An Atmosphere for the Moon

 

N. M. Hoekzema

 

 

 

A thin, semi-stable lunar atmosphere may form within a few centuries. This atmosphere, probably dominated by oxygen, may have a total mass in the order of 1011 kg and a surface pressure of order of 10-2 Pa: i.e., comparable to Earth’s atmosphere at an altitude of about 100 km.

 

It may be feasible to install a thick, moist lunar atmosphere. However, it is at least doubtful whether this can result in the Terra-formation of the Moon in the foreseeable future.

 


Introduction

 

The present extremely thin lunar atmosphere is completely dominated by the solar wind; it contains only about 104--105 kg of hydrogen, and helium, and a small percentage of heavier gases: mainly oxygen, carbon, and nitrogen. If compressed to a pressure of one Barr, this whole atmosphere would fit into a single large Zeppelin. Most of its particles are captured from the solar wind, but part of the oxygen is extracted from the lunar soil by impacts with: energetic photons, solar wind particles, cosmic rays, or (micro) meteorites. The average atmospheric particle is an ion, traveling around in semi-ballistic orbits, and resides in this atmosphere for only a few weeks, before binding with the soil, or being kicked into space by solar wind interactions. 

 

I’ll mention two possible scenarios for the human future and discuss their impact on the lunar atmosphere. The last section deals with speculations about whether it is possible to Terra-form the Moon.


 

 

Humans Altering Atmospheres

 

Humans have significantly altered the Earth atmosphere; e.g., in the past century it has been `enriched' with 1014--1015 kg of carbon dioxide, causing the popular concern about the greenhouse effect. In the recent past humans substantially altered the lunar atmosphere as well; exhaust gases from the Apollo landing and ascend vehicles temporarily thickened it by more then 10%. But these effects were very temporal and within a few months the extra lunar atmosphere had almost completely disappeared through interactions with the solar wind.

 

But what if the human involvement with the Moon were more substantial? If there were ever to be substantial industries on the Moon, what would their impact be on the local atmosphere? To formulate an answer we must consider the possible nature of such industries.

 

Creating them would consume at least hundreds of billions, and more probably a few trillion dollars. It is utterly naive to think that humans would ever make such a huge investment without the certainty of a solid yield that could not be obtained from Earth based industries. Right now I can only think of two slightly plausible scenarios that might initiate extensive lunar settlement in the foreseeable future, together with industries that are important in this respect.

 

1.    Controlled nuclear fusion will finally be mastered and the supply of 32He for fusion reactors will depend on mining the Moon.

 

2.    Humanity runs out of fossil fuels and it turns out to make economic sense to build large Earth orbiting solar power satellites, creating the need for large quantities of lunar ores to construct them.

 

Traveling to the Moon is very expensive, but humanity spends about ten trillion dollars per year to pay for its energy bills, people could easily afford to mine the Moon if it were important for generating a major part of the worlds energy supply.


 

Mining Lunar 32He

 

Future nuclear fusion plants will depend on either 31H (tritium) or 32He. There is no significant natural source for tritium and the cost of manufacturing it could be prohibitive. Its use in fusion generators would generate much radioactive junk making it a less than ideal fuel.

 

32He is a much cleaner fuel, but also not readily available. Though the isotope is stable, it is only present on Earth in extremely minute quantities. Almost all of the helium on Earth originates from alpha decay of natural radioactive isotopes and such decay does not produce 32He.

 

Through the ages about 1015 kg of solar wind particles got stuck in the upper few centimeters of the lunar surface. Most of the captured gases, including about 5% helium, escape when the soil is only slightly heated. Though the lunar resources of helium are smaller than that of Earth, they have a solar origin and therefore contain a much larger fraction of the required isotope; the surface of the Moon contains a few kilograms of 32He per square kilometer. If were required in large quantities today, it would be cheaper to collect it on the Moon than to manufacture it artificially.   

About 100 tons 32He per year could cover the worlds demand for energy; harvesting it on the Moon would not only yield the required isotope, but could free the other one million times more abundant gases as well.


Mining Lunar Ores

 

Once the readily available resources of fossil fuels on Earth are exhausted the Sun may become the main source of energy for humanity. In the late seventies it was calculated that solar power becomes cheaper than fossil energy once the oil price rises above

$70 per barrel (recent years: near $20). At present, placing solar power satellites in Earth orbit would be more efficient than building large scale solar power plants on Earth. In space there are no clouds obscuring the sun, there is no night, the station can always be directed ideally, and there is little weathering that limits the expected lifespan of such installations. Moreover, space is not as crowded as Earth. On Earth it would not be easy to free an area that is several times as large as the surface of the Netherlands, for solar power plants.

 

When building large orbiting solar power satellites, it would be ludicrous to blast all constructing materials up from Earth by rocket. The Moon offers them all, and they can be shipped up from the lunar surface fairly easily with electro-magnetic guns. The building of large solar power satellites thus induces the development of substantial lunar industries of which a blast furnace would probably have the largest impact on the lunar atmosphere.

 

To set the magnitude of such industries: Hoogovens, the Dutch Steel-Works, produced about 6.109 kg of steel in 1994. On the Moon a production facility this size would set free 109--1010 kg of oxygen per year.

