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Buckyballs found in Almahata Sitta meteorites

Buckyballs found in Almahata Sitta meteorites

Small fragments of the Almahata Sitta meteorite
Figure 1: Small fragments of the Almahata Sitta meteorite observed with a digital microscope (credit: H. Sabbah/Univ. Toulouse 3, P. Jenniskens/SETI Institute, and M. H. Shaddad/Univ. of Khartoum).

Buckyballs, or Buckminsterfullerenes (C60), are cage-like molecules made of hexagonal and pentagonal rings of carbon atoms. They are found in astrophysical environments and should have been part of the stuff our solar system was born from, but so far this molecule has defied unambiguous detection in primitive meteorites such as Murchison and Allende. Now, an international group of researchers report in the June 1 issue of the Astrophysical Journal that Buckminsterfullerene and other fullerenes are present in an unusual meteorite called "Alamahata Sitta."

"Almahata Sitta is the name used for the meteorites recovered from the impact of small asteroid 2008 TC3, which was tracked in space before it impacted in the Nubian Desert of Sudan in 2008," says co-author Peter Jenniskens of the SETI Institute and NASA Ames Research Center. Jenniskens collaborated with astronomer Muawia H. Shaddad and his University of Khartoum students in the meteorite recovery.   

Most of the recovered meteorites proved to be ureilites but also included an array of other meteorite types. Ureilites are stony meteorites that represent the mantle of a partially melted, carbon-rich asteroid. Ureilites originated in an early solar system asteroid heated by radioactive decay that experienced a catastrophic impact before it completely melted. The fragments created by this giant impact reassembled into smaller bodies, some of which survive in the asteroid belt where 2008 TC3 originated.

"We are studying these meteorites to better understand the composition and variety of materials found in asteroid 2008 TC3", says Shaddad. "Finding buckyballs in Almahata Sitta is a surprising discovery."

Jenniskens brought small fragments of the Almahata Sitta ureilites from the University of Khartoum collection to laboratory astrophysicists Hassan Sabbah and Christine Joblin in France. The team investigated the pieces with a sophisticated mass spectrometer called Astrochemistry Research of Organics with Molecular Analyzer (AROMA) at the laboratory astrophysics facility at IRAP of the University Toulouse 3. The institute is also part of CNRS and CNES.

The team made the first unambiguous detection in a meteorite of buckminsterfullerene and other fullerenes up to a typical size of about 100 carbon atoms. All samples studied also showed a distribution of polycyclic aromatic hydrocarbons (PAHs) that indicated that PAHs and fullerenes had come from different sources.

"After finding a surprisingly strong signal of these fullerenes in our data, we re-visited some primitive meteorites looked at in the past," says Hassan Sabbah,  the paper’s lead author. "We confirmed that fullerenes could not be detected in the Murchison and Allende meteorites with our instrument, demonstrating that these species are absent or have a lower concentration in these carbonaceous chondrite meteorites."

Mass spectrum of a ureilite fragment of the Almahata Sitta meteorite, AhS #04
Figure 2: Mass spectrum of a ureilite fragment of the Almahata Sitta meteorite, AhS #04, recorded with the AROMA setup. The observed peaks are attributed to fullerenes. Weaker lines near each peak are due to the same compounds containing one or two atoms of the 13C carbon isotope (credit: H. Sabbah/Univ. Toulouse 3). Click to view larger image.

The researchers were left with the question of where the fullerenes in Almahata Sitta originated. One possibility is that the fullerenes formed from the graphite and nano diamonds common in ureilites. However, this process would require very high temperatures and pressures.

"The most catastrophic event experienced by the Almahata Sitta ureilites was the breakup of the ureilite parent asteroid by a giant impact," says co-author and ureilite expert Cyrena Goodrich of the Lunar and Planetary Institute (USRA). "Although this was a high-temperature event, it was still not hot enough to have created fullerenes from pre-existing carbon phases.”

“Another possibility is that of an interstellar heritage,” says Christine Joblin, head of the laboratory astrophysics facility and one of the PIs of the Nanocosmos synergy project founded by the European Research Council that aims to provide a new vision on the origin and evolution of cosmic dust. "The production of fullerenes could be linked to the presence of a massive star close to the molecular cloud at the time when the ureilite parent asteroid formed, early in the history of the solar system."

C60 is the largest molecule identified to date in interstellar and circumstellar environments. It is particularly resilient against processing by ultraviolet photons from stars, thus surviving as a tracer of its molecular origins, but how such primitive molecules could have survived subsequent events on this asteroid remains to be understood.

"Its discovery in Almahata Sitta opens perspectives not only for the formation of the solar system but also for the search for fullerenes in astrophysical environments," says Joblin, "a topic that should progress rapidly with the upcoming observations from the James Webb Space Telescope."

More information
Hassan Sabbah, Mickaël Carlos, Peter Jenniskens, Muawia H. Shaddad, Jean Duprat, Cyrena A. Goodrich , and Christine Joblin (2022) Detection of Cosmic Fullerenes in the Almahata Sitta Meteorite: Are They an Interstellar Heritage? Astrophysical Journal:



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