Researchers suggest molecules traveled to Earth on comets

By Robert Sanders, Public Affairs

Organic molecules may have survived a comet impact with Earth like the one depicted in this NASA drawing. The research finding by Jennifer Blank and her colleagues supports theories that life came from space. NASA drawing/Donald E. Davis

12 April 2001 | Simulating a high-velocity comet collision with Earth, a Berkeley research geologist and her colleagues have shown that organic molecules hitchhiking aboard a comet could have survived such an impact and seeded life on this planet.

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.

“Our results suggest that the notion of organic compounds coming from outer space can’t be ruled out because of the severity of the impact event,” said research geologist Jennifer Blank of the Department of Earth and Planetary Science in the College of Letters and Science.

Blank and her colleagues — Randy Winans and Mike Ahrens of the Chemistry Division of Argonne National Laboratory, and engineer-mathematician Gregory Miller of the Applied Numerical Algorithms Group of Lawrence Berkeley National Laboratory — reported their preliminary findings April 5, at the national meeting of the American Chemical Society in San Diego.

Blank’s team shot a soda-can sized bullet into a nickel-sized metal target containing a teardrop of water mixed with amino acids, the building blocks of proteins. 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.

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.

“The neat thing is that we got every possible combination of dipeptide, many tripeptides and some tetrapeptides,” said Blank, a geochemist. “We saw variations in the ratios of peptides produced depending on the conditions of temperature, pressure and duration of the impact. This is the beginning of a new field of science.”

Freezing the target to mimic an icy comet increased the survival rate of amino acids, she added.

The ballistic test was designed to simulate the type of impact that would have been frequent in Earth’s early history, some four 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 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.

“At very low angles, we think that some water ice from the comet would remain intact as a liquid puddle concentrated with organic molecules,” ideal for the development of life, Blank said. “This impact scenario provides the three ingredients believed necessary for life: liquid water, organic material and energy.”

The best known theory of the origin of life on Earth is that it derived from complex molecules such as amino acids and sugars produced early in the planet’s history by electrical discharges in an atmosphere replete with gases such as methane, hydrogen, ammonia and water. The famous Miller-Urey experiment in 1953, conducted by Stanley Miller and Harold Urey of the University of Chicago, demonstrated that a lightening-like discharge in a test tube filled with these molecules could produce amino acids.

Other scientists, however, have proposed that the building blocks of life arrived from space. Astronomers have detected many kinds of organic molecules in space, floating in clouds of gas or bound up in dust particles. They range from the simplest — water, ammonia, methane, hydrogen cyanide and alcohols, including ethyl alcohol — to more complex molecules, including chains of up to eight carbon atoms.

The next hitchhikers she plans to subject to a shock test are bacterial spores, which some have proposed arrived on Earth via comet to jump-start evolution.