James R. Gaier

Glenn Research Center, Cleveland, Ohio

Regolith Activation on the Lunar Surface 

and Its Ground Test Simulation

NASA/TM—2009-215648

October 2009

2009–01–2337
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James R. Gaier

Glenn Research Center, Cleveland, Ohio

Regolith Activation on the Lunar Surface 

and Its Ground Test Simulation

NASA/TM—2009-215648

October 2009

2009–01–2337

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Regolith Activation on the Lunar Surface and Its Ground Test Simulation 

James R. Gaier 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Activation of the surfaces of lunar regolith particles can 

occur through interactions with solar electromagnetic radiation, 

solar and galactic particle radiation and micrometeoroid 

bombardment. An attempt has been made to quantify the 

relative importance of each of those effects. The effects of 

these activated surfaces may be to enhance the adhesion and 

toxicity of the particles. Also key to the importance of 

activation is the lifetimes of activated states in various 

environments which is controlled by their passivation rate as 

well as their activation rate. Although techniques exist to 

characterize the extent of activation of particles in biological 

system, it is important to be able to quantify the activation state 

on the lunar surface, in ground-test vacuum systems, and in 

habitat atmospheres as well. 

Introduction 

The surface of the Moon is covered by a layer of crushed 

rock from 8 to 48 m deep known as the lunar regolith (Ref. 1). 

The composition of the regolith samples returned from the six 

Apollo sites and the two Luna sites have been studied in great 

detail. These are summarized in the Lunar Sourcebook, (Ref. 2) 

and numerous other references. From these studies it is known 

that 65 to 90 percent of the regolith is composed of silicate 

minerals, depending on location on the lunar surface. The 

largest component is made of plagioclase feldspars [(CaNa)(AlSi)4O8], followed by pyroxenes [(Ca-Fe-Mg)Si12O6] and 

then olivine [(Mg-Fe)2SiO4]. Oxide minerals such as ilmenite 

[(Mg-Fe)TiO3] and spinels [FeCr2O4, FeTiO4, FeAl2O4, 

MgTiO4] are present as well. Although the spinels occur in 

only trace amounts, they are extremely hard and may be 

important abrasive agents. Ilmenite, which can act as an 

oxygen ore, has been found in mare basalts in concentrations as 

high as 20 percent, but only about 0.5 percent in the highlands. 

Volcanic and impact glasses can make up from about 7 to 30 

percent of the regolith. In addition there are small amounts of 

native metals [Fe0

, Ni0, Co0]; phosphate minerals, primarily 

whitlockite (Ca3(PO4)2); and sulfur, primarily in the form of 

troilite [FeS]. Some soils contain small amounts of material 

high in potassium (generally much less abundant on the Moon 

than on Earth), rare Earth elements, and phosphorus referred to 

as KREEP. In addition there are about 100 other minerals 

present in trace amounts, some of which have not been 

observed terrestrially. 

It is thought that the lunar regolith is formed by a series of 

highly energetic processes known as space weathering. 

Energetic solar particles and galactic cosmic rays constantly 

bombard the lunar surface. This results in the implantation of 

atoms, primarily hydrogen and helium, as well as sputtering of 

atoms off of surface of particles. Meteoroids continually strike 

the surface as well. Even though the vast majority of these are 

tiny (<1 mm) they strike the surface with so much kinetic 

energy that they both break up rock and melt it. A series of 

complex processes follows from this that includes the release 

of adsorbed and implanted hydrogen and helium, the release of 

oxygen from the rock structure, the reduction, vaporization and 

redeposition of iron, and the flow of molten rock downward to 

form agglutinates and impact breccias. 

Regolith Activation 

One of the results of these energetic processes is that a 

relatively large number of highly reactive surface atoms are 

thought to be formed. Such surfaces are said to be “activated”. 

