Peering Into Comets, Finding More of the Same

Bruce Dorminey
for National Geographic News
July 11, 2007

Comets may have a more consistent mix of chemicals than previously thought, according to a new study that contradicts assumptions about the fiery celestial bodies.

Ground-based observations of two fragments from the nucleus of a comet called SW 3 indicate a "very homogeneous" composition, said study lead author Neil Dello Russo of Johns Hopkins University Applied Physics Lab in Maryland.

Dello Russo and his team made their observations in 2006 using NASA's Infrared Telescope Facility and the Keck II telescope atop Mauna Kea volcano in Hawaii. That's when SW 3 was a mere 7.5 million miles (10 million kilometers) from Earth.

Secrets of Dirty Snowballs

For scientists, these dirty snowballs of ice, gas, and dust hold the keys to answering how our solar system formed and evolved from a giant cloud of gas and dust some 4.5 billion years ago.

Researchers are tantalized by each new scrap of comet data. For instance, in 2005 scientists smashed a NASA spacecraft into a comet called Tempel 1, which enabled them to study its interior. (See a video of the crash countdown.)

In contrast to SW 3, Tempel 1 was found to be chemically heterogeneous. (Related: "Comets May Not Have Solid Cores, "Impact" Shows" [September 8, 2005].)

However, the work done on SW 3 "does not invalidate the work done on Tempel 1. We see a great compositional diversity in comets throughout the population," Dello Russo said.

The study will appear in today's journal Nature.

Comet Birthplace

For a comet to have a more of the same ingredients, the area where the comet was born must also have had a uniform array of chemicals, said comet scientist Laura Woodney at California State University, San Bernardino. Woodney was not involved in the study.

This may show less mixing of materials in the outer solar nebula—an area where many comets were formed—than previously believed. (Explore a virtual solar system.)

Walter Harris, a planetary scientist at the University of California, Davis, cautioned against drawing definitive conclusions about the young solar nebula from such a limited sampling.

Because short-period comets spend more time in the sun's vicinity, Harris said they've been in the sun's "oven" a lot longer than the comets with incredibly long orbital periods, which travel on the solar system's outskirts.

"For comet researchers," Harris said, "it's a lot like trying to understand how people are made and develop by looking only at a few students in kindergarten—and [then] some residents of a retirement home."

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