will be roughly comparable to the daytime gas pressure for the 20 person base, and will degrade the vacuum to the range of 2.E-9 torr for the 250 person facility. This is higher than the desired pressures for some semiconductor manufacture processes or for a lunar-based particle accelerator.

1. The Lunar Ambient

The existing lunar atmosphere is tenuous and not well characterized.

The ambient pressure is in the range of 5.E4 [1] to 2.E5 [2-4] molecules/cm3 during the lunar night, and 6.1 E5 to <1 E7 molecules/cm3 during the lunar day [5]. This corresponds to pressures from 5.E-13 torr (0.0005 nanotorr) up to 0.4 nanotorr, primarily consisting of hydrogen, helium, argon, and neon at night, with the probable addition of CO and CO2 in the daytime [6]. The mean free path for these pressures are in the range of hundreds to thousands of kilometers; thus, the movement of gas in the atmosphere is primarily via ballistic transport.

The atmospheric escape lifetime from the sunlit side of the moon is approximately 10000 seconds (fifteen minutes) for the lightest molecules (hydrogen and helium), and up to 1 E7 seconds, approximately 100 days, for heavier molecules [7]. 1 E7 seconds is roughly the maximum lifetime of atmosphere constituents; this is approximately the time it takes for the molecules to become ionized by the solar ultraviolet, at which time they are swept away by electric fields associated with the solar wind in times which are typically no more than a few hundred seconds [7]. As noted by Vondrak [8], this mechanism becomes ineffective if the atmosphere is thick, however, the gas input rate (on the order of 250,000 tons/month) required to reach such a level is considerably higher than what is likely to be produced in any near-term industrial facility.

2. Required Vacuum

It seems absurd to expect that the lunar vacuum could be lost by small-scale operations on the moon. However, high-vacuum and ultra-high vacuum is needed for many industrial processes, some of which may be accomplished on the moon. Some processes which require vacuum and thus would be simpler to manufacture or use on the moon include vacuum tubes, semiconductor manufacture, solar cell manufacture, and particle accelerators.

Silicon is a major component of the lunar crust. One likely low-cost process sequence for producing solar cells on the moon [9] is plasma-deposition of amorphous silicon. Such deposition processes typically have base pressures in the very high vacuum range, mid- E-6 torr, to below 1 E-7 torr for some experimental set-ups. It is believed that impurities in the deposited films of concentration greater than 1 E18/cm3 cause (or exacerbate) the deleterious light-induced degradation effect; this corresponds to a base pressure of 2000 nanotorr at deposition pressure 1 torr; 100 nanotorr at deposition pressure 0.05 torr.

Many processes for manufacturing semiconductor products require vacuum.

One process for depositing high-purity layered compound semiconductors is Molecular Beam Epitaxy (MBE). This process requires ultra-high vacuum.

Base pressure for MBE is in the range of 0.1 nanotorr [10,11] and can be as low as 0.03 nanotorr for GaAlAs [12], where C and O contamination are particularly harmful.

"Vacuum" tubes have a different values for the required operating vacuum, depending on the type of tube and the lifetime, noise level, etc. required. This ranges from 1 E-5 torr for the magnetron tubes used in microwave ovens, to ultra-high vacuum of 1 E-10 torr for travelling-wave tubes.

The moon would be a good location for a large, high-energy particle accelerator for several reasons, one of which is the vacuum ambient.

Intersecting Storage Ring (ISR) accelerators require very good vacuum, since any residual gas tends to scatter and defocus the beam. At a pressure of 10 nanotorr the beam lifetime is typically around one hour; and operating pressures of under 0.01 nanotorr are required for long lifetime storage and operation [13]. An additional problem is that whenever the beam tube is vented to atmosphere, gas is adsorbed onto the surfaces which is later desorbed by the beam current.

This vacuum will be degraded by human habitation and industrial processing of materials. It is unlikely that maintaining the lunar vacuum will be an important priority of the occupants of a moonbase. The amount of degradation can be calculated by multiplying the mass of gas exhausted times the gravitational acceleration of the moon and dividing by the lunar surface area. This factor is equal to 3.2.E-13 torr per (metric) ton of gas exhausted. Since the exhausted gas has an average lifetime in the lunar atmosphere of 100 days, the equilibrium contribution to the atmosphere is 1.3.E-12 torr per ton of exhaust gas per month.