Activation includes any process that enhances the chemical 

reactivity of the surface. Activated states can result from three 

basic mechanisms. The first is the ejection of electrons from 

the surface by light in the ultraviolet or x-ray regions (0.01 to 

300 nm wavelengths) resulting in free radicals, that is, atoms 

with unpaired electrons. These are usually chemically unstable 

and will react with the first atomic collision after they are 

formed. The second is the sputtering of atoms off of the 

surface, which often results in the remaining surface atoms 

having “dangling bonds”, or unsatisfied valence shell electrons. 

These are also chemically reactive. Third, high energy 

collisions with micrometeorites or energetic particles ejected 

from the Sun can result in crystal lattice defects, with atomic 

displacements which can lead to highly strained structures. 

These highly strained structures are also chemically very 

reactive. 

Passivation is the process of relaxation of atoms back to the 

ground state. These processes are dominated by collisions with 

low energy atoms, but also include spontaneous emission of 

light, or radiationless relaxation processes. Because of the 

ultrahigh vacuum present on the Moon, 10–12 Torr during the 

day and 10–14 Torr during the night, there are few collisions 

with gas phase atoms to relax or passivate these states. Thus, it 

is expected that the concentration of these activated states can 

become significant. 

The presence of activated surface states may materially 

change some of the properties of regolith particles. This is 

expected to be most significant for the smallest particles (i.e., 

dust), since they will have the highest surface to volume ratios. 

Adhesion of the dust to other materials, including spacecraft 

NASA/TM—2009-215648 1 
components, is expected to increase as the probability of 

forming covalent bonds from free radicals and dangling bonds 

is higher. This would make it more difficult to clean the dust 

from sensitive spacecraft surfaces. If the activated dust is 

brought into contact with the humid air of the cabin, it might 

accelerate corrosion or even might catalyze unwanted reactions 

of trace gases in the atmosphere. Finally, there is also a 

possibility that the toxicity of the dust could be dramatically 

increased, as happens terrestrially with silica (Ref. 3). 

The technologically important question is not whether the 

dust is activated, but rather, if it is activated what are the 

consequences? For example, it is reasonable to assume that the 

adhesion forces between the regolith and a sensitive spacecraft 

surface (such as a radiator or a photovoltaic array) will increase 

since there will be more opportunity for chemical bond 

formation when one or both of those surfaces is chemically 

active, but will it change the performance of the surface or the 

techniques to clean it? 

It is well known that adhesive forces may increase a 

hundredfold or more in ultrahigh vacuum due to clean body 

effects (Ref. 4). But it is not yet known whether activation will 

significantly increase the adhesion even more. But if it does 

and we understand the mechanisms by which the adhesion is 

enhanced, it may point the way to mitigation technologies. For 

example, cleaning off dust in an airlock that has been partially 

pressurized may be much more effective than cleaning in the 

hard vacuum of the Moon because a large fraction of the active 

sites may be passivated before the cleaning process begins. 

If the effects of activated dust on spacecraft components and 

health are to be studied, techniques must be developed to 

measure the degree of activation, and to generate activated 

dust. But the detection and quantification of activated surfaces 

is not a trivial matter. Physical changes, such as the 

displacement of surface atoms and the imaging of defect sites 

can be done using atomic force microscopy (AFM), 

transmission electron microscopy (TEM), field effect 

transmission electron microscopy (FETEM), or electron 

diffraction. However, these techniques can also impart surface 

damage of their own, and it is difficult to unambiguously 

assign the cause to a particular defect. They are also time 

consuming and can only be done after the test is completed. 

What is needed is a diagnostic test that can be carried out in 

situ so that the activation state of the lunar regolith or lunar 

regolith simulant sample can be verified before it is used to test 

hardware components. 

One test that has been proposed is the utilization of a quartz 

crystal microbalance (QCM). These instruments are very 

sensitive to mass changes. So if a surface is activated it will 

react with a reactive gas, such as oxygen, to a greater extent 

than a passivated surface would. Such an additional mass 

uptake may well be measurable using a QCM. 

A novel technique to quantify the activation of lunar dust 

simulants has been explored by Vander Wal et al., who 

monitored the electrical conductivity of the dust as it was 

subjected to a discharge plasma in a vacuum chamber (Ref. 5). 

