• Historical overview: Setting the stageVesta, the Moon, Mercury

• The problem with Asteroids• Lunar samples: a critical test• Convergence of thought• Link between S asteroids & Ordinary Chondrites• Ongoing issues

Space Weathering

Eros @1.1 km

ALHA81018 H5

Sample Analysis vs. Remote Sensing

Very different scales!

How do we relate small scale observations to ‘large’ scale

observations (e.g., spacecraft observations).

Keep in mind that the wavelengths you are using to observe an object/surface will be related to physical

properties at roughly similar length scales.

When photoelectric spectrometers became available, Vesta was the first asteroid to be measured.

Modelers were uncomfortable with Vesta-HED link.McCord et al 1970

Not only was low-Ca pyroxene observed, but Vesta’s spectrum was almost identical to basaltic achondrites [HEDs]. Note that there is little evidence for ‘classic’ space weathering.

1970 Reflectance Spectrum of Vesta

The Adams and McCord collaboration linked telescopic and laboratory measurements.

The distinctly subdued nature of mineral absorptions (Fe absorptions) was noted for lunar soils.

McCord et al 1970

1970 Spectra of Lunar Soils

Spectroscopy of Minerals and Rocks Provide the Basis for

Remote Compositional Analyses

0

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500 1000 1500 2000 2500

15415 Plag<25072415 Ol <4512063 Cpx <4578235 Opx <500

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ance

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Lunar Minerals

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Wavelength nm

Apollo 17Lunar Basalt Samples

79221 (soil)

75035 (rock)

Spectra of Minerals/RocksLab reflectance spectra of minerals and rocks provide the basis for comparison to asteroid and lunar spectra (remote observations).

We know that common Fe-bearing minerals (e.g., pyroxene and olivine) exhibit strong crystal field absorptions....so why don’t we see them for the lunar soils (and why do we see them for Vesta)?

Spectroscopy of Minerals and Rocks Provide the Basis for

Remote Compositional Analyses

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Lunar Minerals

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Apollo 17Lunar Basalt Samples

79221 (soil)

75035 (rock)

Adams & McCord 1972

Mixing with Glass??Early suggestions that glass in the soils may explain the muting of spectral features (not quite right, but points in the right direction).

[Bell et al., 1976]

Role of Oxygen Fugacity

Role of Composition

Mixing with Glass??Can glasses explain the observations?

Glasses cannot account for observed optical properties.

Lunar basaltic glass is relatively “red” and has strong FeO absorption. Ti-rich basaltic soils are relatively “blue”.

Role of Oxygen Fugacity

Role of Composition

FeO

FeO + TiO2

Mixing with Glass??

• Agglutinates - recycled materials• Formed during micrometeorite bombardment• Dark, amorphous “glass” reduces spectral contrast

Agglutinates became the favored explanation for alteration of lunar materials.

Agglutinates?

Simplified VersionWith exposure to the space environment:

• Absorption bands weaken• Albedo decreases• Spectrum takes on a characteristic “red” continuum

(increased reflectance with wavelength)

Lunar Space Weathering Paradigm

Spectroscopy of Minerals and Rocks Provide the Basis for

Remote Compositional Analyses

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ance

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Lunar Minerals

0

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lect

ance

Wavelength nm

Apollo 17Lunar Basalt Samples

79221 (soil)

75035 (rock)

McCord et al 1970

Vilas 1976Clark et al. 1979

Mercury

Apollo 16

Thermalcomponent

TerrestrialAtmosphere

Mercury

Apollo 16

Mercury: Another “Lunar” RegolithReflectance spectrum of visible wavelengths exhibit reddening and lack diagnostic Fe bands, similar to lunar soils.

[Note that data from MESSENGER mission has shown that surface is very Fe-poor]

• Lunar soils contain agglutinates which degrade diagnostic spectral absorptions. [Immature soils (at craters) contain fewer agglutinates and thus stronger features]

• If asteroids have regoliths, they are simply pulverized rocks (the Vesta example) (e.g., comminution).

• Mercury probably has a lunar-like soil.

All of the above have something incorrect.

1970’s View of Space Exposure

Itokawa (Hyabusa)

Asteroids VisitedWhat have spacecraft observations told us about space

weathering on asteroids?