3. Twenty Person Base

The major contribution to the lunar atmosphere from a small exploration base is exhaust gas from the transport. Assuming a specific impulse of 400 seconds (90% of the theoretical specific impulse of a hydrogen/oxygen engine), landing on the moon requires 0.8 tons of propellant per ton of landed material. If we assume a 10 ton lander making one trip per month with 2 tons of supplies landed per person per month (including the personnel rotation, machinery, scientific and exploration equipment, etc.), this results in an equilibrium pressure of 0.06 nanotorr for a 20 person base. This does not assume that the lander is refueled on the moon from lunar oxygen (i.e., it includes the fuel use to relaunch the lander, but does not assume that any payload is carried from the moon).

In actuality, it is not correct to assume that all of the propellant expended from the ship will contribute to the lunar atmosphere. The exhaust velocity of a hydrogen/oxygen engine is 4 km/sec, nearly double the lunar escape velocity. Further, if the trajectory used is an insertion into low lunar orbit followed by a descent burn, for much of the engine burn the exhaust will not be directed toward the lunar surface. However, for a rough calculation here I assume that the entire engine exhaust contributes to the atmosphere.

Another contribution to the generated atmosphere is air leakage from the living quarters. One estimate [14] of air leakage from an advanced long-duration habitat at atmospheric pressure is 1.2 kg of oxygen plus 4.5 kg of nitrogen per person per day. This would result in a pressure contribution of 0.004 nanotorr for a 20 person base. It has frequently been proposed that oxygen be locally generated. If this is done, it is unlikely that nitrogen dilution would be used, since nitrogen is nearly absent on the moon. Thus, the habitat pressure would be proportionately lower, and the leakage rate is expected to be reduced to 23% of that listed above. However, as discussed below, lunar generation of oxygen would itself be likely to be a source of leakage of waste gas.

In addition to this leakage, air will normally be lost during ingress and egress for extra-vehicular (or extra-habitat) activities ("EVA"). The amount of air lost will depend on whether the airlock is simply vented during egress, or if the lock is pumped down and the exhaust air reused.

In the baseline case, I will assume that the lock is simply vented. If there is one EVA per person per day, and the lock volume is 2 cubic meters of air at one atmosphere pressure, this then results in a contribution of 0.0017 nanotorr for the 20 person base, which is somewhat less than the habitat leakage (and, like the leakage, reduced if the base is assumed to have a pure oxygen atmosphere).

Table 1 summarizes the contributions of the various gas sources discussed.

The daytime total atmosphere is in the range of 0.07 nanotorr, comparable to the natural lunar atmosphere. During the lunar night, most of this will be adsorbed into the soil, resulting in considerably lower pressure.

4. 250 Person Industrial Facility

If large-scale industrialization takes place on the moon, it could be expected that the lunar habitat may have hundreds of inhabitants, and considerably more frequent resupply flights. In this case, the vacuum degradation will be correspondingly worse. The baseline calculated here will be for a 250 person base processing oxygen from lunar soil.

I assume here slightly less support material required from Earth, 1 ton of material per person per month; however, since the lander is fueled from lunar-produced oxygen, the fuel for the lander must be delivered into lunar orbit. Total gas contribution to the lunar atmosphere is 720 tons/month, for a pressure contribution of 0.6 nanotorr. Habitat leakage and airlock losses will contribute 0.05 nanotorr.

Lunar oxygen production to fuel the lander will require 400 tons of oxygen per month. A 25% loss rate, which is realistic for a low-cost industrial process, would contribute 0.13 nanotorr. If the trans-lunar injection ship is also to be fueled, this is a additional contribution. It has often been proposed that lunar oxygen production could be used as a cheap source of fuel for spacecraft to be used from Earth orbit. I assume a baseline facility designed to deliver oxygen to Earth orbit at a production rate of 500 tons per month. Lifting this from the moon will require 400 tons of fuel, and leakage losses will be about 200 tons. The contribution to the lunar atmosphere is 0.78 nanotorr. This will be considerably less, however, if the oxygen is to be shipped by mass-driver rather than lifted off the surface by rocket.