Although much work remains to quantify this effect, it is 

relatively straightforward to implement within a vacuum 

chamber, and so holds promise as an in situ activation detector. 

One of the major obstacles is that a quantitative measure of the 

activation state of the dust on the lunar surface itself is 

unknown. This technique lends itself to an instrument that 

could be mounted on a lunar lander to determine the dynamic 

changes in activation state of dust on the lunar surface. 

In all likelihood some fraction of dust will be tracked from 

the lunar surface into the lunar habitat and pressurized rover 

cabins. The activated surface of this dust can react with oxygen 

and water vapor to form hydroxyl (OH) peroxyl radicals 

(OOH). These are very reactive and corrosive agents that if 

present in the cabin atmosphere in high enough concentrations 

over long enough times can corrode metals and embrittle 

polymers. 

Dust activation is also of intense interest to NASA’s 

toxicology community due to the possibility that activation can 

significantly enhance the toxicity of lunar dust. In this case, the 

questions revolve around the passivation mechanisms in the 

humid air within the lung. If the dust is passivated in air 

quickly, then whether activation enhances the toxicity is less 

important than if the passivation times are long enough to 

affect human exposure. If the species persists long enough to 

be transported into the lung, then as they passivate they may 

generate reactive oxygen species such as hydroxyl and peroxyl 

radicals in high enough concentrations to overwhelm the 

body’s defenses and damage biological tissues. 

Wallace and co-workers have developed procedures and 

analytical tests to quantify activation under biological 

conditions (Ref. 6). The tests rely on the activated surfaces 

generating reactive hydroxyl radicals, which are then assayed 

with a fluorescent probe. This is a particularly relevant test as it 

is these secondary products like hydroxyl and peroxyl radicals 

that do the most damage to biological systems. 

Evidence For Regolith Activation 

There is experimental evidence that the lunar regolith exists 

on the lunar surface in an activated state. For example, Hoyt, et 

al., were able to reconstruct the solar flare proton spectrum 

over the past 5000 yr by studying the thermoluminescence 

caused by radiation damage (Ref. 7). They found radiation 

damage (lattice dislocations) in the top 18 mm of lunar rock, 

decreasing towards the bottom. Most of the damage was 

attributable to solar flare protons. Cosmic ray damage was 

found even deeper within the rocks. 

In another example, Grossman, et al., measured the cohesion 

in a porous friable microbreccia in Apollo 11 sample 10065-33 

in a vacuum of 7×10–10 Torr (Ref. 8). Upon cleaving the sample 

the cohesion between the two pieces was measured to be 

8×10–3 N. After just 4 min the cohesion dropped to 2×10–3 N, 

and within 15 min the forces were too weak to be measurable. 

This may be due to quenching of activated sites by gas 

NASA/TM—2009-215648 2 
molecules. Although tested at 10–10 Torr, the pressure is still 

two to four orders of magnitude higher than lunar pressures. 

Further evidence of regolith activation comes from the 

experience of the Apollo astronauts. Several of them remarked 

that when they first took their helmets off after being on the 

lunar surface that the lunar dust had a pungent odor, often 

described as the smell of burnt gunpowder. This smell can be 

the result of olfactory cells responding to free radicals, such as 

NOx (a stable free radical) which has a pungent odor as well. 

Further, the lunar curator reported at the Biological Effects of 

Lunar Dust Workshop in 2005 that “the gunpowder smell went 

away in a few hours (Ref. 9).” This would be consistent with 

the passivation of free radicals on the particle surfaces. 

Lunar Simulants 

In order to simulate the effects of the particle surface 

activation four conditions must be met. First, the particles 

being activated must have the correct bulk composition and 

chemistry. Starting with materials that have a different 

elemental composition, or even with materials with the same 

composition but a different crystal form will not lead to the 

same surface chemistry. Second, activation mechanisms must 

be used that will produce the same types of surface damage or 

modification. This does not imply that the activation must be 

done using the same mechanism. An atom that is a free radical 

or has dangling bonds or is in a strained lattice position will 

react the same regardless of how that excited state is formed. 