Few asteroids are directly comparable to meteorites.[S,Q,V type asteroids exhibit clear 1 & 2 µm bands]

Principle Component Analysis clusters for asteroids do not correspond to meteorites.

VQ

V

Q

S

S

Britt et al. 1992+ Ordinary Chondrites* HEDs

Tholen 1974

The Difficulties of Asteroids

Gaffey 1976; 1993 Gaffey et al 1993

Same mineralogy:

- pyroxene- olivine- metallic Fe

…...but the continuum is different (and not “lunar-like”)

The Ordinary Chondrite -- S-Asteroid Conundrum

• Both have weakened absorption bands and “red” continuum. But….

• Lunar continua are steeper at longer wavelengths.

• Ferrous absorption bands of asteroid soils are stronger than lunar soils.

Lunar and S-Asteroid Soils

Lunar and S-Asteroid soils have similarities but also have distinct differences.

[Pieters et al., 2001]

Gaffey 1976

Free Iron

OC Meteorite Iron

=> Thermal gradient “Big Picture” Bell et al. 1989

S Asteroids

OC

1980’s: S-type Asteroids are Differentiated

S-type asteroids contain core & mantle.Thermal gradient

A. Space Weathering only exists on the Moon

(&Mercury?).• Agglutinates are the cause of

the optical alteration.

• S-asteroids represent differentiated bodies.

A radial thermal event occurred in the early SS.

B. Space Weathering processes occur on all airless bodies.

• The style and degree of weathering vary.

• S-asteroids represent a common type of meteorite (OC) that has been altered.

No major thermal gradient is required.

1990’s Perspectives

New Clues

• Rims ≠ interior for soil grains (Keller)

• Optical properties dominated by surface effects (Pieters)

• npFe0 documented on rims (Keller, McKay)

• npFe0 accounts for characteristic continuum (Keller, Pieters, Noble, Hapke, Taylor)

Clues from ongoing work

• Agglutinate complexity (e.g., McKay, Basu)

• Is/FeO model (e.g., Morris)

• Plagioclase and differential communition (e.g., Hörz, Cintalla)

What Do Lunar Soils Tell Us?

Returned lunar soils can be our ‘ground truth’ to understand space weathering processes (at least those relevant to the Moon).

20

FMR and the Role of npFe0 [Morris, 1976]

Ferromagnetic Resonance (FMR) spectra measured for lunar soils.

FMR spectra are dominated by resonance caused by fine-grained particles that are ferromagnetically ordered.

The particles are constrained to be spherical/nearly spherical in shape.

FMR Curie point measurements indicate they are pure metallic Fe.

Characteristic resonance associated with agglutinates & regolith breccias.

Is = (∆H)2A; ∆H = linewidth, A = amplitude of FMR resonance

R. Morris 1977-1980:

• Is is proportional to npFe° abundance

• npFe° is concentrated in finest grain size fraction

• Abundance of Is varies with grain size

• Abundance of Is varies with composition

Is used as a “maturity index” when normalized to FeO.

The Role of npFe0npFe0 Concentrated in Finest FractionMorris 1977-1980:• Is is proportional to npFe° abundance• Abundance of Is varies with grain size• Abundance of Is varies with composition

• Is used as a “maturity  index”  when  normalized to FeO

0

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wt %

npF

e°

Is

after Morris 1977

• The bulk soil is best modeled by <45 µm fractions (not by agglutinates).

• Particles >45µm have almost no effect.

• Natural soils cannot be duplicated artificially by physical crushing.

Space weathering is surface correlated![Pieters et al., 1993]

The Role of npFe0

The ‘finest fraction’ dominates the optical properties of lunar soils.

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10084<1010084_10-2010084_20-4510084<45 Bulk10084 45-75 [M]10084 <25 from 45-75

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ance

Wavelength (nm)

Ap 11 Soil10084

Prepared"Soil"

The Role of npFe0

X-ray imaging of lunar soils

Fe-rich rims are observed on Fe-poor grains.

(i.e., the host grain does not have to contain Fe in order to produce npFeO)

npFe° observed as (multiple) deposits on rims.Interior Fe° is larger.

[work by L. Keller]

Plagioclase grain

Agglutinate grain

The Role of npFe0

TEM images of soil grains

Optical Properties ofSize Separates for LSCC Soils

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70181(47)

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64801 [71]

Mare Soil Highland Soil

Optical Properties ofSize Separates for LSCC Soils

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70181(47)

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64801 [71]

Mare Soil Highland SoilMare Soil Highland Soil

Lunar Soil Characterization Consortium (LSCC)

Systematic and diverse study of lunar soil properties.