Mining of the lunar regolith for helium 3 (3He) to fuel terrestrial deuterium-helium 3 fusion reactors has recently become a topic of interest [15]. 3He implanted into the lunar regolith by the solar wind would be extracted by baking the soil, and then distilled. For every ton of 3He produced, about 3300 tons of helium 4, 6100 tons of hydrogen, 3000 tons of carbon monoxide and dioxide, and 500 tons of nitrogen will be produced [15]. 10 tons of 3He would be required to be mined per year if half the US electrical consumption of 285 GWe is to be produced. Most of the byproduct gasses produced will be useful to the lunar base. Except for refrigeration and pressurization use, however, the helium produced will not be of great use, and may eventually leak to the atmosphere. This was not assumed in the following analysis.

Since the escape lifetime for hydrogen and helium is much shorter than that for other gasses, these must be considered separately. If 25% of the gas content is lost as waste due to soil agitation during mining plus leakage and waste in the baking and condensation process, production of 10 tons/yr of 3He would produce 23500 tons/yr of waste hydrogen and 4He, plus 12000 tons/yr of heavier gas. Assuming an escape lifetime of 10000 seconds for the hydrogen and helium, this results in a contribution of 0.002 nanotorr for helium and hydrogen, and 0.87 nanotorr for heavier gasses.

The impact of helium 3 mining on the lunar atmosphere has also recently been considered by Duke [1], who concluded that stripping 100,000 tons of regolith per year would release an amount of trapped gas "roughly equivalent" to the existing lunar atmosphere.

In addition, the moonbase is likely to be a place where various other mining, refining and manufacturing operations take place, producing solar cells, aluminum and titanium structures, habitation modules, and probably other objects useful to further colonization. These processes will involve some amount of gas generation and, consequently, wastage. Until the processes are more completely specified, the contributions from this processing is unknown.

Finally, the lunar soil contains trapped gas at a concentration on the order of 50 ppm by weight, primarily hydrogen and helium from the solar wind, plus and carbon compounds and nitrogen. This is only loosely bound to the soil, and physical disturbance, as well as movement of soil by mining, etc., will likely release some of the gas content. This contribution is expected to be negligible compared to other sources.

The total contribution to the lunar atmosphere from the assumed industrial facility producing both oxygen and helium 3 is 2.5 nanotorr (see Table 1), a factor of 5-100 higher than the "natural" daytime atmosphere. This is low enough that manufacture of amorphous silicon solar cells can be performed without any additional vacuum pumping. For other processes discussed, such as MBE, travelling-wave vacuum tube formation, or siting of a large accelerator on the moon, the vacuum is not good enough, and these will require additional pumping.

While the lunar vacuum may not be sufficient for some operations, it must be kept in mind that even after degradation, the ambient remains a very high vacuum. It is much easier to pump a starting ambient of 1 E-9 torr down to ultra-high vacuum levels of 1 E-11 than it is to reach ultra-high vacuum starting from atmospheric pressure. Leaks and virtual leaks will be little problem; there will be almost no problem with desorption of gasses from chamber walls that have been exposed to ambient, and finally, the "vacuum chambers" will not be required to hold up to the large mechanical pressure of 10 tons/m2 imposed by the Earth's atmosphere.

It is an advantageous feature of the moon that the vacuum is self cleansing by the solar ultraviolet and solar wind. "Air pollution" is a temporary effect. If it is decided that a high vacuum is required, a wait of a few hundred days will suffice for the gas to be removed by the solar wind.

However, this is only true as long as the amount of atmosphere present is low enough that there is little shielding of the solar UV. This is likely to be true for the amounts of gas discussed in the present paper. Some amount of gas will be adsorbed by the lunar soil. Cleansing of this gas to restore the original ultrahigh vacuum will take longer, since the soil will take time to outgas.

5. Atmosphere Variation with Position

The calculations have so far assumed that the atmosphere generated can be assumed to be evenly distributed around the moon.

