February 21, 2013
Researchers Searching For Solar System’s Chemical Recipe
April Flowers for redOrbit.com - Your Universe Online
A research team from the University of California, San Diego is hoping to learn how our solar system evolved by studying the origins of different isotope ratios among the elements that make up today´s smorgasbord of planets, moons, comets, asteroids, and interplanetary ice and dust. The scientists are led by Mark Thiemens, Dean of the Division of Physical Sciences, who has worked on this problem for over three decades.
“Mark and his colleagues, Subrata Chakraborty and Teresa Jackson, wanted to know if photochemistry could explain some of the differences in isotope ratios between Earth and what´s found in meteorites and interplanetary dust particles,” says Musahid (Musa) Ahmed of Berkeley Lab´s Chemical Sciences Division, a scientist at the Chemical Dynamics Beamline. “They needed a source of ultraviolet light powerful enough to dissociate gas molecules like carbon monoxide, hydrogen sulfide, and nitrogen. That´s us: our beamline basically provides information about gas-phase photodynamics.”
The Chemical Dynamics Beamline — Beamline 9.0.2 - generates intense beams of VUV — vacuum ultraviolet light in the 40 to 165-nanometer wavelength range, which can be precisely tuned to mimic the radiation from the protosun when the solar system was forming.
Two of the most important elements for life are oxygen and sulfur, third and tenth most abundant in the solar system respectively. The isotopic differences of oxygen and sulfur from Earth's can be clearly observed in many different types of meteorites. The research team used the beamline 9.0.2 in 2008 to test a theory called "self-shielding." They were looking for a reason why oxygen-16 is less prevalent in the meteorites — relics of the primitive solar system — than it is in the sun, which contains 99.8 percent of the mass in the solar system. The results of this experiment showed that that self-shielding could not resolve the oxygen—isotope puzzle.
The self-shielding theory hypothesizes that the outer layers of clouds of carbon monoxide (CO) — abundant when the solar system was formed — would have preferentially absorbed certain wavelengths of the protosun's ultraviolet radiation. This would have broken the CO into carbon and oxygen atoms, with a disproportionate amount of oxygen-16. An excess of oxygen-17 and oxygen-18 would be found deeper in the cloud, both equally excessive compared to the ratios among the three isotopes found on Earth today.
In more recent tests, the group used beamline 9.0.2 to perform the first VUV experiment on sulfur. They used the results of this analysis to build a model of chemical evolution in the primitive solar nebula that could yield the isotopic ratios of sulfur seen in meteorites.
The findings of this study were recently published in Proceedings of the National Academy of Sciences.
Present in air, water and rocks, oxygen is the most abundant element on Earth. 99.762 percent of oxygen is the isotope oxygen-16, which has eight protons and eight neutrons. Another two-tenths of a percent is accounted for by oxygen-18, with ten neutrons; and oxygen-17, with nine neutrons, provides the last four-hundredths of a percent.
Sulfur is less abundant, but just as essential to life. Sulfur-32 makes up 95.02 percent, sulfur 34 4.21 percent, sulfur-33 0.75 percent, and sulfur-36´s mere 0.02 percent brings up the rear.
“One depends on the mass of the isotopes themselves,” Ahmed says, explaining the two basic processes that account for these ratios. “Oxygen-18 is two neutrons heavier than oxygen-16. One effect of this, although not the only one, is that when the temperature rises, oxygen-16 evaporates faster. And when the temperature falls, oxygen-18 condenses faster.”
Different isotope ratios are produced by changes in temperature and other factors. This accounts for the higher proportion of oxygen-18 in raindrops than in the clouds they fall from, for example.
Researchers commonly graph these processes by plotting samples with increasing proportions of oxygen-17 to oxygen-16 along the X axis, while the Y axis shows increasing proportions of oxygen-18 relative to oxygen-16. In almost any sample from Earth to an arbitrary standard called SMOW (standard mean ocean water), the three isotope's proportions diverge at a rate that can be plotted along a line with a distinctive slope; approximately one-half.
Samples with isotope ratios not on the slope-one-half line are not the result of mass-dependent processes. A study in 1973 found the ratios of oxygen isotopes in carbonaceous meteorites, the oldest objects in the solar system, to vary significantly from those on Earth. The graph of the ratios had a slope close to one. In the 1980s, Thiemens and John Heidenreich found that ozone, the three-atom molecule of oxygen, showed a similar isotope trend, with a similar slope of one. This relationship was partly due to the molecule's chemical formation.
The plot for sulfur isotope ratios is similar. The standard is not native to Earth, however. Diablo Canyon Troilite - an iron sulfide mineral — was found in a fragment of the meteorite that created Arizona´s Meteor Crater.
“Mass-independent processes suggest chemical reactions, whether in the lab, the stratosphere, or the early solar system,” says Ahmed. “In the proto-solar system, bathed in intense ultraviolet light, these might have occurred on a grain of rock or ice or dust, or in just plain gas. The goal is to identify distinctive isotopic fractionations and examine the chemical pathways that could have produced them.”
Theimen's laboratory has worked on perfecting methods of recovering primordial samples from dust, meteorites, and the solar wind since his early work with ozone 30 years ago. Chakraborty, who along with Thiemens was a part of the science team for NASA´s Genesis mission, was able to extract mere billionths of a gram of oxygen from particles of the solar wind even after the spacecraft´s collectors were badly damaged when they crashed upon return to Earth.
Sulfur isotopes show up in different fractions in different solar system sources, just like oxygen. Tracing the possible origins of sulfur isotopes, the recent study at beamline 9.0.2 began by flowing hydrogen sulfide gas — the most abundant sulfur-bearing gas in the early solar system — into a pressurized reaction chamber. In this chamber, the synchrotron beam decomposed the gas and deposited elemental sulfur on “jackets” made of ultraclean aluminum foil.
Repeated at four different wavelengths, the experiment resulted in carefully stored aluminum jackets that were taken to the Thiemens lab at UC San Diego. Chakraborty and Jackson chemically extracted the sulfur and then measured its isotopes using Isotope Ratio Mass Spectrometry. The isotope compositions were found to be mass independent in all samples.
One natural source of fractionation is photodissociation of hydrogen sulfide as the gas condensed to iron sulfide in the inner solar system. This process was driven by intense 121.6-nanometer-wavelength ultraviolet light as the young star repeatedly shook with violent flares and upheavals.
Different isotope ratios evolved in different classes of meteorites, as well as different parts of the same meteorite, such as their crust or various inclusions. These ratios depended on where and when in the solar system they formed. Sulfur and oxygen compositions evolved independently from one another.
The Thiemens group is continuing their research using beamline 9.0.2, studying nitrogen. Nitrogen is the seventh most abundant element in the solar system, with 99.63 of nitrogen on Earth expressed as nitrogen-14, and nitrogen-15 being the remaining 0.37 percent. The scientists are using measurements of the solar wind, carbonaceous meteorites, and other sources, which show wide swings in their proportions.
Says Musa Ahmed, “Tracking down how isotopic ratios may have evolved, we basically send these elements back in time. The more we learn about the fundamental elements of the solar system at the Chemical Dynamics Beamline, the more it´s like really being out there when the solar system began.”