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8.5 Comet Chemistry

Progress on the question of comets’ composition came in 1868 when English astronomer William Huggins first studied them through a spectroscope. This instrument spreads visible light into an array of wavelengths, or a spectrum, revealing the amount of light reflected from an object in different colors. The spectrum allows astronomers to measure the material properties of a celestial body. Different materials have distinct spectra, allowing the composition of a distant object to be determined.


Example of an iron emission spectrum. . Click here for original source URL

William Huggins. Click here for original source URL


Huggins found that the comets he observed contained gaseous carbon. Modern astronomers have shown that comets also contain the same atoms we see elsewhere in the Solar System, including hydrogen (H), oxygen (O), nitrogen (N), and carbon (C). Comets also contain many of the molecules that can be formed from these four elements, including CN, CH, OH, H2O+, CN+, CH+, OH+, N2+, CO+, and CO2+. (The raised "+" indicates a positively charged ion, an atom or molecule that has lost one or more electrons. When an atom loses an electron, it acquires a positive charge.) Recent studies have also revealed complex organic molecules, such as CH2CN. This discovery is important because it proves that organic molecules, which are the building blocks of life, form elsewhere in the universe besides on Earth.

There is a connection between the atoms and molecules that make up a comet and its icy composition. Note that all the atoms (H, O, C, N) and molecules detected in comets are just the ones that would be expected if the gas around comets were to form from the sublimation of common Solar System ices, such as H2O (water), CH4 (methane), NH3 (ammonia), and CO2 (carbon dioxide). In other words, the H, O, C, and N atoms and molecules streaming off comets are coming from subliming ice.

In addition to revealing the composition of comet gases, spectroscopes show that some particles in comet tails are much bigger than individual molecules. These particles, or dust grains, form slightly curved tails. Gas tails, by contrast, are straight. Magnetic forces straighten the stream of ions in the gas tails, while the uncharged dust grains form a curved tail.

Nucleus of Halley's comet. Click here for original source URL.

Halley's comet in 1986. Click here for original source URL.

The European Space Agency’s Giotto mission gave us excellent information on the chemical composition of the dust in Halley’s Comet. The carbon-rich dust particles in the nucleus were also found to be rich in hydrogen, oxygen, and nitrogen, resembling meteoritic particles collected on Earth. These interplanetary dust particles are believed to be rich in organic molecules (large, carbon-based molecules) and are different from familiar terrestrial dust, which is richer in silicon, iron, other metals, and their oxides. In 2014, the Philae lander detected carbon-rich organic solids on the comet 69P. Interest in this organic-rich cometary dust is growing. Scientists believe that as comets collided with the forming planets early in Solar System history, they may have provided organic molecules, without which, life on Earth may never have originated.