Recent measurements in solar system oxygen match prediction made in 2002 by chemist Robert ClaytonBy Steve Koppes
Robert Clayton has attended all 39 meetings of the annual Lunar and Planetary Science Conference in Houston.
“You can never beat the first one, when we first had the moon samples come back,” said Clayton, the Enrico Fermi Distinguished Service Professor Emeritus in Chemistry and the College.
But for Clayton, the last one was more memorable than most. At the meeting, the Lunar and Planetary Institute released a book of collected papers, Oxygen in the Solar System, dedicated in his honor. And Kevin McKeegan of the University of California, Los Angeles, reported to his colleagues the highest-priority measurements of NASA’s Genesis mission to study the sun.
McKeegan’s results were exactly as Clayton had predicted in an article published in the Feb. 21, 2002 issue of the journal Nature. The results also brought full-circle an idea that Nobel laureate Harold Urey, a former Chicago chemist, had unsuccessfully pursued in the 1930s.
The primary goal of Genesis, which collected protons and atoms from the sun and returned them to Earth in 2004, was to measure the oxygen isotopic composition of these samples. All oxygen atoms contain eight protons at their core, but the number of neutrons can vary depending on the isotope.
“McKeegan found that the sun has a 5 percent higher ratio of oxygen-16 to the other two oxygen isotopes than is found in the Earth, moon, Mars and presumably the whole inner solar system,” Clayton said. “People have been trying to understand the chemistry of planet formation and leaving out one very important process having to do with the chemistry of oxygen. It’s the main element that makes the planets, and this has been ignored until now.”
Meteorites with calcium aluminum inclusions, the first minerals to form out of the gas cloud that formed the sun, provided Clayton the basis for his prediction.
In 2002, several researchers released oxygen isotopic measurements of CAIs using the ion microprobe technique. The technique enabled scientists to examine smaller samples, but with less precision than the mass spectrometry that Clayton had used for a similar analysis 30 years ago.
The ion microprobe data exactly matched the mass spectrometry results, even though the samples were a million times smaller. The oxygen isotopic data plotted on coordinates that scientists would expect to result from the mixing of two components.
Plotting at one end of the chart were meteorites with CAIs, which are highly enriched in oxygen-16 as compared to the relatively rare oxygen-17 and oxygen-18. Plotting at the other end were samples from the Earth, the moon and Mars, which lacked the excess oxygen-16.
Scientists once thought the sun’s oxygen isotopic composition closely resembled the Earth’s. “It had been assumed, in part, because we all think that we’re the center of the universe, that that was the composition of the sun and everything else in the solar system,” Clayton said.
In 1973, Clayton, along with Lawrence Grossman, Professor in Geophysical Sciences and the College, and the late Toshiko Mayeda, Senior Research Associate in the Enrico Fermi Institute, had documented the seemingly odd isotopic composition of CAIs.
They also proposed that the oxygen-16 came from interstellar dust grains. Stars spewed these grains into interstellar space before the birth of the sun and became mixed into the dust cloud that collapsed to form the solar system. “They had such weird isotopic compositions that they had to have formed out of a different nuclear recipe from solar system matter,” Grossman said.
When Urey saw the paper, he sent a letter to Clayton relaying how pleased he was to see the new oxygen isotopic results. “He had hoped something like that would show up,” Clayton said.
In 1934, Urey received the Nobel Prize in chemistry for his discovery of deuterium, a heavy form of hydrogen. That same year, he also co-authored a paper in which he attempted to measure the oxygen isotopes of meteorites that he thought came from outside the solar system.
But the state of mass spectrometry in those days was too primitive to reveal anything interesting, and scientists later realized that meteorites come strictly from within Earth’s own solar system. “It was a wonderful idea, but it was way too soon,” said Clayton, who inherited Urey’s office and laboratory space. “That was the kind of thing that Urey was noted for.”
When scientists eventually did discover oxygen-containing interstellar grains in meteorites, their composition differed significantly from the CAIs. Then the Genesis results showed that the composition of the CAIs was nothing special after all. “They had the same composition as the sun. That’s where everything started,” Grossman said.
The Earth is what’s different. “And it’s not just the Earth,” Clayton said. “It’s everything in the inner solar system.”
Clayton based his 2002 prediction on a process that astronomers have documented in distant, young star systems. In this process, ultraviolet light of a specific frequency from the young, nearby sun is absorbed by the plentiful molecules containing oxygen-16 as it shines through a vast cloud of gas and dust. The rarer molecules containing oxygen-17 and 18, which react to different frequencies, survive in greater numbers.
Clayton applied the process to Earth’s own solar system for the first time. And now he has the gratifying Genesis mission data in hand. “We are now in a much better position to understand the formation of our planetary system, since we now know the composition of the starting materials,” he said.