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Experimental Test of Self-Shielding in VUV Photodissociation of CO Print

 

One way to test models of the solar system’s formation is to compare the isotopic abundances of the elements found in its constituent bodies. A case in point is oxygen with three stable isotopes dominated by oxygen-16, with minute fractions of oxygen-17 and oxygen-18. Primitive objects whose formation predates the Earth’s, such as the calcium-aluminum-rich inclusions in the Allende meteorite, have relatively lower fractions of the two heavier isotopes than does the Earth’s crust. Among the numerous explanations that have been proposed is the notion that chemical processes within the early solar nebula gave rise to the oxygen ratios, a leading candidate being a process called isotope self-shielding. But researchers at the University of California, San Diego, and Berkeley Lab have now shown that photodissociation of carbon monoxide (CO) caused by vacuum-ultraviolet (VUV) light from the early sun could generate reservoirs of the heavier isotopes in the solar nebula without the help of self-shielding.

Building the Solar System

Before the solar system formed, scientists have suggested, the early Sun emitted intense ultraviolet light that dislodged oxygen atoms, a process called photodissociation, from gas molecules in the primordial solar nebula, one step in the long process by which the planets were built. Small flecks of embedded minerals thought to date from the beginning of the solar system found in chunks of a stony meteorite that exploded over Pueblito de Allende, Mexico, in 1969 have a pattern of oxygen isotopes (oxygen whose nuclei have different numbers of neutrons) that differs from those found in all known planetary rocks that formed much later. Why this is so has proved to be the largest puzzle in unraveling solar system evolution for nearly 40 years.

Now Chakraborty et al. have eliminated a major model proposed to explain the anomaly: the idea that light from the early Sun could have shifted the balance of oxygen isotopes in molecules that formed after it turned on by a phenomenon termed self-shielding. With ultraviolet photodissociation experiments, they found that photodissociation without self-shielding was enough to produce the isotope ratios found in samples from the early solar system. Analysis of samples of the Sun's outer atmosphere captured by the GENESIS spacecraft from the solar wind outside the moon’s orbit with the goal of determining the original composition of the solar nebula will have to be interpreted in light of these results.

The self-shielding observed experimentally in molecular clouds of dust and gas in outer space occurs when energetic VUV light from a nearby star breaks CO molecules into atoms of carbon and oxygen. Different isotopes absorb VUV at slightly different photon energies. Near the edge of the cloud, the CO with the abundant oxygen-16 isotope soaks up as many of the photons as it can, thus shielding the oxygen-16 deeper inside the cloud. However, oxygen-17 and oxygen-18 are not shielded, so that inside the cloud relatively more CO molecules with the heavier isotopes are dissociated. As CO was the most abundant oxygen-bearing molecule in the solar nebula before planetary formation, it is reasonable that a similar process may have been at work early in the development of the solar system, either in a hot region near the young sun or in colder regions farther away.

mineral sample

After breaking into thousands of fragments in the atmosphere, the Allende meteorite scattered widely across Chihuahua, Mexico. Pale specks on the surface of this meteorite are among the oldest minerals in the solar system. An odd mix of oxygen isotopes within these minerals has puzzled scientists for decades.

To test whether VUV self-shielding really works under these conditions and, if so, what effect it has on the resulting ratio of oxygen isotopes, the San Diego researchers turned to ALS Beamline 9.0.2, where they sent ultrahigh-purity carbon monoxide through a test chamber and exposed it to a beam of VUV photons, with runs at four different wavelengths that were important for the self-shielding hypothesis. As the CO dissociated, the oxygen released quickly recombined with undissociated CO to form carbon dioxide (CO2), which was collected and taken to UC San Diego, where O2 was chemically removed from the CO2, after which the isotope ratios were determined by mass spectrometry. While the researchers found differences in the way the CO responded to the different wavelengths used in the experiment, they were not those predicted by the self-shielding hypothesis. Not only were they due to electronic properties of the CO molecules that were unrelated to self-shielding, the ratios themselves were still a good match for those found in samples like the Allende meteorite.

Self-shielding has been observed in molecular clouds. Whether or not self-shielding was involved in creating the oxygen isotope ratios in the early solar nebula, the ratios are preserved as the oxygen dissociation products of CO that combine with hydrogen to form hydroxyl ions and then water, which later reacts with dust grains to form minerals.

The authors concluded that cold regions of the solar nebula were indeed a potential site for the generation of some of the oxygen reservoirs with relatively high amounts of the heavier oxygen isotopes, but not via self-shielding. But identifying a possible source with larger fractions of the heavier isotopes does not end the story. For example, CO photodissociation in the early solar system together with water make for some intricate chemistry that locks the heavier isotopes of oxygen into minerals, both those that make up the oldest meteorites and those that subsequently formed all the other bodies of the solar system. One of the steps in the chemistry of oxygen that the group wants to test next at the ALS is the reaction between oxygen, water, and silicates, which produced the solar system’s first rocks.

Oxygen isotope composition of product CO2 showing a wavelength-dependent fractionation pattern during CO photodissociation. The y-axis measures the variation of the oxygen-17:oxygen-16 ratio and the x-axis the variation of the oxygen-18:oxygen-16 ratio in parts per thousand (0/00) of the product CO2 relative to the starting CO in the experiment chamber. The data for experiments at different wavelengths from 94 to 107 nm at room temperature (RT) and –66 °C reveal that the pattern expected for self-shielding (lines with a slope of 1) is not consistent with experiment and is therefore ruled out.

 



Research conducted by S. Chakraborty, T.L. Jackson, and M.H. Thiemens (University of California, San Diego) and M. Ahmed (Berkeley Lab).

Research funding: NASA and the U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: S. Chakraborty, M. Ahmed, T.L. Jackson, and M.H. Thiemens, “Experimental test of self-shielding in vacuum ultraviolet photodissociation of CO,” Science 321, 1328 (2008).

 

 

ALSNews Vol. 296, March 25, 2009

 

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