Optical properties vary for different size separates.(recall that npFeO also varied with particle size separates)

(LA Taylor, Pieters, McKay, Keller, Morris)

• Nine diverse mare soils; 10 highland

• Emphasis is on the “finer” fraction– <45 µm Bulk– 20-45– 10-20 µm– <10 mm

• Visible - NearIR, MidIR Spectroscopy

• Mineralogy• Bulk Chemistry• Is/FeO• Grain surface,

coatings, texture

Lunar Soil Characterization Consortium (LSCC)

SAMPLES ANALYSES

Optical properties and amount of npFe° are directly related

Noble et al., 2001 after Morris 1977

Optical properties and amount of npFe° are directly related

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10084 7812001 5612030 1415041 9415071 5261221 964801 71

67461 2567701 3968501 8570181 4771501 3579221 81

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ance

Wavelength (nm)

Lunar Soils <10 µm0

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Fe0 R

M

Is

79221

70181

67701

64801

61221

12030

12001 15071

10084 15041

<10µm size fractions

71501

Noble et al., 2001 after Morris 1977

Optical properties and amount of npFe° are directly related

0

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10084 7812001 5612030 1415041 9415071 5261221 964801 71

67461 2567701 3968501 8570181 4771501 3579221 81

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lect

ance

Wavelength (nm)

Lunar Soils <10 µm0

0.1

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t % F

e0 RM

Is

79221

70181

67701

64801

61221

12030

12001 15071

10084 15041

<10µm size fractions

71501

Noble et al., 2001 after Morris 1977

The Role of npFe0

(Noble et al., 2004; 2006) (Hapke, 2000)

0.07

0.130.19

0.30

2.1

0.0050.02

npFe0 in Silica Gel

Effects of npFe0 on Optical Properties

0

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ce

Wavelength (nm)

0.00

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0.5

2.0 Mass Fraction% Fe0

Effects of npFe0 in Silicate Matrix

0.05

Experimental (Noble et al., 2004; 2006) Modeling (Hapke, 2000)

0.07

0.130.19

0.30

2.1

0.0050.02

npFe0 in Silica Gel

Experimental Modeling

The Role of npFe0

• Predictions of npFe° on grain surfaces [Hapke, 1973]

• npFe°and space exposure linked [Housley et al 1973; Morris 1976]

• npFe°observed as deposits on grain rims [Keller and [McKay 1993; 1997; Wentworth et al., 1999, Taylor et al., 2000].

• Abundance of surface npFe° linked with optical properties [Pieters et al., 1993; Noble 2000]

• Reproduction of optical effects of npFe° experimentally and with modeling [Allen 1996, Hapke 2000; Sasaki et al, 2001]

What Have We Learned?

30

New Paradigm

In the space environment

• npFe0 is deposited on the surface of grains

• Reduction of FeO occurs during sputtering and micrometeorite impact vaporization

• Accumulation of npFe0 accounts for the observed optical properties.

Spaceweathering does NOT require

• Formation of agglutinates• Solar wind implanted H

Degree of Spaceweathering depends on

• Host composition• Position in Solar System• Time

New Paradigm

Mercury soils experiences a higher rate of npFe0 accumulation (more intense impact vapor fractionation), but other processes affect steady state (Noble and Pieters 2001).

New Paradigm

Asteroid regoliths of the main belt experience npFe0 accumulation (due largely to sputtering), but at a slower rate than at 1 AU.

OC parent bodies are thus perfectly compatible with the mineralogy of many S-Type asteroids. No major early thermal gradient required.

The observed optical continuum of S-type asteroids (red in visible; flat in nearIR) is a natural consequence of lesser space weathering of semi-transparent minerals.

New Paradigm

S-Asteroids: Not a Problem!

Spectral properties of NEA range continuously from typical S-type to OC-like. [Binzel et al., 1996]

X-ray data from NEAR mission are most consistent with OCs [Trombka et al., 2000].

