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Thermal Infrared (TIR) Emission Spectroscopy 1
Measurements in the Thermal Infrared (TIR) can be used to:
Warmer objects will emit more energy, but the exact amount of
energy emitted at a given wavelength will also depend on the
properties of that material.
The pattern of emission versus wavelength is an emissivity
spectrum.
- estimate the abundances of minerals on the surface, allowing
mapping of rock types (this requires modeling of the measured
signal and can be non-unique!)
- estimate surface temperature and atmospheric temperature
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Thermal Infrared (TIR) Emission Spectroscopy 2
TIR measurements can be made at different times of day over the
same spot.
Different materials will absorb and release energy at different
rates (sand heats up fast, but larger rocks heat up slowly), thus
we can use these types of measurements to estimate the physical
properties of surface materials and better understand how the
surface responds to temperature variations.
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Thermal Infrared (TIR) Emission Spectroscopy 3
As with many spectroscopic techniques, geologists want to use
TIR data (emissivity spectra) to learn about the rock types and
minerals that make up the surface of a planet.
This is a map created from several TIR images acquired by the
THEMIS instrument on the Mars Odyssey satellite.
In this false-color image the bright pink and blue-purple tones
represent olivine-rich deposits, likely olivine-rich basalt
flows.
These maps can then be used to inform us about how the crust of
a planet has evolved.(Is it basaltic? Are there more evolved,
silica-rich, lava flows? Is there evidence of water?)
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Atmospheric Windows 5
In addition to measuring the sunlight that is reflected off of
the surface, we can also measure the energy that is emitted by a
surface.
This occurs at longer wavelengths (planetary objects are colder
than the Sun) and if we want to observe the surface then we need to
make sure the surface is warmer than the atmosphere (otherwise the
warmer atmosphere would emit more energy and dominate the energy
from the surface).
(Earth)
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Important for Mars!
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The radiance froma mineral at one tempis different from that
atanother temp.
Divide a radiance spectrumby that of a blackbody atthe same
temp.
Fractional representation of the amountof energy from a material
vs. the energythat would come from a blackbody at thesame
temperature.
Emissivity
Emissivity: Lsample,T / Lblackbody,T
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Inde
x of
Ref
. or E
xtin
ctio
n C
oeff.
Ref
lect
ance
n: index of refraction is a measure of the speed of light in a
substanceK: extinction coefficient, related to absorption
coefficient (k) by k = 4 pi K/
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Inde
x of
Ref
. or E
xtin
ctio
n C
oeff.
Em
issi
vity
Kirchhoff's law E = 1 – R
n: index of refraction is a measure of the speed of light in a
substanceK: extinction coefficient, related to absorption
coefficient (k) by k = 4 pi K/
-
Inde
x of
Ref
. or E
xtin
ctio
n C
oeff.
Em
issi
vity
Kirchhoff's law E = 1 – R
n: index of refraction is a measure of the speed of light in a
substanceK: extinction coefficient, related to absorption
coefficient (k) by k = 4 pi K/
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Frequency at which the real part of the refractive index of the
sample, n, equals 1
Emissivity Features
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Restrahlen Bands
Emissivity Features: Christiansen Feature
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Restrahlen Bands
What happens to spectral contrast as particle size
decreases?
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CF is an emission maximum and is an indicator of
composition.
The wavelength position of the CF is diagnostic of composition
and changes with the change in bond strength and molecular geometry
associated with changing mineralogy.
Fayalite
Forsterite
Augite
Enstatite
An
Albite
Quartz
Olivine and Pyroxene
High Fe and Mg = CF Long Wavelengths
Plagioclase
Low Fe and Mg = CF Short Wavelengthsmicrons
Emissivity Features
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Emissivity Features: Other Effects!
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Emissivity Features: Orientation Effects
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Emissivity Features: Packing (Porosity) Effects
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Emissivity Features: Effects of Coatings/Rinds
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PyroxenesRock Forming Minerals
Emissivity Spectra
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Emissivity Spectra: Clay Minerals
(Pressed Pellets)
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Emissivity Spectra: Clay Minerals
(Pressed Pellets)
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(Pressed Pellets)
Emissivity Spectra: Clay Minerals