Carbon nanotubes are tiny tubes made of pure carbon. There is evidence that they could be produced in the dust and gas that surround dying stars. The findings suggest a simple but elegant process for the synthesis and survival of complex carbon molecules in space.

In the mid-1980s, the finding of complex carbon molecules drifting through the interstellar medium attracted major interest, with Buckminsterfullerene, or “buckyballs” — spheres composed of 60 or 70 carbon atoms, likely being the most well-known example. Scientists, on the other hand, have struggled to comprehend how these molecules may originate in space.

Researchers from the University of Arizona propose a surprisingly simple answer in a work accepted for publication in the Journal of Physical Chemistry A. The researchers observed the spontaneous formation of carbon nanotubes after exposing silicon carbide – a common component of dust grains in planetary nebulae – to conditions similar to those found around dying stars. Carbon nanotubes are highly structured rod-like molecules made up of multiple layers of carbon sheets. 

The discovery, led by UArizona researcher Jacob Bernal, builds on a study released in 2019 that revealed the group could make buckyballs using the same experimental setup. The findings suggest that buckyballs and carbon nanotubes could evolve when silicon carbide dust from dying stars is subjected to high temperatures, shock waves, and high-energy particles, removing silicon from the surface and leaving carbon.

The findings back with the theory that dying stars could leave nanotubes and other complex carbon compounds in the interstellar medium. The findings have astrobiological implications since they reveal a method for concentrating carbon that might then be delivered to planetary systems.

“We know from infrared observations that buckyballs populate the interstellar medium,” explained Bernal, a postdoctoral research associate at the University of Arizona Lunar and Planetary Laboratory. “The big problem has been explaining how these massive, complex carbon molecules could possibly form in an environment saturated with hydrogen, which is what you typically have around a dying star.”

The laws of thermodynamics make it almost impossible for carbon-rich molecules or species made of only carbon to form when hydrogen is present. Instead of assembling individual carbon atoms, the new research suggests that buckyballs and nanotubes could be made by altering the structure of graphene — single-layered carbon sheets that are known to develop on the surface of heated silicon carbide grains.

This is precisely what Bernal and his co-authors discovered when they examined commercially available silicon carbide samples burned to temperatures similar to those found in dying or dead stars. As the temperature got close to 1050 degrees Celsius, small hemispherical structures about 1 nanometer in size were seen on the surface of the grains. The spherical buds began to expand into rod-like structures after minutes of sustained heating, containing numerous graphene layers with curvature and dimensions indicating a tubular form. The resulting nanotubules were larger than buckyballs, measuring 3 to 4 nanometers in length and width. The largest graphitic carbon specimens that were scanned contained more than four distinct layers of the material. The tubes were seen to jiggle and bud off the surface throughout the heating experiment before being sucked into the vacuum surrounding the sample.

Bernal remarked, “We were surprised we could make these extraordinary structures. Chemically, our nanotubes are very simple, but they are extremely beautiful. “

Fullerenes are the largest molecules currently known to exist in interstellar space, which was thought to be barren of any molecules having more than a few atoms, 10 at most, for decades. They were named by their resemblance to architectural works by Richard Buckminster Fuller. The fullerenes C60 and C70, which contain 60 and 70 carbon atoms, respectively, are now well recognized as common components of the interstellar medium.

The transmission electron microscope located at the Kuiper Materials Imaging and Characterization Facility at UArizona is one of the first of its kind in the world and is ideally suited to recreate the planetary nebula environment. Its 200,000-volt electron beam can probe matter down to 78 picometers, which is the gap between two hydrogen atoms in a water molecule, allowing individual atoms to be seen. The instrument works in a vacuum, which is supposed to be similar to the pressure – or lack thereof – seen in circumstellar regions.

While a spherical C60 molecule has a diameter of 0.7 nanometers, the nanotube structures created in this experiment were several times larger, easily topping 1,000 carbon atoms. According to co-author Lucy Ziurys, a UArizona Regents Professor of Astronomy, Chemistry, and Biochemistry, the study authors are convinced that their experiments accurately matched the temperature and density conditions that would be expected in a planetary nebula.

“We know the raw material is there, and we know the conditions are very close to what you’d see near the envelope of a dying star,” she added. “There are shock waves that pass through the envelope, so the 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 the end products you would expect in our experiments.”

Carbon nanotubes, together with the smaller fullerenes, may be subsequently pumped into the interstellar medium, as suggested by these experimental models. When properly insulated from high-energy cosmic radiation, carbon nanotubes are known to have high radiation stability, and fullerenes can persist for millions of years. These structures could also be found in carbon-rich meteorites like carbonaceous chondrites, according to the researchers.

The difficulty in finding nanotubes in these meteorites, according to study co-author Tom Zega, a professor at the University of Arizona Lunar and Planetary Lab, is due to the very small grain sizes and the fact that the meteorites are a complex mix of organic and inorganic materials, some with sizes similar to nanotubes.

“Nonetheless, our experiments suggest that such materials could have formed in interstellar space,” said Zega. “If they survived the journey to our local part of the galaxy where our solar system formed some 4.5 billion years ago, then they could be preserved inside of the material that was left over.”

Zega said that Bennu, a carbon-rich near-Earth asteroid from which NASA’s UArizona-led OSIRIS-REx mission took a sample in October 2020, is a good example of this kind of leftover material. Scientists are looking forward to the delivery of the sample in 2023.

“Asteroid Bennu could have preserved these materials, so it is possible we may find nanotubes in them,” Zega added.

Image Credit: Jacob Bernal/University of Arizona

You were reading: New Experiment Shows Dying Stars Seeding Interstellar Medium