The gas input mechanisms discussed are either continuous or periodic with a characteristic time less than or equal to the resupply time, assumed to be one month. This is much shorter than the escape time, and so the overall variation with time is expected to be small. However, in the vicinity of intermittant gas sources, such as the exhaust plume of a lander, will be temporary large increases in the gas concentration. Sensitive processes would likely be shut down during such periods.

Gas molecules escape from the atmosphere primarily from the sunlit hemisphere of the moon, where they have higher kinetic energy and also are subject to photoionization by solar UV. Thus, the escape lifetime is determined by the gas distribution on the sunlit hemisphere.

For pressures of nanotorr and below, the gas in the atmosphere can be well modelled by ballistic transport. Gas molecules leave the surface with random direction and a thermal velocity profile, follow a ballistic trajectory until again intersecting the surface, and then may be temporarily adsorbed by the surface before being reemitted, again at a random direction and velocity. Temporary adsorption of gas by the surface is irrelevant to the calculation of equilibrium atmosphere pressure by a steady-state source, since the adsorbed gas neither contributes to the total pressure nor is subject to escape; however, a large amount of gas stored in the adsorption reservoir will proportionately increase the time needed to reach equilibrium pressure, and also increase the time needed to purge the atmosphere after the gas source is discontinued.

A complete transport calculation would integrate over the thermal (Maxwell-Boltzmann) velocity distribution, averaging over the hemispherical angular distribution, and also take into account the spherical lunar geometry and gravitational potential. A more complete calculation would include gas-gas collisions and the variation of temperature over the lunar surface.

For an order of magnitude calculation, however, it is sufficient to assume that all the molecules can be characterized by the average thermal energy of kT/2 per degree of freedom. At a temperature of 365=B0 K, this yields root mean square (RMS) vertical and radial velocities of 300 and 430 m/sec respectively for an O2 molecule. The horizontal d travelled on a parabolic trajectory is thus 160 km, and the time in flight 380 seconds. This distance is sufficiently small compared to the circumference of the moon that the assumption of parabolic trajectories is justifiable.

In a random walk process the expected distance from the origin equals the distance d per step times =88N, the square root of the number of steps; thus, the area covered equals =BCd2N. To cover the surface area of the moon thus requires roughly 450 steps, a flight time of 47 hours. This time is short compared to the escape lifetime of gas in the atmosphere, thus, the assumption of roughly uniform gas distribution is justified, and there will not be a significant difference in the amount of gas near the base compared to far from the base.

For other molecules, the time is proportional to (kT/m)^-3/2. Water vapor, for example, with a molecular weight of 18, will spread across the surface considerably faster. Hydrogen and Helium spread across the full surface area of the moon in a time of roughly an hour. Since the escape time for hydrogen and helium is considerably less than an hour, gas concentrations for hydrogen and helium can not be assumed uniform, and considerable variations in density will exist between areas close to the gas source to areas far away.

On the night side of the moon, the typical temperature is only 100K.

Molecules thus take six times as long to diffuse across the same area, and since any given molecule will spend six times as long on the night hemisphere as on the day hemisphere, the gas reservoir on the night side will be proportionately greater. Again, it should be noted that these times are exclusive of any time spent adsorbed in the soil.

These conclusions are different from those of Burns et al. [16], who particularly discuss column density with respect to opacity of the atmosphere for UV and radio astronomy, and conclude that the local pressures may be many orders of magnitude larger than the equilibrium pressure at locations close to a gas source.

6. Conclusions

Establishment of a lunar base will degrade the lunar vacuum. The time scale for distribution of exhaust gas across the surface of the moon is much less than the excape lifetime of the gas in the lunar atmosphere, and thus exhaust gas can be approximated as uniformly spread across the surface. A 20 person exploration base will contribute an amount of waste gas on the same order of magnitude as the daytime "natural" atmosphere. A 250 person "industrial" facility would be likely to contribute considerably more due to waste gas from various production processes such as lunar oxygen production and mining of helium 3 from the lunar regolith. This could degrade the lunar ambient to levels on the order of 3 nanotorr, replacing the mostly non-reactive gasses hydrogen, helium, and neon with more reactive gasses containing carbon and oxygen. This vacuum is still good enough to perform many important vacuum processes, such as plasma-deposition of amorphous silicon for solar cells, but processes such as molecular beam epitaxy or locating a intersecting beam accelerator on the moon will require additional vacuum pumping. In any case, though, pumping to ultrahigh vacuum will be much easier on the moon than on Earth.