Third, the passivation mechanisms must be slow enough that 

the activated dust has time to react with whatever other 

substances or surfaces. Particles activated in air, for example, 

are likely to react with the surrounding atmosphere long before 

reaching the substance of interest, given that there are about 

30 million collisions per second for a gas at atmospheric 

pressure (Fig. 1). Last, there must be some way to detect the 

presence of activated surface species, so that effects seen can 

be rightly attributed. 

Due to the paucity of lunar material available on Earth, most 

work will necessarily be carried out using lunar simulants. 

There are several lunar simulants that are available, and more 

are being developed. Probably the most used simulant at the 

time of writing is JSC-1A, which mimics lunar mare regolith, 

and the most prominent in development in NU-LHT, which 

mimics lunar highlands regolith. 

Lunar simulants are made from terrestrial rock and minerals, 

and some common components of terrestrial rock are rare or 

absent in lunar materials. Potassium feldspars [(K-Al)Si3O8] 

and silica [SiO2] though abundant in terrestrial rock, are rare in 

the lunar samples. Absent on the Moon (at least in the 

equatorial regions) because they contain water are micas, clays, 

and amphiboles. These minerals should likewise be absent 

from lunar simulants. In addition, the simulant should have a 

substantial glassy fraction, since much of the lunar regolith is 

dominated by glasses produced during space weathering. This 

is important because displacement of surface atoms in a 

Collision Frequency 

Figure 1.—Collision frequency calculated for N2 molecules at 

298 K as a function of pressure from kinetic molecular 

theory (Ref. 10). Other molecules in the lunar temperature 

range give similar values. 

highly ordered crystal lattice is a higher energy process than 

displacement of surface atoms in a glass. For example, if an 

oxygen atom in a disordered glass structure is bound to a single 

silicon atom instead of two, calculations show that the energy to 

remove it from the lattice is decreased by about 5 eV (Ref. 11). 

The surface properties of lunar simulant under terrestrial 

conditions will not mimic the surface properties of lunar 

regolith under lunar conditions. In fact, lunar regolith will not 

react the same under terrestrial conditions as it will under lunar 

conditions. The environment of the Earth is humid, oxidizing 

and protected from high energy radiation by the Earth’s 

magnetic field and atmosphere. In contrast, the lunar 

environment is dry, chemically reducing, and constantly 

bombarded by high energy electromagnetic and particle 

radiation. The surface chemistry of any material will be 

different in these two environments. 

The first step in any process to produce activated lunar 

simulant would be to remove the adsorbed layer of Earth 

atmospheric gases from the surface of the particles. Such 

studies are being carried out in the Lunar Dust Adhesion Bell 

Jar (LDAB). This is a facility built to determine the effects of 

the lunar environment on adhesion to sensitive spacecraft 

surfaces. Details of its philosophy and design have been 

reported elsewhere (Ref. 12). By far the largest contaminant is 

water vapor, which is almost totally absent from the lunar 

environment. Studies suggest that heating the simulant in vacuo

or even in an atmosphere of dry nitrogen to a temperature of 

200 °C can accomplish this (Ref. 13). There is an additional 

problem with water in that most terrestrial rock have inclusions 

that contain water between the crystalline grains deep within 

the rock. This may take very high temperatures to remove, but 

removing it may not be necessary if diffusion of this reservoir 

of water to the surface is sufficiently slow. 

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E+12

1.E-15 1.E-12 1.E-09 1.E-06 1.E-03 1.E+00 1.E+03 1.E+06

Pressure, Torr

Collisions/sec 

NASA/TM—2009-215648 3 
The second largest contaminant is likely organic molecules such 

as methane, ethane, and hydrocarbon oils that are nearly 

ubiquitous in the Earth’s environment. It is easiest to remove these 

organics through oxidation. There are many routes to oxidation, 

but using a gas phase method that will not distort the surface 

composition of the rock is probably best. Either oxygen plasma (as 

is used in the LDAB) or ozone generation can result in reactive 

oxygen that can convert hydrocarbons to volatile carbon dioxide. 

