In our earlier spectroscopy experiment, we explored the way in which atoms and molecules of different substances can emit electromagnetic radiation (typically light) of different wavelengths (often seen as colors), and how this information can help us identify the types of substances found in distant objects without actually visiting them. Along with their emission properties, most substances are also selective in the wavelengths of electromagnetic radiation which they reflect. The study of this property, called reflectance spectroscopy, is a basic tool in the study of the surface properties of the Earth and other planets. With this technique, the light does not originate with the object studied, but comes from somewhere else and reflects off of the object studied. Of course, not all of it reflects- some of it is absorbed or transmitted- and the details of this depend on the substance. In general, if an object appears to be a particular color, it is usually because it reflects light of that color (and absorbs much of the rest of the spectrum).

The instrument we are using is the ALTA Reflectance Spectrometer made by the Lunar and Planetary Institute in Houston, Texas. Examine the instrument carefully. Note the sliding power switch on the side, the nine buttons and digital display on the front, and the hole in the back through which you can see some circuitry. Please do not stick objects into the hole in the back! The circuitry is fragile, and the light sources can be altered if covered with greasy fingerprints! Figure out just what it is that the digital display is showing you- what happens when you point the hole in the back at the room light or out the window?

The basic operating mode for the instrument is to place it against a (flat) surface of the substance to be measured, press the buttons to turn on each of the lamps in turn, and note the readings. To conserve battery life, hold the button down just long enough for the reading to stabilize.

Note that when you place the instrument against a dark surface and turn on NONE of the lights, there is still a small reading. This is called the dark voltage (or current). It should be noted and subtracted from all measurements made with the instrument!

The lights in the instrument do not put out narrow spectral lines like the gas lamps. Instead, they each emit a narrow band of wavelengths centered at the wavelength (or color) marked on the button. You can check this out with the blue "Project STAR Spectrometers" that we used earlier.

Your instrument must be calibrated by reading the reflectance from a standard "white" object (a sheet of paper) which we will assume (risky!) reflects 100% at all wavelengths. This calibration needs to be done only once. Then you will be ready to take readings off of various objects. For each wavelength, calculate the percentage reflectance by comparison with the white paper standard.

For example, if your instrument reads 8 in the dark, and at a given wavelength it reads 840 when against white paper, but 420 when against a certain specimen, then the reflectance of that specimen at that wavelength is:

(420-8)/(840-8) = 412/832 = 0.495 = 49.5%

"Work sheets" are available to record the data and calculate the reflectance. You will also be able to draw a graph of the reflectance as a function of the wavelength.

The Problem- Why is Mars Red?

After landing on Mars in 1997, the Mars Pathfinder camera made a reflectance spectrogram of the scene in front of it. It did this by photographing the scene repeatedly through various filters that were moved in front of the camera one at a time. The filters transmitted narrow bands of wavelength centered at the following values (in nanometers):

470, 530, 600, 670, 750, 800, 860, 900, 930, 970, 1000

These wavelengths are sufficiently similar to those of the ALTA Reflectance Spectrometer that we can use our instrument to examine the reflectance spectrum of various terrestrial rocks and other materials to see if they are "candidates" for the surface of Mars.

The Study

Measure, calculate, and plot the reflectance spectra for at least four substances that you can compare with the results from Mars. Use various rocks around the lab, the purple paint on the door frames, the bricks outside the building, etc. Select one substance to test which is not red at all to improve your confidence in the method. Discuss briefly what you feel your data say about the possible sources of the "redness" of Mars.