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Near-field Optical Microscopy and its application in study of Space Weathering on the asteroids

Near-field Optical Microscopy and its application in study of Space Weathering on the asteroids. A. Longobardo 1,2 , E. Palomba 1 , M. Girasole 3 , G. Longo 3 , G. Pompeo 3 and A. Cricenti 3 1 IFSI-INAF Via Fosso del Cavaliere 100, 00133, Roma, Italy

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Near-field Optical Microscopy and its application in study of Space Weathering on the asteroids

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  1. 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

  2. Outline • Introduction • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  3. 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

  4. 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) • Enstatite Chondrites (ECs) • 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) 2th Young Meeting Researcher, Rome

  5. 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. 2th Young Meeting Researcher, Rome

  6. Outline • Introduction • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  7. 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

  8. 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. 2th Young Meeting Researcher, Rome

  9. Outline • Introduction • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  10. 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. 2th Young Meeting Researcher, Rome

  11. The SNOM technique (1) OPTICAL RESOLUTION BETTER THAN l/2 2th Young Meeting Researcher, Rome

  12. 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 0 0.5 1.0 1.5 2.0 2.5 Images from Speclib

  13. SNOM configurations • The fiber tip acts as both illuminator and collector • The fiber tip acts as illuminator, while collector is external • 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. 2th Young Meeting Researcher, Rome

  14. Outline • Scientific background • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  15. 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). 2th Young Meeting Researcher, Rome

  16. Simulation results With npFe Without npFe The larger the wavelength, the more peaked the reflected light distribution in the presence of a npFe. 2th Young Meeting Researcher, Rome

  17. Outline • Scientific background • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  18. 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: 2th Young Meeting Researcher, Rome

  19. Detection of npFe: a semi-quantitative approach Since we do not have an absolute calibration in reflectance, a semi-quantitative approach for nanophase detection has been developed, based on three steps: • Identification of a region of the sample containing pure silicate • Comparison between the silicatic region and other regions of the sample • Identification of regions of the sample potentially hosting npFe 2th Young Meeting Researcher, Rome

  20. 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: 2th Young Meeting Researcher, Rome

  21. 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 • : standard deviation of reflectance in the silicatic region Ri : mean reflectance of the region I 2th Young Meeting Researcher, Rome

  22. Detection of npFe 2th Young Meeting Researcher, Rome

  23. Outline • Scientific background • Scientific issues • The Scanning Near-field Optical Microscopy (SNOM) • Simulations • Laboratory analysis • Conclusions and future perspectives 2th Young Meeting Researcher, Rome

  24. 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 2th Young Meeting Researcher, Rome

  25. 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 2th Young Meeting Researcher, Rome

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