Characteristics of the Lunar Environment
Calina C. Seybold
During the planning phase of a space mission, a quick reference guide to the expected environmental conditions can be a useful tool for determining design choices. Therefore, relevant facts on the lunar environment have been summarized below from the sources noted at the end of the article.
|Equatorial Radius||1738 km||6378 km|
|Surface Gravity||1.62 m/s2||9.78 m/s2|
|Escape Velocity||2.37 km/s||11.18 km/s|
|Atmospheric Pressure||10-14 atm||1 atm|
|Seismic Activity||500 quakes/yr||10,000 quakes/yr|
|Magnetic Field Strength||3x10-9 - 3.3x10-7 t||3.0x10-5 t|
- The lunar gravity field is not completely uniform due to mass concentrations (mascons) beneath the surface. However, the accelerations experienced due to the irregularities is generally less than one-thousandth of the surface gravity. (Ref. 2)
- Compared to the Earth, the Moon is seismically stable. The majority of the 500 detectable moonquakes every year fall within the 1-2 magnitude range on the Richter scale. Quakes in this range are generally undetected on Earth due to the seismic background noise. (8)
- The magnetic field on the Moon is a remnant from an extinct magnetism source. Additionally, an external field--generated by the solar wind--ranges from 5x10-9 - 10x10-9 t across the lunar surface. (8)
- Although fears that there might be "destructive organisms" on the Moon drove NASA to implement strict quarantine procedures for the first Apollo landings, subsequent studies of the lunar rocks concluded that the Moon is, and probably always has been, lifeless. (3)
The lunar regolith is chemically composed of several elements and compounds in varying concentrations. The carbon, hydrogen, and nitrogen found in the soil are almost entirely due to implantation by the solar winds. Unlike most Earth soils, the lunar soil has high concentrations of sulfur, iron, magnesium, manganese, calcium, and nickel. Many of these elements are found in oxides such as FeO, MnO, MgO, etc. Ilmenite (FeTiO3), most common in the mare regions, is the best source of in situ oxygen. (5)
Although the exact composition of the lunar interior is unknown, the upper few hundred meters are believed to be rubble generated by eons of meteor bombardment. Micrometeorite impacts alone are sufficient to "churn" the entire regolith about every 40 million years. This process, assisted by the effects of radiation and the solar wind, accounts for the weathering of the lunar surface.
Weathering has left the lunar soil with a relatively fine texture, as illustrated by the grain-size distribution on soil taken from a mare region on Apollo 11 (5).
|Grain Size (mm)
|10 - 4||1.67|
|4 - 2||2.39|
|2 - 1||3.20|
|1 - 0.5||4.01|
|0.5 - 0.25||7.72|
|0.25 - 0.15||8.23|
|0.15 - 0.090||11.51|
|0.090 - 0.075||4.01|
|0.075 - 0.045||12.40|
|0.045 - 0.020||18.02|
|less then 0.020||26.85|
Additionally, a more general distribution scale, drawn from soil samples from several missions, is depicted on the graph below (5).
Contrary to popular perceptions, the Moon does have an atmosphere. However, it is extremely thin. To illustrate, the entire lunar atmosphere, compressed to Earth surface temperature and pressure, would fit into a 210 foot cube. Although the research is not complete, helium, argon, sodium, and potassium have been identified as atmospheric elements. Of these, the helium most likely comes from the solar wind, while the argon originates in the lunar interior. The discovery of sodium and potassium gives the lunar atmosphere important (research-wise) similarities to the atmospheres of Mercury and Io. (7)
Because the atmosphere is too weak to provide protection, sunlight strikes the lunar surface with an intensity unknown on Earth. During the month-long lunar day, equatorial temperatures can range from 400 K to 100 K, with rapid (5 K/hr) temperature changes at sunset and sunrise. In the polar regions, where the sun does not rise more than 1.5¡ above the horizon, there are many crater floors which are in permanent shadow. These areas, with their constantly cold (about 80 K) temperatures, are the best hope for locating permafrost (water) on the Moon. (2, 8)
The thin lunar atmosphere also creates radiation and micrometeorite impact concerns for equipment deployed on the surface. Three radiation sources affect the Moon: galactic cosmic rays, solar flare particles, and solar wind particles (see table).
|cosmic rays||1-10 Gev/nucleon||1||few meters|
|solar flares||1-100 Mev/nucleon||100||1 cm|
|solar wind||1000 ev/nucleon||108||10-8 cm|
Of these, the solar flare particles are the most dangerous to electronic instruments.
