Geologists have found new evidence that certain organic compounds found in meteorites -- compounds ordinarily associated with living organisms -- are relics of a time before the solar system formed 4.5 billion years ago.
It has long been known that certain meteorites appear to contain amino acids, organic compounds ordinarily manufactured within living cells to be the building blocks of protein molecules.
Because meteorites originated in deep space far from any living organisms, researchers have been working to determine whether the amino acids are evidence of surprisingly complex but nonliving chemical processes in space or merely the result of contamination after the fall to Earth.
In recent years contamination has been fairly well ruled out through a variety of approaches (carefully extracting samples from deep within meteorites, for example), but positive evidence of a deep-space origin has largely eluded researchers until now.
The new evidence comes from the much-studied Murchison meteorite, which landed in Australia in 1969 and was found to be rich in organic compounds.
A team from the California Institute of Technology and Arizona State University has discovered that many of the hydrogen atoms in the meteorite's amino acids are of a type, called deuterium, that is rare on Earth but relatively abundant in the gas and dust of interstellar space. It was this material whose accretion into a spinning "solar nebula" gave rise to the solar system. Deuterium is a hydrogen isotope whose nucleus has one neutron in addition to the one proton that is in the normal hydrogen atom.
Samuel Epstein, the Caltech geologist who led the research with J.R. Cronin of Arizona State, said the high proportion of deuterium suggests that the Murchison meteorite was formed early in the history of the solar system, capturing a deuterium content typical of interstellar space and preserving it unchanged through the accretion of gas and dust that eventually gave rise to the sun and planets.
Like most meteorites it then whirled about the sun for billions of years, perhaps as part of the asteroid belt or as a comet, until it chanced to encounter enough of Earth's gravitational strength to pull it down.
"It's kind of exciting," Epstein said. "Practically everything that was taken up in that original solar nebula got heated and subjected to pressure and changed around in all sorts of ways. But if we're right, this meteorite is a piece of the original stuff that came through unchanged."