Tons, perhaps tens of tons, of carbon molecules in dust particles and meteorites fall on Earth daily. Meteorites are especially valuable to astronomers because they provide relatively big chunks of carbon molecules that are easily analyzed in the laboratory. In the past few years, researchers have noticed that most meteorite carbon are molecules called polycyclic aromatic hydrocarbons (PAHs), which are very stable compounds and are survivors.
PAHs are the most common carbon-rich compound in the universe. They are found in everything from distant galaxies to charbroiled hamburgers and engine soot. When they are first formed, or found in space, their structures resemble pieces of chicken wire, fused six-sided rings. However, when found in meteorites, these aromatic rings are carrying extra hydrogen or oxygen.
Scientists at NASA Ames Research Center, Moffett Field, Calif. performed laboratory experiments that explain the process by which these meteoritic hydrocarbons attract the extra hydrogen and oxygen. They are very similar to the molecules identified as evidence of alien microbes in an earlier Science paper (McKay et al 1996).
“Our findings are important because it is the first time anybody explained these carbon-rich molecules found in meteorites. They are similar to the molecules that make-up living things,” said Max Bernstein, a space scientist at NASA Ames.
As it happened, their findings were judged significant enough to be award-winning. Published in Science (1999) by Bernstein and fellow NASA Ames scientists Scott Sanford and Louis Allamandola, their paper won the 2008 H. Julian Allen Award at NASA Ames Research Center.
It takes a long time for scientific papers to win awards.
“As scientists, we like to quantify things. Scientific papers are judged by the number of times they are cited in other scientific papers. Other scientists need to say that I couldn’t have written my paper without your paper. Often it takes a few years,” Bernstein explained.
These carbon-rich molecules are produced by carbon-rich, dying, giant red stars. When they are first formed, astronomers observe them as normal PAHs. However, when they are seen in meteorites billions of years later, they almost always have oxygen or heavy hydrogen attached to them. (Heavy hydrogen carries an extra neutron, and is called a deuterium isotope.) Something happened to change them, say scientists.
To study the process by which these carbon compounds change, the Ames Astrochemistry Laboratory studied PAHs in water ices that were exposed to ultraviolet radiation under space-like conditions. Scientists reproduced conditions including an incredibly high vacuum, extremely low temperatures (- 340 degrees Fahrenheit), and harsh radiation. When the extremely cold temperature was reached, these PAHs were exposed to ultraviolet radiation, and they changed. The experiment successfully reproduced the hydrocarbons found in meteorites. For the first time, scientists were able to show how hydrogen was exchanged for deuterium, or heavy hydrogen.
“It turns out, you only need water ice and radiation to change these molecules,” said Bernstein.
Using infrared spectroscopy, the Ames research team proved that the laboratory-produced hydrocarbons were the same hydrocarbons found in meteorites and observed through telescopes. Scientists observed the chemical reaction in a stainless steel chamber as it was happening. The laboratory sample reflected the same infrared colors as the hydrocarbons seen by astronomers using telescopes. Because the techniques used were the same, the results were directly comparable. “We were seeing the same molecules from telescopes as were reproduced in the laboratory,” said Sandford.
Once the molecular-size laboratory sample was retrieved, it was taken to Richard Zare’s laboratory at Stanford University, where researchers weighed the individual molecules. Findings showed that ices, modified by radiation, created new molecules.
These molecules, called quinones, received considerable attention by the astrobiology community because they are common to all life forms. They are potentially significant for the “origin of life” or the habitability of planets. How does a planet become habitable?
“Molecules from space helped to make the Earth the pleasant place that it is today,” said Allamandola, founder of the Ames Astrochemistry Laboratory.
“Our findings were new because we showed how these molecules formed. It was already known that these molecules were in meteorites and delivered to the planets,” said Bernstein.
“We now understand why these life-like carbon compounds are raining down on the Earth and other planets. Knowing this will help us search for life on other worlds by distinguishing these molecules from biomarkers,” said Bernstein.
- Bernstein, Max P., Scott A. Sanford, Louis Allamandola, J. Seb Gillette, Simon J. Clemett and Richard N. Zare. UV Irradiation of Polycyclic Aromatic Hydrocarbons in Ices: Production of Alcohols, Quinones, and Ethers. Science, 1999; 283 (5405): 1135 DOI: 10.1126/science.283.5405.1135
- McKay, David S., et al. Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001. Science, 1996; 273 (5277): 924 DOI: 10.1126/science.273.5277.924
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