 


Thickening the Lunar Atmosphere

 

It is possible that within one century Lunar based industries will dump much gas into the lunar environment. The oxygen output from a substantial mining industry could exceed 109--1010 kg per year. If a fusion industry becomes important, the output from a 32He mining industry could add up to 1010--1011 kg of hydrogen and helium, and 108--109 kg of heavier elements (mostly oxygen, carbon, nitrogen, and neon). Helium and hydrogen will readily escape into space; the gravitational force of the Moon being much too feeble to hold on to such light atoms. The heavier elements will stick to the Moon much longer.

 

At present, particles reside only for very short periods of time in the extremely thin lunar atmosphere because they all interact with the solar wind within a few weeks. A less thin atmosphere could deflect the solar wind, its upper layers protecting the deeper layers from interactions; this would greatly enlarge the stability of the atmosphere. NASA scientist Richard Vondrak calculated that during a year only about 108 kg of atmospheric mass can be carried away by solar wind interactions. There is no other atmospheric loss mechanism that is important on a time-scale of years for gases heavier than helium; dumping 109 kg or more of heavy gases into the lunar environment will thus create a rapidly thickening atmosphere.

 

If the formation of a lunar atmosphere is unwanted, precautions must be taken. The undesirable furnaces could be placed in space, and heavy excess gases could be injected into the Moons crust, where they can react chemically and bind to the soil.

 

If on the other hand the formation of an even thicker atmosphere is desired, a few nuclear bombs could be used to evaporate lunar rocks, which would free most of the oxygen in them. One percent of the US nuclear stockpile could set free as much as 1011 kg of oxygen.

 


A 1011 kg Lunar Atmosphere

 

·       Will mainly consist of O2, but may contain important fractions of N2, CO2, and Ne if partly induced by 32He mining.

 

·       Will be partly ionized, even near the surface.

 

·       Will be stable for hundreds of years

 

·       Will have surface pressure about 10-2 Pa (=10-7 Atm), comparable to Earths atmosphere at an altitude of 100 km.

 

·       Will stop all micro meteorites, and most larger ones as well.

 

·       Will deflect the solar wind, stop almost all cosmic ray particles and roentgen photons, and will thus strongly reduce the radiation level at the Moons surface (now in the order of 10 REM/year, limit for nuclear workers is 5 REM/year)

 

·       Will stop deposition of 32He into the lunar surface. Deposition is a slow process and stopping it will only cause significant effects after thousands of years.

 

·       Apart from some aurora, it will be completely invisible to the naked eye.

 


A Speculative Scenario: Relocating Ice-Dwarfs

 

James Oberg: “Go to the outer solar system, get a small ice-dwarf and dump it onto the Moon. This will create a thick and moist atmosphere, from which the Moon can be truly Terra-formed.”

 

Of course this idea is pretty far over the top for the moment, but is it impossible? If so, why?

 

Is it possible to move around ice-dwarfs with the techniques of the foreseeable future? To set the dimensions of this problem: could humanity ship Mount Everest to Latin America, or could it dump 10% of the Antarctic ice-cap onto the Sahara? Such would be difficult projects, but once properly motivated humans can achieve great enterprises. E.g., near the closing of the last century people discovered gold in a small mountain range somewhere in Southern Africa; within a few years the whole range was shoveled away and all that remained was a tremendously large hole. If a few thousand people can displace several cubic kilometers with not much more than shovels and their bare hands, one may expect that the organized power of Caterpillar, Boskalis, and Smit-Tak could move mildly larger objects like Mount Everest and major parts of the Antarctic ice-cap.

 

But could such undertakings be performed in space? Not right now: of course not. However, if several trillion dollars were already spent on creating large space-based industries, the answer would be different. Of course it would be costly, and it could well take a hundred years or so to transfer the ice-dwarf, but with heavy industries already in space the mission wouldn't really be difficult, not even with present day technology.


Chemistry of the Lunar Soil

 

Is it wise to impact an ice-dwarf onto the Moon? I doubt it.

 

A lunar oxygen atmosphere can exist for many centuries, but only if it is very thin and in the absence of water. What if this atmosphere were moist and oxygen rich?

 

The upper few kilometers of the lunar surface contain several times 1018 kg of iron(II) which in the presence of water would readily react with oxygen to form iron(III). Such an amount of iron(II) could easily absorb all of the oxygen in the Earth atmosphere.

 

A large fraction of the Moons crust consists of oxides of calcium, magnesium, and iron(II), which in the presence of water would react to form hydroxides that would (partly) dissolve in the forming seas to create a poisonously alkaline fluid, with pH 10--11. If enough oxygen were available to oxidize the dissolved iron(II)hydroxides, insoluble iron(III)hydroxides would precipitate on the sea floors and shores, creating vast quantities of slightly poisonous, orange mud. Such reactions would be violent and fast in the upper part of the crust, but their rate would decrease with increasing depth. The oxidizing, hydration, and other processes would continue for ages. In the meantime oxygen and other pressures would not be stable. Most of important all: the absorption of such enormous amounts of oxygen, water, by the upper part of the crust of the Moon would make the rocks expand by perhaps as much as ten percent or more. One can wonder if such expansion would be a tranquil process. It could create strong quakes for possibly many thousands of years.    

 


Conclusions:

 

·       Creation of a thin lunar atmosphere is perfectly feasible, even with present day technology, and in some scenarios even hard to avoid.

 

·       Truly Terra-forming the Moon, if possible at all, could take thousands of years and might yield violent Moon quakes and a highly unstable, chemically aggressive environment during this long period.