Since the dominate minerals are aluminosilicates, silicates, and 

oxides, the surface composition will likely be unchanged. 

The oxidation processes described above will leave the 

surface highly oxidized. This means that transition metals such 

as iron will likely be in very high oxidation states, such as iron 

(III). The lunar surface, on the other hand, is being bombarded 

by a constant stream of hydrogen and helium atoms and ions 

from the solar wind, which generate a highly reduced surface. 

Iron (III) is almost unknown on the lunar surface, with the iron 

being predominantly in the iron (II) state, or even in the iron 

(0) state. So this preponderance of water and oxygen that is 

found adsorbed to the surface of terrestrial materials must 

replaced with a preponderance of hydrogen. 

After a correct bulk material has been obtained, the terrestrial 

surface layer stripped off, and lunar-like surface layer has been 

added, then the simulant can be activated. There are many ways 

this can be accomplished. In the LDAB the dust is subjected to 

a second plasma, where the gas is a 6 percent hydrogen in 

helium mixture. Although the solar wind is about 

96 percent protons and 4 percent α particles, the much higher 

helium content was selected for safety reasons. It has also 

found that the sticking coefficient of hydrogen is so much 

higher than helium that hydrogen dominates the surface 

adsorption despite its lower concentration. Whenever the dust 

is stirred, sifted, or crushed in the LDAB, hydrogen is released 

but helium is not. 

Mechanisms of Activation 

As illustrated in Table I, typical mineral bonds have strengths 

that range from 3 to 9 eV (Ref. 14). Thus any mechanism that can 

impart more than about 3 eV to a surface atom has the potential to 

activate it. 

TABLE I.—ENERGY OF OXIDES FOUND IN LUNAR REGOLITH†

eV ΔHbond ΔHform ΔHlattice ΔHsublimation ΔHionization

SiO2 8.15 –9.44 136 4.66 45.1 

TiO2 7.28 –9.79 126 4.90 43.3 

Al2O3 4.98 –17.4 165 3.42 28.5 

CaO 4.77 –6.58 35.4 1.84 11.9 

Cr2O3 4.37 –11.8 158 4.11 31.0 

MgO 3.90 –8.20 39.3 1.52 15.0 

Na2O 3.34 –4.29 25.7 1.11 5.14 

FeO 1.60 –2.82 39.3 4.32 16.2 

 

†

Although the metals are largely bound to oxygen in the mineral 

crystal lattices, and traditionally expressed as oxides as in this table, it 

is not correct to think of the minerals as being a simple collection of 

metal oxides, as their structure is much more complex. 

One potential energy source is the light energy emitted by 

the Sun. The Sun is a much more potent source of ultraviolet 

radiation on the Moon than the Earth because a substantial 

portion of the solar electromagnetic output is filtered out by the 

atmosphere of Earth, much of it by the ozone layer. A plot of 

the standard “air mass zero” is shown in Figure 2 (Ref. 15). 

This is the spectrum observed just above the Earth’s 

atmosphere, and is what is seen on the lunar surface as well. 

Note that the bulk of the radiation is in the visible and infrared 

regions of the spectrum, between about 0.2 and 7 eV. 

Work by Sternovsky et al., found that the work function 

(energy to extract an electron from the surface) of the lunar 

regolith can be expected to be about 5.8 eV 

(Ref. 16). Analysis of the spectrum of the solar energy reaching 

the lunar surface reveals that about 0.1 percent of the energy 

released by the Sun is in the form of photons that have energy 

greater than 5.8 eV (wavelength <213 nm), enough energy to 

remove an electron and form a free radical. Given that the 

intensity of the solar flux is 1366 W/m2

, on average each 

surface atom will be hit by a photon with more than 5.8 eV of 

energy about once every 2 min. 

