Kevin McKeegan's announcement at the 2008 Lunar and Planetary Science Conference that the pattern of oxygen isotopes on the Sun differs greatly from that of Earth took many planetary scientists by surprise, but the findings may help them explain how Earth and the other planets grew out of the solar nebula, the giant cloud of gas and dust from which the solar system formed.
"I learned that experienced scientists were taking bets on the outcome of McKeegan's measurements," said cosmochemist Robert Clayton, "since many were reluctant to believe that the Earth and Sun could have different isotopic compositions." Clayton triggered the intense interest in oxygen isotopes among planetary scientists with his discovery in 1973 that tiny nuggets (called Calcium-Aluminum-rich Inclusions or CAIs) in the Allende meteorite contained a very different pattern of oxygen isotopes than had previously been seen. The pattern that McKeegan's team found in the solar wind is similar to what Clayton found in the CAIs.
McKeegan heads the UCLA team that is analyzing samples of the solar wind as part of the Genesis mission. Though he cautioned that the results are preliminary, he expressed confidence in the general finding.
"Those little tiny grains in meteorites," McKeegan recalled, "for 35 years we've called them anomalous because they're different than all the other samples of planetary materials that we have. Samples of the Earth, samples of the Moon, the great majority of stuff in meteorites, all are very different (from CAIs), and so the natural thinking was that these little refractory inclusions in meteorites were exceptional."
"The new Genesis results turn this around," observed Clayton, who predicted McKeegan's finding in 2002. Since the Sun holds more than 99 percent of the solar system's oxygen, Clayton pointed out, the Sun's isotopic ratio and that of the similar CAIs should be considered normal. "Earth and everything else in the inner solar system are anomalous and require some process to account for the isotopic difference," he said.
Scientists have also found significant differences between the oxygen isotopic ratios of Earth, Mars, and the asteroid Vesta (based on meteorites believed to have come from those latter locations), but these differences are much less pronounced than the difference between any of these planetary bodies and the ratios found in CAIs and the solar wind.
Explaining the process by which the pattern of oxygen isotopes changed from that of the Sun—and by implication the solar nebula—to that of Earth and the other terrestrial planets may be the first step toward explaining how the planets began to form. Scientists have developed various alternative models of how the isotopic balance could have shifted, but until now they haven't known the starting point for this shift—the isotopic ratios of the solar nebula. As Clayton put it, "How can you possibly understand how to make the objects in the solar system if you don't know what the starting materials are?"
Mark Thiemens, head of the Genesis team at the University of California, San Diego (UCSD) agrees. "If you know that original composition and you have the meteorites, Earth and Mars to compare it to," he said, "you have a crack at really solving what happened and when." The ultimate goal of the Genesis mission is to describe that original composition.
Importance of Oxygen
Collecting and analyzing oxygen in the solar wind tops the list of the Genesis mission's priorities, and with McKeegan's announcement, this objective is well on its way to being fulfilled.
Why do planetary scientists consider oxygen so important? "There are two reasons," McKeegan explained. "One is that in the meteorites, we see this large variation in oxygen isotopes, which persists on every scale that we've been able to measure, from the smallest grains in the meteorites up to things the size of planets." This contrasts with most other elements, which look the same everywhere scientists have analyzed.
"And then the second reason," McKeegan continued, "is that oxygen is the major element in the inner solar system, the major element that forms rocks. It's the most abundant element on Earth, for example. And so it's not something you can just ignore."
Befitting oxygen's importance to the mission, a special concentrator instrument on the Genesis spacecraft was devoted primarily to collecting it. And fortunately, the oxygen collection plates, known as "targets," didn't break during the hard landing in the Utah desert. Although dirt contaminated the surfaces of the targets during the crash, the solar wind atoms, having hit the targets at about a million mph when they were captured in space, were not jarred loose by crashing into the desert at a mere 200 mph. They remained safely embedded about 100 nanometers below the surface of the material in which they were collected.
