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Recipe for a comet

— Thar's gold in them thar comets - fool's gold, that is, also known as iron sulfides. Thar's also clay and chalk, which are far more interesting to the scientists behind the latest effort to figure out the recipe for a comet.

Observations from NASA's Spitzer Space Telescope, gathered during last year's Deep Impact encounter with Comet Tempel 1 and reported this week in Science Express, have turned up a couple of surprise ingredients in the recipe. To explain the presence of clay and chalk, researchers have bought into a theory that may be unorthodox - but isn't at all foolish.

NASA / JPL-Caltech / UMD
A colorized image shows the plume
of ice and dust thrown up from
Comet Tempel 1 during the Deep
Impact encounter.

Chemists have thought that such clays (smectite) and chalky material (carbonates) are formed only in the presence of liquid water. To be sure, Tempel 1 and most other comets have plenty of water, but it's either bound up as ice or liberated in the form of water vapor.

The head of the Spitzer observation team, astronomer Carey Lisse of the Johns Hopkins University Applied Physics Laboratory, told me today that there could be another mechanism for a comet's water-based chemistry. If the material making up the comet spent a lot of time within the inner solar system, billions of years ago, it could have reacted with the water vapor there ... er, here ... to create the telltale compounds.

That fits in with findings from NASA's Stardust mission, which found crystalline silicates in samples brought back to Earth from Comet Wild 2's coma. The only way scientists can explain those silicates would be if the stuff making up Wild 2 spent some time relatively close to the sun - then congealed in the solar system's chillier zone.

Tempel 1, which also contains crystalline silicates, appears to have resulted from the same recipe, Lisse said.

"In the same body, you have material formed in the inner solar system, where water can be liquid, and frozen material from out by Uranus and Neptune," he said in a Johns Hopkins news release. "Except for the lightest elements, the total abundances of atoms in the comet are practically the same as makes up the sun. It implies there was a great deal of churning in the primordial solar system, with high- and low-temperature materials mixing over great distances."

Lisse told me that Stardust's findings provide important "ground truth" for Spitzer's observations from a distance. Stardust's scientists haven't yet detected the presence of clays or carbonates in their samples - but Lisse is betting that they'll eventually be found. Literally.

If the Wild 2 samples eventually yield evidence of carbonates, Lisse said Stardust team member John Bradley will owe him a six-pack of beer. "It's a gentlemen's agreement," he said.

When I asked Bradley about Lisse's wager, the researcher at Lawrence Livermore National Laboratory laughed heartily. "Ask him when the Guinness will be delivered," Bradley said. On a more serious note, Bradley said "the jury is still out" on the detection of carbonates.

In addition to the clays and carbonates, there's yet another intriguing ingredient detected by the Spitzer Space Telescope: polycyclic aromatic hydrocarbons, or PAHs. These are carbon-containing molecules that are also found on Earth in, say, barbecue grills or automobile exhaust.

"I think we have the first definitive detection of PAHs in a comet," Lisse said.

Although PAHs are generally thought to arise through organic chemistry, Lisse said the molecules aren't necessarily indicative of life. After all, they've been detected before in the interstellar medium, around young stellar objects and in the enigmatic Allan Hills meteorite from Mars.

"To me, PAHs can easily be made through inorganic processes," he said. "I think that's a very common process."

One reason scientists study comets is to determine whether they might contain chemicals that served as the ingredients for life. Lisse said the evidence indicates that building blocks are there, but in the form of "very, very basic components" such as methanol or formaldehyde. So far, DNA or amino acids haven't been found in comets (though meteorites may be a different story).

Lisse said he's not surprised to see the basics of organic chemistry show up in comets. It takes a lot more than those raw ingredients to come up with a recipe for life, he explained.

"Given something that's the size of an Appalachian mountain, that weighs about 1014 kilograms, if .001 percent of that is more exotic, or more complex, it's possible you could get something more complicated that might lead to life," he said. "But it's a helluva lot easier if you dumped all those ingredients on Earth, and let all that stuff react on Earth."

There's a certain school of thought that suggests life or its building blocks originated elsewhere in the universe, then came to Earth on comets or asteroids. Does that make sense to you, or is it mere foolishness? I'd love to see your comments on the subject.