References

[1] M. Duke, "Lunar Atmosphere," in Discussion Panel section of NASA Lewis
Research Center, Lunar Helium-3 Fusion Power Workshop, April 25-26 1988.

[2] J.H. Hoffman et al., "Lunar Atmospheric Composition Experiment", in
Apollo 17 Preliminary Science Report, NASA SP-330, p. 17-1 (1973). 

[3] F.S. Johnson et al., "Cold Cathode Gage (Lunar Atmosphere Detector)",
in Apollo 12 Preliminary Science Report, NASA SP-235, p. 93 (1970). 

[4] G. Jeffrey Taylor, "Geological Considerations for Lunar Telescopes,"
Future Astronomical Observatories on the Moon, NASA Conference Publication
2489, 21-28, 1988.

[5] J.H. Hoffman et al., "Lunar Atmospheric Composition Results from Apollo
17," Proc. 4th Lunar Sci. Conf., 2865-2875, 1973.

[6] R.R. Hodges Jr., "The Escape of Solar-Wind Carbon from the Moon,"
Proc. 7th Lunar Sci. Conf., 493-500, 1976.

[7]  F.S. Johnson, "Lunar Atmosphere," Rev. Geophys. and Space Phys., Vol.
9 #3, 813-823 (1971).

[8] R.R. Vondrak, "Creation of an Artificial Lunar Atmosphere," Nature 248,
657-659 (1974).

[9] G.A. Landis, "Lunar Production of Space Photovoltaic Arrays," 20th IEEE
Photovoltaic Specialists Conference, Las Vegas, NV; 874-879 (1988).

[11] S.M. Sze, Semiconductor Devices Physics and Technology, p. 333; Wiley
and Sons, NY (1985).

[12] M. Yokoyama and S-I Ohta, J. Appl. Phys. 59(11), 2929-3921 (1986). 

[13] F.Y. Juang et al., J. Appl. Phys. 58(5), 1986-1989 (1985).  

[14] R. Calder et al., Proc. IX Conf. High Energy Accelerators, p. 70 (1974).

[15] P.O. Quattrane, "Extended Mission Life Support Systems", AAS 81-237,
in Vol. 57, Science and Technology Series, The Case for Mars, P.J. Boston,
ed.  131-162 AAS, 1984.

[16] G.L. Kulcinski and H.H. Schmitt, "The Moon: an Abundant Source of
Clean and Safe Fusion Fuel for the 21st Century," Lunar Helium-3 Fusion
Power Workshop, NASA Lewis Research Center, April 25-26 1988.

[17] J.O. Burns et al., "Artificially Generated Atmosphere Near a Lunar
Base," Lunar Bases and Space Activities in the 21st Century Symposium,
Houston TX, paper LBS-88-024, April 5-7, 1988.

*National Research Council Resident Research Associate, NASA Lewis Research Center


Table 1: Contributions to Lunar Atmosphere


Source            Contribution  Notes
                  (nanotorr)                                     

20 Person Base:

Propellant              0.06    50 ton lander; all exhaust gas contributes
Habitat Leakage         0.004   6.7 Kg/person/day
Airlock losses          0.002   2 m3 vented; less if pumped down for EVA
Total                   0.07 nanotorr

250 Person Industrial Facility:

Propellant              0.6     refueled using lunar oxygen
Habitat Leakage         0.05    6.7Kg/person/day
O2 Production           0.9     500 tons/month; 25% leakage
3He Mining              0.9     10 tons/year; 25% leakage
Industrial Processing           unknown
Total                   2.5 nanotorr


Geoffrey A. Landis, mercenary scientist
Nyma, Inc., at NASA Lewis Research Center
GLANDIS@LERC.NASA.GOV