But collecting atoms and analyzing them are two different things. As important and as difficult as it was to get the samples, an equally important and difficult part of the Genesis mission has been to design and build instruments that can count how many atoms the samples contain of each oxygen isotope. NASA has sponsored the development and construction of two different instruments for this purpose—one built by McKeegan's team at UCLA and the other by Thiemens' group at UCSD—and established a collaboration with a laboratory at Open University in England that is using yet another instrument. McKeegan's announcement reflected only his team's findings; the other groups have not yet analyzed their oxygen samples as of this writing.
The Genesis mission's principal investigator, Don Burnett, explained, "An important advantage of sample-return missions is that, for important measurements, we are able to have results replicated by different techniques." Thiemens elaborated, "In a measurement that's this hard, the more ways you can confirm the measurement isn't some goofy artifact of your machine, the more confidence you have (in the results). And given what it takes to collect the samples, you really want as many measurements as possible, especially on something like oxygen, where it's not only hard, it's important."
Beyond Needle in a Haystack
Measuring the amount of each oxygen isotope in the Genesis sample is far more challenging than finding a needle in a haystack. It's more like finding a particular needle in a giant stack of identical needles. Oxygen's ubiquitousness, one of the characteristics that make it so important to planetary scientists, also makes it extremely challenging to analyze.
"The difficulty of oxygen is that in the solar wind sample, it's essentially a trace amount, it's a few nanograms," said Peter Mao, who was a key member of the UCLA team. "But oxygen is a very abundant element (here on Earth). It's 20 percent of what we're breathing right now, it's two-thirds of the rocks that we stand on, it's a major component of water, it's everywhere." So the great challenge of analyzing the oxygen atoms from the solar wind is keeping oxygen atoms from the Earthly environment out of the instrument. "There's water and oxygen everywhere, and it gets into your measurements," Thiemens said. "It took us a year to solve that problem. It was basically a matter of engineering special vacuum systems and finding the sources of where oxygen can sneak in."
The instruments at the three institutions differ in a number of respects, but the basics of how they work are quite similar. In each case, they need to separate the solar-wind oxygen atoms from all the other atoms in the sample, and then separate and count the individual isotopes.
"Our measurement puts limits on what ideas can fly about how we go from gas and dust to planets," Mao said. "That's the big picture of what we do. But in order to build up that big picture, you have to get all your details right. Hopefully, sometime in the future, there'll be an ‘aha!' moment for someone. What matters to us is that we get a measurement that's good, that we get a measurement that stands the test of time."
McKeegan pointed out that there are several suspected sources of error for which his team will need to correct before their results can be deemed final. Two of the most challenging are the skewing believed to have been done by the concentrator instrument and by the solar wind itself. According to McKeegan, the concentrator bent the paths of the lighter isotopes more than that of the heavier isotopes, much like the mass spectrometers that are being used to analyze the samples, and therefore focused each of the three kinds of oxygen isotopes in a slightly different place on the target.
The nature of the solar wind complicates the problem considerably. "Things were coming in from all different angles, with different charges and different energies," McKeegan said, "so it's quite complex to model." Nevertheless, the group at Los Alamos National Laboratory that designed and built the concentrator is working on modeling this effect, and experiments are planned at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland to test the model.
There is also a big question about how the process by which the Sun forms the solar wind may affect the distribution of oxygen isotopes. "I hope in the not too distant future, we will start pushing people to come up with better theories," Burnett said. "We've been undergoing discussions with people who do the appropriate kind of solar physics for this."
Still, McKeegan said, "none of this should really affect the fact that the there's more 16O (the lightest and most abundant of the three oxygen isotopes) in the target than in terrestrial samples." So outstanding issues aside, this first look at the nursery that gave rise to our solar system is a landmark achievement.
"The most exciting part of the project," Mao said, "is to be able to make a measurement that people have been waiting so long for. The way Don Burnett put it when he sold me on coming over here was that the oxygen numbers will go in the textbooks. It's rare to have an opportunity to go for a textbook number."
"Personally," McKeegan said, "what I find amazing is that we have a piece of the Sun. A physical piece of the Sun is in our laboratory at UCLA. It's bizarre, right?"