PLANETARY PHOTOMETRY AND SPECTROSCOPY
As a reminder from Chapter 5, in photometry astronomers measure the amount of radiant energy, while'in spectroscopy they study the composition by wavelength of white light. Photometric measurements provide information about the nature of reflecting materials, such as clouds in a planet's atmosphere or the surface of a planet or its satellite. Photography through color filters, which restricts the light to a narrow spectral region, and conventional spectroscopy give us clues about the chemical composition of a planet's surface, clouds, and atmosphere. Atmospheric constituents for the planet may be revealed by absorption lines or bands that are superimposed on the spectrum of the sunlight reflected from within the atmosphere. These planetary absorption features are sometimes difficult to separate in wavelength from absorption lines originating in the sun's atmosphere.
With Polaroid filters and other devices for measuring the polarization of light, astronomers can analyze a planet's surface and atmosphere by the manner in which reflected sunlight is polarized. To understand the significance of polarization of light in this context, you should remember what happens when sunlight reflected from, say, a car windshield is observed through Polaroid sunglasses. The Polaroid lens does not transmit all the reflected light that is polarized, and thus the glare is less. From this example we can go on to study reflection from a variety of surfaces to show that the degree of polarization of reflected light is indicative of the nature of the reflecting material, such as a planet's surface.
