1. Evolving perspectives - a historical prologue
Ingredients of the Sun and planetary atmospheres
The ingredients of the Sun and planetary atmospheres can be determined when the intensity of their radiation is shown as a function of its wavelength. Such a display is called a spectrum, and the study of spectra is known as spectroscopy. Each chemical element or compound produces a unique set, or pattern, of spectral signatures at certain specific wavelengths and only at those wavelengths. They resemble a barcode or a fingerprint that can be used to identify the element or compound.
The technique of astronomical spectroscopy was first developed using the bright light of the Sun. When its spectrum is examined carefully, with fine wavelength resolution, numerous fine, dark absorption lines are seen crossing the rainbow-like display. The separate colors of sunlight are somewhat blurred together when coarser resolution is used, and the dark places are no longer found superimposed on its spectrum.
We now know that hydrogen accounts for 92.1 percent of the number of atoms in the Sun, and that hydrogen is the most abundant element in most stars, in interstellar space, and in the entire Universe. Helium, the second-most abundant element in the Sun, is so rare on Earth that it was first discovered in the Sun. The next-most abundant elements in the Sun are carbon, nitrogen and oxygen, as well as the inert element, neon.
Strong red and infrared absorption lines were also detected in early spectroscopy of the giant planets. They were interpreted in the 1930s as absorption by methane, CH4, and ammonia, NH3. The presence of methane and ammonia would be expected if the planets formed together with the Sun. The massive giant planets would then approximate the solar composition, which would explain their low mean mass densities. The overwhelmingly abundant hydrogen, H, would combine with the abundant carbon, C, nitrogen, N, and oxygen, O, in the low-temperature environment far from the Sun, to form stable molecules of methane, CH4, ammonia, NH3, and water vapor, H2O. Nevertheless, the exact composition of the atmospheres of the giant planets had to await space-age infrared spectroscopy, which showed that Jupiter and Saturn are mainly composed of hydrogen.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University