35

S-Asteroids: Not a Problem!Results from Hayabusa mission to asteroid Itokawa

over each region. The spectra are plotted in Fig. 3 and compared withan LL5 chondrite spectrum after continuum removal. Consistentwith earlier studies3,7, these NIRS spectra exhibit similar bandfeatures around 1.0, 1.25 and 1.95mm to those of this LL5 chondrite.After continuum removal, all three spectra look very similar to oneanother except for the depths of absorption bands. The mismatcharound 1.7 mm in wavelength is due to the known calibration error.This suggests that they are made of the same material (similar to LL5or LL6 chondrite), with different degrees of space weathering and/ormean optical path length (MOPL). The MOPL can be the effectivegrain size if the material consists of particulate, monomineralic grains,but the MOPL of a rock may depend not only the mineral grain sizesbut also physical conditions such as porosity.To estimate the degrees of space weathering in these two regions,

we used Hapke’s space weathering model4. He proposed a formulafor the absorption coefficient a(l) of a material containing nano-phase metallic iron (npFe0) particles:

aðlÞ ¼ ahðlÞ þ36p

lf zðlÞ ð1Þ

where

zðlÞ ¼ n3hnFekFe

ðn2Fe 2 k2Fe þ 2n2hÞ2 þ ð2nFekFeÞ2ð2Þ

where ah is the absorption coefficient of the host material, l is thewavelength of light, nh is the real part of the refractive index ofthe host material, nFe and kFe are the real and imaginary parts of therefractive index of iron, and f is the volume fraction of npFe0

particles in the host material. If we assume that the absorbancespectrum of the Alta’ameem sample and the surface material ofItowaka can be approximated by the negative natural logarithm of itsreflectance spectrum, we can obtain, from equation (1):

lnRðlÞø2 ahðlÞ þ36p

lfzðlÞ

! "de ð3Þ

where R(l) denotes reflectance at l, and d e denotes the MOPL.The absorption coefficient spectrum of the Alta’ameem sample

(,125 mm) can be obtained using equation (3) with no npFe0

(f ¼ 0) from its laboratory reflectance spectrum: for example, byassuming its effective grain size to be the median 62.5 mm. Assumingthe host material of Itokawa’s surface has the same optical properties

as does the Alta’ameem sample, we use the above-obtained absorp-tion coefficient as ah(l) in equation (3). From a previous study8 thatfound that LL5-6 chondrites havemineral assemblages dominated byolivine (about 55% in modal abundance as opposed to 25% ofpyroxene abundance), which has an average chemical composition ofFe/(Fe þ Mg) ¼ ,0.4 (Fa 40: fayalite 40% and forsterite 60% in theolivine solid-solution series), we can derive the real part of therefractive index of the host material nh to be 1.76 at the visiblewavelength (0.55mm)9. The corresponding spectral shape is assumedto be nh(l) ¼ 1.6328 þ 0.06998/l (ref. 10). Then, the z(l) functionin equation (3) can be calculated using equation (2) and by using thedata on the real and imaginary parts of the refractive index ofmetalliciron11 extrapolated to the longer wavelength, as was done in Hapke’sspace weathering model4.We are now ready to estimate the percentage volume of npFe0 (f)

andMOPL (d e) in equation (3) for the natural log reflectance spectraof the two areas (dark and red, and bright and blue) by optimizingthese two parameters for the best fit between the measured spectrumfor each region on the left hand and the model spectrum calculatedon the right hand of equation (3). The results are shown in Fig. 4. Asis expected, both areas require some amounts of npFe0 and smallerMOPL than those for the Alta’ameem sample, where the dark and redarea, having 0.069 vol.% npFe0, is more space-weathered than thebright and blue area, having 0.031 vol.% npFe0. The average of thesetwo amounts of npFe0 is consistent with the result of a ground-basedtelescopic work7, which estimated it as 0.05 vol.%.We note that the natural log reflectance values used in the

optimization process and shown in Fig. 4 are offset to 0 atl ¼ 1.54 mm. Owing to the surface roughness and incident anglevariation, a wavelength-independent factor always makes the appar-ent reflectance spectrum different from the standard spectrummeasured in a laboratory even if the materials are the same. Such afactor appears as an offset in the natural logarithm of reflectance

Figure 2 | Variations in brightness and redness of NIRS spectra nearTsukuba and Sagamihara. NIRS at Itokawa on 25 October 2005. Thebrightness is defined as reflectance at 0.76 mm (open and filled circles) andthe redness as the 1-mmband continuum slope (1.54 mm/0.76mm) (open andfilled triangles). Continuum slopes are plotted by scaling with a factor of0.1. The NIRS observations were performed at intervals of 17 s. Datapoints plotted in filled triangles and circles indicate spectra which are darkerthan 12.3% and redder than 1.33 (dark and red area), or brighter than 15.2%and bluer than 1.23 (bright and blue area) used for spectral analyses.

