A Comet's Chemical Composition

What do you get when you kick up a whole lot of debris from an icy comet under the watchful eye of a powerful infrared telescope-spectrometer? An up-close look at the chemical composition of the heavenly body.

A large team of astronomers and other scientists has just published the first detailed analysis of infrared emission spectra recorded during last year's Deep Impact mission to comet Tempel 1. The results point to an assortment of minerals, water, and other inorganic and organic materials as the stuff of which comets are made (Science, DOI: 10.1126/science.1124694). The study provides an unprecedented examination of the chemical nature of the interior of comets, which are thought to be composed of leftover debris from the formation of the solar system, and may help answer questions regarding the evolution of celestial objects.

In a spectacular conclusion to a six-month, 80 million-mile journey, NASA's Deep Impact vehicle sent a massive "impactor" smashing into comet Tempel 1 on July 4, 2005. The 800-lb flying probe rammed headlong into the comet at nearly 30,000 miles per hour and burrowed deep into the comet's nucleus. The crash released millions of pounds of subsurface material into space as a flyby spacecraft, in addition to Earth- and spaced-based telescopes, recorded the collision and its aftermath in detail (C&EN Online Latest News, Sept. 12, 2005).

Among the probes zooming in on the comet last year, NASA's Spitzer Space Telescope focused on infrared light emitted from fine dust particles that were ejected from the impact site. According to the research team, which was led by Carey M. Lisse of the Applied Physics Laboratory at Johns Hopkins University and includes 16 other scientists at the University of Maryland, College Park; California Institute of Technology; and elsewhere, analysis of the spectra reveals the IR signatures of a host of amorphous and crystalline inorganic powders. Included are minerals such as magnesium-rich forsterite and iron-rich fayalite (both in the olivine family); ferrosilite, an iron-rich pyroxene; and nontronite, a smectite clay containing iron, aluminum, and sodium. The spectra also feature telltale signs of other minerals, amorphous carbon, water ice, sulfides, and polyaromatic hydrocarbons.

The researchers point out that previous comet studies relied on data collected from material that migrated from the comet surface to the atmosphere surrounding the comet—the coma. But current thinking in the field suggests that a comet's exterior evolves because of the effects of solar radiation and hides pristine solar system material under a surface layer. The present study probes 20 to 30 meters beneath that layer.

"Now we have some important clues as to how our solar system formed by digging into one of the leftover relics of its formation—Tempel 1," Lisse says.

One surprising observation is that comets contain a mixture of materials that form at widely varying temperatures. The finding suggests that the materials were created separately and somehow mixed together while forming a comet.

Lisse remarks that "it's really neat to see that the materials we find are all simple and what one would expect if you vaporized everything in the solar system today, then let it cool slowly, while stirring."