Extraterrestrial gases, including helium, are trapped in "buckyball" molecules in a layer of sedimentary clay found in many places on Earth, according to a paper to be published March 28, 2000, in the Proceedings of the National Academy of Sciences.

The discovery provides a new tool for tracing asteroid and comet impacts in Earth's geological and biological records. A University of Hawaii geochemist and her colleagues, including a NASA scientist, found gases that did not originate on Earth inside buckyballs, or fullerene carbon molecules.

Window mechanism for the encapsulation of extraterrestrial noble gases (such as helium) atoms inside the fullerene cage, or buckminsterfullerene (buckyball). One view shows a broken bond, or open "window," with atom moving out through window. Photo: NASA/Ames.
 

The fullerene molecule is a hollow, cage-like structure typically made of 60 or more carbon atoms; it is also referred to as a "buckyball," in honor of Buckminster Fuller, designer of the geodesic dome that resembles the molecule.

"We discovered extraterrestrial noble gases trapped inside buckyballs in a one-inch thick sedimentary layer of clay that is exposed at several locations on Earth," said Ted Bunch, a scientist at NASA's Ames Research Center in California's Silicon Valley. "The buckyballs containing the gases arrived on Earth about 65 million years ago during an asteroid impact that scientists theorize ended the age of the dinosaurs. The clay layer that formed from fallout of the impact debris was globally distributed," Bunch explained.

Luann Becker, of the University of Hawaii, Honolulu, HI; Robert Poreda, of the University of Rochester, Rochester, NY; and Ted Bunch of NASA Ames, discovered the extraterrestrial gases in the fullerenes.

"Helium from different sources on Earth, like our atmosphere or the emissions from volcanoes, have a very different isotopic signature from the helium in a meteorite," Becker said. An isotopic signature is the ratio of the isotopes of an element; for example, terrestrial helium consists of a small amount of helium 3 (whose nucleus has two protons and one neutron), and mostly helium 4 that has 2 protons and 2 neutrons. Cosmic helium is mostly helium 3.

"The helium we found within the fullerene cages of Australia's Murchison meteorite, for example, is similar to the helium that existed when our Solar System first formed," Becker stated. That finding points to a cosmic source for the fullerenes, the researchers say. In contrast, molecules formed in the high pressure and temperature of an earthly impact or the heat of wildfires that followed would have encapsulated terrestrial helium, according to the researchers.

They say the finding also supports the theory that atmospheric gases and organic compounds arrived on the Earth's surface during asteroid and comet strikes early in the planet's history when impacts were very numerous. The discovery relates to previous work by Becker and Bunch, published in Nature in July 1999 that first identified naturally occurring fullerenes in a meteorite. The scientists found significant quantities of very large fullerene molecules, some containing as many as 400 carbon atoms, in samples from the 4.6-billion-year-old Allende meteorite that landed in Mexico three decades ago.

The subsequent work examined several Cretaceous/Tertiary boundary clay sediments distributed worldwide, including deposits in Denmark, New Zealand and North America. In each case, the researchers found fullerenes that encapsulated noble gases with unmistakable extraterrestrial and possibly extra-solar isotopic signatures.

The scientists examined the one-inch clay layer because it is a well-studied sediment that contains extraterrestrial iridium and highly shocked minerals resulting from an asteroid impact 65 million years ago. A highly shocked mineral is one that has experienced temperatures of more than 2,000 C and pressures of about 400,000 atmospheres from impact shock. The clay layer documents a period of abrupt change in biological evolution, including mass extinction of the dinosaurs, now generally attributed to the impact of a carbonaceous asteroid with the Earth.

Becker said she hopes to expand the research to examine other periods of mass extinction such as the even more devastating event that formed the 250-million-year-old Permian/Triassic layer of sediment. She added that she hopes to determine if impacts with Earth trigger global change, including whether fullerenes of extraterrestrial origin delivered gases and carbon necessary to establish life on Earth.

"We now have a powerful new tracer to look at sediment layers very carefully," Becker said. "It opens new possibilities in looking at the problem of how planetary atmospheres evolved and maybe even how life evolved on Earth and perhaps on other moons and planets." She said she also hopes to work with astronomers to study the formation of fullerenes. "We have yet to learn why these things are there and what they tell us about carbon in the universe. We need to figure out how to establish their existence and how to search for it."

Grants from the NASA Cosmochemistry and Exobiology programs supported the research.