These days astronomers do very little naked-eye study of the planets with a telescope. The primary function of a telescope is for direct photography, spectroscopic studies, or photometric work because these techniques leave a record that astronomers can study as needed. Also using a radiation detector like a photographic emulsion or a photoelectric device enables astronomers to make quantitative measurements, something generally not possible with eye observations. Planetary photographs have been collected from various observatories worldwide in an international center in France that now contains over 33,000 pictures.
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.
THERMAL AND NONTHERMAL RADIATION
Thermal radiation from the planets, which is due to the fact that the planets are hot, can be studied far into the infrared with today's heat-sensing instruments. This radiation is blackbody radiation, caused by the random thermal motion of the particles that compose the outer parts of the planets. Thus astronomers may apply Planck's law, the Stefan-Boltzmann law, and Wi en's law to such radiation. The data obtained from infrared radiation provide important information on surface and atmospheric temperatures and, indirectly, chemical composition.
Nonthermal radiation is radiation due to physical processes other than that involved in producing blackbody radiation. That is, it owes its explanation to some other fact than that the planet is hot. For example the light produced in lightning is nonthermal radiation. The planets' normal thermal radiation and any nonthermal radiation present are often observable with radio telescopes in the regions of millimeter, centimeter, and meter wavelengths.
RADAR MAPPING OF PLANETARY SURFACES
Pulsed signals sent and received by radar have given us relief maps of Mercury, Venus, and Mars. This is important and somewhat remarkable for Venus since a thick cloudy atmosphere prevents visual observations of its surface. The signals are reflected from the planet's surface and return to the earth (or an orbiting satellite) as an echo modified by the surface terrain. The returning wave train from different portions of the surface is analyzed by a computer and then can be used to construct a topographic map of the planet's surface.
Analyzing the Doppler shift in the reflected radar signal caused by the rotation of the planet has given us the period of rotation for Mercury and Venus; it could not have been found for Venus by optical methods because of cloud cover. The radio wave reflected from the part of the surface that is approaching the earth has its wavelength slightly decreased because of the Doppler effect. The echo from the side of the planet that is receding from us is slightly increased in wavelength.
