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Life's Building Blocks are Common in Space.
NASA researchers have found that many of the basic building blocks for life here on Earth are common throughout the Universe. Using the Spitzer Space Telescope, the researchers observed that complex organic molecules called polycyclic aromatic hydrocarbons (PAHs) are everywhere they looked: in the Milky Way and in the most distant observable galaxies. Most of these molecules contain nitrogen, which is the key requirement for life.
Duplicating the harsh conditions of cold interstellar space, scientists from NASA's Ames Research Center have shown that nitrogen containing aromatic molecules, chemical compounds that could be important for life's origin, are widespread throughout space.
Combining laboratory experiments with computer simulations, this team had earlier shown that complex organic molecules known as polycyclic aromatic hydrocarbons (PAHs) are widespread throughout space. PAHs, large, flat, chicken-wire shaped molecules made up of Hydrogen and carbon are extremely stable and can withstand the hostile radiation environment of interstellar space. The Ames team showed that PAHs are responsible for the mysterious Infrared radiation that Astronomers first called the Unidentified Infrared Emission. NASA's Spitzer Space Telescope, an instrument of unprecedented sensitivity, has now detected the PAH tell-tale signature throughout our Galaxy the Milky Way and in Galaxies very far away, Galaxies nearly as old as the universe itself. Now the Ames team has found that these PAHs contain nitrogen, a key biochemical element (Figure 1). Doug Hudgins, the lead author of the study, points out "Not only are nitrogen containing aromatic hydrocarbons the information carrying molecules in the DNA and RNA that make up all living matter as we know it, they are found in many biologically important species. For example, caffeine and the main ingredient in chocolate are among these kinds of molecule (Figure 2). Seeing their signature across the universe tells us they are accessible to young, habitable planets just about everywhere."
This is the first direct evidence for the presence of complex, prebiotically important, biogenic compounds in space and brings us a step closer to assessing if life's origin on Earth may have had a helping hand from infalling stardust. The bulk of the astronomical evidence points to the formation of these nitrogen containing PAHs in the winds of dying stars which inject them into interstellar space. Eventually they become incorporated into the clouds of material that give birth to stars and planets. Freshly formed planets continue to collect infalling material (dust, asteroids, meteorites, and comets) from the star formation process and life on Earth is thought to have emerged from this primordial chemical soup.
The most common scientific theory for the Origin of Life on Earth is that somewhere in the vast, but simple, chemical resources available on the early Earth, conditions favored the formation of more complex chemical compounds and chemical processes which eventually led to life. However, this theory was conceived at a time when it was thought space was barren of complex organics because interstellar radiation is too harsh, the distances too great, and violent shocks too frequent to support complex chemistry, let alone survival of large molecules and their transport to planetary surfaces. In sharp contrast to that picture, this new work shows that the early chemical steps believed to be important for the Origin of Life do not require a previously formed planet to occur. Instead, some of the chemicals are already present throughout space long before planet formation occurs and, if they land in a hospitable environment, can help jump-start the origin of life.
The NASA Ames team developed the techniques to measure the PAH Infrared signature under conditions found in space - no small feat. While on Earth these compounds are in the solid form; in space they are in the gas, under vacuum, electrically charged and very cold (near absolute zero -441 oF/ -263 oC). "The terrestrial PAH IR fingerprint hardly resembles the emission from space. However, when we prepare the PAHs as they are in space the IR signature changes dramatically and the match is pretty good" said Lou Allamandola, space scientist and team leader. It was this good overall match that largely established the acceptance of PAHs in space and justified digging deeper and bringing powerful new tools to bear on the problem. Chief among these is computational chemistry. "Given Ames is NASA's Information Technology Center for Excellence, it was a natural to see if we could calculate the Infrared signature of these very complex molecules. It had never been done before and, now with the lab data available, we could test and sharpen the accuracy of our methods" said Charles Bauschlicher, a renowned computational chemist. "Now that we know the computational methods work very well, the great advantage computational Chemistry brings to this effort is the ability to calculate the IR spectrum of PAHs and related species for which there is no lab counterpart. You can imagine that stars don't eject only chemicals that can be put in a bottle and stored on a shelf. We can now calculate the spectra of those very elusive molecules" stressed Bauschlicher. This ability is key to the new work reported here.
