In this work, we report on the discovery of naturally occurring fullerenes (C60 to C400) in the Allende and Murchison meteorites and some sediment samples from the 65 million-year-old Cretaceous/Tertiary boundary layer (KTB). Unlike the other pure forms of carbon (diamond and graphite), fullerenes are extractable in an organic solvent (e.g., toluene or 1,2,4-trichlorobenzene). The recognition of this unique property led to the detection and isolation of the higher fullerenes in the Kratschmer/Huffmann arc evaporated graphite soot and in the carbon material in the meteorite and impact deposits. By further exploiting the unique ability of the fullerene cage structure to encapsulate and retain noble gases, we have determined that both the Allende and Murchison fullerenes and the KTB fullerenes contain trapped noble gases with ratios that can only be described as extraterrestrial in origin.
Since the synthesis, isolation, and characterization of fullerenes, there has been considerable interest in ascertaining whether or not this family of carbon molecules occurs naturally on the Earth and in the cosmos (e.g., circumstellar shells, interstellar medium, etc.). For example, it has been suggested that the fullerene molecule, C60, might be widely distributed in the universe, particularly in the outflows of carbon stars, due to its exceptional thermal stability and photochemical properties. Despite an intense effort carried out by the scientific community to verify this hypothesis, only a few natural occurrences of fullerenes have been reported. Terrestrial discoveries have been limited to the detection of mostly C60 and C70 whereas extraterrestrial evidence is based on trace findings of C60 and a single mass spectrum of a high mass carbon envelope from the Allende meteorite.
On the Earth, trace amounts of fullerenes have been reported in some unusual rocks such as shungite, a highly metamorphosed coal, and fulgurite, a glassy rock, that forms when lightning hits the ground. Fullerenes have also been detected in deposits associated with two separate events involving the impact of a large bolide (asteroid or comet) with the Earth. Fullerenes have been extracted from shock-produced breccias (Onaping Formation) associated with the 1.85 billion-year-old Sudbury Impact Crater and in clay sediments within the 65 million-year-old Cretaceous/Tertiary (KTB) boundary. Possible scenarios for the presence of fullerenes in the Sudbury and KTB impact deposits are as follows: (i) Fullerenes were synthesized within the impact plume from the carbon contained in the bolide or target rocks; and/or (ii) fullerenes were already present in the bolide and survived the impact event (e.g., Sudbury) based on their unique noble gas isotopic signature; or (iii) fullerenes in the KTB clays formed as a result of global wildfires triggered by the impact event.
The survival of fullerenes over geologic time (millions to billions of years) and in two very different geological environments raises many interesting questions about the importance of fullerenes on the early Earth and, by implication, on the surfaces of other planets. A potential problem for fullerenes produced in the KTB wildfires is that, although fullerenes have been detected in high-temperature benzene flames, fullerenes have not been detected in modern day wildfires. This raises the possibility that the fullerenes found in the KTB clays were either synthesized in the impact plume or that fullerenes were already present in the bolide and somehow survived the impact event. Do fullerenes form as result of an impact event or by the transformation of other carbon compounds in the gas phase? How can we determine the origin of fullerenes in natural samples? One possibility we have explored is to search for trapped noble gases inside the fullerenes.