Figure 3 | Comparison of reflectance spectra of two areas on Itokawa withan ordinary chondrite spectrum. NIRS at Itokawa on 25 October 2005.Plotted here are natural logarithm of average reflectance spectra of the darkand red area and the bright and blue area (filled markers), their continuum-removed spectra (open markers), and the continuum-removed and scaledspectrum of Alta’ameem (LL5) chondrite14 powder sample (,125mm).Error bars are 1j. Both the NIRS spectra and the Alta’ameem spectrum arefor the same viewing geometry of 308 incidence and 08 emergence angles.The continuum (broken line) is defined as a linear-to-wavenumber curvewhich passes the shortest-wavelength data point (at 0.76 mm) and istangential to the spectrum around 1.55mm, connected with a constant (flatline) longward of that contact point. The continuum-removed Alta’ameemspectrum (solid line) was scaled by factors of 0.43 and 0.63 to fit with thespectra of the dark and red area and the bright and blue area, respectively.

NATURE|Vol 443|7 September 2006 LETTERS

57

over each region. The spectra are plotted in Fig. 3 and compared withan LL5 chondrite spectrum after continuum removal. Consistentwith earlier studies3,7, these NIRS spectra exhibit similar bandfeatures around 1.0, 1.25 and 1.95mm to those of this LL5 chondrite.After continuum removal, all three spectra look very similar to oneanother except for the depths of absorption bands. The mismatcharound 1.7 mm in wavelength is due to the known calibration error.This suggests that they are made of the same material (similar to LL5or LL6 chondrite), with different degrees of space weathering and/ormean optical path length (MOPL). The MOPL can be the effectivegrain size if the material consists of particulate, monomineralic grains,but the MOPL of a rock may depend not only the mineral grain sizesbut also physical conditions such as porosity.To estimate the degrees of space weathering in these two regions,

we used Hapke’s space weathering model4. He proposed a formulafor the absorption coefficient a(l) of a material containing nano-phase metallic iron (npFe0) particles:

aðlÞ ¼ ahðlÞ þ36p

lf zðlÞ ð1Þ

where

zðlÞ ¼ n3hnFekFe

ðn2Fe 2 k2Fe þ 2n2hÞ2 þ ð2nFekFeÞ2ð2Þ

where ah is the absorption coefficient of the host material, l is thewavelength of light, nh is the real part of the refractive index ofthe host material, nFe and kFe are the real and imaginary parts of therefractive index of iron, and f is the volume fraction of npFe0

particles in the host material. If we assume that the absorbancespectrum of the Alta’ameem sample and the surface material ofItowaka can be approximated by the negative natural logarithm of itsreflectance spectrum, we can obtain, from equation (1):

lnRðlÞø2 ahðlÞ þ36p

lfzðlÞ

! "de ð3Þ

where R(l) denotes reflectance at l, and d e denotes the MOPL.The absorption coefficient spectrum of the Alta’ameem sample

(,125 mm) can be obtained using equation (3) with no npFe0

(f ¼ 0) from its laboratory reflectance spectrum: for example, byassuming its effective grain size to be the median 62.5 mm. Assumingthe host material of Itokawa’s surface has the same optical properties

as does the Alta’ameem sample, we use the above-obtained absorp-tion coefficient as ah(l) in equation (3). From a previous study8 thatfound that LL5-6 chondrites havemineral assemblages dominated byolivine (about 55% in modal abundance as opposed to 25% ofpyroxene abundance), which has an average chemical composition ofFe/(Fe þ Mg) ¼ ,0.4 (Fa 40: fayalite 40% and forsterite 60% in theolivine solid-solution series), we can derive the real part of therefractive index of the host material nh to be 1.76 at the visiblewavelength (0.55mm)9. The corresponding spectral shape is assumedto be nh(l) ¼ 1.6328 þ 0.06998/l (ref. 10). Then, the z(l) functionin equation (3) can be calculated using equation (2) and by using thedata on the real and imaginary parts of the refractive index ofmetalliciron11 extrapolated to the longer wavelength, as was done in Hapke’sspace weathering model4.We are now ready to estimate the percentage volume of npFe0 (f)

