5:15 PM - 7:15 PM
[PPS03-P11] A Thermal Infrared Emission Spectral Morphology Study of Serpentinite – Preliminary Results
Keywords:thermal infrared spectroscopy, fine particulate, planetary surfaces, asteroids
One of the biggest questions scientists face today regards the origins of life on Earth, and a way to investigate this is through studying asteroids – debris from the birth of the Solar System. Research into asteroid compositions contributes to finding the origin of water and organics on Earth [1], and places constraints on planetary dynamics and migration models [2] that can help understand how planetary systems around other stars may form and evolve.
Remote sensing observations and laboratory measurements in the thermal infrared (TIR; 5-25µm) are particularly valuable in finding asteroid compositions due to the rich diagnostic information in this region. A way to interpret TIR spectra is with the linear mixing assumption, under which spectral features from the individual mineral end-members sum together linearly (weighted by their abundances) to form the spectrum of their mixture [3,4]. However, this assumption breaks down as the particle size of the material decreases and approaches the scale of the photon wavelength, leading to an increase in multiple/volume scattering. Despite the known issues of this model for fine particulates (<60µm), linear mixing is still the most widely used model of choice. Therefore, the overarching aim of this work is to parameterise the linear mixing model to incorporate the non-linear scattering behaviour of fine particulates, or derive a different method altogether, such that composition may be more accurately estimated from TIR emission spectra for a wider range of particle sizes.
A decrease in particle size has well-known effects on the morphology of TIR spectra, with feature positions remaining largely unchanged, thus capturing the non-linear scattering behaviour. Previous work investigating these effects have largely been qualitative in nature or quantified different effects [5-8], whereas the work presented here quantifies changes in the Christiansen Feature (CF) “roll-off” and Transparency Feature (TF) as particle size decreases. These features arise as a result of scattering and so tend to be common across several mineral species.
The results presented will show how the gradient of the CF roll-off and depth of the TF change with particle size for the size fractions: <5µm, 5-10µm, 10-15µm, 15-20µm, 20-25µm, 25-30µm, 30-35µm, 35-40µm, 40-45µm, 46-51µm, 51-55µm, 55-62µm, 62-73µm, 73-120µm, 120-200µm, 200-250µm, 250-350µm, and 350-500µm. All were separated out from the same bulk powdered natural serpentinite sample (mostly lizardite with trace amounts of spinel, magnetite and olivine impurities). Size fractions up to 45µm were separated via centrifugation and filtration, and the sizes from 46µm separated via sieving. The thermal infrared emission spectra were collected using the PASCALE instrument at the University of Oxford’s Planetary Spectroscopy Laboratory.
The overall goal of the spectral morphology study is to model trends of CF roll-off gradient and TF depth with particle size, using these in the parameterisation of linear mixing, or derivation of a new method. From this, more accurate estimations of planetary surface compositions can be obtained, which is vital for future planetary exploration and understanding the history of our Solar System.
References:
[1] Morbidelli, A., et al. (2000). Meteoritics & Planetary Science 35.6, pp. 1309–1320.
[2] Walsh, K. J., et al. (2012). Meteoritics & Planetary Science 47.12, pp. 1941–1947.
[3] Lyon, R. J. P. (1964). NASA Contractor Report CR-100.
[4] Ramsey, M. S. and Christensen, P. R. (1998). Journal of Geophysical Research: Solid Earth 103.B1, pp. 577–596.
[5] Salisbury, J. W., Walter, L. S., and Vergo, N. (1987). Mid-Infrared (2.1-25 µm) Spectra
of Minerals: First Edition.
[6] Mustard, J. F. and Hays, J. E. (1997). Icarus 125.1, pp. 145–163
[7] Ito, G., Arnold, J. A., and Glotch, T. D. (2017). Journal of Geophysical Research: Planets 122.5, pp. 822–838.
[8] Shirley, K. and Glotch, T. (2019). Journal of Geophysical Research: Planets 124.4, pp. 970–988.
Remote sensing observations and laboratory measurements in the thermal infrared (TIR; 5-25µm) are particularly valuable in finding asteroid compositions due to the rich diagnostic information in this region. A way to interpret TIR spectra is with the linear mixing assumption, under which spectral features from the individual mineral end-members sum together linearly (weighted by their abundances) to form the spectrum of their mixture [3,4]. However, this assumption breaks down as the particle size of the material decreases and approaches the scale of the photon wavelength, leading to an increase in multiple/volume scattering. Despite the known issues of this model for fine particulates (<60µm), linear mixing is still the most widely used model of choice. Therefore, the overarching aim of this work is to parameterise the linear mixing model to incorporate the non-linear scattering behaviour of fine particulates, or derive a different method altogether, such that composition may be more accurately estimated from TIR emission spectra for a wider range of particle sizes.
A decrease in particle size has well-known effects on the morphology of TIR spectra, with feature positions remaining largely unchanged, thus capturing the non-linear scattering behaviour. Previous work investigating these effects have largely been qualitative in nature or quantified different effects [5-8], whereas the work presented here quantifies changes in the Christiansen Feature (CF) “roll-off” and Transparency Feature (TF) as particle size decreases. These features arise as a result of scattering and so tend to be common across several mineral species.
The results presented will show how the gradient of the CF roll-off and depth of the TF change with particle size for the size fractions: <5µm, 5-10µm, 10-15µm, 15-20µm, 20-25µm, 25-30µm, 30-35µm, 35-40µm, 40-45µm, 46-51µm, 51-55µm, 55-62µm, 62-73µm, 73-120µm, 120-200µm, 200-250µm, 250-350µm, and 350-500µm. All were separated out from the same bulk powdered natural serpentinite sample (mostly lizardite with trace amounts of spinel, magnetite and olivine impurities). Size fractions up to 45µm were separated via centrifugation and filtration, and the sizes from 46µm separated via sieving. The thermal infrared emission spectra were collected using the PASCALE instrument at the University of Oxford’s Planetary Spectroscopy Laboratory.
The overall goal of the spectral morphology study is to model trends of CF roll-off gradient and TF depth with particle size, using these in the parameterisation of linear mixing, or derivation of a new method. From this, more accurate estimations of planetary surface compositions can be obtained, which is vital for future planetary exploration and understanding the history of our Solar System.
References:
[1] Morbidelli, A., et al. (2000). Meteoritics & Planetary Science 35.6, pp. 1309–1320.
[2] Walsh, K. J., et al. (2012). Meteoritics & Planetary Science 47.12, pp. 1941–1947.
[3] Lyon, R. J. P. (1964). NASA Contractor Report CR-100.
[4] Ramsey, M. S. and Christensen, P. R. (1998). Journal of Geophysical Research: Solid Earth 103.B1, pp. 577–596.
[5] Salisbury, J. W., Walter, L. S., and Vergo, N. (1987). Mid-Infrared (2.1-25 µm) Spectra
of Minerals: First Edition.
[6] Mustard, J. F. and Hays, J. E. (1997). Icarus 125.1, pp. 145–163
[7] Ito, G., Arnold, J. A., and Glotch, T. D. (2017). Journal of Geophysical Research: Planets 122.5, pp. 822–838.
[8] Shirley, K. and Glotch, T. (2019). Journal of Geophysical Research: Planets 124.4, pp. 970–988.