Reflectance Spectroscopy

These labs in the attached word docs require reflectance spectrometers. We use these: https://www.vernier.com/products/sensors/spectrometers/alta/. The following lab is a modified version of this used in our online ASTR 150 course. I recommend opening an accompanying discussion forum so students can discuss the activity online with each other, your TAs or you to mimic the interaction they would have if they were working on the lab in-person - I have this activity setup as a canvas quiz:

Introduction

In the first half of this course, you learned how important samples are to our understanding of the surface composition, geologic history, and age of a world. We are now encountering worlds in the outer Solar System that we do not have samples from. Therefore we need to use a different method to investigate the surfaces of these new worlds.

One way we can determine the composition of a surface is to measure the light reflected by the surface. The total amount of light reflected by a surface is typically referred to as the albedo of the object and is given as a number between 0 and 1. By itself, the albedo can tell us whether the surface is composed of light or dark material, but we would like to be able to say more about a surface. We would like to know what that light or dark material actually is.

As it turns out, every material reflects light in a specific way – every material has essentially a “spectral fingerprint” that uniquely identifies it, just like your fingerprint is a unique way of identifying you. We get the unique fingerprints of various materials by measuring their reflectance spectrum or the amount of light reflected by the surface across a variety of wavelengths in a laboratory. We then measure the reflectance spectra of the surfaces of worlds in the Solar System and compare their reflectance spectra to the reflectance spectra of materials here on Earth.

Goals

Toady you will learn to use the reflectance spectra of four known samples (anorthosite, olivine, green leaves and basalt) and to use this information to determine the composition of two different surfaces. You will compare the reflectance of the known samples to the reflectance of the two surfaces using two different filters.

When you take an image with a camera, the film records an image of the entire visible spectrum. Most cameras on spacecraft take images only in a small wavelength range, some far beyond the visible part of the electromagnetic spectrum. To take images of just a piece of the spectrum, filters are placed in front of the camera. Two such filters are indicated by the blue shaded regions labeled “1” and “2” on the graph below.

The light that is reflected by any object (or emitted by any source like the Sun and other hot objects) tends to cover a broad range of wavelengths (energies, frequencies). Many times the light we receive is NOT in the visible portion of the electromagnetic spectrum, or in other words, in the range of wavelengths our eyes are sensitive to. That (very narrow) visible range for most humans is roughly between 400-750 nm. This is the range where we see color or the "rainbow" of colors our eyes have evolved to be sensitive to. The figure below shows the color versus wavelength for the visible spectrum along with all of the other wavelengths detected in the universe. Keep in mind, that we only see in the visible portion of the spectrum. If some object emits or reflects light in any wavelength outside of the visible we can not see it; it would essentially appear to not be emitting or reflecting any light whatsoever (even though it is). It would appear to be dark or black to our eyes. Each wavelength has a unique energy and in the visible, a unique color. Objects that reflect a lot of light around 450 nm would appear blue, those reflecting a lot of light at 700 nm would appear red to your eyes. Our Sun is yellow because at its surface temperature of 5700 K, it primarily emits light with a wavelength around 600 nm. Study the figure of the electromagnetic spectrum below. It will be useful in answering some of the questions that will follow.

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Once you are comfortable with the spectrum (wavelengths, colors and energies), use that knowledge along with the following graph and filters to answer the questions.