In the LDAB there are two processes that generate UV light 

with more than 5.8 eV. The first is through two radio frequency 

plasmas used to prepare the dust. The first plasma is an air 

plasma used to oxidize away organic matter contaminating the 

dust. The second plasma is a hydrogen-helium plasma used to 

chemically reduce the surface states. Analysis of the emission 

spectrum reveals that about 1 percent of the light given off has 

an energy greater than 5.8 eV. Each plasma has an input power 

of 200 W applied for 1 hr, or a total of 720 kJ. However, it is 

not known how the energy is partitioned between ionization 

and excitation of electronic states, or what fraction of the light 

impinges on the dust, so the flux cannot be calculated. 

The second UV generating process is through heating the 

dust coated samples with a Xe arc lamp. The intensity of the 

LDAB arc lamp is about 20 Suns, so following from the 

Air Mass Zero Solar Spectrum 

Figure 2.—Air mass zero solar spectrum over its 

most intense energy region (Ref. 13). 

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

02468

Energy, eV

W/m2-um 

10

NASA/TM—2009-215648 4 
estimation above, each atom should be exposed to a phonon 

with 5.7 eV energy or higher about every 6 sec. It should be 

noted that the dust is only exposed to that flux for about 15 min 

during the test. 

In addition to electromagnetic radiation, the Sun also 

bombards the lunar surface with high energy particles, largely 

protons, but α particles as well. By far the greatest number of 

these come from the solar wind, but solar storms and solar 

flares bombard the surface with less frequent, but higher energy 

particles. In addition, the lunar surface is also bathed in a low 

flux of very high energy cosmic rays. 

Figure 3, adapted from Wilson, et al., shows the particle 

radiation flux density and energy emitted by the Sun to the 

Moon (Ref. 17). A typical mineral crystal will have about 

1015 atoms/cm2

 of surface. Extrapolation of the solar wind 

protons to that value leads to the conclusion that a solar wind 

proton will strike a surface atom carrying a keV or more of 

energy each (24 hr) day. Similarly, it will interact with a 1 

MeV solar storm proton about once per millennia, a 100 MeV 

proton, and a GeV cosmic ray about once every 100 million 

years. These energetic particles may have several collisions 

within the lattice before they come to rest, and will often 

become implanted, as evidenced by their release as hydrogen 

gas from the Apollo samples (Ref. 18). They also sputter atoms 

from the surfaces of the particle, resulting in dangling bonds 

and lattice dislocations. 

High energy particle radiation is simulated in the LDAB by 

the radio frequency plasmas described previously. Again, it is 

difficult to know how the total energy is partitioned between 

electromagnetic radiation and the ion formation and 

accelerations. Further, it is not known what fraction of the ions 

strike the dust, nor with what energy. An in situ instrument that 

could quantify the extent of activation would be useful in 

determining how long the dust should be exposed to the plasma. 

The meteoroid flux that the International Space Station is 

exposed to has been quantified by Christiansen (Ref. 19). A 

log-linear extrapolation of his Figure 1-2 to smaller particles 

sizes shown in Figure 4 has been used to estimate the 

micrometeoroid flux on the lunar surface. 

Figure 3.—Impacts per lunar surface atom per second for 

energetic radiation. (Adapted from Wilson, et al. (Ref. 3).) 

Estimated Micrometeoroid Flux on the Moon 

Figure 4.—Impacts on lunar surface per year per m2. (Adapted 

from Christiansen (Ref. 16).) 

It is assumed that the flux in low Earth orbit is similar to that 

of the lunar surface. It is recognized that because of the greater 

size of Earth’s gravity well, the flux on the lunar surface is 

probably somewhat lower. The estimate is that each year there 

will be about ten 100-μm meteoroids and about ten thousand 

10-μm meteoroids strike each square meter of surface. 

Each of these high energy collisions with the regolith will 

result in the fracture of a large number of particles. The 

surfaces of freshly ground minerals are characterized by the 

presence of highly reactive free radicals (Ref. 3), and since the 

regolith was formed by impact fracture, they probably form on 

the lunar surface as well. These fracture surfaces often include 

“shock structures” which include dislocations and dangling 

bonds which will be chemically active. Thermal excursions 

which can be as large as 300 K near the equator will induce 

thermal stresses which may create new dislocations and 

fracture surfaces. 

