Mar 3, 2008
Meteorites contain graphite "whiskers"
Wisps of graphite, hundreds of nanometres long, have been found in several meteorites – a discovery that might have implications for stars called supernovae and "dark energy". Marc Fries and Andrew Steele of the Carnegie Institute of Washington in the US say that the whiskers were created when our solar system was formed some 4.5 billion years ago and the nanomaterials were then blown out into interstellar space. The researchers obtained their results by examining meteorite residues using Raman imaging and electron microscopy.
Several allotropes of carbon have already been found in carbon-containing meteorites. These include macromolecular carbon and diamond. Now, Fries and Steele have added graphite whiskers to the list.
The whiskers are needle-like nanomaterials made from rolled-up graphite spindles. They can be made by condensing high-temperature plasmas in the laboratory at temperatures of over 1200 K. Fries and Steele detected the whiskers in several meteorites – dating from the formation of the solar system – using confocal Raman imaging spectroscopy and scanning electron microscopy. The Raman spectrum allowed individual whiskers to be detected, even when other carbon phases were present.
According to the team, the whiskers were probably created from carbon-rich gas in the high temperatures of the early solar system. The nanomaterials were found in calcium-aluminium inclusions in the meteorites.
Whiskers in interstellar space
"During this time, when the Sun was young, the solar wind was very strong," explained Fries. "So graphite whiskers formed near the Sun could have blown into interstellar space. The same thing might have happened around other young stars as well." Supernovae explosions might also produce and disperse graphite whiskers out into space, he added.
Such a thin interstellar "haze" of graphite whiskers would affect how different wavelengths of light, especially those in the near infrared, pass through space. For example, near infrared light from "Type 1a" supernovae would be dimmed.
Supernovae and dark energy
Type 1 supernovae are some of the brightest objects in the universe and astronomers use them as "standard candles" to estimate cosmological distances – brighter supernovae are closer and dimmer ones further away. At the end of the 1990s, researchers observed that some supernovae seemed too dim (and thus too far away) to be explained by conventional cosmology theories. This led them to conclude that the universe's expansion was accelerating and that a force called dark energy must be responsible for this acceleration.
However, researchers had previously suggested that graphite or other whisker-like materials might also explain the observations, but such whiskers had never been seen in space until now.
"If graphite whiskers in space are absorbing supernovae's light, then this could affect measurements of the rate of the universe's expansion," said Steele. "While we cannot comment further on the effects of whiskers on the dark energy hypothesis it is important to study the characteristics of this form of carbon carefully so that we can understand its impact on dark energy models."
The results were published in Science.