London, March 14 : Scientists have made a measurement of oxygen composition at the birth of the solar system by capturing the stream of ionized particles known as the solar wind - which, because it emanates from the relatively unchanged outer layers of the Sun, is believed to carry primordial oxygen among its elements.
According to a report in Nature News, the particles were trapped from the solar wind by the NASA Genesis probe.
The finding that the Sun is relatively richer than Earth in oxygen-16, the most common oxygen isotope, contradicts the conventional wisdom that Earth has the same oxygen isotope composition as the Sun.
The discovery also gives researchers a reference point for the oxygen composition at the origin of the Solar System.
Oxygen-16, with eight protons and eight neutrons, comprises 99.8% of the oxygen on Earth. There are smaller amounts of oxygen 17 and oxygen 18, whose proportions vary throughout the Solar System.
Scientists have measured slightly different proportions on Earth, Mars, the Moon and in meteorites, as if each place has its own oxygen fingerprint.
"We had a map for oxygen isotopes," said Kevin McKeegan, a cosmochemist at the University of California, Los Angeles. "But we didn't know which way was up," he added.
For the analysis, McKeegan and his group used a mass spectrometer on a 3-millimetre-square section of a silicon wafer containing oxygen from the solar wind.
Using a beam of caesium ions, the researchers eroded the top 20 nanometres of the sample to remove any contamination by Earth-based oxygen. Then, in a vacuum, they measured the isotopic composition of the Sun's oxygen, using the ion beam to knock the atoms loose from the silicon trap, and found a greater proportion of oxygen-16 than on Earth.
According to Marc Chaussidon, one of the researchers, scientists now need to understand why Earth's oxygen composition is different from the Sun's, and what chemical processes caused the change.
Whatever the process, it would have sucked out oxygen-16 while the gas of the proto-Solar System condensed into solid grains that coalesced into the planets.
"It would also have been one of the very first things to happen in the 4.5685-billion-year-old Solar System," said Chaussidon. "The mystery process would have stripped away the oxygen-16 within the system's first few million years of existence," he added.