Micrometeor impacts can be simulated by fracturing the 

simulant using high impact milling. As the grains fracture, 

defect sites will be present that are induced by the strain before 

fracture. Although this method may be used to activate dust or 

test of reactivity in the habitat or toxicological environment, it 

would be difficult to generate this type of activation in a 

vacuum environment. That being said, Grossman et al.’s study 

referred to above induced a single fracture of a relatively large 

rock to study the nature of fracture activation (Ref. 7). 

Mechanisms of Passivation 

While there are many processes on the lunar surface to 

activate particle surfaces, there are not many passivation 

mechanisms. Under the rarified atmosphere (actually a ballistic 

exosphere) with pressures of 10–12 Torr on the day side and 

10–14 Torr on the night side, there are very few collisions with 

gaseous atoms that have a chance to relax the active site. Such 

atoms as do collide will have such low collisional energy that 

1.E-08

1.E-06

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1.E+04

0.001 0.01 0.1 1 10

Particles size, cm

Flux, meteoroids/yr-m2

NASA/TM—2009-215648 5 
NASA/TM—2009-215648 6 

few will be able to heal a lattice defect, as is illustrated in 

Hoyt’s study referred to above (Ref. 6). As a result, passivation 

in the lunar environment is expected to be a very slow process. 

The work to produce the highly activated surfaces in ground 

test systems will be for naught if passivation occurs before the 

active surface can react with the experimental surface in 

question. One possibility is auto-passivation, that is energy can 

be emitted to return an excited atom to the ground state, or a 

lattice defect may spontaneously correct itself. But more often 

passivation will be the result of collisions with other atoms. So 

the most effective way to limit passivation is to limit the 

exposure of the activated material to external atomic collisions. 

Figure 1 shows a plot of molecular collisions/sec as a function 

of pressure. Examination of the collision frequency reveals that 

effectively, this means by operating in a vacuum. 

Each type of activation site will have its own passivation 

energy. Free radicals have such a low passivation energy that 

they are usually passivated by the first collision, whereas that 

of lattice defects is so high that they can be used to track solar 

activity over thousands of years (Ref. 3). The probability that a 

collision will be by an atom or molecule with sufficient energy 

is provided by the Boltzmann distribution, which is very 

sensitive to temperature. So the best condition to extend the 

lifetime of the activated stated is low pressure and low 

temperature. 

The LDAB tests are carried out in a vacuum of 10–8 Torr. At 

these pressures the passivation of the particle surfaces will be 

slowed, but the pressure is still 10,000 times higher than even 

the dayside pressure on the Moon. Tests must be carried out in 

UHV facilities at10–12 Torr to determine how these passivation 

rates compare with those at lunar pressures. 

Conclusions 

The lunar surface is an energetic environment. 

Electromagnetic and particle radiations emanating from the 

Sun contain a small fraction of photons and protons that are 

energetic enough to create free radicals, dangling bonds, and 

lattice defects. There is circumstantial evidence from the 

Apollo record that these activation processes do occur. These 

high energy activation sites can affect the reactivity of the dust 

particle surface enough to affect their adhesion to spacecraft 

surfaces and human toxicology. 

Avenues are being explored to generate activated dust 

surfaces using mechanical, thermal, and chemical means. 

Progress is being made on the quantification of the activation 

state under high vacuum conditions. Methods to quantify their 

potential for toxicological effects are already well known. 

It is important to quantity the passivation rates in a variety of 

environments. Since they are dependent on molecular collisions 

they will vary widely depending on whether their environment 

of interest is the lunar surface, standard ground-test vacuum 

systems, airlocks, habitats, or within the lung. Overall it is 

important to understand all aspects of activated states on dust 

particles; how they are formed, how the effect mechanical and 

biological systems, and how they are passivated. 

References 

1. B.B. Wilcox, et al., “Constraints on the Depth and Variability of the 

Lunar Regolith,” Meteoritics & Planetary Science, 40(5) (2005) 

pp. 695–710. 

