Using Reflectance Spectrometry to Identify Compositions

Credit: NASA/JPL-CalTech

Credit: NASA/JPL-CalTech/LANL

How Do We See Mars?

Our eyes, cameras, and mission instruments use the sunlight reflected off its surface.

Spectra Are “Fingerprints”

Fingerprints can be used to identify individual people

Credit: http://math-blog.com Credit: http://www.vetmed.vt.eduCredit: http://www.moillusions.com

Spectra Are “Fingerprints”

Spectra are unique to elements and minerals

Used to identify minerals; rocks

Credit: Univ. of Toronto

Credit: ChemCam/LANL/IRAP/CNES

Types of Spectra

Credit: http://csep10.phys.utk.edu/astr162/lect/light/spectra2.gif

Using the ALTA to take a Spectrum

These rocks are “Mars” rocks that you—like a rover on a mission—are analyzing. Your task it to determine their possible compositions, by comparing the Mars rocks’ spectra to known Earth rocks’ spectra.

Credit: Ward’s Scientific

Using the ALTA to take a Spectrum

These rocks are “Mars” rocks that you—like a rover on a mission—are analyzing. Your task it to determine its possible composition, by comparing the Mars rocks’ spectra to known Earth rocks’ spectra.

Buttons on front, LEDs & detector on the back

Push each button on the front and see what happens on the back

Using the ALTA to take a Spectrum

What is the ALTA doing?

Light from diode strikes the surface of the sample

Light reflected off the sample enters the detector

Read-out on the front indicates how much of the light has been reflected (voltage)

Because each ALTA has its own sensitivities, the read-out should be compared to the reflectance of white paper

Taking the Dark Voltage

What happens if you place the ALTA down flat and don’t turn on any of the lights?

You still have a number - ALTA’s photodetector and electronics don’t go to zero. This is your “Dark Voltage”

Using the ALTA to take a Spectrum

Need a large flat surface if possible.

Try not to let outside light into the detector.

Work in groups of 4; one can hold the sample and ALTA, one can press the buttons, one to record the data, etc.

Begin with taking the data for two stacked pieces of white paper.

Terrestrial Spectrum #1

Terrestrial Spectrum #2

Terrestrial Spectrum #3

Terrestrial Spectrum #4

Terrestrial Spectrum #5

Terrestrial Spectrum #6