Studying the Chemical Composition of a Planet

Probably the most important single instrument available for determining the chemical composition of a planet has long been the 

Each element and compound produces a unique, characteristic line spectrum when it is heated. Hence an element can be identified by its line spectrum. Scientists can analyze electromagnetic radiation emitted by or reflected from a planet s surface or its atmosphere with a spectrometer to identify the line spectra present. By analyzing those line spectra, a scientist can determine which elements, compounds, and other chemical species are present on the planet s surface or in its atmosphere. 

One of the earliest and most famous examples of the use of spectrometry to study the solar system occurred in 1868. While observing a solar eclipse in India, the French astronomer Pierre Janssen (1824-1907) found a new spectral line in sunlight very close to one found in the spectrum of sodium. Janssen was able to show, however, that the new yellow light was different from the sodium line. Indeed, it was a spectral line that had never before been observed on Earth. Janssen hypothesized that the presence of the line could be explained only if the Sun s atmosphere contained an element that had not yet been discovered on Earth. The British astronomer Sir Norman Lockyer (1836-1920) later suggested the name helium for the element, a name based on the Greek word for sun, helios. It was almost 30 years later that Janssen s bold hypothesis was confirmed. In 1895, the British chemist and physicist Sir William Ramsay (1852-1916) first detected helium on Earth, during a series of experiments on an ore of uranium called clevite. 

Today astronomers routinely study the chemical composition of a planet by analyzing sunlight reflected off its surface and atmosphere. The same method is used to analyze the chemical composition of other bodies in the solar system, such as comets, meteors, and planetary satellites. This process is challenging since, in some cases, relatively modest amounts of light are reflected from a planet or other body. Also, the spectrum observed is likely to be very complex, with the lines of many elements and compounds present in the pattern. 

One of the great breakthroughs for astrochemists in the past half century has been the development of rockets and space vehicles that can carry spectrometers into space, far above the Earth s at- 

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