andMOPL (d e) in equation (3) for the natural log reflectance spectraof the two areas (dark and red, and bright and blue) by optimizingthese two parameters for the best fit between the measured spectrumfor each region on the left hand and the model spectrum calculatedon the right hand of equation (3). The results are shown in Fig. 4. Asis expected, both areas require some amounts of npFe0 and smallerMOPL than those for the Alta’ameem sample, where the dark and redarea, having 0.069 vol.% npFe0, is more space-weathered than thebright and blue area, having 0.031 vol.% npFe0. The average of thesetwo amounts of npFe0 is consistent with the result of a ground-basedtelescopic work7, which estimated it as 0.05 vol.%.We note that the natural log reflectance values used in the

optimization process and shown in Fig. 4 are offset to 0 atl ¼ 1.54 mm. Owing to the surface roughness and incident anglevariation, a wavelength-independent factor always makes the appar-ent reflectance spectrum different from the standard spectrummeasured in a laboratory even if the materials are the same. Such afactor appears as an offset in the natural logarithm of reflectance

Figure 2 | Variations in brightness and redness of NIRS spectra nearTsukuba and Sagamihara. NIRS at Itokawa on 25 October 2005. Thebrightness is defined as reflectance at 0.76 mm (open and filled circles) andthe redness as the 1-mmband continuum slope (1.54 mm/0.76mm) (open andfilled triangles). Continuum slopes are plotted by scaling with a factor of0.1. The NIRS observations were performed at intervals of 17 s. Datapoints plotted in filled triangles and circles indicate spectra which are darkerthan 12.3% and redder than 1.33 (dark and red area), or brighter than 15.2%and bluer than 1.23 (bright and blue area) used for spectral analyses.

Figure 3 | Comparison of reflectance spectra of two areas on Itokawa withan ordinary chondrite spectrum. NIRS at Itokawa on 25 October 2005.Plotted here are natural logarithm of average reflectance spectra of the darkand red area and the bright and blue area (filled markers), their continuum-removed spectra (open markers), and the continuum-removed and scaledspectrum of Alta’ameem (LL5) chondrite14 powder sample (,125mm).Error bars are 1j. Both the NIRS spectra and the Alta’ameem spectrum arefor the same viewing geometry of 308 incidence and 08 emergence angles.The continuum (broken line) is defined as a linear-to-wavenumber curvewhich passes the shortest-wavelength data point (at 0.76 mm) and istangential to the spectrum around 1.55mm, connected with a constant (flatline) longward of that contact point. The continuum-removed Alta’ameemspectrum (solid line) was scaled by factors of 0.43 and 0.63 to fit with thespectra of the dark and red area and the bright and blue area, respectively.

NATURE|Vol 443|7 September 2006 LETTERS

57

“Bright” and “darker” regions on Itokawa exhibit different degrees of

reddening and all spectra are consistent with LL chondrites.

Space weathering materials do accumulate on small asteroids!

[Hiroi et al., 2006]

What is the rate of npFe0 accumulation and how does it vary with...

• bulk composition of host?• character (m, v) of

micrometeorites?• solar wind ions?

What is the role of native Fe in the host?

Outstanding Questions

McCord et al 1970

Outstanding QuestionsThe lunar samples were critical to understanding

the problem!

Additional samples from other bodies (asteroids) will likely be just as informative.

What is space weathering like on C-type objects?

Promising Simulations Producing npFe0

• Japanese

Sasaki et al.Nature

Hiroi et al.,NIPR2001

Pulsed laser in vacuum

Promising Simulations Producing npFe0

• Japanese

Sasaki et al.Nature

Hiroi et al.,NIPR2001

Pulsed laser in vacuum

[Sasaki et al., Nature; Hiroi et al., 2001]

Pulsed laser in vacuum

Simulated Space

Weathering

Outstanding Questions

Regolith Breccias & Impact-Darkened Meteorites Muddy the Story

Lower albedo, weaker bands, but no characteristic “red” continuum

Britt 1994

Pervomaisky

Darkening is due to dispersed micron-size metallic particles

Outstanding Questions