Near-field Optical Microscopy and its
application in study of Space
Weathering on the asteroids
A. Longobardo1,2, E. Palomba1, M. Girasole3, G. Longo3 , G. Pompeo3 and A. Cricenti3
1 IFSI-INAF Via Fosso del Cavaliere 100, 00133, Roma, Italy
2 Dipartimento di Fisica, Università Sapienza, Piazzale Aldo Moro 5, 00185, Roma, Italy
3 ISM-CNR Via Fosso del Cavaliere 100, 00133, Roma, Italy
Outline
i.
Introduction
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
2
Asteroids
Asteroids are minor bodies of the inner
Solar System.
According to their orbital characteristics,
they are grouped in:
• Main Belt Asteroids
• Trojans (in 1:1 resonance with a planet)
• Mars Crossers (perihelion between 1.3
UA and 1.666 UA)
• Near Earth Asteroids (NEA)
According to Chapman (1975) classification, they are assigned a type based
on spectral shape, color and albedo:
• C-type: dark carbonaceous asteroids
• S-type: silicaceous asteroids
• U-type: asteroids that do not fit either C or S
Meteorites
Meteorites are objects originating in the outer space that survive
impact with Earth’s surface.
They can derive from small astronomic objects or from impact of
these latter with planets or asteroids.
According to their composition (linked to their parent body), they are
classified in:
• Stony meteorites
• Iron meteorites
• Stony-iron meteorites
Stony meteorites are divided in Chondrites (primitive) and Achondrites
(differentiated). Chondrites are classified in:
• Carbonaceous Chondrites (CCs)
• Ordinary Chondrites (OCs) [parent body: S-type asteroids] (Gaffey, 1976)
• Enstatite Chondrites (ECs)
Longobardo A.
2th Young Meeting Researcher,
Rome
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Space Weathering
Space Weathering is the ensemble of processes that act on a body
exposed to the space environment.
It includes:
• collisions with cosmic rays
• interactions with solar wind particles
• meteorites and micro-meteorites bombardment
Asteroid surfaces are more subject to SW compared to planetary
surfaces, because of the absence of atmospheric screening.
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2th Young Meeting Researcher,
Rome
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Outline
i.
Introduction
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
6
Reddening
Reddening, i.e. the red-IR reflectance increase at increasing
wavelength, is observed especially in S-type asteroids.
Image from Chapman (2004)
Related to Space Weathering
The iron nanoparticles (npFe)
The iron nanoparticles are identified as the main responsible for reddening.
Two scenarios about npFe formation have been proposed:
• sputtering from solar wind particles (Sasaki et al, 2001)
• shock-induced phase transformation on Fe-Ni alloys caused by collisions
(Moretti et al, 2005)
The aim of the work is to clarify the role, the characteristics and the
formation of npFe on S-type asteroids, by means of OCs analysis.
A high-resolution nanoimaging technique would give a fundamental
contribute to reveal morphology and optical properties of both silicates
and metallic inclusions in the analysed samples.
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2th Young Meeting Researcher,
Rome
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Outline
i.
Introduction
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
9
High-resolution imaging techniques
•
•
•
•
•
Atomic Force Microscopy (AFM)
Scanning Electron Microscopy (SEM)
Transmission Electron Microscopy (TEM)
Scanning Tunnelling Microscopy (STM)
Scanning Near-field Optical Microscopy (SNOM)
AFM gives topographic information about the analysed sample; SEM, TEM and
STM provide topography and electric properties; only SNOM allows to obtain
both morphological and optical properties.
For our purpose, SNOM analysis on Ordinary Chondrites has been performed.
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The SNOM technique (1)
OPTICAL RESOLUTION BETTER THAN l/2
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The SNOM technique (2)
This work is the first multi-colour SNOM experiment ever performed on
extraterrestrial samples.
The wavelengths of interest are contained in bands of maximum or minimum
reflectance of olivine and pyroxenes.
488 nm – 516 nm - 908 nm – 1300 nm – 1500 nm
0 0.5 1.0 1.5 2.0 2.5
Longobardo A.
0 0.5 1.0 1.5 2.0 2.5
Images from Speclib
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SNOM configurations
1.
The fiber tip acts as both illuminator and collector
2.
The fiber tip acts as illuminator, while collector is external
3.
The fiber tip acts as collector, while illuminator is external
In the second and in the third case, substitution of the external element
(required to work at different wavelengths) causes variations of illumination
(or collection) angle, hence the configuration n. 1 has been adopted.
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2th Young Meeting Researcher,
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Outline
i.
Scientific background
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
14
Simulations
Simulations support the laboratory analysis.
They have been performed using a software that models the interaction
between electromagnetic wave and the sample and calculates the energy
reflected by the sample, solving Maxwell Equations through a variant of Finite
Integration Technique (FIT).
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Simulation results
Without npFe
With npFe
The larger the wavelength, the more peaked the reflected light
distribution in the presence of a npFe.
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2th Young Meeting Researcher,
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Outline
i.
Scientific background
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
17
Data reduction
Images of the same sample collected at different wavelengths
have a slight different spatial scale (due to mechanical drifts)
Spatial shift between optical images has been calculated applying
a cross correlation analysis on the respective topographies.
If I(x,y) and J(x,y) are topographies relative to two images of the same sample, the
shift between them is the vector (X,Y) which maximizes the following expression:

Longobardo A.
x, y
I ( x, y ) J ( x  X , y  Y )
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Detection of npFe: a semi-quantitative approach
Since we do not have an absolute calibration in reflectance, a semiquantitative approach for nanophase detection has been developed,
based on three steps:
1. Identification of a region of the sample containing pure silicate
2. Comparison between the silicatic region and other regions of the sample
3. Identification of regions of the sample potentially hosting npFe
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Identification of silicatic regions
Simulation results show that npFe cause reflectance peaks at the longer
wavelengths and have no influence at the shorter ones.
Regions of the sample presenting an uniform reflectance distribution AT ALL
THE ANALYSED WAVELENGTHS can be associated with pure silicate:
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Identification of metallic inclusions
The region I is considered as potentially hosting metallic inclusions if:
•
at 1300 nm amd 1500 nm Ri-Rs > 2s
•
at 904 nm Ri > Rs
•
at 476 nm and 516 nm Ri~Rs
Rs : mean reflectance of the silicatic region
s: standard deviation of reflectance in the silicatic region
Ri : mean reflectance of the region I
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2th Young Meeting Researcher,
Rome
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Detection of npFe
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2th Young Meeting Researcher,
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Outline
i.
Scientific background
ii.
Scientific issues
iii.
The Scanning Near-field Optical Microscopy (SNOM)
iv.
Simulations
v.
Laboratory analysis
vi.
Conclusions and future perspectives
Longobardo A.
2th Young Meeting Researcher,
Rome
23
Conclusions
• SNOM guarantees the best setup reproducibility when the fiber
tip acts as both illuminator and collector. However, also in this
case small mechanical drifts are unavoidable
• According to simulations, npFe contribute to create reflectance
peaks more intense the larger wavelength
• Simulation results have been taken into account to develop a
technique of semi-quantitative comparison to detect npFe
• Some regions potentially hosting metallic inclusions have been
identified
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2th Young Meeting Researcher,
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Next steps
• Absolute calibration in reflectance, collecting optical images of a
material of known reflectivity
• More quantitative technique to detect npFe
• Link between presence of npFe and amount of shock degree
• Comparison with remote sensing data
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2th Young Meeting Researcher,
Rome
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