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

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.

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?)

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)

Important for Mars!

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

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/

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/

Frequency at which the real part of the refractive index of the sample, n, equals 1

Emissivity Features

Restrahlen Bands

Emissivity Features: Christiansen Feature

Restrahlen Bands

What happens to spectral contrast as particle size decreases?

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

Emissivity Features: Other Effects!

Emissivity Features: Orientation Effects

Emissivity Features: Packing (Porosity) Effects

Emissivity Features: Effects of Coatings/Rinds

PyroxenesRock Forming Minerals

Emissivity Spectra

Emissivity Spectra: Clay Minerals

(Pressed Pellets)

Emissivity Spectra: Clay Minerals

(Pressed Pellets)

(Pressed Pellets)

Emissivity Spectra: Clay Minerals