Isotopic analysis: Solar system revelations

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  • Published: Mar 1, 2013
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Isotopic analysis: Solar system revelations

Cosmic beamline studies

Tracing the possible origins of sulfur isotopes involved exposing a stream of hydrogen sulfide gas to the intense ultraviolet beamline and capturing the products in

Researchers have used UV beamline studies and other techniques to analyse the origins of different isotope ratios among the chemical elements that make up the planets, moons, comets and asteroids, as well as the interplanetary ice and dust in our solar system. Their studies could provide new insights as to how the solar system evolved and what might be its ultimate fate.

Mark Thiemens of the University of California, San Diego, has worked on this problem for more than thirty years. Now, with colleagues Subrata Chakraborty and Teresa Jackson, and Musahid Ahmed of Berkeley Lab, he has focused attention on the ALS's Chemical Dynamics Beamline to try and explain the differences in isotopic ratios between terrestrial and extraterrestrial rocks. The ultraviolet light from the synchrotron beamline is powerful enough to dissociate gas molecules such as carbon monoxide, hydrogen sulfide, and nitrogen allowing the team to carry out gas-phase photodynamics studies and can be tuned by wavelength (in this case 40-165 nanometres) to simulate radiation from the proto-sun when the solar system was forming.

Sulfur sussed

Oxygen and sulfur are the third and tenth most abundant elements in the solar system but their isotopic differences are clearly seen in meteorites. In their earlier studies they were unable to explain why oxygen-16 is less prevalent in meteorites than it is in the sun, which contains 99.8%, of the solar system's total mass. Now, they have carried out experiments on sulfur, using the results to help them build a model of chemical evolution in the primitive solar nebula that might lead to new clues to explain the oxygen-16 deficit and enlighten them as to the evolution of the solar system. Sulfur has four stable isotopes: sulfur-32 accounts for 95.02%, sulfur 34 4.21%, sulfur-33 0.75%, and sulfur-36 just 0.02%.

These proportions could be accounted for by two processes - different rates of condensation and evaporation with falling and rising temperatures.

Original isotopes

Tracing the possible origins of sulfur isotopes involved exposing a stream of hydrogen sulfide gas to the intense ultraviolet beamline and capturing the products in "jackets" of ultraclean aluminium foil. The contents were analysed at the San Diego laboratory using isotope ratio mass spectrometry. For all samples, the isotope compositions were mass independent.

"Mass-independent processes suggest chemical reactions, whether in the lab, the stratosphere, or the early solar system," explains 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."

One source of such fractionation might be photodissociation of hydrogen sulfide as intense ultraviolet shockwaves from the young Sun repeatedly ripped through the debris in its orbit. Different classes of meteorites, and even different parts of the same meteorite, might thus have different isotope ratios depending on where and when they formed in the solar system.

"Photochemistry is very important in the early evolutionary stage of our solar system," Chakraborty told SpectroscopyNOW. "We have gas and lots of lots of light from our own sun and other stellar sources...photochemistry ought to happen." He adds that the question to ask then becomes: how are these photochemical processes changing the proportions of different isotopes of an element, in this particular case sulfur. The composition of meteorites varies widely and the organic components show a different isotopic blend than the inorganic parts. "I think it is important to understand that as they may offer some hint about the processing of organic material of our solar system...which finally gave rise to life on earth."

Related Links

Proc Natl Acad Sci 2013, online: "Sulfur isotopic fractionation in vacuum ultraviolet photodissociation of hydrogen sulfide: potential relevance to meteorite analysis"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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