News – In the mid-1980s, the discovery of complex carbon molecules driving through the interstellar medium attracted much attention, with perhaps the most famous examples being the Buckminster Fuller, or “buckyballs” – spheres consisting of 60 or 70 carbon atoms. But scientists have struggled to understand how these molecules can form in space.
In a paper accepted for publication in Journal of Physical Chemistry A, Researchers from the University of Arizona suggest a surprisingly simple explanation. After exposing silicon carbide – a common ingredient in dust grains in planetary nebulae – to conditions similar to those found around dying stars, the researchers observed spontaneous formation of carbon nanotubes, which are highly structured rod-like molecules composed of several layers of carbon layers. The results were presented June 16 at the 240th meeting of the American Astronomical Society in Pasadena, California.
Led by UArizona researchers Jacob Bernalthe work is based on research published 2019, when the group showed that they could create buckyballs with the same experimental design. The work suggests that buckyballs and carbon nanotubes can form when silicon carbide dust from dying stars is hit by high temperatures, shock waves and high-energy particles, which leaks silicon from the surface and leaves carbon behind.
The findings support the idea that dying stars can sow the interstellar medium with nanotubes and possibly other complex carbon molecules. The results have implications for astrobiology, as they provide a mechanism for concentrating carbon that can then be transported to planetary systems.
“We know from infrared observations that buckyballs populate the interstellar medium,” said Bernal, a postdoctoral research assistant at Arizona Lunar and Planetary Laboratory. “The big problem has been explaining how these massive, complex carbon molecules can possibly be formed in an environment saturated with hydrogen, which is what you usually have around a dying star.”
The formation of carbon-rich molecules, even smaller species that contain pure carbon, in the presence of hydrogen is practically impossible due to thermodynamic laws. The new study results offer an alternative scenario: Instead of assembling individual carbon atoms, buckyballs and nanotubes can be the result of simply reorganizing the structure of graphene – single-layered carbon disks known to form on the surface of heated silicon carbide grains.
This is exactly what Bernal and his co-authors observed when they heated commercially available silicon carbide samples to temperatures found in dying or dead stars and imaged them. When the temperature approached 1,050 degrees Celsius, small hemispherical structures with an approximate size of about 1 nanometer were observed at the grain surface. Within minutes of continued heating, the spherical buds began to grow into rod-like structures, containing several graphene layers with curvature and dimensions indicating a tubular shape. The resulting nanotubes ranged from about 3 to 4 nanometers in length and width, larger than buckyballs. The largest specimens depicted consisted of more than four layers of graphite carbon. During the heating experiment, the tubes were observed to wobble before budding off the surface and were sucked into the vacuum surrounding the sample.
“We were amazed that we could make these extraordinary structures,” Bernal said. “Chemically, our nanotubes are very simple, but they are extremely beautiful.”
Named after their resemblance to architectural works by Richard Buckminster Fuller, fullerenes are the largest molecules currently known to exist in interstellar space, which for decades were thought to lack molecules containing more than a few atoms, at most 10. It is now well established that the fullerenes C60 and C70, which contain 60 and 70 carbon atoms, respectively, are common ingredients in the interstellar medium.
One of the first of its kind in the world, the transmission electron microscope at the Kuiper Materials Imaging and Characterization Facility in UArizona is uniquely suited for simulating the environment of planetary nebulae. Its 200,000-volt electron beam can examine matter down to 78 picometers – the distance between two hydrogen atoms in a water molecule – making it possible to see individual atoms. The instrument operates in a vacuum that closely resembles the pressure – or lack thereof – that is believed to exist in circulatory environments.
While a spherical C60 molecule measures 0.7 nanometers in diameter, the nanotube structures formed in this experiment several times the size of C60, and easily exceeded 1,000 carbon atoms. The study authors are convinced that their experiments accurately replicated the temperature and density conditions that would be expected in a planetary nebula, said the co-author Lucy Ziurysa UArizona Regents Professor of Astronomy, Chemistry and Biochemistry.
“We know the raw material is there, and we know the conditions are very close to what you would see near a dying star’s shell,” she said. “There are shock waves passing through the envelope, so temperature and pressure conditions have been shown to exist in space. We also see buckyballs in these planetary nebulae – in other words, we see the beginning and end products you can expect in our experiments.”
These experimental simulations suggest that carbon nanotubes, along with the smaller fullerenes, are then injected into the interstellar medium. Carbon nanotubes are known to have high radiation stability, and fullerenes can survive for millions of years when sufficiently shielded from high-energy cosmic radiation. Carbon-rich meteorites, such as carbonaceous chondrites, may also contain these structures, the researchers suggest.
According to study co-authors Tom ZegaProfessor at the UArizona Lunar and Planetary Lab, the challenge is to find nanotubes in these meteorites, due to the very small grain sizes and because the meteorites are a complex mixture of organic and inorganic materials, some with sizes similar to nanotubes.
“Nevertheless, our experiments suggest that such materials could have formed in interstellar space,” Zega said. “If they survived the journey to our local part of the galaxy where our solar system was formed about 4.5 billion years ago, then they could be preserved inside the material that was left over.”
Zega said that an excellent example of such residual material is Bennu, a carbonaceous terrestrial asteroid from which NASA’s UArizona-led OSIRIS-REx mission took a sample in October 2020. Scientists are eagerly awaiting the arrival of that sample, scheduled for 2023.
“Asteroid Bennu could have preserved these materials, so it is possible that we can find nanotubes in them,” said Zega.
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