Picking Pieces of Supernova Grit Out of Meteorites
|A silicon carbide grain is only a few microns across, smaller than a yeast cell or red blood cell, but it has traveled across space and time bearing the secrets of its parent star within it. Courtesy of Scott Messenger
Ernst K. Zinner, PhD, research professor of physics and earth and planetary sciences in Arts & Sciences has received a three-year, $1,380,000 grant from the National Aeronautics and Space Administration (NASA) to study presolar grains in a sample of the Murchison meteorite, a primitive meteorite that fell to Earth near the town of Murchison, Australia, in 1969.
Presolar grains are literally tiny bits of stars — stardust — that were born and died billions of years ago, before the formation of the solar system.
From a generous chunk of the meteorite, Zinner hopes to extract exceptionally large grains that came from supernovae, giant stars that exploded at the ends of their lives. The larger grains will allow him to make more comprehensive measurements and, in turn, achieve a clearer understanding of what happened in these long-extinct stars — where most of the elements that make up our bodies and our Earth were forged.
Until the 1960s, most scientists believed that the early solar system got so hot that presolar material could not have survived intact. However, in the mid-1960s, researchers started finding unusual isotopic ratios of the noble gases neon and xenon in certain types of meteorites. The fact that these volatile gases were still there suggested that they were trapped in very refractory (heat-resistant) mineral grains.
In 1987, Ed Anders and his co-workers at the University of Chicago and Zinner and his colleagues at WUSTL succeeded in identifying diamond and silicon carbide as the noble gas carriers. This was achieved by dissolving meteorites in acid, a method described by Anders as “burning down the haystack to find the needle.”
Presolar grains are very small, typically only a few millionths of a meter across, so sophisticated instruments are needed to study them. Zinner will be using an ion microprobe, a type of Secondary Ion Mass Spectrometer, or SIMS, instrument that achieves high spatial resolution by using a finely focused ion beam. Zinner himself developed many of the techniques that allow the microprobe to perform such precise analytical work.
SIMS works by sandblasting a sample and passing the electrically charged debris that comes flying off through electric and magnetic fields that sort it by mass. The masses, in turn, identify individual elements and their isotopes.
The isotopic compositions of the grains allow the scientists to understand the evolution of the stars from which the grains originated, especially the nuclear processes that created the elements of which the grains consist.
“What I want to do in this project,” Zinner says, “is to locate as many supernova grains as possible that are large enough that we can do measurements of many different elements.
“Presolar grains have survived in the Murchison meteorite,” Zinner says, “because it is primitive, or unprocessed. It is a piece of an asteroid that was small enough that the rock never melted or separated according to density.
“We’ll extract the silicon carbide grains by using a series of acids to dissolve away the rest of the meteorite. It’s a simple process,” he says, “but it took 20 years to figure out it was possible.
“We’ll start with half a kilogram of Murchison, which is a lot,” he says. “Usually people don’t want to give you more than a few grams of a meteorite. But fortunately quite a lot of material fell at Murchison, about 200 kilograms, so we could obtain a large amount of it.”
The silicon carbide grains are only a small fraction of the meteorite, and Zinner wants to select only the biggest of them, those that are five microns in diameter or bigger. Once he has his big grains, he’ll separate those originating from supernova from those originating in red giants.
This will be done by isotopic analysis, he explains. One of the silicon isotopes is mostly made in supernovae, he says, and by looking at the silicon isotopic composition, the ion probe can sort the grains automatically.
“In short,” he says, “the process will be to dissolve the meteorite, then collect the silicon carbide grains, then separate the silicon carbide grains according to size, and finally analyze the isotopic composition of the big grains to find the supernova grains.”
“We hope to end up with roughly 10,000 five-micron grains and of those perhaps 100 will be supernova grains,” Zinner says.
Only then will the real work begin. Because supernova grains are rare and most are very small, studies of supernovae grains have so-far yielded only patchy information. By working with large grains Zinner hopes to be able to make much more comprehensive measurements.
If he’s lucky, he says, his findings will narrow the constraints on theoretical models of supernovae and the production of elements within them. The Big Bang created only the lightest elements, hydrogen and helium, he says. Red giants make the elements up through oxygen, but the oxygen remains in the star when it ejects its outer layers and becomes a white dwarf. Most of the oxygen in the solar system and all of the elements heavier than oxygen were created in the bellies of supernovae, some in the death throes of these stars by a process called explosive nucleosynthesis.
It is hard to imagine science with more profound implications; if the elements made in the nuclear furnaces of supernovae did not exist, we would not exist. We are all, as has been famously said, made of stardust.