Radiative Transfer Modelling of Martian Surface Regolith Emissivity

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Radiative Transfer Modelling of Martian Surface
Regolith Emissivity

• Karly Pitman
• September 25, 2003

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Martian Surface Emissivity

• Emissivity its definition and importance
• How Mars emissivity measurements are taken
• Directional emissivity
• Radiative transfer model development
• Theory lt---gt lab comparisons
• Implications for Martian regolith particles

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Emissivity Defined

• e (power radiated by material) / (power
  radiated by BB _at_ same temperature)
• L sT4 for blackbody L esT4 for everything
• e 1.0 gt perfect radiator/absorber

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The Importance of Emissivity

• 250 GB of raw emissivity spectral data returned
  by THEMIS alone
• Emissivity can tell us about mineralogy and, in
  theory, physical properties of grains

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Reading Emissivity Plots
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Remote Sensing Emissivities of Mars
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Possible Explanations

• Emissivity does depend on e
• 4 possible competing effects
• Particle shape
• Particle size
• Packing fraction
• Surface roughness

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Before Interpreting Orbiter e Data

• Computer modelling of e for thermal IR l (3-30
  mm)
• Measure e for particulates in lab
• Measure e in field (desert terrain)
• Self-consistent checks

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How To Densely Pack Particles

• Filling factor f 1 porosity
• Static structure factor S 1 / ( 1 nK(x) )

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How S Affects Emissivity
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Lab Measurements of Emissivity

• Instrument transmission spectrometer
• Grain size fractions from 0-1000 mm
• Pros easily calibrated Cons size bins,
  compositions

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RT Model / Lab Comparison

• RT Material quartz (Wenrich Christensen 1996)
• RT Shape sphere
• RT Size 10, 30, 50, 100, 1000 mm radius
• RT Bin Size 5 mm

• Lab Material synthetic SiO2 (SIL-CO-SIL
  53)
• Lab Shape sphere
• Lab Size 0-53 mm diameter
• Lab Bin Size 5 mm ltlt in progress gtgt

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Preliminary Lab Measurements for S
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Preliminary Lab Measurements for e
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Field Measurements of Emissivity

• Identify Martian terrestrial analog sites
• Instrument portable mFT-IR
• Pros inexpensive device
• Cons difficult to operate in field

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Summary

• Emissivity is a fundamental physical quantity for
  planetary science that deals with surface
  radiation
• NASA expects geologists to infer Mars mineralogy
  using thermal IR emissivity spectra we can also
  infer Mars regolith dust physical properties from
  these data
• Neither lab nor field data should be directly
  compared to Mars remote sensing emissivity data
• Unexplained directional emissivity effects are
  under investigation as part of a systematic
  theoretical laboratory -
  field comparison project

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References Resources

• Christensen, P. R., et al., A thermal emission
  spectral library of rock-forming minerals, J.
  Geophys. Res., 105, E4, 9735-9739, 2000.
• Jakosky, B. M., G. W. Finiol, and B. G.
  Henderson, Directional variations in thermal
  emission from geologic surfaces, Geophys. Res.
  Lett., 17, 7, 985-988, 1990.
• Mackowski, D. W., J. Opt. Soc. Am. A, 11,
  2851-2861, 1994.
• Mishchenko, M. I., JQSRT, 52, 1, 95-110, 1994.
• Mishchenko, M. I., and Macke, A., JQSRT, 57, 1,
  767-794, 1997.
• Moersch, J. E., and P. R. Christensen, Thermal
  emission from particulate surfaces A comparison
  of scattering models with measured spectra, J.
  Geophys. Res., 100, E4, 7465-7477, 1995.
• Stamnes, K., et al., Appl. Opt., 27, 2502-2507,
  1988.
• Wenrich, M. L., and Christensen, P. R., JGR, 101,
  B7, 15921-15931, 1996.

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References Resources

• ASU Mars Space Flight Facility Thermal Emission
  Spectral Library http//tes.asu.edu/speclib/index
  .html
• Blackbody radiation http//www.omega.com/literatu
  re/transactions/volume1/theoretical2.html
  http//www.cis.rit.edu/drp9420/background.html
• Graphics courtesy http//imagers.gsfc.nasa.gov/em
  s/infrared.html (Slide 3)http//mars.jpl.nasa.gov/
  odyssey/gallery/instruments/browse/themis-sm.jpg
  (Slide 4), Josh Bandfield Ariz. State Univ.
  Mars Space Flight Facility (Slides 5, 7), Karly
  Pitman (Slides 5, 14), Hapke 1993 p. 185 (Slide
  6, item 1), Clancy Lee 1991 (Slide 6, item 2)

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Radiative Transfer Model Quantities

• Phase function p S11 / (Csca k2)
• Asymmetry parameter g lt cosq gt
• Scattering cross section Csca
• Extinction cross section Cext
• Single-scattering albedo w Csca / Cext
• Emissivity e 1 R (R depends on I / F)

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Natural (Angular) Quartz
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Natural (Angular) Quartz
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Synthetic (Spherical) Silica
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Selected Field Sites
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Directional e on Earth
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How S Affects RT Quantities