COURTESY OF NASA/JPL-CALTECH/R. HURT

The Fourth of July last year had some extra fireworks. NASA's Deep Impact spacecraft sent a hefty projectile--more than 800 pounds--into the body of the comet known as Tempel 1. The collision delivered 19 gigajoules of energy--the equivalent of nearly five tons of explosive TNT--into the wandering comet and ejected a plume of its innermost secrets. Roughly 10 million kilograms of comet stuff (more than 22 million pounds) spread out into space, giving scientists a rare glimpse of the ingredients that go into making a comet. Now researchers observing with the Spitzer Space Telescope have revealed their findings: comets contain a mix of materials that formed under widely divergent conditions.

Carey Lisse of Johns Hopkins University and his colleagues studied the collision through 12 infrared spectrographs taken by Spitzer from July 2 through July 5. Prior to impact, Tempel 1 displayed the same streaming dust as any other comet, pushed back from the cometary body by the sun's radiation. But after the early-morning impact, Tempel 1 revealed itself to be made of water ice and gas, carbonates, polyaromatic hydrocarbons, silicates, sulfides and other elements.

This mix of components does not match current models of comet dust. Some of the minerals detected require temperatures between 1,100 and 1,400 degrees Kelvin--only found as close to the sun as Mercury--as well as volatile gases such as methane that only remain stable at temperatures below 100 K. This means that there must have been some form of mixing over large distances going on in the nebula that gave birth to the sun billions of years ago.

The spectra also hint that water must have been abundant in the area where the comet formed and that Tempel 1 is not as carbon-rich as some of its peers; carbon-based materials appear to make up only 20 percent of this comet compared to as much as 50 percent of others. Nevertheless, the material in Tempel 1 matches that ejected by Comet Hale-Bopp in 1995 and that means that these comets formed in broadly similar ways, the researchers argue. Science published the paper analyzing the spectra online yesterday.