How comets may have 'seeded' life on Earth

Some scientists have long held the notion that comets delivered many of the chemical building blocks of organic life. NASA's Deep Impact mission to comet Tempel 1 has substantially strengthened their case.

This week, at a meeting in London and in results published in Thursday's edition of Science Express, Deep Impact scientists say they have found high levels of organic chemicals beneath the surface of Tempel 1's core.

They have yet to identify all of the chemicals present in the material, which was ejected on July 4, when the comet collided with a projectile the Deep Impact spacecraft released.

But what they've seen so far makes it "more likely" that comets seeded Earth with the chemical precursors for organic life, says Michael A'Hearn, a University of Maryland planetary scientist and the mission's lead researcher.

Past studies from ground-based observatories and comet flybys have identified many of the chemicals in the halo of dust and gas that surrounds the core, as well as in the bright tails of comets.

But these sightings have had little to say directly about the amounts and relative abundance of the compounds comets contain in their cores.

As the team continues to pour through its data, researchers expect to identify all of the chemicals "that comets brought in abundance to the early Earth," Dr. A'Hearn says. "That will be our biggest contribution" to understanding comets' roles in the story of life on Earth.

One surprise: The team has detected an unexpectedly high concentration of methyl cyanide. Biologists say methyl cyanide is a key player in reactions that form DNA.

"If methyl cyanide is a particularly abundant component, it would suggest that comets could have delivered an abundance of these highly reactive compounds to the early Earth," notes Tom McCollom, a researcher at the University of Colorado at Boulder's Laboratory of Atmospheric and Space Physics.

Methyl cyanide's abundance may also confirm that comets like Tempel 1 can open a window on conditions from which the sun and solar system formed some 4.6 billion years ago.

In the dust and gas that inhabits the distances between stars, hydrogen cyanide is more abundant than methyl cyanide, notes Diane Wooden, an astrophysicist at the NASA Ames Research Center at Moffett Field, Calif.

But once frigid clouds of dust and gas begin to condense, methyl cyanide begins to dominate hydrogen cyanide in the center of the cloud where sun-like stars and planets will form.

Thus, Deep Impact may be looking at the heart of the cloud that gave birth to our solar system.

Deep Impact also has given planetary scientists the first direct measurements of a comet's density. The results imply that the nearly four-mile-wide nucleus is a loose ball of rubble. Global pictures of the nucleus suggest that it is built from two smaller sections that merged early in the object's history.

Moreover, when the impact struck, it burrowed into a layer of loosely bound, fine icy dust that is at least 30 feet deep. The surface appears to offer little support to anything that would land on it.

These observations hold troubling implications for Rosetta, a European Space Agency spacecraft en route to another comet.

If Rosetta's target has a surface as weak, dusty, and thick as Tempel 1, "the spacecraft will sink into the comet," laments Horst Keller, a researcher with the Max Planck Institute for Astronomy in Katlenburg-Lindau, Germany.