Microcraters, known as zap pits, will be fairly common on the exposed surfaces of deployed equipment. Based on data from lunar rocks, Apollo windows, and the Surveyor 3 camera shroud, the expected numbers of craters per square meter per year are given below. (8)
|Crater Diameter (10-6 m)
||Craters (m-2 yr-1)
|> = 0.1||30 000|
|> = 1.0||1 200|
|> = 10||300|
|> = 100||0.6|
|> = 1000||0.001|
Selected Design Considerations
- Each Apollo mission increased the mass of the lunar atmosphere by about 30%, and it took several weeks for the atmosphere to return to its natural state (7). Highly sensitive measurement or communication instruments may be affected by the additional atmospheric particles.
Do any of our instruments depend on the lunar atmosphere to be in its natural state? If so, how long will it take the atmosphere to return to normal?
- Deployment of penetrators to the lunar surface will cause both compaction and cratering of the lunar regolith. Therefore, the soil in the landing area may not be representative of the natural surface. Additionally, the structure and payload will have to withstand the shock of impact.
How much stress will the spacecraft experience on impact? How deep will the spacecraft penetrate into the soil? How far will the resulting ejecta blanket extend?
Do we need "pure" soil samples for our experiments? If so, how will we get beyond the ejecta blanket? Will there be any chemical reactions in the soil due to the friction/heat associated with the landing?
- It is expected that about 30 000 zap pits per year will be formed on a given square meter of the lunar surface (8). Exposed surfaces on the spacecraft and external experiments must be able to absorb this type of bombardment for the lifetime of the mission.
What is the lifetime of the mission? How much exposed surface is there? Given the answers to the previous questions, what is the risk of zap pits being a problem? What are our contingency plans (if any)?
- Solar flare particles have high energies (1 - 100 Mev/nucleon) and high fluxes (100 cm-2 s-1) (8). Electronic devices, e. g. for experiments or communications, must be able to withstand this environment.
Is the mission being conducted during a period of high solar flare activity? Are any of our instruments sensitive to radiation? If so, what are our shielding plans? What are our contingency plans (if any) if a strong solar flare or other galactic radiation event occurs during the mission?
- The fourteen-day lunar days and nights have correspondingly extreme hot and cold temperatures with rapid adjustments from one to the other. Spacecraft instruments may need thermal protection to cope with these adjustments.
Will our spacecraft be in permanent sunlight/shadow? If not, what thermal protection system (if any) do we have for the instruments?
Lunar Environment References
Bernold, Leonhard E. "Compaction of Lunar-Type Soil." Journal of Aerospace Engineering, Vol. 7, No. 2, pp. 175-187, April 1994.
Bowell, E.L.G. "Surface of the Moon." The Earth and Its Satellite, ed. John Guest, New York, pp. 100-111, 1971.
Horowitz, Norman H. "To Utopia and Back". New York, pp. 65-67, 1986.
Lin, Chuan-Ping, et. al. "Model Studies of Effects on Lunar Soil of Chemical Explosions." Journal of Geotechnical Engineering, Vol. 120, No. 10, pp. 1684-1703, October 1994.
McKay, David S. and Douglas W. Ming. "Mineralogical and Chemical Properties of the Lunar Regolith." Lunar Base Agriculture: Soils for Plant Growth, ed. D.W. Ming and D. L. Henninger, Madison, WI, pp. 45-68, 1989.
Moore, Diane F. The Harper Collins Dictionary of Astronomy and Space Science, New York, 1992.
Stern, Alan. "Where the Lunar Winds Blow Free." Astronomy, Vol. 21, No. 11, pp. 36-42, November 1993.
Taylor, G. Jeffrey, "The Environment at the Lunar Surface." Lunar Base Agriculture: Soils for Plant Growth, ed. D. W. Ming and D. L. Henninger, Madison, WI, pp. 37-44, 1989.
CSR/TSGC Team Web