While the PAH model appeared to satisfy many observations made through most of the 90's, the higher quality IR spectra that were beamed back to Earth from The Infrared Space Observatory, ISO, posed new challenges. In analyzing these spectra, Belgian Astronomer Els Peeters found small but real mismatches with the Ames spectra. "We measured the complete Infrared spectra of over 55 different astronomical objects, many which couldn't be detected before. We found that none of the spectra in the Ames database could reproduce the regular changes we saw that occurred between very old interstellar regions and very young astronomical objects known as planetary nebulae," said Peeters. "That difference showed something important was missing in the Ames dataset and that something told us about PAH evolution" explained Peeters.
"This was about the time we realized that chemically, a nitrogen atom could easily replace a carbon in a PAH's hexagonal skeleton" recalled Hudgins, "but we didn't have a clue as to how that might alter the PAH spectrum." This was also the time when experimental physical chemist and Oklahoman Andrew Mattioda joined the group. "Those were exciting days" Mattioda remembered, "the PAH spectra we had were being used as new tools to analyze regions thousands of light years away and, incredibly, new observations were giving us feedback on the structures of these distant molecules and conditions in the astronomical objects themselves. We geared up to measure the spectra of all the nitrogen containing PAHs (PANHs) we could find, but there weren't many and they are much smaller than those we believe are in space. There are probably hundreds of different PANHs in space and we only had six or seven of the smaller ones." Ultimately, Mattioda's experiments showed that the simple PANHs could not resolve the problem Peeters uncovered.
This was when the computational power came to the fore. Bauschlicher determined the spectra of a variety of species involving PAHs to understand the changes Peeters had found. "Because I can compute the spectra of PAHs much larger than anything that has been synthesized and also vary the placement of nitrogen within these large molecules, something impossible for the lab, we can now investigate a very large number of PAH varieties and sizes." Bauschlicher explained. "With this we have shown we can reproduce both the range in spectral shift Els measured and the relative intensities she found by incorporating N deep into the PAH skeleton" he explained further.
This discovery is profound at several levels. "First, this resolves part of a longstanding mystery about the distribution of nitrogen in space, second, PANHs have signatures in the optical and radio wavelengths that can account for unexplained astronomical phenomena and third, these compounds are of biogenic interest" summed Hudgins. "Most people will take notice of their possible role in the origin of life, the point in our history when Chemistry became biology, but there are other serious implications as well" he continued.
There are hundreds if not thousands of these species in space and it is beginning to look like these types of compounds are strikingly similar to many of those brought to Earth today by infalling meteorites and their smaller cousins, the interplanetary dust particles. Every year more than a hundred tons of extraterrestrial stuff falls on the Earth, and much of it is in the form of organic material. In the early life of our Solar System, before the debris from its formation was fully cleared away, these materials were deposited on the Earth in far greater quantities than we see today. Thus, much of the organic material found on the primordial Earth likely included a strong dose of interstellar PANHs.
Allamandola reiterated, "The spell is now breaking that interstellar Chemistry is only a Chemistry of relatively small and simple molecules. Twenty years ago the notion of abundant, gas phase, polycyclic aromatic hydrocarbons anywhere in interstellar space was considered impossible. Now we know better. PANHs/PAHs dwarf all other known Text Box: Figure 3. Infrared image of spiral Galaxy M-81 taken by the Spitzer Space Telescope. The red traces the emission from PANHs. interstellar molecules in size and, as a class, they are more abundant than all other known interstellar polyatomic molecules combined. We are only seeing the tip of the iceberg in terms of extraterrestrial molecular complexity. Spitzer has detected the PAH IR signature across the Universe, even back to only a few billion years after the Big Bang. When the universe is looked at through PAH filtered glasses (Figure 3) it is clear that PAHs are indeed everywhere and we live in a molecular Universe."
These results are published in the current, issue of the Astrophysical Journal. The authors and team members include Drs. Hudgins, Bauschlicher, Mattioda, Peeters, and Allamandola of NASA's Ames Research Center.
This research is supported by the Space Science Division at NASA Ames Research Center and the Offices of Exobiology, Long Term Space Astrophysics, and Astrobiology at NASA Headquarters, Washington, D.C.
The recent development of Exobiology and Astrobiology as interdisciplinary research fields has brought together Astronomers and chemists, enabling the type of interdisciplinary work described here by created funding opportunities in a way that wasn't possible ten years ago.
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