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Studying the Solar System's Chemical Recipe Print

To study 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, a team of scientists from the University of California, San Diego is using ALS Chemical Dynamics Beamline 9.0.2 to mimic radiation from the protosun when the solar system was forming. For more than three decades, Mark Thiemens, Dean of the Division of Physical Sciences at UCSD, has worked to learn how our solar system evolved. Now he and his colleagues Teresa Jackson and Subrata Chakraborty (who won the David A. Shirley Award for Outstanding Scientific Achievement at the ALS in 2011) are using ALS Beamline 9.0.2 to see if photochemistry can explain the differences in isotope ratios between elements on Earth and what’s found in meteorites and interplanetary dust particles. The Chemical Dynamics Beamline generates intense beams of vacuum ultraviolet light (VUV) that can be precisely tuned to mimic the radiation from the protosun. It is powerful enough to dissociate gas molecules like carbon monoxide, hydrogen sulfide, and nitrogen, providing information about gas-phase photodynamics.

Ratios Reveal
Elements' Origins

Understanding oxygen isotope ratios is keys to deciphering past climates. Inside the nucleus of every atom of oxygen lies 8 protons and either 8, 9, or 10 neutrons. “Light” oxygen, which has 8 neutrons and is known as oxygen-16, is the most common isotope in nature. Oxygen-18, with 10 neutrons, is less common, and oxygen-17 the least. The relative amount of these varieties, or isotopes, of oxygen diverges at a rate with a distinctive slope of one-half—unless the sample is from outer space.

In 1973 the ratios of oxygen isotopes in carbonaceous meteorites, the oldest objects in the solar system, were found to vary significantly from those on Earth. Their graph line had a slope close to one. Ozone (a three-atom molecule of oxygen) showed a similar isotope trend with a slope of one when studied a decade later. The slope is at least partly due to the molecule’s chemical formation. Sulfur isotope ratios are plotted in a similar way; the standard is an iron sulfide mineral called Diablo Canyon Troilite, which is not native to Earth, but is found in a fragment of the meteorite that created Arizona’s Meteor Crater.

Schematic diagram of the solar nebula on top of an artist's rendering. The protosun evolved in a hot nebula of infalling gas and dust that formed an accretion disk (green) of surrounding matter. Visible and ultraviolet light poured from the sun, irradiating abundant clouds of carbon monoxide, hydrogen sulfide, and other chemicals. Temperatures near the sun were hot enough to melt silicates and other minerals, forming the chondrules found in early meteoroids (dashed black circles). Beyond the “snowline” (dashed white curves), water, methane, and other compounds condensed to ice. Numerous chemical reactions contributed to the isotopic ratios seen in relics of the early solar system today.

Oxygen and sulfur are the third and tenth most abundant elements in the solar system, respectively, and two of the most important for life. The isotope ratios of these elements are different whether found on Earth or a meteorite. Five years ago, the UCSD team used Beamline 9.0.2 to test a theory called “self-shielding” about why oxygen-16 is less prevalent in these relics of the primitive solar system than it is in the sun, which contains 99.8 percent of all the mass in the solar system. To their surprise, the experimental results showed that self-shielding could not resolve the oxygen-isotope puzzle. (Read more about this work.)

In their most recently published work, the group performed VUV experiments on sulfur, using the results to build a model of chemical evolution in the primitive solar nebula that could yield the isotopic ratios of sulfur seen in meteorites.

Oxygen is the most abundant element on Earth; 99.762 percent of it is the isotope oxygen-16, with eight protons and eight neutrons. Oxygen-18 has two additional neutrons and accounts for another two-tenths of a percent; oxygen-17 (one extra neutron) provides less than four-hundredths of a percent. Sulfur, with four stable isotopes, is less abundant but essential to life. Sulfur-32 accounts for 95.02 percent of sulfur on Earth, 4.21 percent is sulfur‑34, 0.75 percent is sulfur-33, and a mere 0.02 percent is sulfur-36.

Changes in temperature and other physical factors can produce different isotope ratios. For example, the mass of the isotopes themselves—oxygen-18 is two neutrons heavier than oxygen-16. When the temperature rises, oxygen-16 evaporates faster, and when the temperature falls, oxygen-18 condenses faster. That’s why there’s a greater proportion of oxygen-18 in raindrops than in the clouds they fall from, for example.

Isotope-ratio researchers commonly graph these processes by plotting samples with increasing proportions of oxygen-18 relative to oxygen-16 along the Y axis; the X axis shows increasing proportions of oxygen-17 to oxygen-16. When comparing these three isotopes in almost any sample from Earth to an arbitrary standard called SMOW (standard mean ocean water), the proportions of the three always diverge at a rate that can be plotted along a line with a distinctive slope: about one-half.

Samples whose isotope ratios don’t fall on the slope-one-half line didn’t result from mass-dependent processes (see sidebar for examples). Mass-independent processes suggest chemical reactions, whether in the lab, the stratosphere, or the early solar system. In the proto-solar system, these reactions might have occurred on a grain of rock or ice or dust, or in just plain gas, bathed in intense ultraviolet light. The goal is to identify distinctive isotopic fractionations and examine the chemical pathways that could have produced them.

A three-isotope plot of oxygen begins with the Earth-standard mean ocean water ratio (SMOW). Departures from SMOW in the fractions of oxygen-17 (vertical axis) and oxygen-18 (horizontal axis) fall along a slope of about one-half, a sign of processes solely dependent on the differing isotope masses. In meteorites and other interplanetary sources, differing oxygen ratios fall on a different slope, signaling chemical processes as well as physical ones.

Like oxygen, sulfur isotopes show up in different fractions in different solar system sources. Tracing their possible origins, the recent study of sulfur isotopes at Beamline 9.0.2 began by flowing hydrogen sulfide—the most abundant sulfur-bearing gas in the early solar system—into a pressurized reaction chamber, where the synchrotron beam decomposed the gas and deposited elemental sulfur on “jackets” made of ultraclean aluminum foil.

The experiment was performed at four different VUV wavelengths, and the aluminum jackets were carefully taken to Thiemens’s lab at UCSD, where Chakraborty and Jackson chemically extracted the sulfur and measured its isotopes. In all samples the isotope compositions were found to be mass independent.

Tracking down how isotopic ratios may have evolved, learning the process behind each elements’ fractionation, adds to the knowledge base about the fundamental elements of the solar system. Sulfur compositions evolved independently from the way oxygen isotope compositions evolved. Sulfur’s fractionation in nature was due to the photodissociation of hydrogen sulfide as the gas condensed to iron sulfide in the inner solar system, driven by intense 121.6-nanometer-wavelength ultraviolet light as the young star repeatedly shook with violent flares and upheavals.

The most recent target of this research is nitrogen, the seventh most abundant element in the solar system. On Earth, 99.63 of nitrogen is nitrogen-14, and nitrogen-15 comprises the remaining 0.37 percent. Measurements of the solar wind, carbonaceous meteorites, and other sources show wide swings in their proportions. The work is ongoing.

 

Watch a video about molecular beam mass spectrometry with tunable VUV synchrotron radiation, published by JoVE.


 

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

Research funding: This work was funded by NASA’s Origins and Cosmochemistry programs. Operation of the ALS is supported by the DOE Office of Basic Energy Sciences.

Publication about this research: S. Chakraborty, T.L. Jackson, M. Ahmed, and M.H. Thiemens, "Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulfide and its potential relevance to meteorite analysis," PNAS (2013).

ALS Science Highlight #267

 

ALSNews Vol. 340