2. G.H. Heiken, D.T. Vaniman, and B.M. French, The Lunar 

Sourcebook: A User’s Guide to the Moon, (Cambridge University 

Press, 1991). 

3. B. Fubini, E. Giamello, M. Volante, V. Bolis, “Chemical 

functionalities at the silica surface determining its reactivity when 

inhaled. Formation and reactivity of surface radicals,” Toxicology 

and Industrial Health 6 (1990) pp. 571–598. 

4. J.J. Grossman and J.A. Ryan, “Comments on Lunar Surface 

Adhesion,” Proc. 7th Annual Working Group on Extraterrestrial 

Resources, NASA SP-229 (1970) pp. 113-115. 

5. R.L. Vander Wal, et al., manuscript submitted for publication (2009). 

6. W.T. Wallace, D.K. Hammond, and A.S. Jeevarajan, “Lunar Dust 

and Lunar Simulant Activation and Monitoring,” NASA Human 

Research Program Investigators’’ Workshop, League City, TX 

(2008). 

7. H.P. Hoyt, Jr., et al., “Solar Flare Proton Spectrum Averaged Over 

the Last 5×103

 Years,” Proc. Fourth Lunar Science Conf., vol. 3, 

(1973) pp. 2489–2502. 

8. J.J. Grossman, et al., “Microchemical, microphysical and adhesive 

properties of lunar material,” Proc Apollo 11 Lunar Sci Conf., vol. 3, 

pp. 2171–2181. 

9. Carl Allen, at the Biological Effects of Lunar Dust Workshop, March 

29–31, 2005. 

10. P. Atkins, Physical Chemistry, 6th ed. (W.H. Freeman and Co., New 

York, 1999) p. 29. 

11. F. Mota, et al., “Radiation Damage Modeling of Fused Silica in 

Fusion Systems,” IAEA2006, paper ITERP1–30 (2006). 

12. J.R. Gaier and E.A. Sechkar, “Lunar Simulation in the Lunar dust 

Adhesion Bell Jar,” NASA/TM—2007-214704, 2007. 

13. K.W. Street, Jr., NASA Glenn Research Center, private 

communications (2008). 

14. CRC Handbook of Chemistry and Physics, 76th Ed. (1995), D.R. 

Lide, Ed., pp. 9-51–9-74. 

15. 2000 ASTM Standard Extraterrestrial Spectrum Reference E-490-00, 

at http://rredc.nrel.gov/solar/ spectra/am0/, accessed Feb. 26, 2009. 

16. Z. Sternovsky, et al., “Contact charging of lunar and Martian dust 

simulants,” J. Geophy Res 107 (E11) (2002) pp. 15-1–15-8. 

17. J.W. Wilson et al., “Transport Methods and Interactions for Space 

Radiations,” NASA Reference Publication 1257 (1991). 

18. G.H. Heiken, D.T. Vaniman, and B.M. French, The Lunar 

Sourcebook: A User’s Guide to the Moon, (Cambridge University 

Press, 1991), p. 444. 

19. E.L. Christiansen, “Meteoroid/Debris Shielding,” NASA/TP—2003-

210788. 
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Regolith Activation on the Lunar Surface and Its Ground Test Simulation 

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Gaier, James, R. 

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14. ABSTRACT

Activation of the surfaces of lunar regolith particles can occur through interactions with solar electromagnetic radiation, solar and galactic 

particle radiation and micrometeoroid bombardment. An attempt has been made to quantify the relative importance of each of those effects. 

The effects of these activated surfaces may be to enhance the adhesion and toxicity of the particles. Also key to the importance of activation 

is the lifetimes of activated states in various environments which is controlled by their passivation rate as well as their activation rate. 

Although techniques exist to characterize the extent of activation of particles in biological system, it is important to be able to quantify the 

activation state on the lunar surface, in ground-test vacuum systems, and in habitat atmospheres as well.

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Lunar soil; Lunar dust 

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