Researchers discovered a mechanism for forming complex carbon molecules in a simulated planetary nebula environment.
Scientists have long been confused about the existence of so-called "buckyballs" – complex carbon molecules with soccer-like structure – throughout the interstellar space. Now, a research team from the University of Arizona has proposed a mechanism for their formation in a study published in the Astrophysical Journal Letters.
Carbon 60, short C 60 whose official name is Buckminsterfullerene occurs in spherical molecules consisting of 60 carbon atoms arranged in five- and six-membered rings. The name "Buckyball" is derived from the similarity with the architecture of Richard Buckminster Fuller, who designed many dome structures that resemble C 60 . Their creation should only be possible in the laboratory until their discovery in space challenged that assumption.
For decades, humans thought that interstellar space was only sparse with light molecules: mostly single atoms, two- atoms, and occasionally 9- or 10-atom molecules. This was until several years ago massive C 60 and C 70 molecules were detected.
The researchers were also surprised that they were made of pure carbon. The laboratory produces C 60 by blasting pure carbon sources such as graphite. In space, C 60 was detected in planetary nebulae, which are the wreckage of dying stars. In this environment are about 10,000 hydrogen molecules for each carbon molecule.
"Every hydrogen should destroy fullerene synthesis," said senior author of the work, doctoral student in astrobiology and chemistry, Jacob Bernal. "If you have a box of bullets and have a carbon bullet per 10,000 hydrogen balls and shake them constantly, how likely is it that 60 carbons stick together? It is very unlikely. "
Bernal and his co-authors began investigating the C 60 mechanism after they realized that the transmission electron microscope (TEM) located at the Kuiper Materials Imaging and Characterization Facility in UArizona can handle the planetary nebulous environment quite well simulate.
The TEM, funded by the National Science Foundation and NASA, has the serial number "1" because it is the world's first of its kind with its exact configuration. Its 200,000 volt electron beam can study matter up to 78 picometers – a scale that the human brain can not capture – to detect individual atoms. It works under a vacuum with extremely low pressures. This or a lack of pressure in the TEM comes very close to printing in circumstellar environments.
"It's not as though we've necessarily tailored the instrument to this kind of pressure," said Tom Zega, Associate Professor, Arizona, Lunar and Planetary Lab and co-author of the study. "These instruments work at very low pressures, not because they should be like stars, but because molecules in the atmosphere interfere with trying to create high-resolution images with electron microscopes."
The team worked with the US Department of Energy's Argonne National Lab near Chicago, whose TEM is able to study radiation responses from materials. They placed silicon carbide, a dust-like star in the low-pressure environment of TEM, exposing it to temperatures of up to 1,830 degrees Fahrenheit and irradiating it with high-energy xenon ions.
Then it was brought back to Tucson for researchers to take advantage of the higher resolution and better analysis capabilities of the UArizona TEM. They knew that their hypothesis would be confirmed if they observed the deposition of silicon and the exposure of pure carbon.
"Certainly the silicon has dissolved, and you had carbon layers in six-membered ring sets called graphite," said co-author Lucy Ziurys, a professor of astronomy, chemistry and biochemistry. "And when the grains had an uneven surface, five- and six-membered rings formed spherical structures that corresponded to the diameter of C 60 . So we think we see C 60 .
This work suggests that C 60 is derived from dying stars' silicon carbide dust, which is then hit by high temperatures, shock waves, and high-energy particles that drain silicon from the surface and leave carbon behind , These large molecules are scattered as dying stars eject their material into the interstellar medium – the interstices between the stars – thus explaining their presence outside the planetary nebulae. Buckyballs are highly radiation stable and can survive billions of years when shielded from the harsh environment of space.
"The conditions in the universe under which we would expect complex things to be destroyed are in fact the conditions under which they arise. "Bernal added that the implications of the results are endless."
"If this mechanism forms C60, it probably forms all sorts of carbon nanostructures," Ziurys said, "and if one reads the chemical literature, it is believed that these are all synthetic materials made only in the laboratory, and yet they seem to be naturally produced by interstellar space. "
Rather, the universe must tell us how chemistry really works.
Reference: "Formation of Interstellar C60 from Circumstellar Silicon Carbide Grains" by JJ Bernal, P. Haenecour, J. Howe, TJ Zega, S. Amari and LM Ziurys, October 1, 2019, Astrophysical Journal Letters
DOI: 10.3847 / 2041-8213 / ab4206
This work was supported by the National Science Foundation (AST-1515568), 1531243 and AST-1907910), NASA (NNX15AD94G, NNX15AJ22G, NNX16A31G, NNX12AL47G and 80NSSC19K0509), the National Institutes of Health (R25GM062584), the US Energy Foundation (DE-AC07-051D14517).