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Interstellar Space a Likely Source of Organic Molecules
June 7, 2001

Astrochemistry Laboratory
Ames Research Center
Moffett Field, CA

Scientists today described how the interaction of hard radiation and ices in space leads to the production of complex organic molecules. The report is being presented to the American Astronomical Society meeting in Pasadena, CA by Drs. Louis Allamandola, Max Bernstein, Jason Dworkin, and Scott Sandford of the Astrophysics Branch of NASA's Ames Research Center, in Moffett Field, CA, Dr. David Deamer of the Biochemistry Department of the University of California, in Santa Cruz, CA, and Dr. Richard Zare and Ms. Jamie Elsila of the Department of Chemistry at Stanford University, in Stanford, CA. The production of organic compounds in space is of special interest to scientists since these molecules may have played a role in the origin of life on Earth.

These scientists have been studying the chemistry of organic carbon compounds that occurs in dense molecular clouds in interstellar space, the locations where new stars and planetary systems are born. Such clouds consist of concentrations of dust, ice, and gas that screen out much of the light produced by outside stars. As a result, the interiors of these clouds can become very cold, sometimes attaining temperatures as low as 10 Kelvin (-263 C). At these temperatures, many of the molecules and atoms that are normally present as gases condense to form ice mantles surrounding the dust particles in the cloud, much as your breath condenses into frost on a cold window. These ices are primarily made up of simple molecules like water (H2O), methanol (CH3OH), carbon dioxide (CO2), carbon monoxide (CO), ammonia (NH3), and methane (CH4).

At such low temperatures, these molecules would not normally be expected to react with each other, particularly when they are embedded in ice. However, the ice mantles are exposed to low levels of ionizing radiation in the form of cosmic rays and ultraviolet photons. This radiation can break apart the molecules in the ice and produce highly reactive ions and radicals that can recombine to form larger, more complex molecules.

At NASA-Ames, Allamandola, Sandford, Bernstein, and Dworkin use cryogenically cooled vacuum chambers and UV lamps in their laboratory to form and irradiate interstellar ice analogs under conditions that simulate those found in dense interstellar clouds. "Basically, we freeze mixed gases onto an extremely cold window and then give the ices the equivalent of a good suntanning," says Allamandola. "After the sample is warmed up, we can remove any remaining organic materials from the sample chamber and study them using a variety of analytical techniques," he continued.

One of these is the technique of two step laser-desorption laser-ionization mass spectrometry. "That's quite a mouthful," says Stanford graduate student Elsila, "but essentially this is an analytical technique that allows us to measure the masses of the various compounds in the organic residue that results from the ice irradiation." "The surprise," says Zare, leader of the Stanford group, "is just how complex the population of organics is. Generally we see a peak at virtually every mass up to and beyond 500 atomic mass units!" This means that the residue must contain hundreds of distinctly different molecules, the vast majority of them being considerably larger than the molecules that made up the original ice.

"We are only just beginning to identify all the compounds that are present," notes Dworkin. "One of the more interesting classes of compound we have identified in the residues are amphiphiles. These molecules have the interesting property that, if you add them to water, they can spontaneously form vesicles, that is, walled structures reminiscent of cells." This raises the possibility similar materials could have fallen on the early Earth and played a role in the formation of the first cellular structures. "There is some precedence for this idea," notes Deamer, a biochemist from UCSC and an expert on membranes. "Primitive meteorites are also known to contain amphiphiles that, when added to water, make structures that are very similar to those we make from the simulated interstellar residues," he continued.

Other chemical compounds the team has been studying is a class of molecules called "polycyclic aromatic hydrocarbons," or PAHs for short. These molecules consist of small sheets of carbon atoms arranged in hexagons with hydrogen atoms around their edges, much like the shapes you would get if you cut out pieces of a chicken wire fence. PAHs are common molecules on the Earth and are a major component of auto exhaust and soot. PAHs are also very abundant in space, where they are thought to originate primarily in the outflows of gas given off by stars like our own Sun when they reach the end of their normal lives. Like the other molecules in space, PAHs should be frozen into the ice mantles that surround dust grains in interstellar clouds.

When the team examined the chemistry that occurred when PAH-containing H2O ices were irradiated with ultraviolet light, they discovered that the PAHs were not destroyed, but that many of them did have their edges modified by the addition of extra oxygen and hydrogen atoms. The addition of oxygen atoms results in the formation of aromatic alcohols and ketones, i.e., PAHs where a peripheral H atom is replaced by an -OH group or a doubly bonded oxygen, respectively. The aromatic ketones are of particular interest. This class of compounds includes quinones, molecules that currently play critical metabolic roles in the biochemistry of all living organisms on Earth. "As with the amphiphiles, this raises the interesting possibility that the infall of materials made in the interstellar medium may have played a significant role in getting life started on Earth," notes Bernstein, who along with Dworkin, makes most of the residues.

"However," Allamandola added, "the production of organics in space can't play a role in the origin of life on planets if the material is unable to safely survive transportation from the interstellar medium to the surface of a newly formed planet. Fortunately, meteorites provide us with evidence that organic materials can survive this transition." This evidence comes primarily from the detection of deuterium enrichments in many meteoritic organics.

Deuterium is one of the heavier isotopes of hydrogen, having one extra neutron. "It turns out that most of the chemical processes that we think occur in the interstellar medium favor the heavier deuterium over normal hydrogen," says Sandford. "As a result, the presence of excess deuterium in meteoritic organics strongly suggests an interstellar connection. One of our current research activities is to try to understand how deuterium behaves during our ice chemistry simulations. We are discovering patterns to the placement of deuterium in the resulting organics and one of our plans for the future is to compare our results to meteoritic organics to see if the same patterns appear in them."

Perhaps the most important point of all this, notes Sandford, is that this type of chemical activity is a universal process that should be happening in all interstellar dense clouds. "It appears that the universe is, in some sense, 'hardwired' to produce relatively complex organics," he quips. "Furthermore, since it is from these clouds that new planetary systems are made, it is reasonable to expect that essentially all new planets should have some of this material fall on them. Thus, interstellar organics may play a wider role in the formation of life on other planets, not just the Earth."

This work was funded by the National Aeronautics and Space Administration.

More information:

Related material which has appeared in the scientific literature:

Images of vesicle research:

[Image 1: (286KB)]
These droplets (~10 microns across) show structures reminiscent of cells (although they are not alive). They are from a chemically separated fraction of the bulk residue.

[Image 2: (167KB)]
These droplets (small ones are ~10 microns across) glowing under black light in the microscope show internal structure and suggest chemical complexity. They are from a chemically separated fraction of the bulk residue.

[Image 3: (85KB)]
This is a vesicle (~10 microns across) glowing under black light in the microscope made from the bulk residue. Proof that it is a hollow vesicle, rather than a simple drop of oil, is the green pyranine dye which we have trapped inside of it.

Contact information:

Dr. Louis J. Allamandola
NASA-Ames Research Center

Dr. Max P. Bernstein
Astrophysics Branch, NASA-Ames Research Center

Dr. David Deamer
Dept. of Chemistry and Biochemistry, UC Santa Cruz
831-459-5158, deamer@hydrogen.UCSC.EDU

Dr. Jason P. Dworkin
Astrophysics Branch, NASA-Ames Research Center

Ms. Jamie Elsila
Dept. of Chemistry, Stanford University
650-723-4318, jelsila@Stanford.EDU

Dr. Scott A. Sandford
Astrophysics Branch, NASA-Ames Research Center

Dr. Richard N. Zare
Dept. of Chemistry, Stanford University

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