Nearfield Optical Microscopy and its application in study

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 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. 2 th Young Meeting Researcher, Rome 2

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 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. 2 th Young Meeting Researcher, Rome 4

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. Longobardo A. 2 th Young Meeting Researcher, Rome 5

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. 2 th Young Meeting Researcher, Rome 6

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 (np. Fe) The iron nanoparticles are identified as the main responsible for reddening. Two scenarios about np. Fe 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 np. Fe 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. Longobardo A. 2 th Young Meeting Researcher, Rome 8

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. 2 th 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. Longobardo A. 2 th Young Meeting Researcher, Rome 10

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

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 12

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. Longobardo A. 2 th Young Meeting Researcher, Rome 13

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. 2 th Young Meeting Researcher, Rome 14

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). Longobardo A. 2 th Young Meeting Researcher, Rome 15

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

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. 2 th Young Meeting Researcher, Rome 17

Data reduction Images of the 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 sample, the shift between them is the vector (X, Y) which maximizes the following expression: Longobardo A. 2 th Young Meeting Researcher, Rome 18

Detection of np. Fe: 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 np. Fe

Identification of silicatic regions Simulation results show that np. Fe 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: Longobardo A. 2 th Young Meeting Researcher, Rome 20

Identification of metallic inclusions The region I is considered as potentially hosting metallic inclusions if: • at 1300 nm amd 1500 nm Ri-Rs > 2 s • 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 Longobardo A. 2 th Young Meeting Researcher, Rome 21

Detection of np. Fe Longobardo A. 2 th Young Meeting Researcher, Rome 22

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. 2 th 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, np. Fe 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 np. Fe • Some regions potentially hosting metallic inclusions have been identified Longobardo A. 2 th Young Meeting Researcher, Rome 24

Next steps • Absolute calibration in reflectance, collecting optical images of a material of known reflectivity • More quantitative technique to detect np. Fe • Link between presence of np. Fe and amount of shock degree • Comparison with remote sensing data Longobardo A. 2 th Young Meeting Researcher, Rome 25