Did molecular hitchhikers ride a comet to earth?

Hitchhiking molecules could have survived fiery comet collisions with Earth, according to a major new experiment done by researchers at Argonne, the University of California at Berkeley and Lawrence Berkeley National Laboratory.

The experiments simulated a high-velocity comet collision with Earth. The results give credence to the theory that the raw materials for life came from space and were assembled on Earth into the ancestors of proteins and DNA.

Argonne chemists Randy Winans and Mike Ahrens were members of the research team, which shot a soda-can sized bullet into a quarter-sized metal target containing a teardrop of water mixed with amino acids, the building blocks of proteins.

The ballistic test was designed to simulate the type of impact that would have been frequent in Earth’s early history, some 4 billion years ago, when rocky, icy debris in our solar system accreted to form the planets in what must have been spectacular collisions. Much of the debris would have resembled comets — dirty snowballs thought to be mostly slushy water surrounding a rocky core — slamming into Earth at velocities greater than 16 miles (25 kilometers) per second.

The severity of the laboratory impact was akin to an oblique collision with the rocky surface of the Earth — a comet coming in at an angle of less than 25 degrees from the horizon, rather than head on perpendicular to the Earth’s surface.

More than 70 varieties of amino acids have been found in meteorites — many the suspected cores of comets that smashed to earth — and are presumed to exist in interstellar dust clouds.

"Comets are all frozen, so amino acids could be preserved within them," Winans said. "Assuming that the comets did not directly hit the Earth, but glanced the surface, they could have survived the fall."

Not only did a good fraction of the amino acids survive the simulated comet collision, but many polymerized into chains of two, three and four amino acids, so-called peptides. Peptides with longer chains are called polypeptides, while even longer ones are called proteins.

After the experiment, Winans and Ahrens analyzed the materials at Argonne, taking advantage of the laboratory’s liquid chromatography and mass spectroscopy expertise, to determine the species and concentrations of molecules present.

The experimental results suggest that some ice from the comet would remain intact as a liquid puddle concentrated with organic molecules ideal for the development of life. This impact scenario provides the three ingredients believed necessary for life: water, energy and organic material.

The work was sponsored by the National Science Foundation, NASA and the U.S. Department of Energy.

For more information please contact Catherine Foster